The presence of liquid water on Mars could be at the origin of a rare phenomenon observed on the dunes of the Red Planet: this is the finding of the study "Geomorphological observations and physical hypotheses about Martian dune gullies" conducted by researchers from the National Institute of Geophysics and Volcanology (INGV) and recently published in the journal Geosciences of MDPI. At least 3.7 billion years ago, Mars had a much denser atmosphere than it does today and was home to lakes and oceans: over time, much of the planet's atmosphere has been lost, making the presence of stable liquid water on its surface nearly impossible due to very low atmospheric pressure. The INGV-led research analyzed the leeward slope of the Russell Dune, the largest of the wind-formed dunes within the Martian crater of the same name, focusing on the behavior of water under atmospheric conditions of temperature and pressure on the Red Planet. "The analysis of 110 very high-resolution images (up to 25 cm/pixel) collected over 8 Martian years (or about 16 Earth years) by the U.S. Mars Reconnaissance Orbiter probe has allowed us to highlight for the first time the possible presence on Mars of water at its triple point, that is, in an equilibrium capable of the coexistence of solid, liquid and vapor physical states, highlighting a recurring seasonal cycle," explains Adriano Nardi, INGV researcher and first author of the paper. "Albeit for brief periods, in the early days of the Martian spring and during wind gusts, water may appear on this dune every year under atmospheric conditions of temperature and pressure that allow its transient appearance in the liquid state". The water, in this case, would be produced by a meteorological phenomenon typical of the Martian environment that occurs near the surface of the dunes due to their aerodynamic shape, which is impossible to reproduce on Earth where, moreover, the characteristic dune gullies of Mars have never been observed.