While most of the scientific community believes that at one point, billions of years ago, there was water on Mars, it has been a difficult task to prove true. Two researchers at the University of Colorado at Boulder, Brian Hynek and Gaetano Di Achille have uncovered some new evidence that might shed light onto the subject.
While analyzing 52 different river deltas on the Martian surface, they discovered an interesting connection between 17 of them. Deltas are piles of sediment that form at the outlets of a river, similar to the one that the Mississippi River has been creating at the Gulf of Mexico. What Hynek and Achille have discovered is that 17 of the deltas on Mars are at a similar elevation within one standard deviation of each other, never straying more than 177 meters above or below the elevation they believe to be the surface of an ancient Martian ocean.
During a period of time 3.5 billion years ago when Mars is thought to have been warmer, wetter and have a more hospitable climate, these deltas were formed around the same ocean. When overlayed on maps with the shorelines from the same ocean that other researchers have conceived and drawn out, there is a very high consistency and overlap. It seems like a convincing argument then, that an ocean did exist at that time.
However, other scientists in the field remain skeptical because of inconsistencies with the remaining 35 deltas. Some of them lay well below the cluster supposedly at the level the proposed ocean. To have deltas and deep networks of valleys well below the surface of water is highly improbable if an ocean was formed at that time. Despite those inconsistencies though, the same skeptical researchers do believe that the abundant evidence presented from many varying sources points to water on Mars.