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Register a new userVisibility and Line-Of-Sight Extinction Estimates in Gale Crater during the 2018/MY34 Global Dust Storm
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Northern line-of-sight extinction within Gale Crater during the 2018 global dust storm was monitored daily using MSLs Navcam. Additional observations with Mastcam (north) and Navcam (all directions) were obtained at a lower cadence. Using feature identification and geo-referencing, extinction was estimated in all possible directions. Peak extinction of $>1.1$ km$^{-1}$ was measured between sols 2086 and 2090, an order of magnitude higher than previous maxima. Northern and western directions show an initial decrease, followed by a secondary peak in extinction, not seen in column opacity measurements. Due to foreground topography, eastern direction results are provided only as limits, and southern results were indeterminable. Mastcam red and green filter results agree well, but blue filter results show higher extinctions, likely due to low signal-to-noise. Morning results are systematically higher than afternoon results, potentially indicative of atmospheric mixing.
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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.
We study the 2018 Martian Global DustStorm (GDS 2018) over the Southern Polar Region using images obtained by the Visual Monitoring Camera (VMC) on board Mars Express during June and July 2018. Dust penetrated into the polar cap region but never covered the cap completely, and its spatial distribution was nonhomogeneous and rapidly changing. However, we detected long but narrow aerosol curved arcs with a length of 2,000-3,000 km traversing part of the cap and crossing the terminator into the night side. Tracking discrete dust clouds allowed measurements of their motions that were towards the terminator with velocities up to 100 m/s. The images of the dust projected into the Martian limb show maximum altitudes of around 70 km but with large spatial and temporal variations. We discuss these results in the context of the predictions of a numerical model for dust storm scenario.
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.
Studying absorption and scattering of X-ray radiation by interstellar dust grains allows us to access the physical and chemical properties of cosmic grains even in the densest regions of the Galaxy. We aim at characterising the dust silicate population which presents clear absorption features in the energy band covered by the Chandra X-ray Observatory. Through these absorption features, in principle, it is possible to infer the size distribution, composition, and structure of silicate in the interstellar medium. In particular, in this work, we investigate the magnesium and silicon K-edges. By using newly acquired synchrotron measurements, we build X-ray extinction models for fifteen dust candidates. These models, adapted for astrophysical analysis, and implemented in the Spex spectral fitting program, are used to reproduce the dust absorption features observed in the spectrum of the bright low mass X-ray binary GX 3+1 which is used as a background source. With the simultaneous analysis of the two edges we test two different size distributions of dust: one corresponding to the standard Mathis-Rumpl-Nordsieck model and one considering larger grains ($n(a) propto a_i^{-3.5}$ with $0.005<a_1<0.25$ and $0.05<a_2<0.5$, respectively, with $a$ the grain size). These distributions may be representative of the complex Galactic region towards this source. We find that up to $70%$ of dust is constituted by amorphous olivine. We discuss the crystallinity of the cosmic dust found along this line of sight. Both magnesium and silicon are highly depleted into dust ($delta_{Z} = 0.89 rm{and} 0.94$, respectively) while their total abundance does not depart from solar values.
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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