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#New evidence suggests giant ocean on Mars CALTECH (US) esearchers have found signs of an ancient delta on Mars where a river might have emptied once into a vast ocean. This ocean, if it existed, could have covered much of Mars northern hemispheretretching over as much as a third of the planet. cientists have hypothesized long that the northern lowlands of Mars are a dried-up ocean bottom, but no one yet has found the smoking gun, says Mike Lamb, an assistant professor of geology at California Institute of technology (Caltech). Although the new findings are far from proof of the existence of an ancient ocean, they provide some of the strongest support yet, says postdoctoral scholar Roman Dibiase. Most of the northern hemisphere of Mars is flat and at a lower elevation than the southern hemisphere, and thus appears similar to the ocean basins found On earth. The border between the lowlands and the highlands would have been the coastline for the hypothetical ocean. Researchers used new high-resolution images from the Mars Reconnaissance Orbiter (MRO) to study a 100-square-kilometer area that sits right on this possible former coastline. Previous satellite images have shown that this areaart of a larger region called Aeolis Dorsa which is about 1, 000 kilometers away from Gale crater where the Curiosity rover is now roamings covered in ridge-like features called inverted channels. These inverted channels form when coarse materials like large gravel and cobbles are carried along rivers and deposited at their bottoms, building up over time. After the river dries up, the finer materialuch as smaller grains of clay, silt, and sandround the river erodes away, leaving behind the coarser stuff. This remaining sediment appears as today ridge-like features, tracing the former river system. When looked at from above, the inverted channels appear to fan out a configuration that suggests one of three possible origins: the channels could have once been a drainage system in which streams and creeks flowed down a mountain and converged to form a larger river; the water could have flowed in the other direction, creating an alluvial fan, in which a single river channel branches into multiple smaller streams and creeks; or the channels are actually part of a delta, which is similar to an alluvial fan except that the smaller streams and creeks empty into a larger body of water such as an ocean. Sediment history To figure out which of these scenarios was most likely, the researchers turned to satellite images taken by the Hirise camera on MRO. By taking pictures from different points in its orbit, the spacecraft was able to make stereo images that have allowed scientists to determine the topography of the martian surface. The Hirise camera can pick out features as tiny as 25 centimeters long on the surface and the topographic data can distinguish changes in elevation at a resolution of 1 meter. Using this data the researchers analyzed the stratigraphic layers of the inverted channels, piecing together the history of how sediments were deposited along these ancient rivers and streams. The team was able to determine the slopes of the channels back when water was still coursing through them. Such slope measurements can reveal the direction of water flown this case, showing that the water was spreading out instead of converging, meaning the channels were part of an alluvial fan or a delta. But they also found evidence for an abrupt increase in slope of the sedimentary beds near the downstream end of the channels. That sort of steep slope is most common when a stream empties into a large body of wateruggesting that the channels are part of a delta and not an alluvial fan. No boundaries Scientists have discovered martian deltas before, but most are inside a geological boundary, like a crater. Water therefore would have flowed most likely into a lake enclosed by such a boundary and so did not provide evidence for an ocean. But the newly discovered delta is not inside a crater or other confining boundary, suggesting that the water likely emptied into a large body of water like an ocean. his is probably one of the most convincing pieces of evidence of a delta in an unconfined regionnd a delta points to the existence of a large body of water in the northern hemisphere of Mars, says Dibiase, lead author of the paper published in the Journal of Geophysical Research. This large body of water could be the ocean that has been hypothesized to have covered a third of the planet. At the very least, the researchers say, the water would have covered the entire Aerolis Dorsa region, which spans about 100,000 square kilometers. Of course, there are still other possible explanations. It is plausible, for instance, that at one time there was a confining boundaryuch as a large craterhat was erased later, Lamb adds. But that would require a rather substantial geological process and would mean that the martian surface was more geologically active than has been thought previously. The next step the researchers say, is to continue exploring the boundary between the southern highlands and northern lowlandshe hypothetical ocean coastlinend analyze other sedimentary deposits to see if they yield more evidence for an ocean. n our work and that of othersncluding the Curiosity rovercientists are finding a rich sedimentary record on Mars that is revealing its past environments, which include rain, flowing water, rivers, deltas, and potentially oceans, Lamb says. oth the ancient environments on Mars and the planet sedimentary archive of these environments are turning out to be surprisingly Earthlike. This research was supported by the National Science Foundation NASA, and Caltech

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