Figure: Distributary Fan: "Smoking Gun" Evidence for Persistent Water Flow and Sediment Deposition on Ancient Mars (Eberswalde Delta)

Mars Global Surveyor (MGS) has been operating in Mars orbit longer than any other spacecraft. The Mars Orbiter Camera (MOC) began taking pictures in September 1997, shortly after MGS arrived. Since that time, it has only imaged about 3% of the martian surface with its high resolution (1.5 to 12 meters per pixel), narrow angle (NA) camera system. Thus, an important discovery from MOC can--and does--come at any time, even five and six years into the mission.

This week, the journal Science has published online (in Science Express) the most recent MOC discovery: an ancient, eroded, and exhumed sedimentary distributary fan located in a crater at 24.3 ^oS, 33.5 ^oW. A distributary fan is a generic term used by geologists to describe a family of deposits that includes river deltas and alluvial fans. Sometime in the distant past, when it was still possible for liquid water to flow across the martian surface, sediments transported through valleys by water formed a fan-shaped deposit in a 64 km diameter crater northeast of Holden Crater.

What is important about this discovery? First, it provides clear, unequivocal evidence that some valleys on Mars experienced the same type of on-going, or, persistent, flow over long periods of time as rivers do on Earth. Second, because the fan is today a deposit of sedimentary rock, it demonstrates that some sedimentary rocks on Mars were, as has been suspected but never clearly demonstrated, deposited in a liquid (probably water) environment. Third, the general shape, pattern of its channels, and low topographic slopes provide circumstantial evidence that the feature was actually a delta--that is, a deposit made when a river or stream enters a body of water. In other words, the landform discovered by MOC may be the strongest indicator yet that some craters and other depressions on Mars once held lakes. Although hundreds of other locations on Mars where valleys enter craters and basins have been imaged by MOC, this is the first to show landforms like those presented here.

This picture is a map showing the location of the distributary fan. It is at the west end of a ~64 km wide crater ("Holden NE" Crater) northeast of the much larger, Holden Crater. The white box indicates the location of the fan. To the west of the fan are several valleys that fed water and sediment to the "Holden NE" Crater. This map is a mosaic of Mars Odyssey daytime thermal infrared images released by the THEMIS team. The contours are topography, relative to the martian datum (the elevation 0 contour) derived from observations acquired by the MGS Mars Orbiter Laser Altimeter (MOLA) experiment. Sunlight illuminates the mosaic from the upper left.

The MGS MOC continues to return new pictures every day, and will provide critical support to the Mars Exploration Rover (MER) mission beginning in January 2004. Together with the MGS Mars Relay (MR) system, MOC will provide near-realtime coverage of engineering telemetry during the critical entry, descent, and landing (EDL) of both MER-A ("Spirit") and MER-B ("Opportunity"). After landing, MOC/MR will relay rover science data from the surface of Mars back to Earth, while simultaneously continuing its on-going science investigations, in particular searching for further evidence of the environmental conditions both in early martian history and more recently. Given the results to date, exciting, unexpected discoveries are certain to follow.

Credit: NASA/JPL/Malin Space Science Systems

http://www.msss.com/mars_images/moc/2003/11/13/