The weather forecast on early Mars was pretty extreme: asteroids, with a chance of scalding rain. A study of the Martian climate billions of years ago suggests that this rain would have been enough to erode craters on the surface 鈥� which could help us learn more about the history of Mars.
Some of the asteroids and comets that hit Mars more than 3.8 billion years ago are thought to have been large enough to vaporise ice on the planet鈥檚 surface, creating vast clouds of hot water that would fall as rain over decades. This rain has been suggested as the cause of a network of valleys on the planet that date to around the same time, with the liquid carving out the canyons.
But Martin Turbet at Sorbonne University in France and his colleagues aren鈥檛 so sure. They applied an Earth-like climate model to Mars following each of its three largest impacts. These led to the formation of the largest basins on Mars: Hellas Planitia at about 2300 kilometres across, Argyre Planitia at 1800 kilometres and Isidis Planitia at 1500 kilometres. All three are believed to have formed more than 3.8 billion years ago.
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Each impact is thought to be the result of an object 100 kilometres or so in diameter smashing into the surface. The researchers found that such a strike would produce enough energy to vaporise a huge amount of ice into water clouds 鈥� an amount comparable to all the water locked in ice on Mars today.
The team found that each of the impacts would produce an average of 2.6 metres of global rainfall per Earth year for decades. But this precipitation wasn鈥檛 limited to a particular location, meaning it was unlikely to be responsible for carving the planet鈥檚 valley networks, the team says. Instead, the rain would have had a much larger effect on the surface than previously thought.
The widespread rain could solve a long-standing puzzle about Martian craters. More than a decade ago, researchers discovered that craters on Mars dating back more than 4 billion years were much more eroded than those aged 3 billion years or less, even accounting for the additional erosion of an extra billion years.
Turbet and his team suggest rainfall from these large impacts could have been the cause, in effect erasing some of the history of the planet from its surface.
鈥淥ne of the speculations that has been proposed in the past is that maybe there was some precipitation that took place between 3 to 4 billion years ago that eroded most of the old craters,鈥� says Turbet. 鈥淲hat we propose is maybe the large impacts on the surface of Mars are good candidates.鈥�
The findings could help us delve more into the history of Mars, says Ashley Palumbo at Brown University in Rhode Island. 鈥淚t鈥檚 a really important result,鈥� she says. Learning more about how these large impact-induced rainfalls were localised could help us understand the effect they had on shaping the Martian surface, she says.
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