Gullies in Terra Sirenum

MGS MOC Release No. MOC2-406, 29 June 2003

Sending a very high resolution camera to Mars--the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC)--has resulted in some startling discoveries about the red planet since 1997. One of the MOC discoveries, announced three years ago in June 2000, is the presence of middle- and polar-latitude gullies that were cut by a fluid that behaves as water does. Since their discovery, gullies have generated considerable discussion and debate in the Mars science community. Some speculate that they are caused by groundwater, others suggest melting of subsurface ice or surface accumulations of snow. Still others debate whether the fluid was water, or something more exotic like gaseous carbon dioxide. Regardless, thousands of MOC images obtained since 1997 show that the gullies, while occurring at middle and polar latitudes, do not show a particular preference for poleward-facing slopes, as had been originally determined from the smaller sampling available in 1999 and 2000. Gullies tend to occur in regional clusters and they tend to be associated with layers exposed on the walls of craters, troughs, and valleys. This example shows gullies in the wall of an impact crater in Terra Sirenum near 39.1°S, 166.1°W. The picture was taken on June 10, 2003, the same day as the recent Mars Exploration Rover, Spirit, launch. The scene is illuminated from the upper left and covers an area 3 km (1.9 mi) across.