THE MYSTERY WATER OF MARS (GALE CRATER)

Since August, 2014, the Mars Science Laboratory has been investigating layered rocks in Gale Crater on Mars. These were interpreted, initially from orbital data, as mainly ancient alluvial fan and lake deposits, dating from an early warm and wet era. A problem, as stated by H. Y. McSween, Jr. for Mars in general (2015, Am. Mineral., p. 2387), is: “The compositions of sedimentary rocks are very similar to basalt at the same locations, pointing to their...lack of chemical changes during weathering...The resulting trend is notably unlike terrestrial basaltic sedimentary rock compositions...” In other words, why didn’t the Martian water weather the basalt? It wasn’t too cold, inasmuch as ice-related features (moraines, ice wedges, etc.) were interpreted to be lacking at the time of deposition. At Gale (and other rover sites), the mystery is worse, because there is no direct evidence of transport or deposition by water either - no paleo-channels, no lateral facies variations related to streamflow, no evaporites, no mud cracks, no de-watering textures (e.g., load casts), and so on. Note that rounded clasts indicate abrasion, not water. There are no laminated shales, although especially fine-grained clastic sediments (of composition similar to that of Martian dust) have been interpreted as lacustrine, despite cross-bedding.

During the billions of years that followed deposition, these sediments underwent limited interaction with water. Some diagenetic alteration was pervasive, and much fracture-controlled. It mainly involved the formation of primitive clays (smectites), iron oxides, and amorphous materials (although some of this could be inherited glass). In addition, the sediments are veined by Ca-sulfates, and some paleo-surfaces and veins show evidence of post-depositional acid alteration (e.g., silica-rich or Fe-sulfate bearing compositions), probably representing acid fluids descending from the surface. Sandy, cross-bedded sediments interpreted as eolian have undergone very similar post-depositional alteration.

Unless ancient liquid water on the surface of Mars was chemically inert, perhaps it is time to look at other origins for these layered sedimentary rocks. The observed grain sizes and sedimentary structures could easily be created by impacts on a basaltic planet with an atmosphere and ice.