Achievement: Three martian years of systematic re-imaging of sites, producing knowledge about rates of modern surface processes.
Significance: Mars continues to evolve in ways we are only beginning to understand.
Image: NASA/JPL/Caltech/University of Arizona
February 19, 2008: Martian polar caps contain a semi-permanent residual cap beneath a surface seasonal cap that waxes and wanes. The residual caps are largely water, but each year as the winter cold deepens the water caps become surfaced with frozen CO2, which was thought to sublime gently in spring. This photograph proved otherwise. The closeups show two separate CO2/dust avalanches cascading down the side of a single 700-meter scarp. Four different avalanches were observed in the same MRO HiRISE shot; the upper fall is 160m wide. Analysis suggests the falls originated on the sides of the scarp rather than the top and were triggered by spring sublimation of dry ice. Mars is still a planet of dramatic changes.
Russell, P. et al. (2008). Seasonally active frost-dust avalanches on a north polar scarp of Mars captured by HiRISE, Geophys. Res. Lett., 35, L23204.
Bombardment by solar system debris continues, marked by impact craters that appear on MOC and HiRISE re-imaging of sites. (right) Red dots are MOC-observed craters made from 2001 to 2006, green are craters observed by MRO HiRISE since 2006.
Malin, M.C. et al (2006). Present day impact crater rate and contemporary gully activity on Mars. Science 317, 1573.
Image: NASA/JPL/MSSS, Image: NASA/MEPAG
Southern winters are deeper than northern (see Climate and Weather) and the south residual cap’s upper layers, like the seasonal cap, are frozen CO2. (left) For reasons not well understood, not only the seasonal but parts of the southern residual CO2 cap are subliming in spring, producing landforms with no analogue on Earth. (right) Dendritic “spiders” on the southern cap. These may be dust deposited by jets of subliming CO2 gas – another process with no terrestrial counterpart. By re-imaging sites over time, MGS’s MOC and MRO’s HiRISE cameras have catalogued hundreds of (stunning) instances of polar and other seasonal processes.
Thomas, N. (2009). HiRISE observations of gas sublimation-driven
activity in Mars’ southern polar regions: II. Surficial deposits and their
origins, Icarus,doi: 10.1016/j.icarus.2009.05.030 (in press).
Kieffer, H.H. et al (2006) CO2 jets formed by sublimation beneath translucent slab ice in Mars’ seasonal south polar ice cap. Nature 442, 793.
Byrne, S., and A. P. Ingersoll (2003), Martian climatic events on timescales of centuries: Evidence from feature morphology in the residual south polar ice cap, Geophys. Res. Lett., 30(13), 1696.
Malin, M.C. and Edgett, K.S. (2001). Mars Global Surveyor Mars Orbital Camera: interplanetary cruise through primary mission. J. Geophys. Res. 106 (E10), 23,429—23,570.
A few gullies are known to have formed or extended since 2001. The mechanism(s) are hotly debated, and more than one may be involved, especially since new gullies have formed both on dunes and in crater walls.
Dickson, J.L. and J.W. Head (2008). Global synthesis of Mars gully
observations: evidence for climate controlled formation from morphology,
distribution, topography and terrestrial analogs. In: 39th Lunar and Planetary
Science Conference, March 10-14 2008, Houston TX, Abstract #1181. Sponsored by
Lunar and Planetary Institute, Houston TX. Retrieved from http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1181.pdf
Fan, C. et al. (2009). Investigation of water signatures at gully-exposed sites on Mars by hyperspectral image analysis. Planetary and Space Science 57 (1), 93-104.
McEwan, A.S. et al. (2007). A closer look at water-related geologic activity on Mars. Science 317, 1706-1709.
Pelletier, J.D. et al. (2008) Recent bright gully deposits on Mars: Wet or dry flow? Geology 36 (3), 211-214.
Images: MGS MOC Release, MOC2-1618, NASA/JPL/MSSS
In a very common process, avalanches on dust-covered slopes create slope streaks, which usually are dark at first and then slowly fade.
Aharonson, O. et al.(2003). Slope streak formation and dust deposition rates on Mars, J. Geophys. Res., 108(E12), 5138 and references cited therein.