We knew there was water in abundance in 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 an ocean. However, this is the first time that the remnants of an ocean on Mars have been shown in all their 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 across the entirety of the ocean's area, 60 to 80 metres 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, 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 in 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 for life formation though.
According to the ESA's Mars Express Project Scientist Olivier Witasse, there's little doubt now that there were oceans. But the biggest question of them all remains a mystery: "Where did all the water go?" [ESA]