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Compact Reconnaissance Imaging Spectrometer for Mars

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A NASA engineer and the CRISM instrument.

CRISM has found a characteristic layering pattern of aluminum-rich clays overlying iron- and magnesium-rich clays in many areas scatteredars to search for evidence for past life.

Nili Fossae on Mars - largest known carbonate deposit.

One of the leading hypotheses for why ancient Mars was wetter than today is that a thick, carbon dioxide-rich atmosphere created a global greenhouse, that warmed the surface enough for liquid water to occur in large amounts. Carbon dioxide ice in today's polar caps is too limited in volume to hold that ancient atmosphere. If a thick atmosphere ever existed, it was either blown into space by solar wind or impacts, or reacted with silicate rocks to become trapped as carbonates in Mars' crust. One of the goals that drove CRISM's design was to find carbonates, to try to solve this question about what happened to Mars' atmosphere. And one of CRISM's most important discoveries was the identification of carbonate bedrock in Nili Fossae in 2008.[1] Soon thereafter, landed missions to Mars started identifying carbonates on the surface; the Phoenix Mars lander found between 3–5 wt% calcite (CaCO3) at its northern lowland landing site,[2] while the MER Spirit rover identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater.[3] Later CRISM analyses identified carbonates in the rim of Huygens crater which suggested that there could be extensive deposits of buried carbonates on Mars.[4] However, a study by CRISM scientists estimated that all of the carbonate rock on Mars holds less that the present Martian atmosphere worth of carbon dioxide.[5][6] They determined that if a dense ancient Martian atmosphere did exist, it is probably not trapped in the crust.

Crustal composition

Understanding the composition of Mars' crust and how it changed with time tells us about many aspects of Mars' evolution as a planet, and is a major goal of CRISM. Remote and landed measurements prior to CRISM, and analysis of Martian meteorites, all suggest that the Martian crust is made mostly of basaltic igneous rock composed mostly of feldspar and pyroxene. Images from the Mars Orbiter Camera on MGS showed that in some places the upper few kilometers of the crust is composed of hundreds of thin volcanic lava flows. TES and THEMIS both found mostly basaltic igneous rock, with scattered olivine-rich and even some quartz-rich rocks.

The first recognition of widespread sedimentary rock on Mars came from the Mars Orbiter Camera which found that several areas of the planet - including Valles Marineris and Terra Arabia - have horizontally layered, light-toned rocks. Follow-up observations of those rocks' mineralogy by OMEGA found that some are rich in sulfate minerals, and that other layered rocks around Mawrth Vallis are rich in phyllosilicates.[7] Both class of minerals are signatures of sedimentary rocks. CRISM has used its improved spatial resolution to look for other deposits of sedimentary rock on Mars' surface, and for layers of sedimentary rock buried between layers of volcanic rock in Mars' crust.

Modern climates

To understand Mars' ancient climate, and whether it might have created environments habitable for life, first we need to understand Mars' climate today. Each mission to Mars has made new advances in understanding its climate. Mars has seasonal variations in the abundances of water vapor, water ice clouds and hazes, and atmospheric dust. During southern summer, when Mars is closest to the Sun (at perihelion), solar heating can raise massive dust storms. Regional dust storms - ones having a 1000-kilometer scale - show surprising repeatability Mars-year to Mars-year. Once every decade or so, they grow into global-scale events. In contrast, during northern summer when Mars is furthest from the Sun (at aphelion), there is an equatorial water-ice cloud belt and very little dust in the atmosphere. Atmospheric water vapor varies in abundance seasonally, with the greatest abundances in each hemisphere's summer after the seasonal polar caps have sublimated into the atmosphere. During winter, both water and carbon dioxide frost and ices form on Mars' surface. These ices form the seasonal and residual polar caps. The seasonal caps - which form each autumn and sublimate each spring - are dominated by carbon dioxide ice. The residual caps - which persist year after year - consist mostly of water ice at the north pole and water ice with a thin veneer (a few 10's of meters thick) of carbon dioxide ice at the south pole.

Mars' atmosphere is so thin and wispy that solar heating of dust and ice in the atmosphere - not heating of the atmospheric gases - is more important in driving weather. Small, suspended particles of dust and water ice - aerosols - intercept 20–30% of incoming sunlight, even under relatively clear conditions. So variations in the amounts of these aerosols have a huge influence on climate. CRISM has taken three major kinds of measurements of dust and ice in the atmosphere: targeted observations whose repeated views of the surface provide a sensitive estimate of aerosol abundance; special global grids of targeted observations every couple of months designed especially to track spatial and seasonal variations; and scans across the planet's limb to show how dust and ice vary with height above the surface.

The south polar seasonal cap has a bizarre variety of bright and dark streaks and spots that appear during spring, as carbon dioxide ice sublimates. Prior to MRO there were various ideas for processes that could form these strange features, a leading model being carbon dioxide geysers.[8][9][10][11][12][13][14][15][16] CRISM has watched the dark spots grow during southern spring, and found that bright streaks forming alongside the dark spots are made of fresh, new carbon dioxide frost, pointing like arrows back to their sources - the same sources as the dark spots. The bright streaks probably form by expansion, cooling, and freezing of the carbon dioxide gas, forming a "smoking gun" to support the geyser hypothesis.

See also

References

  1. ^ Ehlmann; Mustard, JF; Murchie, SL; Poulet, F; Bishop, JL; Brown, AJ; Calvin, WM; Clark, RN; et al. (2008). "Orbital identification of carbonate-bearing rocks on Mars". Science. 322 (5909): 1828–1832. Bibcode:2008Sci...322.1828E. doi:10.1126/science.1164759. PMID 19095939.
  2. ^ Boynton, WV; Ming, DW; Kounaves, SP; Young, SM; Arvidson, RE; Hecht, MH; Hoffman, J; Niles, PB; et al. (2009). "Evidence for Calcium Carbonate at the Mars Phoenix Landing Site" (PDF). Science. 325 (5936): 61–64. Bibcode:2009Sci...325...61B. doi:10.1126/science.1172768. PMID 19574384. S2CID 26740165.
  3. ^ Morris, RV; Ruff, SW; Gellert, R; Ming, DW; Arvidson, RE; Clark, BC; Golden, DC; Siebach, K; et al. (2010). "Identification of carbonate-rich outcrops on Mars by the Spirit rover" (PDF). Science. 329 (5990): 421–4. Bibcode:2010Sci...329..421M. doi:10.1126/science.1189667. PMID 20522738. S2CID 7461676. Archived from the original (PDF) on 2011-07-25.
  4. ^ Some of Mars' Missing Carbon Dioxide May be Buried
  5. ^ "Mars' Missing Atmosphere Likely Lost in Space".
  6. ^ Edwards, C.; Ehlmann, B. (2015). "Carbon sequestration on Mars". Geology. 43 (10): 863–866. Bibcode:2015Geo....43..863E. doi:10.1130/G36983.1.
  7. ^ Bibring, JP; Langevin, Y; Mustard, JF; Poulet, F; Arvidson, R; Gendrin, A; Gondet, B; Mangold, N; Pinet, P; Forget, F (2006). "Global mineralogical and aqueous mars history derived from OMEGA/Mars express data". Science. 312 (5772): 400–404. Bibcode:2006Sci...312..400B. doi:10.1126/science.1122659. PMID 16627738.
  8. ^ Piqueux, Sylvain; Byrne, Shane; Richardson, Mark I. (2003). "Sublimation of Mars's southern seasonal CO2 ice cap formation of spiders". Journal of Geophysical Research: Planets. 180 (E8): 5084. Bibcode:2003JGRE..108.5084P. doi:10.1029/2002JE002007.
  9. ^ Manrubia, S. C.; O. Prieto Ballesteros; C. González Kessler; D. Fernández Remolar; C. Córdoba-Jabonero; F. Selsis; S. Bérczi; T. Gánti; A. Horváth; A. Sik; E. Szathmáry (2004). "Comparative Analysis of Geological Features and Seasonal Processes in Inca City and PittyUSA Patera Regions on Mars" (PDF). European Space Agency Publications (ESA SP): 545.
  10. ^ Kieffer, H. H. (2000). "Mars Polar Science 2000 - Annual Punctuated CO2 Slab-ice and Jets on Mars" (PDF). Retrieved 6 September 2009. {{cite journal}}: Cite journal requires |journal= (help)
  11. ^ Kieffer, Hugh H. (2003). "Third Mars Polar Science Conference (2003)- Behavior of Solid CO" (PDF). Retrieved 6 September 2009. {{cite journal}}: Cite journal requires |journal= (help)
  12. ^ Portyankina, G., ed. (2006). "Fourth Mars Polar Science Conference - Simulations of Geyser-Type Eruptions in Cryptic Region of Martian South" (PDF). Retrieved 11 August 2009. {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ Sz. Bérczi; et al., eds. (2004). "Lunar and Planetary Science XXXV (2004) - Stratigraphy of Special Layers – Transient Ones on Permeable Ones: Examples" (PDF). Retrieved 12 August 2009. {{cite journal}}: Cite journal requires |journal= (help)
  14. ^ Kieffer, Hugh H.; Christensen, Philip R.; Titus, Timothy N. (2006). "CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap". Nature. 442 (7104): 793–6. Bibcode:2006Natur.442..793K. doi:10.1038/nature04945. PMID 16915284. S2CID 4418194.
  15. ^ "NASA Findings Suggest Jets Bursting From Martian Ice Cap". Jet Propulsion Laboratory. NASA. 16 August 2006. Retrieved 11 August 2009.
  16. ^ C.J. Hansen; N. Thomas; G. Portyankina; A. McEwen; T. Becker; S. Byrne; K. Herkenhoff; H. Kieffer; M. Mellon (2010). "HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: I. Erosion of the surface" (PDF). Icarus. 205 (1): 283–295. Bibcode:2010Icar..205..283H. doi:10.1016/j.icarus.2009.07.021. Retrieved 26 July 2010.
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