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Register a new userPacing Early Mars fluvial activity at Aeolis Dorsa: Implications for Mars Science Laboratory observations at Gale Crater and Aeolis Mons
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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 lesssim 20-300 {mu}m/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 Craters mound (Aeolis Mons) took at least 10-100 Myr to accumulate, which is testable by the Mars Science Laboratory.
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The Mars Science Laboratory (MSL) Rover Environmental Monitoring Station (REMS) has now made continuous in-situ meteorological measurements for several martian years at Gale crater, Mars. Of importance in the search for liquid formation are REMS measurements of ground temperature and in-air measurements of temperature and relative humidity, which is with respect to ice. Such data can constrain the surface and subsurface stability of brines. Here we use updated calibrations to REMS data and consistent relative humidity comparisons (i.e., w.r.t. liquid vs w.r.t. ice) to investigate the potential formation of surface and subsurface liquids throughout MSLs traverse. We specifically study the potential for the deliquescence of calcium perchlorate. Our data analysis suggests that surface brine formation is not favored within the first 1648 sols as there are only two times (sols 1232 and 1311) when humidity-temperature conditions were within error consistent with a liquid phase. On the other hand, modeling of the subsurface environment would support brine production in the shallow subsurface. Indeed, we find that the shallow subsurface for terrains with low thermal inertia ($Gammalesssim 300$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$) may be occasionally favorable to brine formation through deliquescence. Terrains with $Gammalesssim175$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ and albedos of $gtrsim0.25$ are the most apt to subsurface brine formation. Should brines form, they would occur around Ls 100$^{circ}$. Their predicted properties would not meet the Special nor Uncertain Region requirements, as such they would not be potential habitable environments to life as we know it.
We analyze thermal emission spectra using the 2001 Mars Odyssey Thermal Emission Imaging System (THEMIS) and the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) to characterize grain size and mineralogical composition of dunes at Hargraves crater, Mars. Thermal inertia and bulk composition of the dunes were compared to inferred provenances from the thermal infrared response of surface constituent materials. We use a Markov Chain Monte Carlo (MCMC) technique to estimate the bulk amount of mineralogy contributed by each inferred provenance to the dune field composition. An average thermal inertia value of 238+/-17 Jm-2K-1s-0.5 was found for the dunes corresponding to a surface composed of an average effective grain size of ~391+/-172 um. This effective particle size suggests the presence of mostly medium sand-sized materials mixed with fine and coarse grain sands. The dunes are likely comprised of a weakly indurated surface mixed with unconsolidated materials. Compositional analysis specifies that the dunes are comprised of a mixture of feldspar, olivine, pyroxene, and relatively low bulk-silica content. Dune materials were likely derived from physical weathering, especially eolian erosion, predominantly from the crater ejecta unit at the crater, mixed with a small amount from the crater floor and crater rim and wall lithologies - indicating the dune materials were likely sourced locally.
The Curiosity rover investigated a topographic structure known as Vera Rubin ridge, associated with a hematite signature in orbital spectra. There, Curiosity encountered mudstones interpreted as lacustrine deposits, in continuity with the 300 m-thick underlying sedimentary rocks of the Murray formation at the base of Mount Sharp. While the presence of hematite ($alpha$-Fe2O3) was confirmed insitu by both Mastcam and ChemCam spectral observations and by the CheMin instrument, neither ChemCam nor APXS observed any significant increase in FeO$_T$ (total iron oxide) abundances compared to the Murray formation. Instead, Curiosity discovered dark-toned diagenetic features displaying anomalously high FeO$_T$ abundances, commonly observed in association with light-toned Ca-sulfate veins but also as crystal pseudomorphs in the host rock. These iron-rich diagenetic features are predominantly observed in grey outcrops on the upper part of the ridge, which lack the telltale ferric signature of other Vera Rubin ridge outcrops. Their composition is consistent with anhydrous Fe-oxide, as the enrichment in iron is not associated with enrichment in any other elements, nor with detections of volatiles. The lack of ferric absorption features in the ChemCam reflectance spectra and the hexagonal crystalline structure associated with dark-toned crystals points toward coarse grey hematite. In addition, the host rock adjacent to these features appears bleached and show low-FeO$_T$ content as well as depletion in Mn, indicating mobilization of these redox-sensitive elements during diagenesis. Thus, groundwater fluid circulations could account for the remobilization of iron and recrystallization as crystalline hematite during diagenesis as well as color variations observed in the Vera Rubin ridge outcrops.
The climate on early Mars is one of the major unsolved problems. It is unclear whether early Mars was warm and wet or cold and icy. Morphological features on Mars such as sinuous ridges could provide critical constraints to address this issue. Here we investigate several sinuous ridges to the east of Tempe Terra, Mars and find they may have recorded persistent fluvial activity on early Mars. Our analysis indicates that these ridges may represent exhumation of the channel belts and overbank deposits formed from meander rivers over significant geologic time. Layered smectite-bearing minerals, distributed along the ridge flanks, could be detrital or authigenic floodplain clays. Our interpretation of the stratigraphic relationships indicates that the layered smectite-bearing materials lie between channel belt deposits, supporting the floodplain interpretation. Our results suggest that a persistent warming event has persisted for a geologically significant interval (>1500 yr) during the Noachian period of Mars.
The dynamics of Mars obliquity are believed to be chaotic, and the historical ~3.5 Gyr (late-Hesperian onward) obliquity probability density function (PDF) is high 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{deg}) and enhancing the potential for desiccation of deep aquifers at low obliquities (< 25{deg}). We developed a new technique using 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 Mars impactors and ~3.5 Gyr-long Mars obliquity simulations. In our model, the inclinations and speeds of Mars crossing objects bias the preferred orientation of elliptic craters which are formed by low-angle impacts. Comparison of our simulation predictions with a validated database of elliptic crater orientations allowed us to invert for best-fitting obliquity history. We found that since the onset of the late-Hesperian, Mars mean obliquity was likely low, between ~10{deg} and ~30{deg}, and the fraction of time spent at high obliquities > 40{deg} was likely < 20%.
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