Active gullies on Mars

Planetary Geomorphology Image of the Month, January 1, 2018: Colin Dundas (U.S. Geological Survey). Martian “gullies” are a class of landforms on steep slopes, characterized by an upper alcove and a depositional apron, linked by a channel. On Earth, similar features would likely be termed ravines or alluvial fans. The Martian features usually appear geomorphically fresh, with sharply defined channels and no superposed impact craters. Changes were first detected in Martian gullies over a decade ago, and such observations have become more common as high-resolution repeat image coverage has expanded. This current activity correlates with seasonal frost (which is mostly CO₂ on Mars) and has resulted in substantial modification of some gullies, leading to a debate over whether CO₂ alone is sufficient to form them without liquid water. [More at link]