PAST WATER ACTIVITY IN ARIADNES COLLES
The floor is characterized by mesas and knobs that are about 1 to 10 km, generally larger the central than in the outer part of the chaos. The knobs mesas typically contain light-toned materials that display numerous fractures filled by light material. Both phyllosilicates and sulfates are recognized in the light toned materials indicating formation in both a water-rich environment and an evaporative environment. The light-toned materials are often covered by a darker rubbly layer interpreted to be lava flows. These morphologies could be the remainder of previously more extended materials, which are more resistant to erosion probably due to a process of cementation by fluid circulation and/or a more resistant capping layer. The presence of dendritic valley networks near the rims of the basin, that run towards its center (for example near 173°E - 38°N) and which confirm past water activity and suggest Ariadnes may have probably hosted a lake, consistent with previous interpretations of a lake in this region. Phyllosilicates record sustained weathering perhaps associated with the valley networks; sulfates may represent periods of evaporation within the lake lifetime.