We knew there was water in abundance on Mars, but we never saw its ocean. This is it, as uncovered by strong new evidence found over the course of two years by the MARSIS radar on board ESA's Mars Express.
Before this discovery, scientists suspected what could have been the shorelines of such ocean. However, this is the first time that this Mars' ocean has been shown in all its magnitude. According to ESA, Mars Express "has detected sediments reminiscent of an ocean floor within the boundaries of previously identified, ancient shorelines on Mars."
The sediments are low-density granular material that have been eroded away by water. They have low radar reflectivity, and were detected through all the ocean's area, 60 to 80 meters (197 to 262 feet) under the surface of the Red Planet.
According to Jérémie Mouginot, from the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) and the University of California, Irvine, the sedimentary deposit they have discovered may be ice-rich. They are "a strong new indication that there was once an ocean here."
Scientists now believe that there were two oceans on Mars. The first was there four billion years ago, when the weather was warm. The second one was formed three billion years ago, "when subsurface ice melted following a large impact, creating outflow channels that drained the water into areas of low elevation." The team believes that this ocean didn't exist long enough to serve as a environment to life formation.
According to ESA's Mars Express Project Scientist Olivier Witasse, there's little doubt now that there were oceans there now. But the biggest question of them all remains a mystery: "Where did all the water go?" [ESA]