The northern hemisphere of Mars is lower than the southern hemisphere, and, when Mars was warmer and wetter more than three billion years ago, it has been theorized, the northern hemisphere was covered by an ocean. However, one big problem with this hypothesis is that it has been difficult to find evidence for an ancient shoreline. A team of scientists led by J. Alexis P. Rodriguez of the University of Arizona published a paper on May 19, 2016, claiming to have solved this problem and presenting evidence of ancient tsunamis.
Unlike Earth tsunamis (such as the Indian Ocean tsunami of 2004)—which are usually caused by quakes, landslides, or volcanoes—tsunamis on ancient Mars would have been caused by asteroid impacts. The resulting waves would have been about 50 meters high (about 160 feet). Rodriguez and his team showed pictures of a place where water seems to have flowed uphill and deposited boulders when it reached its highest height. In a subsequent event, which happened when Mars was cooler, the water would have frozen into ice floes as it pushed its way inland. If tsunamis were a regular occurrence, the resulting messy flows of sediment would have disguised an obvious shoreline. (And, of course, for Mars to have had tsunamis, it needs to have had an ocean.)
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