2. Complex Surface Geology
Discovery: The Martian surface is geologically diverse and has evolved from ancient into recent times.
Significance: Comparing the geological evolutions of Earth, the Moon, and Mars gives insight into all three, while the Mars surface’s evolution determines whether life could have evolved there.
Mars through the 200” Palomar telescope during the 2003 close approach. Image courtesy Stan Mechev and Palomar Observatory. |
Knowledge of the martian surface has evolved from Percival Lowell’s “canal” drawings (which illustrated books until the 1960s) to detailed geologic maps from orbital observations. Lowell’s sketch of Mars, prior to 1914. Image: public domain. |
Mars Global Geologic Map 1802ABC, prepared from Viking orbital photographs and Mars Global Surveyor MOLA data. Legend with original at http://webgis.wr.usgs.gov/pigwad/down/mars_geology.htm |
1. eroding Arabia Terra layers. 2. Fossilized
distributary fan in Melas Chasma, thought to be the remains of an ancient lake –
one proposed site for the Mars Science Laboratory. 3. Ancient craters emerging
from beneath and within eroding sediments. 4. Sandstone layers at Burns Cliff,
Meridiani Planum.
Sedimentary rock: About two-thirds of Mars’s surface appears to date from its earliest (Noachian) geologic era, roughly four billion years ago, when Mars was wetter than today. Many areas display complex sedimentary formations.
Photos and annotations 1-3: Mars Odyssey MOC camera; NASA/JPL/Malin Space Science Systems. Photo 4: approximately true-color mosaic, Opportunity Pancam: NASA/JPL/Cornell.
From [SCIENCE 324:736 (2009)]. Reprinted with permission from AAAS. |
Basalt is the most abundant rock type on the surface of Mars, but a wide range of compositions has been found besides just the typical basalt. Plots of sodium oxide plus potassium oxide (referred to as “total alkalis”) versus silica are typically used for geochemical classification of volcanic rocks on Earth. Not all of the martian materials measured are volcanic, but such a plot is still useful for examining compositional range. This plot of measurements of Mars materials demonstrates the extraordinary diversity of the composition of martian surface materials, from very low to very high silica and from low to high alkalis. The red outlined areas are Mars Pathfinder soil compositions measured by Sojourner rover and Meridiani soil compositions measured by Opportunity rover, the orange rectangle shows compositions measured by Odyssey’s Gamma Ray Spectrometer (GRS), the rainbow colored point density shows Odyssey’s Thermal Emission Spectrometer (TES) estimated compositions, the small blue and red dots are measurements of Gusev rocks from Spirit rover, the large blue dots are compositions of Pathfinder rocks, and the other small symbols are martian meteorites found on Earth. |
From McSween et al. Igneous rock: McSween, H.V. et al. Science
324, 736-739 and references cited therein.
At its Gusev crater landing site, Spirit discovered eroded volcanic outcroppings of amorphous hydrated silica (SiO2•nH2O), probably produced by hydrothermal or fumarolic activity (on Earth, hydrothermal silica is a bacterial habitat.) |
Clays containing entrained water occur in distributary fans; these again are potential ancient habitats and several are suggested sites for the Mars Science Laboratory. (above) False-color image of distributary fan in Jazero crater, showing clays in green, from Ehlmann et al. |
Water alteration: Few water-affected rocks are found on Mars compared to Earth, but they occur in a wide range of environments. Water-altered minerals (such as sulfates) are widespread in sedimentary rock. Hydrated silicate and hydrated sulfate minerals are exposed at many points on the Martian surface, where rocks were altered by groundwater and volcanism-produced sulfuric acid; some of these appear geologically middle-aged or even younger, indicating that groundwater activity occurred that recently.
Squyres, S. W. et al. Detection of silica-rich deposits on Mars,
Science 320, 1063-1067 (2008).
Milliken, R.E. et al, Opaline silica in young
deposits on Mars, Geology 36 (11), 847-850 (2008).
Mustard, J.M. et al.,
Hydrated silicate minerals on Mars observed by the CRISM instrument on Mars
Reconnaissance Orbiter, Nature 354, 305-309 (2008).
Ehlmann, B.E. et al. Clay
minerals in delta deposits and organic preservation potential on Mars, Nature
Geoscience, 1, 355 (2008).
