Evidence for an Ancient Sea Level on Mars

Mars shares many similarities and characteristics to Earth including various geological features and planetary structure. The remarkable bimodal distribution of elevations in both planets is one of the most striking global features suggesting similar geodynamic processes of crustal differentiation on Earth and Mars. There also exists much evidence, based on geographic features resembling ancient shorelines, for the existence of an ancient Martian ocean in the northern hemisphere that covers nearly one-third of the planet's surface. However, the interpretation of some features as ancient shorelines has been thoroughly challenged, which left the existence of a primordial Martian ocean controversial. Moreover, if oceans were formerly present on Mars, there is still significant ambiguity about the volume of water with the estimations ranging over 4 orders of magnitude. Here we map the Martian sea level problem onto a percolation model that provides strong evidence that the longest isoheight line on Mars that separates the northern and southern hemispheres acts as a critical level height with divergent correlation length and plays the same role as the present mean sea level does on Earth. Our results unravel remarkable similarities between Mars and Earth, posing a testable hypothesis about the level of the ancient ocean on Mars that can be answered experimentally by the future investigations and spacecraft exploration.