A TWO-DECADE JOURNEY, LEARNING TO RECOGNIZE THAT MARS HAS A VAST AND ACCESSIBLE SEDIMENTARY ROCK RECORD (Invited Presentation)

A foundational step in the study of the rock record of another world is to learn to recognize occurrences of sedimentary rock. When the Curiosity rover landed on Mars in 2012, it seemed possible that this step had already been completed, even though, just 12–15 years earlier, features on Mars were rarely interpreted as sedimentary rock. Sedimentary deposits, yes. Rock, no.

Grotzinger and Milliken (2012, doi:10.2110/pec.12.102.0001) summarized the first decade of discovery. By then, a canonical view had emerged that most Martian sedimentary rocks are stratified, light-toned, and erodible such that they retain few impact craters. Interpretation of data from orbiting infrared spectrometers showed that some include phyllosilicates, sulfates, hematite, chlorides, and opaline silica. The Opportunity rover team studied light-toned, mafic, sulfate- and hematite-bearing sandstone. Mafic clastic rocks, interpreted by team members as tephra and wind-reworked tephra, were observed via the Spirit rover.

Observations of the Curiosity site, coupled with 99.6% coverage of Mars at 0.25 to 6.0 m/pixel by orbiter cameras and continued data analysis, have up-ended the canonical view. The sedimentary rocks studied before 2012 were a sub-set of a greater whole. Updates include:

Rock type - sandstones were known but mudstones and conglomerates were not certain before 2012–2013.

Tone - some sedimentary rocks are dark-toned; light tone can be a property of the rock or it can be a product of minimal roughness and eolian dust retention.

Impact crater retention - firmly cemented sedimentary rock retains small craters as well as do lava flows.

Composition - mafic clastic rocks might dominate; spectroscopy cannot be used, alone, to distinguish between sedimentary, igneous, and shock-metamorphic rocks.

Depositional setting mimicry - Some occurrences are distinguished by erosional expressions that mimic patterns of original depositional settings (e.g., lithified delta, eolian dunes). Some impact crater ejecta deposits are part of the rock record.

The Martian sedimentary rock record includes strata older than the oldest examples on Earth. The archive presents an opportunity for the earth and life sciences through records of analog depositional and diagenetic environments that are time-correlative with Earth’s Hadean and Archean eons.