HiRISE: Gullies and bedrock in Nirgal Vallis

The gullies in this image are within the valley wall of an ancient channel — Nirgal Vallis — a testament to flowing water in Mars’ ancient past. However, the formation of gullies are still the subject of much debate with respect to their formation: “wet” vs. “dry” or even “dry” with the aid of some lubricating fluid… [More at link]