HEAVILY-CRATERED SEDIMENTARY ROCK OCCURRENCES AT THE SURFACE OF MARS

The terms “sedimentary” and “rock” were rarely used together to describe materials on Mars before examples were seen in Mars Orbiter Camera and Opportunity rover data in 1999–2004. An attribute of these occurrences is that they are relatively uncratered, especially with sub-km-diameter craters. These rocks are interpreted as being less resistant to recent erosive processes and thus easily disaggregate into constituent grains that are removed by wind. In contrast, surfaces formed of lava usually display many sub-km-diameter craters (e.g., Arsia Mons caldera floor; Gusev plain investigated by Spirit in 2004). These igneous rocks not only retain small craters, they retain the impact-produced boulder and cobble ejecta. Such lava surfaces can be more cratered than older materials they embay or superpose, highlighting the challenge presented by resistance to erosion in assessing small crater populations for age dating of the surfaces. Observations made using Curiosity rover cameras in Gale crater show that not all Martian sedimentary rocks are lightly- or un-cratered. Some are as heavily-cratered as lava surfaces seen elsewhere on Mars, suggesting they are similarly resistant to erosion. The examples in Gale are mafic sandstones; images indicate they are well cemented and erosion-resistant but not un-erodible. Outside of Gale, a few examples of similar materials were known since 1999, such as a heavily cratered unit, interpreted to be sandstone, that retains the morphology of an eolian dune field (at 12.8°S, 181.9°W). With more than 91% of the planet covered at resolutions of 6 m/pixel or better, images from the Mars Reconnaissance Orbiter cameras show many more previously unrecognized examples of heavily-cratered materials of similar setting and/or morphology. Some of these occur at the distal ends of intracrater sedimentary fans (e.g., Nicholson crater); others examples are interpreted to be inverted stream channels and ponds in intercrater terrain (e.g., at 26.3°N, 339.0°W and 10.9°N, 313.2°W). The recognition of sedimentary rock occurrences, as seen in high-resolution orbiter images, has evolved from the criteria first described by Malin and Edgett (2000; doi:10.1126/science.288.5475.2330), to a broader range of attributes that now can include surfaces that are heavily cratered at sub-km-diameter scale.