A MORE VAST EARLY MARS SEDIMENTARY ROCK RECORD

Mars presents a natural laboratory where the “experiment” of sediment diagenesis and preservation of the rock record ran differently than on Earth. Mars has sedimentary rocks older than the oldest on our planet, corresponding to Earth’s Hadean and early Archean Eons. This is not to say that the planets were alike at that time, but, owing to the tectonic conditions of Earth and Venus, Mars is the only terrestrial world in our solar system, possessive of a substantial atmosphere, that has preserved an environmental record so ancient. Thus, there is potential to gain insight into the nature of early Earth environments through study of the Martian sedimentary rock record. Evidence that sedimentary rocks occur on Mars was first published in December 2000 (doi:10.1126/science.290.5498.1927). Before then, the words “sedimentary” and “rock” rarely occurred together in the Mars scientific literature. Over the next 16 years, many related discoveries ensued, including detection, in some of these rocks, of clay and sulfate minerals. The most readily recognized occurrences, which received the most attention, were those viewed via orbiting instruments in which the rocks exhibit stratification, a light tone, and have very few superimposed impact craters. Some occurrences, such as a lithified delta in Eberswalde crater and a lithified dune field near the Apollinaris Sulci, have erosional expressions that mimic their depositional setting. Imaging observations made possible by the landing of the Curiosity rover in Gale crater in August 2012, combined with the 2006 to 2017 mapping of the entire planet at 6 m per pixel by the Mars Reconnaissance Orbiter Context Camera, show that the sedimentary rocks studied since December 2000 are just a sub-set of a much greater whole. They are now known to include lithified impact ejecta deposits; bodies of rock that are dark-toned, heavily cratered, and have mafic compositions; rocks for which “light tone” is a function of mm-scale roughness and dust cover rather than bedrock color or tone; and Curiosity rover observations showed that Mars has mudstones, conglomerates, fluvial as well as eolian sandstones, and the data facilitate understanding of the connection between landscape expression and rock type. Overall, Mars has more sedimentary rock exposed at its surface than recognized just a few years ago.