On Mars there are scars that seem too big even for an entire planet. Shalbatana Vallis is one of them: a huge valley, stretching near the Martian equator for approx 1,300 kilometersmore or less the same as Italy from north to south. Today it appears dry, rough, crossed by craters, plains smoothed by lava, collapsed areas and dark deposits brought by the wind. Yet, looking at it with the right tools, it stops looking like just a dead landscape. It becomes a kind of open archive, full of clues about a Mars that billions of years ago may have been much warmer, more unstable, wetter.
The new image obtained from the high resolution camera HRSC on board the Mars Express mission shows a portion of the northern part of the valley, where the channel is wedged into the ground and preserves the signs of a rather eventful geological history. Water, volcanoes, impacts, underground ice, slow landslides, materials deposited and then reshaped: everything seems to have accumulated in the same point, as if the landscape had continued to rewrite itself over its own ruins.
A wound dug by the water
Shalbatana Vallis would have formed approximately 3.5 billion years agowhen large quantities of underground water would have found their way to the surface. Imagining Mars as a red and immobile desert helps little here. The original scene must have been more brutal: aquifers or reserves trapped under the ground, pressure, subsidence, then the water breaking the invisible banks and rushing downwards, tearing up material, opening deep furrows, carving out a kilometer-wide channel.
In the observed section, the main valley enters the image from the south-west and bends towards the north, with a width of approx 10 kilometers and depths that reach around the 500 meters. Numbers that would already be enough to show the scale of the phenomenon, even without comparison with our geography. A valley as wide as a city and as deep as an overturned skyscraper speaks of an energy that is difficult to reconcile with the idea of an ever-arid planet.
Scientists believe that Shalbatana Vallis was even deeper in the past. Over billions of years, parts of the channel have filled with different materials, deposits arriving from subsequent processes, layers that have smoothed and covered part of the original shape. In a more irregular area, a dark spot also appears, tending towards blue-black in the chromatic reconstructions, interpreted as volcanic ash then moved and redistributed by Martian winds. This detail also counts: the water opened the wound, volcanism and the atmosphere retouched it.
Where the ocean remains a possibility
Shalbatana Vallis belongs to a larger family of Martian outflow channels, gigantic structures that are concentrated in a particularly interesting area of the planet: the passage between the southern highlands, which are older and cratered, and the northern plains, which are much smoother and lower. This geological boundary is one of the great fractures of Martian history, also because many of the main outflow channels end towards Chryse Planitiaone of the most depressed regions of Mars.
Chryse Planitia itself has fueled a fascinating and still discussed hypothesis for some time: in a remote past, when the planet’s climate could have been warmer and more humid, that area could have hosted a large mass of water, perhaps a ancient ocean. Shalbatana Vallis thus becomes an important piece within a larger map. If enormous volumes of water really flowed towards the low plains, their arrival could have contributed to filling extensive basins, leaving traces that are difficult to read today because Martian time slowly erases, covers, splits, fills.
A virtual flyover of the region had already been released in October 2025, following the path of the valley from its source in the highlands of Xanthe Earth up to the more uniform zone of Chryse Planitia. Seen in this way, Shalbatana Vallis resembles a gigantic natural conduit, a transfer route between high and low areas, between an older and more rugged Mars and a smoother, almost sedimented Mars.
Collapsing land
The strangest part of the landscape is what is called chaotic terrain. The name almost sounds too convenient, but it gives a good idea: broken blocks, irregular reliefs, ridges, mounds, portions of ground that seem to have lost coherence. Around Shalbatana Vallis this terrain appears near dark deposits of volcanic ash, within a larger, more disturbed section of the valley.
The most likely explanation involves underground ice. When ice trapped beneath the surface begins to melt or disappear, the ground above loses support, subsides, shatters, and collapses in a haphazard manner. On Mars, similar landscapes have also been observed in other areas, such as Pyrrhae Regio, Iani Chaos, Ariadnes Colles, Aram Chaos and Hydraotes Chaos. The places change, the same impression remains: the Martian subsoil has been working for a long time, even when the surface already seemed still.
Numerous impact craters then appear around the valley. Some still have clear edges, others have been worn, buried or altered. Several show blankets of ejecta, i.e. material thrown outward at the moment of impact. They are signs of yet another time scale: meteorites arriving after or during the transformations of the region, sharp blows on a surface already full of fractures.
The lava over the old tracks
The smooth surface of many areas instead tells of the passage of the lava. Ancient flows would have covered parts of the landscape, filling depressions and making the terrain more uniform. As the lava cooled and contracted, it formed folds and ripples called wrinkle ridgeslow and sinuous reliefs that look like wrinkles left on a mineral skin.
Here and there isolated reliefs also resist, the mesasremains of once higher surfaces that erosion has worn away around them, leaving them to emerge like rock tables. They are less spectacular details than the large canals, but they help to read the landscape as a set of overlapping times. First the water, then the filling, then the lava, then the wind, then the impacts, then other collapses and other erosions. Mars, seen up close, has less immobility than it appears in clean space-postcard images.
The Mars Express mission has been observing the Red Planet since 2003. In over twenty years it has mapped the Martian surface in color and in three dimensions with a level of detail that has significantly changed the way we look at its geological history. The HRSC camera, developed and managed in Germany, made it possible to build terrain models and perspective views capable of restoring depth, slopes, buried or partially masked shapes.
