- Holo and Kite, The spatial signature of a changing impactor population for Mars, Icarus, 2019.
Using (i) a global database of Mars impact craters and (ii) angular two-point correlation statistics to quantify local to regional crater clustering, we found that degraded craters on low-lying middle-Noachian-highland terrains were subject to spatially-patchy crater obliteration by surface processes, while older, higher-standing craters on the early Noachian highlands remain consistent with being drawn from a spatially uniform distribution. This result supports the hypothesis that the multi-sloped Noachian CSFD results from an early impactor population that changed during the Noachian, rather than from extensive obliteration of craters < 32 km in diameter by surface processes such as fluvial erosion, volcanic flooding, and aeolian infilling. The cause of the change in impactor populations remains unknown.
- Warren, Kite, Williams, and Horgan, Through the thick and thin: New constraints on Martian paleopressure history 3.8-4 Ga from small exhumed craters, JGR-Planets, 2019.
Mars' climate history depends in part on its atmospheric pressure evolution, but most existing constraints on atmospheric pressure are indirect. Thin atmospheres allow small objects to reach the surface and form impact craters, therefore ancient impact craters can constrain past atmospheric pressure. To identify ancient craters preserved in sedimentary rocks and exhumed by wind erosion we use HiRISE orthoimages, anaglyphs, and digital terrain models (DTMs). We compare measured crater populations from two sites to predictions from an atmosphere-impactor interaction model for atmospheres of different pressures. Our upper limits on continuous atmospheric pressure are 1.9±0.1 bar around 4 Ga and 1.5±0.1 bar at 3.8±0.2 Ga. We demonstrate that atmospheric pressure cannot have been continuously above these upper limits. During the interval 3.8±0.2 Ga, our crater counts require that atmospheric pressure was less than 5% of Earth's modern pressure for at least 10^4 yrs, or at higher pressure for a correspondingly longer duration of time (at least 10^5-10^6 yrs at 1.5 bar for our Mawrth phyllosilicates and Meridiani Planum data respectively). Therefore, atmospheric pressure around 4 Ga was either continuously 1.9±0.1 bar, or varied between higher (>1.9 bar) and lower (<1.9 bar) pressures. Similarly, atmospheric pressure at 3.8±0.2 Ga was either continuously 1.5±0.1 bar, or varied between higher (>1.5 bar) and lower (<1.5 bar) pressures. Finally, we synthesize all available paleopressure estimates for early Mars to constrain a 2-component model of Mars' long-term atmospheric pressure evolution. In our model, atmospheric pressures <1 bar early in Mars' history best fit existing paleopressure constraints.
- Stucky de Quay, Kite, and Mayer, Prolonged fluvial activity from channel-fan systems on Mars, JGR-Planets, 2019.
Alluvial fans on Mars, which are primarily sourced from erosional alcoves incised into crater rims, record a period of increased surface runoff which ended >1 Ga. However, we lack quantitative constraints on the frequency and duration of river-forming processes and the climatic conditions which accompanied these long-term habitable episodes. Here we use bedrock erosion and sediment transport models to show that cumulative time span of wet activity was between 100 yr - 1 Myr. We use Context Camera (CTX) digital elevation models to compile a dataset of >200 channels upstream of depositional fans and determine key fluvial geometry metrics. Results from calculating Mars stream power parameters are compared to great escarpment channels on Earth and globally distributed terrestrial bedrock rivers. Although Martian channel profile morphologies fall within the range of those on Earth, they are slightly less concave-up and steeper for a given drainage area. Timescales depend strongly on poorly constrained variables such as erodability and grain size. Channel morphologies, intermittencies, spatial distributions, and orientations collectively suggest an arid climate and a source from snowmelt on steep crater rims, possibly from obliquity-paced insolation variations or orographic accumulation. Derived timescales are consistent with erosion rates and intermittencies observed on Earth and do not support short-lived or catastrophic sources, instead attesting to long-lived, recurring wet activity.
- Kite, Fegley, Schaefer, and Ford, Superabundance of exoplanet sub-Neptunes explained by fugacity crisis, in review.
Transiting planets with radii 2-3 Earth radius are much more numerous than larger planets. We propose that this drop-off is so abrupt because at approximately 3 Earth radius, base-of-atmosphere pressure is high enough for the atmosphere to readily dissolve into magma, and this sequestration acts as a strong brake on further growth. The viability of this idea is demonstrated using a simple model. Our results support extensive magma-atmosphere equilibration on sub-Neptunes, with numerous implications for sub-Neptune formation and atmospheric chemistry.
- Kite, Fegley, Schaefer, and Ford, Atmosphere origins for exoplanet sub-Neptunes, in review.
Planets with radii 2-3 Earth radius and orbital period <100 d are abundant; these sub-Neptune exoplanets are not well understood. For example, Kepler sub-Neptunes are likely to have deep magma oceans in contact with their atmospheres, but little is known about the effect of the magma on the atmosphere. Here we study this effect using a basic model. We consider the two atmosphere sources -- nebular gas, and volatiles outgassed from solids -- and assume that atmosphere equilibrates with magma at T ~ 3000 K. For our Fe-Mg-Si-O-H model system, we find that chemical reactions between the magma and the atmosphere and dissolution of volatiles into the magma are both important. Thus, magma matters. Examples include: (1) Most H goes into the magma, so accreted volatile mass exceeds atmospheric mass. (2) Turning a sub-Neptune into a Super-Earth requires more H loss than is usually assumed. The extra demand on H loss may stress-test H loss models. (3) Atmosphere H2O/H2 ratio is proportional to magma FeO content. (4) An atmosphere mass of 0.3 wt% can be explained by a range of total volatile masses varying by 100-fold. Thus, planet radius is a proxy for atmospheric composition but not for total volatile content. The partitioning of volatiles between atmosphere and magma varies depending on formation processes and especially on magma oxidation state. (5) >10^3-km thick outgassed atmospheres have high mean molecular weight, so outgassing probably cannot explain sub-Neptune radii. These conclusions would be strengthened by including other volatile element ratios, such as C/O. The hypothesis of magma-atmosphere equilibration makes novel, testable predictions. Our model's accuracy is limited by the lack of experiments (lab and/or numerical) that are specific to sub-Neptunes; we~advocate for such experiments.
- Kite, Mayer, Wilson, Davis, Lucas, and Stucky de Quay, Persistence of intense, climate-driven runoff late in Mars history, Science Advances, 2019.
Mars dry today, but numerous precipitation-fed paleo-rivers are found across the planet's surface. These rivers' existence is a challenge to models of planetary climate evolution. We report results from a global survey indicating that for a given catchment area, rivers on Mars were wider than rivers on Earth today. This difference is not the result of Mars' lower surface gravity. We use the scale (width and wavelength) of Mars paleo-rivers as a proxy for past runoff production. Using multiple methods, we infer that intense >(3-20) kg/m2/day runoff production persisted until <3 Ga and probably <1 Ga, and was globally distributed. Therefore, the intense runoff production inferred from the results of the Mars Science Laboratory rover was not a short-lived or local anomaly. Rather, precipitation-fed runoff production was globally distributed, intense, and persisted intermittently over a time span of >1 Gyr. Our improved history of Mars river runoff places new constraints on the unknown mechanism that caused wet climates on Mars.
- de Kleer, Nimmo, and Kite, Variability in Io's volcanism on timescales of periodic orbital changes, Geophysical Research Letters, 2019.
[New York Times]
The widespread volcanism on the jovian moon Io is powered by tidal heating, yet we lack a deep understanding of how this distinctive heating process affects the locations, timing, or intensities of Io's eruptions. We show that the quasiperiodic behavior of the volcano Loki Patera in 1987-2018 matches the timescales for the evolution of Io's eccentricity and semimajor axis (~480 and ~460 days). If this orbital forcing is driving Loki Patera's variability, a low-pass geophysical filter such as poroelastic flow, or a resonant amplification of Io's wobble, could account for the importance of these long-period orbital variations despite their small amplitudes. The peak volcanic response is predicted to roughly coincide with Io's maximum eccentricity, consistent with the observations. High cadence observations over the next several years have the potential to conclusively discriminate between orbital versus geophysical control of Loki Patera's variability.
- Kite, Geologic constraints on Early Mars climate, Space Science Reviews, 2019.
Early Mars climate research has well-defined goals (Mars Exploration Program Analysis Group 2018). Achieving these goals requires geologists and climate modelers to coordinate. Coordination is easier if results are expressed in terms of well-defined parameters. Key parameters include the following quantitative geologic constraints. (1) Post-3.4 Ga precipitation-sourced water runoff in some places exceeded > 1 km column. (2) There is no single Early Mars climate problem: the traces of ≥ 2 river-forming periods are seen. Relative to rivers that formed earlier in Mars history, rivers that formed later in Mars history are found preferentially at lower elevations, and show a stronger dependence on latitude. (3) The duration of the longest individual river-forming climate was >(102 - 103) yr, based on paleolake hydrology. (4) Peak runoff production was >0.1 mm/hr. However, (5) peak runoff production wasitermittent, sustained (in a given catchment) for only <10% of the duration of river-forming climates. (6) The cumulative number of wet years during the valley-network-forming period was >105 yr. (7) Post-Noachian light-toned, layered sedimentary rocks took >107 yr to accumulate. However, (8) an "average" place on Mars saw water for <107 yr after the Noachian, suggesting that the river-forming climates were interspersed with long globally-dry intervals. (9) Geologic proxies for Early Mars atmospheric pressure indicate pressure was not less than 0.012 bar but not much more than 1 bar. A truth table of these geologic constraints versus currently published climate models shows that the late persistence of river-forming climates, combined with the long duration of individual lake-forming climates, is a challenge for most models.
- Kite and Melwani Daswani, Geochemistry constrains global hydrology on Early Mars, Earth and Planetary Science Letters, 2019.
Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars' Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from 2004-2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars' CO2 atmosphere implies subsurface formation of carbonate. We find that the CO2 sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30-5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO2 sequestration is 0.3-400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.
- Mansfield, Kite, Hu, Koll, Malik, Bean, and Kempton, Identifying atmospheres on rocky exoplanets through inferred high albedo, Astrophysical Journal (in press), 2019.
The upcoming launch of the James Webb Space Telescope (JWST) means that we will soon have the capability to characterize the atmospheres of rocky exoplanets. However, it is still unknown whether such planets orbiting close to M dwarf stars can retain their atmospheres, or whether high-energy irradiation from the star will strip the gaseous envelopes from these objects. We present a new method to detect a rocky exoplanet atmosphere, by using thermal emission during secondary eclipse to infer a high dayside albedo that could only be explained by bright clouds. Based on calculations for plausible surface conditions, we calculate that a high albedo could be unambiguously interpreted as a signal of an atmosphere for planets with substellar temperatures of Tss = (420-1250) K. This range corresponds to equilibrium temperatures of Teq = (300-880) K. We compare the inferred albedos of eight possible planet surface compositions to cloud albedo calculations. We determine that a layer of small-particle (<0.1 μm radius) clouds with optical depth greater than τ= 0.1 - 0.9, or larger-particle clouds with optical depths greater than τ = 3-6, would have high enough albedos to be distinguishable from a bare rock surface. This method of detecting an atmosphere on a rocky planet is complementary to existing methods for detecting atmospheres,
because it provides a way to detect atmospheres with pressures below 1 bar (e.g. Mars), which are too tenuous to transport significant heat but thick enough to host high-albedo clouds.
- Koll, Malik, Mansfield, Kempton, Kite, Abbot, and Bean, Identifying candidate atmospheres on rocky M dwarf planets via eclipse photometry, Astronomical Journal (in press), 2019.
Most rocky planets in the galaxy orbit a cool host star, but there is large uncertainty among theoretical models whether these planets are able to retain an atmosphere. The James Webb Space Telescope
(JWST) might be able to settle this question empirically, but most proposals for doing so are based
on spectroscopy and require large observational effort. Here we show that infrared photometry of
secondary eclipses could quickly identify 'candidate' atmospheres, by searching for rocky planets with
atmospheres thick enough that atmospheric heat transport noticeably reduces their dayside thermal
emission compared to that of a bare rock. Assuming a nearby rocky planet amenable to atmospheric
characterization, we find that JWST should be able to confidently detect the heat redistribution signal
of an O(1) bar atmosphere with a single eclipse. A single eclipse is generally much less than the effort
needed to infer an atmosphere on the same planet via transmission or emission spectroscopy. Candidate atmospheres can be further validated via follow-up spectroscopy or phase curves. However, even
without further validation, infrared photometry with JWST could enable a first atmospheric survey
of rocky exoplanets that are too hot to be habitable. Knowing whether hot, rocky planets around
M dwarfs have atmospheres is important not only for understanding the evolution of uninhabitable
worlds: if atmospheres are common on hot planets, then cooler, potentially habitable planets around
M dwarfs are also likely to have atmospheres.
- Malik, Kempton, Koll, Mansfield, Bean, and Kite, Analyzing Atmospheric Temperature Profiles and Spectra of M-Dwarf Rocky Planets, 2019, accepted by Astrophysical Journal.
The highly anticipated launch of the James Webb Space Telescope (JWST) will open up the possibility of comprehensively measuring the thermal emission spectra of rocky exoplanets orbiting M dwarfs to detect and characterize their atmospheres. In preparation for this opportunity, we present model atmospheres for three terrestrial M-dwarf planets particularly amenable to secondary eclipse spectroscopy --- TRAPPIST-1b, GJ 1132b, and LHS 3844b.
Using three limiting cases of candidate atmospheric compositions (pure H2O, pure CO2 and solar abundances) we calculate temperature-pressure profiles and emission and reflection spectra in radiative-convective equilibrium, including the effects of a solid surface at the base of the atmosphere. Our results differ appreciably from simpler parameterized models of super-Earth atmospheres in terms of the overall temperatures and the temperature gradient, which has important observational consequences. We find that the atmospheric radiative transfer is significantly influenced by the cool M-star irradiation; H2O and CO2 absorption bands in the near-infrared are strong enough to absorb a sizeable fraction of the incoming stellar light at low pressures, which leads to temperature inversions in the upper atmosphere. The non-gray band structure of gaseous opacities in the infrared is hereby an important factor. Opacity windows are muted at higher atmospheric temperatures, so we expect temperature inversions to be common only for sufficiently cool planets. We also find that pure CO2 atmospheres exhibit lower overall temperatures and stronger reflection spectra compared to models of the other two compositions. We estimate that for GJ 1132b and LHS 3844b we should be able to distinguish between different atmospheric compositions with JWST. The emission lines from the predicted temperature inversions are currently hard to measure, but high resolution spectroscopy with future Extremely Large Telescopes may be able to detect them.
- Kite, Steele, and Mischna, Aridity enables warm climates on Mars, in revision.
A key unknown in the search for habitable planets is the minimum insolation for sustained surface liquid water. Despite receiving just 30% of the Earth's present-day insolation, Mars had water lakes early in the planet's history, due to an unknown warming mechanism. Most proposed warming mechanisms fail to match the geologic record of >>10^2 yr-long lake-forming climates that persisted as late as <3 Ga (Kite 2019, Haberle et al. 2017). A possible exception is warming by water ice clouds (Segura et al. 2008, Urata & Toon 2013). But this cloud greenhouse has proven difficult to replicate, and has been argued to require unrealistically high-altitude optically thick clouds (Wordsworth 2016, Ramirez & Kasting 2017). Here we use a global climate model (GCM) to show that a water ice cloud greenhouse can warm a Mars-like planet to area-averaged temperature >290 K from a cold, dry start, and stay warm for centuries or longer, but only if the planet is arid. Arid, warm, stable climates involve vapor equilibrium with surface ice only at locations much colder than the planet average, so that the high altitudes of clouds elsewhere maximize warming. Cloud coverage is close to complete because ice sublimates as fall streaks, allowing modest updrafts to sustain volumetrically tenuous, but optically thick clouds. In a warm, arid climate, lakes could be fed by groundwater upwelling, or by melting of ice following a cold-to-warm transition. Our result closes the gap between GCMs and the warm and arid climate favored by interpretation of geologic data (Grotzinger et al. 2014, Ramirez & Craddock 2018). Unexpectedly, partial drying-out of Mars' surface may have been a pre-requisite for the planet's habitability under the faint young Sun.
- Heard and Kite, A probabilistic case for a large missing carbon sink on Mars after 3.5 billion years ago, minor revisions requested.
Mars has a thin (6 mbar) CO2 atmosphere currently. There is strong evidence for paleolakes and rivers formed by warm climates on Mars, including after 3.5 billion years (Ga) ago, which indicates that a CO2 atmosphere thick enough to permit a warm climate was present at these times. Since Mars no longer has a thick CO2 atmosphere, it must have been lost. One possibility is that Martian CO2 was lost to space. Oxygen escape rates from Mars are high enough to account for loss of a thick CO2 atmosphere, if CO2 was the main source of escaping O. But here, using H isotope ratios, O escape calculations, and quantification of the surface O sinks on Mars, we show for the first time that O escape from Mars after 3.5 Ga must have been predominantly associated with the loss of H2O, not CO2, and therefore it is unlikely that >=250 mbar Martian CO2 has been lost to space in the last 3.5 Ga, because such results require highly unfavored O loss scenarios. It is possible that the presence of young rivers and lakes on Mars could be reconciled with limited CO2 loss to space if crater chronologies on Mars are sufficiently incorrect that all apparently young rivers and lakes are actually older than 3.5 Ga, or if climate solutions exist for sustained runoff on Mars with atmospheric CO2 pressure <250 mbar. However, our preferred solution to reconcile the presence of <3.5 Gya rivers and lakes on Mars with the limited potential for CO2 loss to space is a large, as yet undiscovered, geological C sink on Mars.
- Kite, Mischna, Gao, Yung, and Turbet, Methane release on Early Mars by atmospheric collapse and atmospheric reinflation, in review.
A candidate explanation for Early Mars rivers is atmospheric warming due to surface release of H2 or CH4 gas. However, it remains unknown is how much gas could be released in a single event. We model the CH4 release by one mechanism for rapid release of CH4 from clathrate. By modeling how CH4-clathrate release is affected by changes in Mars' obliquity and atmospheric composition, we find that a large fraction of total outgassing from CH4 clathrate occurs following Mars' first prolonged atmospheric collapse. This atmosphere-collapse-initiated CH4-release mechanism has three stages. (1) Rapid collapse of Early Mars' carbon dioxide atmosphere initiates a slower shift of water ice from high ground to the poles. (2) Upon subsequent CO2-atmosphere re-inflation and CO2-greenhouse warming, low-latitude clathrate decomposes and releases methane gas. (3) Methane can then perturb atmospheric chemistry and surface temperature, until photochemical processes destroy the methane.
Within our model, we find that under some circumstances a Titan-like haze layer would be expected to form, consistent with transient deposition of abundant complex abiotic organic matter on the Early Mars surface. We also find that this CH4-release mechanism can warm Early Mars, but special circumstances are required in order to uncork 10^17 kg of CH4, the minimum needed for strong warming. Specifically, strong warming only occurs when the fraction of the hydrate stability zone that is initially occupied by clathrate exceeds 10%, and when Mars' first prolonged atmospheric collapse occurs for atmospheric pressure > 1 bar.
- Yin, Deng, Kite, and Day, Formation of honeycomb-like landform on Mars explained by horizontal compression, in review.
- Kite and Ford, Habitability of exoplanet waterworlds, Astrophysical Journal, 2018.
Many habitable zone exoplanets are expected to form with water mass fractions higher than that of the Earth. For rocky exoplanets with 10-1000x Earth's H2O but without H2, we model the multi-Gyr evolution of ocean temperature and chemistry, taking into account C partitioning, high-pressure ice phases, and atmosphere-lithosphere exchange. Within our model, for Sun-like stars, we find that: (1) the duration of habitable surface water is strongly affected by ocean chemistry; (2) possible ocean pH spans a wide range; (3) surprisingly, many waterworlds retain habitable surface water for >1 Gyr, and (contrary to previous claims) this longevity does not necessarily involve geochemical cycling. The key to this cycle-independent planetary habitability is that C exchange between the convecting mantle and the water ocean is curtailed by seafloor pressure on waterworlds, so the planet is stuck with the ocean mass and ocean cations that it acquires during the first 1% of its history. In our model, the sum of positive charges leached from the planetary crust by early water-rock interactions - coincidentally - often within an order of magnitude of the early-acquired atmosphere+ocean inorganic C inventory overlaps. As a result, pCO2 is frequently in the "sweet spot" (0.2-20 bar) for which the range of semimajor axis that permits surface liquid water is about as wide as can be. Because the width of the HZ in semimajor axis defines (for Sun-like stars) the maximum possible time span of surface habitability, this effect allows for Gyr of habitability as the star brightens. We illustrate our findings by using the output of an ensemble of N-body simulations as input to our waterworld evolution code. Thus (for the first time in an end-to-end calculation) we show that chance variation of initial conditions, with no need for geochemical cycling, can yield multi-Gyr surface habitability on waterworlds.
- Holo, Kite, and Robbins, Mars obliquity history constrained by elliptic crater orientations, Earth and Planetary Science Letters, 2018.
[news coverage in Science]
The dynamics of Mars' obliquity are chaotic, and thus the historical ~3.5 Gyr obliquity probability density function (pdf) is highly uncertain and cannot be inferred from direct simulation alone. Obliquity is also a strong control on post-Noachian Martian climate, enhancing the potential for equatorial ice/snow melting and runoff at high obliquities (>40°) and enhancing the potential for wicking groundwater to the surface at low obliquities (<25°). We used the orientations of elliptical craters to constrain the true late-Hesperian onward obliquity pdf. To do so, we developed a forward model of the effect of obliquity on elliptic crater orientations using ensembles of simulated Martian impactors and ~ 3.5 Gyr-long Mars obliquity simulations. Comparison with a verified global database of elliptic crater orientations allowed us to invert the model for the best fit obliquity pdf. While several obliquity pdf's are consistent with the data, we computed weighted estimates for the mean late-Hesperian onward obliquity and for the fraction of that time spent with obliquity > 40°. Mean obliquity was likely low ~30°, and obliquity was high < 30% of the time. We rejected at the p = 0.025 level that mean obliquity was > 37° and that the obliquity was high > 41% of the time since the beginning of the late-Hesperian. We failed to reject low (~ 15°) mean obliquity solutions.
- Seybold, Kite, and Kirchner, Branching geometry of valley networks on Mars and Earth and its implications for early Martian climate, Science Advances, 2018.
["Research Highlight" in Nature]
Mars' surface bears the imprint of valley networks formed billions of years ago and their relicts can still be observed today. However, whether these networks were formed by groundwater sapping, ice melt, or fluvial runoff has been continuously debated. These different scenarios have profoundly different implications for Mars' climatic history, and thus for its habitability in the distant past. Recent studies on Earth revealed that channel networksi arid landscapes with more surface runoff branch at narrower angles, while in humid environments with more groundwater flow, branching angles are much wider. We find that valley networks on Mars generally tend to branch at narrow angles similar to those found in arid landscapes on Earth. This result supports the inference that Mars once had an active hydrologic cycle and that Mars' valley networks were formed primarily by overland flow erosion with groundwater seepage playing only a minor role.
- Mansfield, Kite, and Mischna, Effect of Mars atmospheric loss on snow melt potential in a 3.5-Gyr climate evolution model, JGR-Planets, 2018.
Post-Noachian Martian paleochannels and fluvial deposits suggest the presence of liquid water on the surface of Mars after about 3.5 Gya, but by this time conditions most favorable to melting no longer existed. We created a zero-dimensional surface energy balance model to explore what conditions could have led to surface liquid water. We combine the energy balance model with 3.5-Gyr physically consistent orbital histories to track melting conditions over the last 3.5 Gyr of Martian history. We find that melting is only allowed for high atmospheric pressures corresponding to exponential loss rates of dP/dt \propto (1/t)^-4.2 or faster, but that small amounts of melting are produced for a rate of atmospheric loss that is within one standard deviation of the rate calculated from initial measurements made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. For loss rates within two standard deviations of the initial MAVEN results, enough melting is produced to match geologic constraints on the formation of Hesperian river networks, assuming optimal surface properties such as albedo and thermal inertia, and assuming meting only occurs during the warmest part of the warmest season each Mars year. We also find that the rate of atmospheric loss has a larger effect on the surface energy balance than changes in Mars's mean obliquity.
- Steele, Kite, and Michaels, Crater mound formation by wind erosion on Mars, Journal of Geophysical Research - Planets, 2018.
Most of Mars' ancient sedimentary rocks by volume are in wind-eroded sedimentary mounds, but the connections between mound form and wind erosion are unclear. In particular, no numerical model has shown whether, or how, mounds can be formed. We perform mesoscale simulations of different crater and mound morphologies to understand the formation of sedimentary mounds. As crater depth increases, slope winds produce increased erosion near the base of the crater wall, forming mounds. Peak erosion rates occur when the crater depth is 2 km. Thus, for the first time, our results use a physically self-consistent model to show how a mound can emerge from initially flat infill. Mound evolution depends on the size of the host crater. In smaller craters mounds preferentially erode at the top, becoming more squat, while in larger craters mounds become steeper-sided. This agrees with observations where smaller craters tend to have proportionally shorter mounds, and larger craters have mounds encircled by moats. If a large-scale sedimentary layer blankets a crater, then as the layer recedes across the crater it will erode more towards the edges of the crater, resulting in a crescent-shaped moat. When a 160 km diameter mound-hosting crater is subject to a prevailing wind, the surface wind stress is stronger on the leeward side than on the windward side. This results in the mound 'marching upwind' over time, and forming a 'bat-wing' shape, as observed for Mt. Sharp in Gale crater.
- Gabasova and Kite, Compaction and sedimentary basin analysis on Mars, Planetary & Space Science, 2018.
Many of the sedimentary basins of Mars show patterns of faults and off-horizontal layers that, if correctly
understood, could serve as a key to basin history. Sediment compaction is a possible cause of these patterns.
We quantified the possible role of differential sediment compaction for two Martian sedimentary basins: the
sediment fill of Gunjur crater (which shows concentric graben), and the sediment fill of Gale crater (which
shows outward-dipping layers). We assume that basement topography for these craters is similar to the
present-day topography of complex craters that lack sediment infill. For Gunjur, we find that differential
compaction produces maximum strains consistent with the locations of observed graben. For Gale, we were
able to approximately reproduce the observed layer orientations measured from orbiter image-based digital
terrain models, but only with a >3 km-thick donut-shaped past overburden. It is not immediately obvious
what geologic processes could produce this shape.
- Kite, Gaidos and Onstott, Valuing life detection missions, Astrobiology, 2018.
Recent discoveriesiply that Early Mars was habitable for life-as-we-know-it (Grotzinger et al. 2014); that Enceladus is habitable (Waite et al. 2017); and that many stars have Earth-sized exoplanets whose insolation favors surface liquid water (Dresig & Charbonneau 2013, Gaidos 2013). These exciting discoveries make it more likely that spacecraft now under construction - Mars 2020, ExoMars rover, JWST, Europa Clipper - could find habitable, or formerly habitable, environments. Did these environments see life? Given finite resources ($10bn/decade for the US ), how could we best test the hypothesis of a second tree of life? Here, we first state the case for and against flying life detection missions soon. Next, we assume that life detection missions will happen soon, and propose a framework for comparing the value of different life detection missions:
Scientific value = (Reach x grasp x certainty x payoff) / $
After discussing each term in this framework, we conclude that scientific value is maximized if life detection missions are set up as hypothesis tests. With hypothesis tesig, even a nondetection outcome is scientifically valuable.
- Spencer, Nimmo, Ingersoll, Hurford, Kite, Rhoden, Schmidt, and Howett, Plume Origins and Plumbing (Ocean to Surface), in Schenk et al. (eds.) Enceladus and the Icy Moons of Saturn, Univ. Ariz. Press, 2018.
The plume of Enceladus provides a unique window into subsurface processes the ice shell and ocean of an icy world. Thanks to a decade of observations and modeling, a coherence picture is emerging of a thin ice shell extending across the south polar region, cut through by fractures directly connected to the underlying ocean, and at least partially filled with water. The plume jets emerging from the fractures directly sample this water reservoir. The shell undergoes daily tidal flexing, which modulates plume activity by opening and closing the fractures. Dissipation in the ice and conduit water components due to this flexing is likely to generate the several Gigawatts of obsevred power, which is lost from the south pole as infrared radiation and plume latent heat.
- Archer, Kite, and Lusk, The ultimate social cost of carbon, in review.
We estimate the potential ultimate social cost of fosi-fuel carbon to all future human
generations throughout the 200-kyr duration of the climate impacts from fossil carbon
combusin. Costs are integrated through time assuming continued human reliance on
our biological habitat, and that each generation of humanity valuesis world as much
as we do. The impact of climate on near-future GDP from economic models, combined
with long-term temperature estimates from climate and geochemical cycle models,
integrates to an eventual cost of about $20,000 per ton of CO2, up to $100,000 per ton
if future human population and economic activity eventually scales with Earth's
agricultural production capacity. Eventual sea level rise of order 50 meters has the
potential to reduce the extent of human habitat by about 10%, for an interval of time
that will be determined by soil formation and isostatic rebound response time scales, of
order 10 kyr. Assuming economic activity ultimately scales with habitat, the ultimate
cost of sea level rise could be of order $5,000 per ton. These costs together are three
orders of magnitude higher than the present-day value of the social cost of carbon
($20-50 per ton), and almost two orders more costly than the cost of atmospheric
removal of CO2 ($600 per ton for direct air capture).
- Kite, Gao, Goldblatt, Mischna, Yung, and Mayer, Methane bursts as a trigger for intermittent lake-forming climates on post-Noachian Mars, Nature Geoscience, 2017.
[News & Views]
["Research Highlight" in Nature]
[Los Angeles Times]
Build-up of relatively young (<3.6 Ga) deltas and alluvial fans on Mars required lakes to persist for >3 Kyr (assuming dilute flow), and the watersheds' little-weathered sisidicate a climate history that was >99% dry. However, the lake-forming climates' trigger mechanism remains unknown. Here we show that these intermittency constraints, while inconsistent with many previously-proposed triggers for lake-forming climates, are consistent with a novel CH4-burst mechanism. In this scenario, chaotic transitions in mean obliquity drive latitudinal shifts in temperature and ice loading that destabilize CH4 clathrate. We shows that outgassed CH4 builds up to levels whose radiative forcing is sufficient to modulate lake-forming climates for past clathrate hydrate stability zone occupancy fractions >0.04. Such occupancy fractions are consistent with CH4 production by >3 Ga water-rock reactions. Individual lake-forming climate are curtailed to <106 yr duration, consistent with data, by UV-limited CH4 photolysis. Our results identify a new pathway for early Mars to undergo intermittent excursions to a warm, wet climate state.
- Melwani Daswani and Kite, Paleohydrology on Mars constrained by mass balance and mineralogy of pre-Amazonian sodium chloride lakes, JGR-Planets, 2017.
Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars' wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 deg C system, we find: (1) individual lakes were >100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source wasineous chlorapatite, then Cl-leaching events would require a (cumulative) time of >10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 - 50 kg Cl/m2, and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.
- Kite, Sneed, Mayer, and Wilson, Persistent or repeated surface habitability on Mars during the Late Hesperian - Amazonian, Geophysical Research Letters, 2017.
Large alluvial fan deposits on Mars record the most recent undisputed habitable window of surface conditions (less than or similar to 3.5 Ga, Late Hesperian - Amazonian). We find net sedimentation rate <(4-8) microns/yr in the alluvial-fan deposits, using the frequency of craters that are interbedded with alluvial-fan deposits. Considering only the observed interbedded craters sets a lower bound of >20 Myr on the total time interval spanned by alluvial-fan aggradation, >103-fold longer than previous lower limits. A more realistic approach that corrects for craters fully entombed in the fan deposits raises the lower bound to >(100-300) Myr. Several factors not included in our calculations would further increase the lower bound. The lower bound rules out fan-formation by a brief climate anomaly. Therefore, during the Late Hesperian - Amazonian on Mars, persistent or repeated processes permitted habitable surface conditions.
- Kite and Mayer, Mars sedimentary rock erosion rates constrained using crater counts, with applications to organic-matter preservation and to the global dust cycle, Icarus, 2017.
Small-crater counts on Mars light-toned sedimentary rock are often inconsistent with any isochron; these data are usually plotted then ignored. We show (using an 18-HiRISE-image, >104 crater dataset) that these non-isochron crater counts are often well-fit by a model where crater production is balanced by crater obliteration via steady exhumation. For these regions, we fit erosion rates. We infer that Mars light-toned sedimentary rocks typically eroded at 102 nm/yr, when averaged over 10 km2 scales and 107-108 yr timescales. Crater-based erosion-rate determination is consistent with independent techniques, but can be applied to nearly all light-toned sedimentary rocks on Mars. Erosion is sit enough that radiolysis cannot destroy complex organic matter at some locations (e.g. paleolake deposits at SW Melas), but radiolysis is a severe problem at other locations (e.g. Oxia Planum). The data suggest that the relief of the Valles Marineris mounds is currently being reduced by wind erosion, and that dust production on Mars <3 Gya greatly exceeds the modern reservoir of mobile dust.
- Kite, Sneed, Mayer, Lewis, Michaels, Hore, and Rafkin, Evolution of major sedimentary mounds on Mars, JGR-Planets, 2016.
We present a new database of >300 layer-orientations from sedimentary mounds on Mars. These layer orientations, together with draped landsies, and draping of rocks over differentially-eroded paleo-domes, indicate that for the stratigraphically-uppermost ~1 km, the mounds formed by the accretion of dfraping strata in a mound-shape. The layer-orientation data further suggest that layers lower down in thestratigraphy also formed by the accretion of draping strata in a mound-shape. The data are consistent with terrain-influenced wind erosion, but inconsistent with tilting by flexure, differential compaction over basement, or viscoelastic rebound. We use a simple landscape evolution model to show how the erosion and deposition of mound strata can be modulated by obliquity. The model is driven by multi-Gyr calculations of Mars' chaotic obliquity and a parameterization of terrain-influenced wind erosion that is derived from mesoscale modeling. Our results suggest that mound-spanning unconformities with kilometers of relief emerge as the result of chaotic obliquity sits. Our results support the interpretation that Mars' rocks record intermittent liquid-water runoff during a >>100 Myr interval of sedimentary rock emplacement.
- Kite, Fegley, Schaefer, and Gaidos, Atmosphere-interior exchange on hot rocky exoplanets, Astrophysical Journal, 2016.
We provide estimates of atmospheric pressure and surface composition on short-period rocky exoplanets (with dayside magma pools and silicate vapor atmospheres). Atmospheric pressure tends toward vapor-pressure equilibrium with the surface composition, and surface composition is set by the balance between fractional vaporization and surface-interior exchange. We use basic models to show how surface-interior exchange is controlled by the planet's temperature, mass, and initial composition. We find:- (1) atmosphere-interior exchange is fast when the planet's bulk-silicate FeO concentration is low, and slow when FeO concentration is high; (2) magma pools are compositionally well-mixed for substellar temperatures less than or similar to 2400K, but compositionally variegated and rapidly variable for temperatures greater than or similar to 2400K; (3) currents within the magma pool tend to cool the top of the solid mantle (``tectonic refrigeration''); (4) contrary to earlier work, many magma planets have time-variable surface compositions.
- Kite and Rubin, Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes, Proceedings of the National Academy of Sciences, 2016.
Spacecraft observations suggest that the the geysers on Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. The prevailing view is that the 100km-long fissures ("tiger stripes") sourcing the geysers are clamped shut by tidal stresses for much of Enceladus' 1.3 day orbit, and that liquid-water conduits should freeze over within weeks, so that eruptions should be intermittent. However, observations show sustained (though tidally modulated) geysering throughout each orbit, and since the 2005 discovery of the plumes. Peak geyser flux lags peak tidal extension by 1 radian, suggestive of resonance. Here we show that a simple model of the tiger stripes as tidally-flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, the maintenance of fissure eruptions over geological timescales, and the total power output of the tiger stripe terrain. The delay associated with flusig and refilling of O(1) m-wide slots with ocean water generates a phase lag relative to tidal forcing and helps to buttress slots against closure, while tidally pumped in-slot flow leads to heating and mechanical disruption that staves off slot freeze-out. Much narrower and much wider slots cannot be sustained. In the presence of long-lived slots, the 106-yr average power output of the tiger stripe is buffered by a feedback between ice melt-back and subsidence to 5 GW, which is equal to the observed power output, suggesting long-term stability. Turbulent dissipation makes testable predictions for upcoming flybys by the Casii spacecraft. Turbulent dissipation in long-lived slots helps maintain the ocean against freezing, maintains access by future Enceladus missions to ocean materials, and is plausibly the major energy source for tiger stripe activity.
- Richter, Chaussidon, Mendybaev, and Kite, Reassessing the cooling rate and geologic setting of Martian nakhlite meteorites with special emphasis on MIL 03346 and NWA 817, Geochimica et Cosmochimica Acta, 2016.
Lithium concentration and isotopic fractionation profiles across augite grains from two
Martian meteorites - MIL 03346 and NWA 817 - were used to determine their thermal
history and implications for their geologic setting. The iron-magnesium zoning and
associated magnesium isotopic fractionation of olivine grains from NWA 817 was also
measured and provide a separate estimate of the cooling rate. The observed correlation of
concentration with isotopic fractionation provides the essential evidence that the zoning
of these grains was in fact due to diffusion and thus can be used as a measure of their
cooling rate. The diffusion rate of lithium in augite depends on the oxygen fugacity,
which has to be taken into account when determining a cooling rate based on the lithium
zoning. The Fe-Mg exchange in olivine is much less sensitive to oxygen fugacity, but it is
significantly anisotropic and for this reason we determined the direction relative to
crystallographic axes of the line along which the Fe-Mg zoning was measured. We found
that the cooling rate of NWA 817 determined from the lithium zoning in augite grains
and that based on the Fe-Mg zoning of olivines are in good agreement at an oxygen
fugacity close to that of quartz-fayalite-magnetite oxygen buffer, but not at the nickel-
nickel oxide buffer that was previously assumed and resulted in what we now believe was
a far too fast previous estimate of the cooling rate of NWA 817 based on the lithium
zoning. The cooling rate of MIL 03346 was found to be resolvably faster than that of
NWA 817 - of the order of 1 degrees/hr for the former and of the order of 0.1 degrees/hr for the latter.
An important observation regarding the history of MIL 03346 and NWA 817 is that the
lithium and Fe-Mg zoning are only observed where the augite or olivine is in contact with
the mesostasis, which implies that they were already about 80% crystallized at the time
diffusion began. The augite and olivine core compositions while very homogeneous are
not in equilibrium with each other, which we interpret to imply that prior to the rapid
cooling there must have been a protracted period of the order of years above the solidus,
during which the much faster Fe-Mg exchange in olivine compared to that in augite
allowed the olivine to maintain equilibrium with a changing melt composition while the
augite was not significantly affected. We suggest two possible geological settings for the
origin and evolution of MIL 03346 and NWA 817: (1) A slow cooling stage in a
crystallization front in a crustal magma chamber, followed by eruption of melt plus
portions of the crystallization front onto the surface where the final fast cooling took
place at the bottom of a lava flow or melt pond, and (2) Eruption of a crystal laden melt
as a thick long-lived lava flow where the crystals continued to grow as a cumulate and
were rapidly cooled when the overlying lava layer was suddenly drained.
- Ehlmann, and others including Kite, The sustainability of habitability on terrestrial planets: insights, questions, and needed measurements from Mars for understanding the evolution of Earth-like worlds, JGR-Planets, 2016.
- Borlina, Ehlmann, and Kite, Modeling the thermal and physical evolution of Mount Sharp's sedimentary rocks, Gale Crater, Mars, JGR-Planets, 2015.
Gale Crater, landing site of the Mars Science Laboratory (MSL), contains a central mound with 5 km of sedimentary stratigraphy (Aeolis Mons/Mt. Sharp). Understanding mound sedimentation, erosion, and diagenesis can constrain past geologic processes and the mound's organic preservation potential. Scenarios for mound fromation include: (1) complete filling of Gale followed by partial removal of sediments; (2) building of a central deposit with morphology controlled by slope winds and only incomplete sedimentary fill. Here we model sediment temperature-time paths for both scenarios, compare results with past MSL analyses, and provide scenario-dependent predictions of diagenesis along MSL's future traverse. Modeled erosion and deposition rates are 5-42 microns/yr, consistent with previously-published esiates. Evidence of diagenesis is expected, though spatial patterns and mineralogical predictions depend on Mars surface paleotemperature and sedimentation scenario. For (1), temperatures experienced by sediments should decrease monotonically over the traverse and up Mt. Sharp stratigraphy, whereas for (2) maximum temperatures are reached in the lower units of Mt. Sharp and thereafter decline or hold roughly constant. If early Mars surface temperatures were similar to modern Mars (mean: -50 degrees C), only select locations under select scenarios permit diagenetic fluids (T>0 degrees C). In contrast, if early Mars surface temperatures averaged 0 degrees C, diagenesis is predicted in most locations with maximum temperatures up to 150 degrees C. Comparing our predictions with future MSL results on diagenetic textures, secondary mineral assemblages, and the age and spatial variability of authigenic phases could constrain both mound formation processes and the physical context for liquid water on early Mars.
- Kite, Howard, Lucas, and Lewis, Resolving the era of river-forming climates on Mars using stratigraphic logs of river-deposit dimensions, Earth and Planetary Science Letters, 2015.
River deposits are one of the main lines of evidence that tell us that Mars once had a climate
different from today, and so changes in river deposits with time tell us something about how Mars
climate changed with time. In this study, we focus in on one sedimentary basin - Aeolis Dorsa - which
contains an exceptionally high number of exceptionally well-preserved river deposits. We use changes
in the river deposits' scale with stratigraphic elevation as a proxy for changes in river paleodischarge.
Meander wavelengths tighten upwards and channel widths narrow upwards, and there is some
evidence for a return to wide large-wavelength channels higher in the stratigraphy. Meander
wavelength and channel width covary with stratigraphic elevation. The factor-of-1.5 to factor-of-2
variations in paleochannel dimensions with stratigraphic elevation correspond to ~2.6-fold variability
in bank-forming discharge (using standard wavelength-discharge scalings and width-discharge
scalings). Taken together with evidence from a marker bed for variability at ~10m stratigraphic
distances, the variation in the scale of river depositsidicates that bank-forming discharge varied at
both ~10m stratigraphic (102-106 yr) and ~100 m stratigraphic (103 - 109 yr) scales. Because these
variations are correlated across the basin, they record a change in basin-scale forcing, rather than
smaller-scale internal feedbacks. Changing sediment input leading to a change in characteristic slopes
and/or drainage area could be responsible, and another posiility is changing climate (± 50 W/m2 in
peak energy available for snow/ice melt).
- Kite, Howard, Lucas, Armstrong, Aharonson, and Lamb, Stratigraphy of Aeolis Dorsa, Mars: stratigraphic context of the great river deposits, Icarus, 2015.
Unraveling the stratigraphic record is the key to understanding ancient climate change and past climate changes on Mars (Grotzinger et al. 2011). Stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers. Therefore, river deposits when placed in stratigraphic order could constrain the number, magnitudes, and durations of the wettest (and presumably most habitable) climates in Mars history.
We use crosscutting relationsis to establish the stratigraphic context of river and alluvial-fan deposits in the Aeolis Dorsa sedimentary basin, 10° E of Gale crater. At Aeolis Dorsa, wind erosion has exhumed a stratigraphic section of sedimentary rocks consisig of at least four unconformity-bounded rock packages, recording three or more disict episodes of surface runoff. Early deposits (>700m thick) are embayed by river deposits (>400m thick), which are in turn unconformably draped by fan-shaped deposits (<100m thick) which we interpret as alluvial fans. Yardang-forming layered deposits (>900 m thick) unconformably drape all previous deposits.
River deposits embay a dissected landscape formed of sedimentary rock. The river deposits are eroding out of at least two disiguishable units. There is evidence for pulses of erosion during the interval of river deposition. The total interval spanned by river deposits is >(1 x 106 - 2 x 107) yr, and this is extended if we include alluvial-fan deposits. Fan-shaped deposits unconformably postdate thrust faults which crosscut the river deposits. This relationships suggests a relatively dry interval of >4 x 107 yr after the river deposits formed and before the fan-shaped deposits formed, based on probability arguments. Yardang-forming layered deposits unconformably postdate all of the earlier deposits. They contain rhythmite and their induration suggests a damp or wet (near-)surface environment. The time gap between the end of river deposition and the onset of yardang-forming layered deposits is constrained to >1 x 108 yr by the high density of impact craters embedded at the unconformity. The time gap between the end of alluvial-fan deposition and the onset of yardang-forming layered deposits was at least long enough for wind-induced saltation abrasion to erode 20-30m into the alluvial-fan deposits. We correlate the yardang-forming layered deposits to the upper layers of Gale crater's mound (Mt. Sharp / Aeolis Mons), and the fan-shaped deposits to Peace Vallis fan in Gale crater. Alternations between periods of low mean obliquity and periods of high mean obliquity may have modulated erosion-deposition cycling in Aeolis. This is consistent with the results from an ensemble of simulations of Solar System evolution and the resulting history of the obliquity of Mars. Almost all of our simulations produce one or more intervals of continuously low mean Mars obliquity that are long enough to match our Aeolis Dorsa unconformity data.
- Kite, Williams, Lucas and Aharonson, Low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters, Nature Geoscience, 2014.
[News & Views]
The decay of the martian atmosphere - which is dominated by carbon dioxide - is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient cratersiterbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity. Here we use high-resolution images and digital terrain models from the Mars Reconnaissance Orbiter to identify ancient craters in deposits in Aeolis Dorsa that date to about 3.6 Gyr ago and compare their size distribution with models of atmospheric filtering of impactors. We obtain an upper limit of 0.9±0.1 bar for the martian atmospheric palaeopressure, rising to 1.9±0.2 bar if rimmed circular mesas - interpreted to be erosionally-resistant fills or floors of impact craters - are excluded. We assume target properties appropriate for desert alluvium: if sediment had rock-mass strength similar to bedrock at the time of impact, the paleopressure upper limit increases by a factor of up to two. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing - as suggested by our results - then a warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing.
Kite, Lewis, Lamb, Newman, and Richardson, Growth and form of the mound in Gale Crater, Mars: Slope-wind enhanced erosion and transport, Geology, 2013.
[pdf] [si] [MATLAB code] [Red Planet Report] [news coverage in Science] [news coverage in Nature] [New York Times]
Ancient sediments provide archives of climate and habitability on Mars. Gale Crater, the landing site for the Mars Science Laboratory (MSL), hosts a 5 km high sedimentary mound. Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and aeolian processes, but the origin and original extent of Gale's mound is unknown. Here we show new measurements of sedimentary strata within the mound that indicate ~3° outward dips oriented radially away from the mound center, inconsistent with the first three hypotheses. Moreover, although mounds are widely considered to be erosional remnants of a once crater-filling unit, we find that the Gale mound's current form is close to its maximal extent. Instead we propose that the mound's structure, stratigraphy, and current shape can be explained by growth in place near the center of the crater mediated by wind-topography feedbacks. Our model shows how sediment can initially accrete near the crater center far from crater-wall katabatic winds, until the increasing relief of the resulting mound generates mound-flank slope-winds strong enough to erode the mound. Our resultsidicate mound formation by airfall-dominated deposition with a limited role for lacustrine and fluvial activity, and potentially limited organic carbon preservation. Morphodynamic feedbacks between wind and topography are widely applicable to a range of sedimentary mounds and ice mounds across the Martian surface, and posily other planets.
Kite, Halevy, Kahre, Wolff, and Manga, Seasonal melting and the formation of sedimentary rocks on Mars, Icarus, 2013.
[pdf] [journal version] [astrobites]
[Red Planet Report] [Planetary Society]
A model for the formation and distribution of sedimentary rocks on Mars proposed. In this model (ISEE-Mars), the rate--limiting step is supply of liquid water from seasonal melting of snow or ice. The model is run for a 102 mbar pure CO2 atmosphere, dusty snow, and solar luminosity reduced by 23%. For these conditions snow melts only near the equator, when obliquity and eccentricity are high, and when perihelion occurs near equinox. These requirements for melting are satisid by 0.01-20% of the probability distribution of Mars' past spin-orbit parameters. This fraction is small, consistent with the geologic record of metastable surface liquid water acting as a "wet-pass filter" of Mars climate history, only recording orbital conditions that permitted surface liquid water. Total melt production is sufficient to account for observed aqueous alteration of the sedimentary rocks. The pattern of seasonal snowmelt isitegrated over all spin-orbit parameters and compared to the observed distribution of sedimentary rocks. The global distribution of snowmelt has maxima in Valles Marineris, Meridiani Planum and Gale Crater. These correspond to maxima in the sedimentary-rock distribution. Higher pressures and especially higher temperatures lead to melting over a broader range of spin-orbit parameters. The pattern of sedimentary rocks on Mars most consistent with a model Mars paleoclimate that only rarely produced enough meltwater to precipitate aqueous cements (sulfates, carbonates, phyllosilicates and silica) and indurate sediment. This is consistent with observations suggesting that surface aqueous alteration on Mars was brief and at low water/rock ratio. The results suggest intermittency of snowmelt and long globally-dry intervals, unfavorable for past life on Mars. This model makes testable predictions for the Mars Science Laboratory Curiosity rover at Gale Crater's mound (Mount Sharp, Aeolis Mons). Gale Crater's mound is predicted to be a hemispheric maximum for snowmelt on Mars.
Kite, Lucas, and Fassett, Pacing Early Mars river activity, Icarus, 2013.
[pdf] [code] [supplementary table]
The impactor flux early in Mars history was much higher than today, so sedimentary sequencesiclude many buried craters. In combination with models for the impactor flux, observations of the number of buried craters can constrain sedimentation rates. Using the frequency of crater-river interactions, we find net sedimentation rate ≤ ~20-300 μm/yr at Aeolis Dorsa. This sets a lower bound of 1-15 Myr on the total interval spanned by fluvial activity around the Noachian-Hesperian transition. We predict that Gale Crater's mound (Aeolis Mons) took at least 10-100 Myr to accumulate, which is testable by the Mars Science Laboratory.
- Šrámek, McDonough, Kite, Lekić, Dye, and Zhong, Geophysical and geochemical constraints on geoneutrino fluxes from Earth's mantle, Earth and Planetary Science Letters, 2013.
[pdf] [si] ["Research Highlight" in Nature]
Knowledge of the amount and distribution of radiogenic heating in the mantle is crucial for understanding the dynamics of the Earth, including its thermal evolution, the style and planform of mantle
convection, and the energetics of the core. Although the flux of heat from the surface of the planet is
robustly estimated, the contributions of radiogenic heating and secular cooling remain poorly defined.
Constraining the amount of heat-producing elements in the Earth will provide clues to understanding
nebula condensation and planetary formation processes early Solar System. Mantle radioactivity
supplies power for mantle convection and plate tectonics, but estimates of mantle radiogenic heat
production vary by a factor of more than 20. Recent experimental results demonstrate the potential for
direct assessment of mantle radioactivity through observations of geoneutrinos, which are emitted by
naturally occurring radionuclides. Predictions of the geoneutrino signal from the mantle exist for
several established estimates of mantle composition. Here we present novel analyses, illustrating
surface variations of the mantle geoneutrino signal for models of the deep mantle structure, including
those based on simic tomography. These variations have measurable differences for some models,
allowing new and meaningful constraints on the dynamics of the planet. An ocean based geoneutrino
detector deployed at several strategic locations will be able to discriminate between competing
compositional models of the bulk silicate Earth.
- Kite and Howard, Commentary: Let's send the DoE to Alpha Centauri, Physics Today, September 2013. [pdf]
Mangold, Kite, Kleinhans, Newsom, Ansan, Hauber, Kraal, Quantin-Nataf and Tanaka, The origin and timing of fluvial activity at Eberswalde Crater, Mars, Icarus, 2012.
The fan deposit in Eberswalde crater has been interpreted as strong evidence for sustained liquid water on
early Mars with a paleolake formed during the Noachian period (>3.7 Gy). This location became a key region
for understanding the Mars paleo-environment. Eberswalde crater is located 50 km north of the rim of the
150 km diameter crater Holden. Stratigraphic relationsis and chronology obtained using recent Mars
Express High Resolution Stereo Camera and Mars Reconnaissance Orbiter Context Camera images show
that Eberswalde fluvial activity crosscuts Holden ejecta and thus postdates Holden crater, whose formation
age is estimated from crater counts as Late Hesperian (3.5 Gy, depending on models). fluvial modeling
shows that short term activity (over several years to hundreds of years) involving dense flows (with sediment:water ratio between 0.01 and 0.3) may be as good an explanation of the fluvial landforms as dilute
flow over longer durations. Modeling of the thermal effect of the Holden impact in the Eberswalde
watershed is used to evaluate its potential role in aqueous activity. The relative timing of the Holden impact
and Eberswalde's fan is a constraint for future studies about the origin of these landforms. Holden ejecta
form a weak and porous substrate, which may be easy to erode by fluvial incision. In a cold climate scenario,
impact heating could have produced runoff by melting snow or ground ice. Any attempt to model fluvial
activity at Eberswalde should take into account that it may have formed as late as the Late Hesperian,
after the great majority of valley network formation and aqueous mineralization on Mars. This suggests
that hypotheses for fan formation at Eberswalde by transient and/or localized processes (i.e. impact,
volcanism, unusual orbital forcing) should be considered on a par with globally warmer climate.
Rappaport, Levine, Chiang, El Mellah, Jenkins, Kalomeni, Kite, Kotson, Nelson, Rousseau-Nepton, and Tran, Possible disintegrating short-period Super-Mercury orbiting KIC 12557548, Astrophysical Journal, 2012.
We report on the discovery of stellar occultations, observed with Kepler, which recur periodically at 15.685 hr
intervals, but which vary in depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star that
is apparently being occulted is KIC 12557548, a V = 16 mag K dwarf with T_eff = 4400 K. The out-of-occultation
behavior shows no evidence for ellipsial light variations, indicating that the mass of the orbiting object is less than
(for an orbital period of 15.7 hr). Because the eclipse depths are highly variable, they cannot be due solely to
transits of a single planet with a fixed size. We discuss but disis a scenario involving a binary giant planet whose
mutual orbit plane precesses, bringing one of the planetsto and out of a grazing transit. This scenario seems ruled
out by the dynamical instability that would result from such a configuration. We also briefly consider an eclipsing
binary, posily containing an accretion disk, that either orbits KIC 12557548 in a hierarchical triple configuration
or is nearby on the sky, but we find such a scenario inadequate to reproduce the observations. The much more likely
explanation - but one which still requires more quantitative development - involves macroscopic particles escaping
the atmosphere of a slowly disintegrating planet not much larger than Mercury in size. The particles could take the
form of micron-sized pyroxene or aluminum oxide dust grains. The planetary surface is hot enough to sublimate
and create a high-Z atmosphere; this atmosphere may be loaded with dust via cloud condensation or explosive
volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind, dragging dust grains with it.
We infer a mass-loss rate from the observations of order 1 Earth mass / Gyr
, with a dust-to-gas ratio posily of order
unity. For our fiducial 0.1 MEarth planet (twice the mass of Mercury), the evaporation timescale may be ~0.2 Gyr.
Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because
they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation
profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present
simulations of such a tail.
Kite, I. Climate change on ancient Mars. II. Exoplanet geodynamics and climate, PhD thesis, University of California, Berkeley.
Kite, Gaidos and Manga, Climate instability on tidally locked exoplanets, Astrophysical Journal, 2011.
[pdf] [htm] [Space.com]
Feedbacks that can destabilize the climates of synchronously rotating rocky planets may arise on planets with strong
day-night surface temperature contrasts. Earth-like habitable planets maintain stable surface liquid water over
geologic time. This requires equilibrium between the temperature-dependent rate of greenhouse-gas consumption
by weathering, and greenhouse-gas resupply by other processes. Detected small-radius exoplanets, and anticipated
M-dwarf habitable-zone rocky planets, are expected to be in synchronous rotation (tidally locked). In this paper,
we investigate two hypothetical feedbacks that can destabilize climate on planets synchronous rotation. (1) If
small changes in pressure alter the temperature distribution across a planetfls surface such that the weathering rate
goes up when the pressure goes down, a runaway positive feedback occurs involving increasing weathering rate
near the substellar point, decreasing pressure, and increasing substellar surface temperature. We call this feedback
enhanced substellar weathering instability (ESWI). (2) When decreases in pressure increase the fraction of surface
area above the melting point (through reduced advective cooling of the substellar point), and the corresponding
increase in volume of liquid causes net dissolution of the atmosphere, a further decrease in pressure will occur.
This substellar dissolution feedback can also cause a runaway climate sit. We use an idealized energy balance
model to map out the conditions under which these instabilities may occur. In this simplified model, the weathering
runaway can shrink the habitable zone and cause geologically rapid 103
- fold atmospheric pressure sits within
the habitable zone. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually
a negative feedback or weak positive feedback on changes in atmospheric pressure. It can only cause runaway
changes for small, deep oceans and highly soluble atmospheric gases.
Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant
for atmospheres that are thin, have low greenhouse-gas radiative efficiency, and have a principal greenhouse gas
that is also the main consiuent of the atmosphere. ESWI also requires land near the substellar point, and tectonic
resurfacing (volcanism, mountain-building) is needed for large jumps in pressure. These results identify a new
pathway by which habitable-zone planets can undergo rapid climate sits and become uninhabitable.
Kite, Michaels, Rafkin, Dietrich, and Manga, Chaos terrain, storms, and past climate on Mars, JGR-Planets, 2011.
[Red Planet Report]
["Research Highlight" in Nature Geoscience]
We model the atmospheric response to a chaos-forming event at Juventae Chasma,
north of Valles Marineris, Mars, using the Mars Regional Atmospheric Modeling System
(MRAMS). Interactions between lake-driven convergence, topography, and the regional
wind field steer lake-induced precipitation to the southwest. Mean snowfall reaches a
maximum of 0.9 mm/h water equivalent (peak snowfall 1.7 mm/h water equivalent) on the
SW rim of the chasm. More than 80% of vapor released by the lake is trapped in or next to
the lake as snow. Radiative effects of the thick cloud cover raise mean plateau surface
temperature by up to 18 K locally. We find that the area of maximum modeled
precipitation corresponds to the mapped Juventae plateau channel networks. At Echus
Chasma, modeled precipitation maxima also correspond to mapped plateau channel
networks. This is consistent with the earlier suggesin that Valles Marineris plateau
layered deposits and interbedded channel networks result from localized precipitation.
However, snowpack thermal modeling shows temperatures below freezing for the 12 mbar
atmosphere used in our MRAMS simulations. This is true even for the most
favorable orbital conditions, and whether or not the greenhouse effect of the lake storm
isicluded. Moderately higher CO2(g) pressures, or non-CO2(g) greenhouse forcing, is very
likely required for melting and plateau channel network formation under a faint young Sun.
Required warming is ≤10 K: global temperatures need not be higher than today. In
these localized precipitation scenarios, the rest of the planet remains dry.
- Kite and Lekić, Feasibility of mantle radiogenic power determination with geoneutrinos, in revision.
Geoneutrinos (gν) offer a new probe of deep Earth structure, and hold
the potential for directly measuring Earth's mantle Urey number. Here, we
use a 3D model of antineutrino emission from candidate structures to assess
the feasibility of gν-based characterization of simically-imaged lower mantle features. We focus on disict classes of structures: (1) large, low shear
velocity provinces (LLSVPs or superplumes) that may be compositionally
disict from the rest of the mantle; (2) ultra-low velocity zones (ULVZs) at
the base of the mantle that may be partially molten. Both superplumes and
ULVZs have been proposed to be reservoirs of radiogenic and other incompatible elements, as a means of resolving geochemical and heatflow paradoxes,
yet determining their composition is difficult with only simic constraints.
We find that a sea-transportable gν detector could place constraints on the
radiogenic power of the superplumes and on the total radiogenic-element
budget of the mantle, provided that measurements are made from more than one site and that the radiogenic-element concentration of the continental
crust is adequately measured by today's land-based gν detectors. However,
constraining the radiogenic power of the ULVZs is more difficult, probably
requiring a directional detector. A directional detector could also quickly
determine the power density of the superplumes from a single oceanic location. We show that gν signal emanating from excess radiogenics within the
LLSVPs and/or ULVZs can strongly bias mantle Urey number constraints
from an arbitrarily situated non-directional detector, and map out locations
that minimize this source of uncertainty. Conversely, locations at which gν
counts are highly sensitive to the distribution of lower mantle radiogenics
have the potential for constraining the relative radiogenic power of LLSVPs
versus ULVZs and thus would offer a unique constraint on their composition.
Manga, Patel, Dufek and Kite, Wet surface and dense atmosphere on early Mars inferred from the bomb sag at Home Plate, Mars, Geophysical Research Letters, 2011.
We use the Mars Exploration Rover Spirit observation
of a bomb sag produced by an explosive volcanic eruption
to infer the atmospheric density at the time of eruption. We
performed analogue experiments to determine the relationship between the wetness of the substrate and the velocity
and density of impacting clasts and 1) the formation (or
not) of bomb sags, 2) the morphology of the impact crater,
and 3) the penetration depth of the clast. The downward
deflection of beds seen on Mars consistent with watersaturated sediment in the laboratory experiments. Collision
angles <20° from vertical are needed to produce bomb
sags. From the experiments we infer an impact velocity up to
m/s, lower than ejection velocities during phreatic
and phreatomagmatic eruptions on Earth. If this velocity
represents the terminal subaerial impact velocity, atmospheric density exceeded 0.4 kg/m3
at the time of eruption,
much higher than at present.
- Kite, Rafkin, Michaels, Manga, and Dietrich, Localized precipitation and runoff on Mars, JGR-Planets, 2011.
[as reported by "New Scientist"]
We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake
storms on Mars, finding that intense localized precipitation will occur for lake size ≥103
Mars has a low-density atmosphere, so deep convection can be triggered by small amounts
of latent heat release. In our reference simulation, the buoyant plume lifts vapor above
condensation level, forming a 20 km high optically thick cloud. Ice grains grow to 200μm
radius and fall near (or in) the lake at mean rates up to 1.5 mm h-1
(maximum rates up to 6 mm h-1
water equivalent). Because atmospheric temperatures
outside the surface layer are always well below 273 K, supersaturation and condensation
begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect storms, lake
storms on Mars involve continuous precipitation, and their vertical velocities and plume
heights exceed those of tropical thunderstorms on Earth. For lake sizes 102.5
plume vertical velocity scales linearly with lake area. Convection does not reach above the
planetary boundary layer for lakes <103
or for atmospheric pressure >O(102) mbar.
Instead, vapor is advected downwind with little cloud formation. Precipitation occurs as
snow, and the daytime radiative forcing at the land surface due to plume vapor and storm
clouds too small to melt snow directly (<+10 W m-2). However, if orbital conditions
are favorable, then the snow may be seasonally unstable to melting and produce runoff to
form channels. We calculate the probability of melting by running thermal models over all
possible orbital conditions and weighting their outcomes by probabilities given by long-term
integrations of the chaotic diffusion of solar system orbital elements. With this approach,
we determine that for an equatorial vapor source, sunlight 15% fainter than at present and
snowpack with albedo 0.28 (0.35), melting may occur with 4% (0.1%) probability. This rises
to 56% (12%) if the ancient greenhouse effect was modestly (6 K) greater than today.
- Kite, Manga and Gaidos, Geodynamics and rate of volcanism on massive Earth-like planets, Astrophysical Journal, 2009.
We provide estimates of volcanism versus time for planets with Earth-like composition and masses 0.25-25 MEarth,
as a step toward predicting atmospheric mass on extrasolar rocky planets. Volcanism requires melting of the
silicate mantle. We use a thermal evolution model, calibrated against Earth, in combination with standard melting
models, to explore the dependence of convection-driven decompresin mantle melting on planet mass. We
show that (1) volcanism is likely to proceed on massive planets with plate tectonics over the main-sequence
lifetime of the parent star; (2) crustal thickness (and melting rate normalized to planet mass) is weakly dependent
on planet mass; (3) stagnant lid planets live fast (they have higher rates of melting than their plate tectonic
counterparts early in their thermal evolution), but die young (melting shuts down after a few Gyr); (4) plate
tectonics may not operate on high-mass planets because of the production of buoyant crust which is difficult
to subduct; and (5) melting is necessary but insufficient for efficient volcanic degassing - volatiles partition
into the earliest, deepest melts, which may be denser than the residue and sink to the base of the mantle
on young, massive planets. Magma must also crystallize at or near the surface, and the pressure of overlying
volatiles must be fairly low, if volatiles are to reach the surface. If volcanism is detected in the 10 Gyr old Tau Ceti system, and tidal forcing can be shown to be weak, this would be evidence for plate tectonics.
- Chiang, Kite, Kalas, Graham and Clampin, Fomalhaut's debris disk and planet: Constraining the mass of Fomalhaut b using disk morphology, Astrophysical Journal, 2009.
Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to
other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of
the belt, then to produce the observed disk morphology it must have a mass M_pl < 3_MJ, an orbital semimajor
axis a_pl > 101.5 AU, and an orbital eccentricity e_pl = 0.11-0.13. These conclusions are independent of Fom b's
photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the
disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction,
can be used to determine the mass more precisely. The inner edge of the debris disk at a ~ 133 AU lies at the
periphery of Fom b's chaotic zone, and the mean disk eccentricity of e ~ 0.11 is secularly forced by the planet,
supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk
morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a
global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk
parent bodies for dynamical stability over the system age of ~ 100 Myr, and model them separately from their
dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to
~ 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be
apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties
may be large. If the apsidal misalignment proves real, our calculated upper mass limit of 3 MJ
still holds. If the orbits
are aligned, our model predicts M_pl = 0.5 MJ
, a_pl = 115 AU, and e_pl = 0.12. Parent bodies are evacuated from
mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter.
The belt contains at least 3 MEarth of solids that are grinding down to dust, their velocity dispersions sired so strongly
by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ.
- Kite, Matsuyama, Manga, Perron and Mitrovica, True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars, Earth and Planetary Science Letters, 2009.
The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar
deposits by 5-10°. We test the hypothesis that the offset is the result of True Polar Wander (TPW), the
motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the
surface of Mars. In particular, we consider the posiility that TPW is driven by late-stage volcanism during
the Late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both
north and south, observed offsets lie close to a great circle 90° from Tharsis, as expected for polar wander
after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas
for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events.
We find that if Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located
close to the equator leads to predicted TPW of 2°, too small to account for observed offsets. If, however,
Tharsis formed far from the equator, late-stage TPW driven by low-latitude, late-stage volcanism would be 6-
33°, similar to that inferred from the location of paleopolar deposits. A volume of 4.4 ± 1.3 x 1019
kg of young
erupted lava can account for the offset of the Dorsa Argentea Formation from the present-day south rotation
pole. This volume is consistent with prior mapping-based estimates and would imply a mass release of CO2(g)
by volcanic degassing similar to that in the atmosphere at the present time. The South Polar Layered Deposits
are offset from the present rotation pole in a direction that is opposite to the other paleopolar deposits. This
can be explained by either a sequential eruption of late-stage lavas, or an additional contribution from a
plume beneath Elysium. We predict that significant volcanic activity occurred during the time interval
represented by the Basal Unit/Planum Boreum unconformity; Planum Boreum postdates the Promethei
Lingula Lobe; and that the north polar deposits span a substantial fraction of Solar System history. If the
additional contribution to TPW from plumes small, then we would also predict that Tharsis Montes
Formation postdates the Promethei Lingula Lobe of the South Polar Layered Deposits. We conclude with a list
of observational tests of the TPW hypothesis.
- Kalas, Graham, Chiang, Fitzgerald, Clampin, Kite, Stapelfeldt, Marois and Krist, Optical images of an exosolar planet 25 light years from Earth, Science, 2008
[#2 Breakthrough of the Year]
Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU of the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet's mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt, matching predictions of its location. The flux detected at 0.8 μm is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 μm and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 μm.
- Kite and Hindmarsh, Did ice streams shape the largest channels on Mars?, Geophysical Research Letters, 2007.
The largest channels on Mars are the Northwestern
Slope Valleys (NSVs) of Tharsis, which have previously
been interpreted as the probable erosional trace of
catastrophic flooding. It is argued here that ice-streaming
within ancient ice sheets emplaced by atmospheric
precipitation at high mean obliquity may instead account
for these channels, explaining similarities between the
region and terrestrial Pleistocene subglacial landscapes. An
ice-sheet model shows extensive basal melting in and only
in the NSV region, and ice streams which have significant