A new gully has appeared on a sloped crater wall on Mars. The channel, which was absent from images in November 2010 but showed up in a May 2013 photo, does not appear to have been formed by water. Exactly what caused this Red Planet rivulet remains a mystery.

The winding gully seems to have poured out from an existing ribbon channel in a crater in Mars' Terra Sirenum region. The leading hypothesis on how the gully formed is that debris flowed downslope from an alcove and eroded a new channel. Though it looks water-carved, the gully is much more likely to have been formed when carbon dioxide frost accumulated on the slope and grew heavy enough to avalanche down and drag material down with it.

Because the pair of images, taken by the orbiting HiRISE camera onboard NASA's Mars Reconnaissance Orbiter, were taken more than a year apart, scientists don't know in exactly which season the new gully formed. Similar activity has been seen to occur during the Martian winter at temperatures too cold for water, which is why researchers think carbon dioxide is a likelier cause. While the formation of these gullies on Mars is well documented, scientists have yet to work out exactly how they work.