- Kite and Ford, Habitability of exoplanet waterworlds, in review.
We model the evolution of ocean temperature and chemistry for rocky exoplanets with 10-1000x Earth's H2O but without H2, for Sunlike stars, taking into account C partitioning, high-pressure ice phases, and atmosphere-lithosphere exchange. Within our model, for Sunlike stars, we find that: (1) habitability is strongly affected by ocean chemistry; (2) possible ocean pH spans a wide range; (3) exsolution-driven climate instabilities are possible; (4) surprisingly, many waterworlds stay habitable for >1 Gyr, and (contrary to previous claims) this longevity does not neccessarily involve geochemical cycling.
We further demonstrate, using an ensemble of N-body simulations that include volatile loss during giant impacts, that >~10% of habitable-zone rocky planets emerge after the giant impact era with deep, ice-free water envelopes. This outcome is sensitive to our assumptions of low initial abundances of Al-26 and Fe-60 in protoplanetary disks, plus H2-free accretion. We track the evolution of these worlds through our look-up table. Thus, for the first time in an an end-to-end calculation, we show that chance variation of initial conditions, with no need for geochemical cycling, can yield multi-Gyr habitability on waterworlds.
- Kite, Gaidos and Onstott, Valuing life detection missions, submitted.
Recent discoveries imply 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 (Dressing & 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 testing, even a nondetection outcome is scientifically valuable.
- Kite, Mischna, Gao, and Yung, Valley network formation at the Noachian/Hesperian boundary on Mars initiated by atmospheric collapse, submitted.
The progressive drying-out of Mars' surface was punctuated by a dramatic transient increase in fluvial erosion around the Noachian-Hesperian boundary (approximately 3.7 Ga). Standard explanations of this climate optimum appeal to volcano- or impact-triggered climates and imply that individual runoff episodes were brief, apparently inconsistent with evidence for persistent runoff. We examine a scenario in which the duration, intensity and uniqueness of the Noachian-Hesperian climate optimum result from degassing of CH4
-clathrate consequent to atmospheric collapse. Atmospheric collapse causes low-latitude surface water ice to sublimate away, depressurizing and thus destabilizing CH4
clathrate in subglacial pore space. Subsequent atmospheric re-inflation leads to warming that further destabilizes CH4
-induced warming is efficient, permitting strong positive feedbacks, and possibly raising Mars into a climate optimum. The optimum is brought to a close by photochemical destruction of CH4
or by a new atmospheric collapse, and drawdown of the CH4
-clathrate reservoir prevents recurrence. This scenario predicts a 105
yr climate optimum, transient connections between the deep hydrosphere and the surface, mud volcanism, and strong surface weathering, all of which are consistent with recent observations. Crustal hydrothermal circulation very early in Mars history could yield CH4
that would be incorporated into clathrate on approach to the cold surface. The scenario explains why regional watershed integration on Mars occurred relatively late and apparently only once, and suggests that the contrasts between Noachian versus Hesperian climate-sensitive deposits on Mars correspond to a transition from a never-collapsed atmosphere to a collapse-prone climate, ultimately driven by slow loss of CO2
(The core idea is described in this [LPSC abstract]
- 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 soils indicate 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.
- Seybold, Kite, and Kirchner, The role of surface water in the geometry of Mars' valley networks and its climatic implications, submitted.
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 networks in 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.
- 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 was igneous 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.
- Mansfield, Kite, and Mischna, Effect of Mars atmospheric loss on snow melt potential in a 3.5-Gyr climate evolution model, in review.
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, in press.
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 is observed for Mt. Sharp in Gale crater.
- Holo, Kite, and Robbins, Mars obliquity history constrained by elliptic crater orientations, in review.
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.
- Archer, Kite, and Lusk, The ultimate social cost of carbon, in review.
We estimate the potential ultimate social cost of fossil-fuel carbon to all future human
generations throughout the 200-kyr duration of the climate impacts from fossil carbon
combustion. Costs are integrated through time assuming continued human reliance on
our biological habitat, and that each generation of humanity values its 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).
- Gabasova and Kite, Compaction and sedimentary basin analysis on Mars, Planetary & Space Science, in press.
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, 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 swift 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 landslides, 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 ofmound strata can be modulated by shifts in 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 shifts. 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 flushing 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 Cassini 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.
- 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 estimates. 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.
- 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.
- 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 deposits indicates 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 possibility 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 relationships 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 consisting of at least four unconformity-bounded rock packages, recording three or more distinct 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 distinguishable 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 relationship 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 craters interbedded 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] [supplementary information] [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 results indicate 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 possibly 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 is 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 satisfied 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 is integrated 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 is 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 sequences include 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] [supplementary information] ["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 in 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 seismic 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.