Vast deposits of salt indicate that Mars' surface and liquid water basins have been present on the planet for about a billion years longer than previously believed, according to a new study.
As of 2008, NASA's Mars Odyssey orbiter found hundreds of sodium chloride deposits (table salt) covering hundreds of square kilometers. Not only do they show that Mars was much wetter a long time ago, but they also provided a method for determining the last period when water on Mars was present on the surface.
"Salt is incredibly soluble," he says Ellen Leask, lead author of a research recently published on AGU Advances. "These deposits must have formed during one last major evaporation of water from the planet."
How did these deposits form? And when?
To answer these questions and understand when the water on Mars disappeared from the surface, Leask and colleagues analyzed the salt deposits, observing on what types of morphologies they formed and how they deposited on the soil of the red planet.
Data from the Mars Reconnaissance Orbiter (MRO), combined with samples found on Earth, led the researchers to discover that the salt deposits are surprisingly thin (less than 3 meters thick and located in topographical minima) in the Martian subsoil.
The closest terrestrial analog we can find are the chains of lakes that form in Antarctica during the winter, when the snow seasonally melts over the permafrost.
Martian salt deposits are often found in shallow depressions, sometimes perched above much larger craters that are devoid of deposits. This orientation would seem to indicate that the salt comes from the surface runoff of water on Mars during an ice freeze-thaw cycle, according to the paper. In this case, Leask and colleagues found the chloride deposits atop a volcanic soil that formed only 2,3 billion years ago. Previously, water on Mars in liquid and large-scale form was believed to have disappeared about 3 billion years ago.
Water on Mars: new perspectives
"This discovery gives us new targets for future missions to Mars," says Leask. “Some of these deposits are located on land that is a billion years younger than the land the Perseverance Rover is currently riding on. This greatly expands our knowledge of water on Mars ”.
Future missions could analyze the chloride structures to verify that they are due to evaporation - and the data, at that point, could also reveal which organic chemicals were present in the water on Mars when it was at the surface and in a liquid state.