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°S, 33.5°W. 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-kilometer (40 miles) -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.