Grotzinger, and 10 co-authors, reported in 2011 that processes that operate at planetary surfaces have the potential to record a history of planetary evolution in the form of sedimentary rocks. This is important because our experience on Earth shows that sediments and sedimentary rocks are the dominant archive of high-resolution proxies of present and past tectonic, climatic, and biological processes.
Our understanding of the evolution of Earth’s very ancient climate and paleobiological records derives from detailed examination of the mineralogical, textural, and geochemical signatures preserved in the sedimentary rock record. Sedimentary rocks were first recognized on Mars less than a decade ago (Malin and Edgett, 2000). Recent interpretations of data collected by the Mars Express and Mars Reconnaissance Orbiter spacecraft have confirmed the surprising abundance of these sedimentary rocks, the past role of water on the Martian surface, and the similarity—in some cases—to sedimentary rocks formed on Earth. Thick sulfate-rich deposits invite comparison to terrestrial evaporites (Grotzinger et al., 2005). In other cases, clay-rich strata are interpreted as the terminal deposits of source-to-sink systems with well-developed fluvial networks in the upper reaches of watersheds that date back to a much wetter period in Mars’ earliest history (Ehlmann et al., 2008; Metz et al., 2009). However, these Earth-like depositional systems contrast with other deposits that may be unique in the Solar System: for example, vast terrains as large as Earth’s continents covered by thick veneers of strata that may derive entirely from settling out of wind-transported dust (Bridges et al., 2010).
Whatever their origin, it is now clear that the sedimentary rocks of Mars represent a new frontier for research. Mars science is in its golden era of exploration—the past decade of orbiter and landed missions has produced an extraordinary amount of new data relevant to the analysis of sediments and sedimentary rocks, and robust international programs exist for future missions. To help stimulate discussion of these data, the First International Conference on Mars Sedimentology and Stratigraphy was convened in El Paso, Texas, in April 2010. The contents of this white paper represent the most significant findings of the conference, with additional information provided by the coauthors, and focus on seven key questions for future investigation by the sedimentary geology community.
The authors concluded that Mars has preserved sedimentary rocks that bear a record of the evolution of its surface environments. The preserved record is highly complementary to the terrestrial stratigraphic record. The oldest Martian sedimentary rocks likely exceed 4 billion years; and, on average, the Martian record is much older than the Earth's record. The younger part of the Martian stratigraphic record, which likely overlaps with Earth’s Archean history, shows a dramatic divergence in the evolution of Mars’ surface environments relative to Earth. Perhaps surprisingly, the record on Mars may be better preserved, given the absence of overprinting thermal/metamorphic effects, or penetrative deformation, as is so common in the ancient terrains on Earth that have been affected by plate tectonic processes, including crustal recycling. Accordingly, Martian sedimentary geology provides strong potential for the study of a well-preserved record of a dramatically different pathway in the early environmental evolution of a terrestrial planet.
It is during this key time interval that life underwent early evolution on Earth. At this time, Mars may have experienced a global change from a wet, pH-neutral planet to a drier, more acidic environment. Martian sediments and sedimentary rocks thus provide the fascinating possibility of studying the divergent evolution of two planetary systems whose surface processes were subject to different initial and boundary conditions, including gravitational constant, crustal differentiation, atmospheric properties, and liquid water composition. Considered in this context, sedimentary geology would achieve its broadest significance.
[Here is an update on what is now known about Mars: https://i2mportal.com/search_resource.php?search_value=Mars&sort=date#page=1]
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