1. Paleo-surface Long/lat: from 341.35E, 24.21N to 341.50E, 23.95N Rational: A layer of the clay unit remained at the surface for a longer time than the rest of the sequence, during the depositional process, and henced recorded impacts before being covered by later layers. This surface has likely recorded different conditions than the rest of the clay unit. This paleo-surface is similar to but smaller than the paleo-surface identified on the western and northern parts of the region of Mawrth Vallis by Loizeau et al. (Icarus, 2010). Morphology & mineralogy: the dark flat surface displays many circular features. It has long outcrops along the flanks of Mawrth Vallis. The morphology is detailed in the next slides. The surface bears Fe/Mg-smectites, but likely contains also pyroxene, by analogy with the paleo-surface of the western part of the region (Poulet et al., A&A, 2008) MSL tasks: At large scale, study the relation of the paleosurface to upper and lower layers, the circular features, its fracturation. At small scale, the grain structure, the mineralogy and chemistry of the rocks, in the circular features and in between. HRSC mosaic with HRSC nadir & color images

2. 2 km CTX & HiRISE mosaic with HiRISE color images

3. 2 km CTX & HiRISE mosaic with HiRISE color images The paleo-surface is exhumed in long outcrops along the flank of Mawrth Vallis. It has a similar albedo than the dark cap, but has a first and strong difference: it is altered (Fe/Mg- smectites).

4. 2 km CTX & HiRISE mosaic with THEMIS thermal inertia (Fergason, R. L.) Next slide The paleo-surface has a strong thermal inertia compared to the rest of the site.

5. 500 m HiRISE mosaic Next slide The paleo-surface appears as relatively flat (compare to the rest of the clay-unit). Many circular features are the most striking morphological feature of this surface: they are interpreted as the record of ancient craters (from impacts at the time of the formation of this layer), later filled by upper layers, and now eroded and exposed. After next slide

6. 100 m HiRISE mosaic Faint circular features are visible on most of the surface (the largest are indicated by white arrows), with sizes from ~100 m to less than 10 m.

7. 100 m HiRISE mosaic On the eastern, most eroded part of the outcrop, only the largest circular features remain.