Scientists say mega-tsunamis shaped the surface of MarsParis, May 19(AZINS) Monster tsunamis caused by meteor impacts swept across the northern plains of Mars more than three billion years ago, radically resculpting the edges of the Red Planet's ancient seas, according to a study published today.

The findings, based on geological mapping, could provide new clues in the search for life. They also bolster the theory that massive floods 3.4 billion years ago transformed Mars' northern lowlands into a sprawling ocean, the study said. Some scientists had challenged that notion, pointing out that the alleged shoreline of this long-gone sea is today ragged and uneven -- not a landscape one would expect to find ringing an ocean. "Our discoveries reconcile the ocean hypothesis with the puzzling absence of shorelines distributed along a constant elevation," lead author Alexis Rodriguez, a researcher at the Planetary Science Institute in Tucson, Arizona, told AFP.

These mega-tsunamis probably numbered in the dozens over hundreds of millions of years , but the study, published in Nature's Scientific Reports, focused on two that happened a few million years apart. The first carried boulders and debris tens, even hundreds, of kilometres inland. The second rose up during a much colder period, tossing off huge blocks of ice as the waves froze in mid-air. Rodriguez and his team traced the ground zero of the tsunamis to two craters each about 30 kilometres across.

The giant waves would have averaged about 50 metres in height, but likely rose up some 120 metres -- visualise a 30-story building -- as they ripped up shorelines and rushed inland. They each submerged areas roughly the size of France and Germany combined before retreating back into the primordial Martian sea. No other explanation can account for the formations uncovered, said Rodriguez. "The shapes of the deposits we mapped are indicative of upward flows" powerful enough to transport boulders hundreds of kilometres, he explained by email.

Mars is surely the most scrutinised planet in our Solar System (besides our own), and yet no one seems to have noticed the evidence of mega-waves in the Red Planet's past. "My guess is that we were trying to find shorelines on Mars like those we typically see on Earth," Rodriquez said. The second tsunami could provide a new hunting ground for signs of life early in Mars history.