Evidence that water was once present on Mars has been accumulating for decades and this month NASA’s Perseverance rover collected rock samples that have all but confirmed that the groundwater has flowed on the red planet.
But scientists at the space agency were unsure whether this water had been present for tens of thousands of years or millions of years. Today, more and more scientists believe they can help to understand why this water has disappeared.
Mars is conspicuously located in the ‘habitable zone’, at a distance from the sun which is neither too hot nor too cold to support liquid surface water and therefore life – but it seems to support neither the one nor the ‘other for the moment.
The most popular hypothesis as to why Mars is sterile today is based on its lack of a magnetosphere.
Unlike Earth, where the molten iron in the heart of the planet has created a protective magnetic shield around us, Mars’ magnetic field is too weak to shield its atmosphere from cosmic forces.
Now, a study published in the Proceedings of the National Academy of Science suggests that Mars was created with a fatal flaw that meant it would never be able to protect this atmosphere and its liquid water.
“The fate of Mars was decided from the start,” said Kun Wang, an assistant professor at the University of Washington and lead author of the study.
“There is probably a threshold on the size requirements of rocky planets to hold enough water to allow habitability and plate tectonics, with a mass exceeding that of Mars,” he explained.
Wang’s team analyzed isotopes of potassium on Mars, something that can be used to trace volatile elements such as water, and compared them to the Earth, the Moon, and an asteroid called 4-Vesta.
They discovered that there was a clear correlation between the size of the cosmic body and the presence of isotopes of potassium.
Since the only Martian samples available on Earth are those from meteorites, they also often function much like capsules showing what the planet looked like at different stages in its history.
“This study highlights that there is a very limited size range for planets to have just enough but not too much water to develop a habitable surface environment,” said co-author Klaus Mezger of the Center for Space and Habitability. from the University of Bern. “These results will guide astronomers in their search for habitable exoplanets in other solar systems. “
Wang now thinks that, for planets that are in habitable zones, the size of the planet should probably be emphasized more and routinely taken into account when considering whether an exoplanet could support life.
“The size of an exoplanet is one of the easiest parameters to determine,” Wang added. “Based on size and mass, we now know whether an exoplanet is a candidate for life, as a first-order determinant for volatile retention is size. ”