Scientists studying the complex topography of a region of Mars known as Arabia Terra think they have identified the source of a tsunami that may have crashed into its shore billions of years ago, at a time when many think Mars had ocean covering much of its northern hemisphere.
The idea of a Martian ocean dates back to the 1990s, when Timothy Parker, now at NASA’s Jet Propulsion Laboratory in Pasadena, California, used Viking images to identify what appears to be an ancient shoreline along the edges of the terrain dichotomy that separates the Red Planet’s northern lowlands from its southern highlands.
Since then, scientists have realised that if one of the many asteroids that pocked the Martian surface happened to crash into that ocean, a tsunami was a likely possibility. One of these teams, led by Alexis Rodriguez of the Planetary Science Institute in Tucson, Arizona, has even found what it believes to be tsunami deposits in flat-bottomed channels along the crustal dichotomy.
But now, in research presented at the 2017 Lunar and Planetary Science Conference in The Woodlands, Texas, and to be published this week in the Journal of Geophysics Research: Planets, a team led by Francois Costard, a planetary geomorphologist from the Université Paris Sud in France, believes it has identified the possible sources for one or more of these tsunamis.
Costard and colleagues began by looking for craters that might reveal the locations of tsunami-producing impacts. Then they modeled how the resulting tsunamis would propagate, using what team member Stephen Clifford, a planetary scientist from the Lunar and Planetary Institute in Houston, Texas, calls “well-verified terrestrial models”.
Not that tsunami processes on Mars would be exactly the same as on Earth. Just to begin with, explains Costard, the lower gravity on Mars should increase the amount of sediment a tsunami can carry. That means that for any given-size tsunami, the resulting sediment deposits would be relatively thicker.
Also, a tsunami generated by an asteroid impact of the size examined by Costard’s and Clifford’s team would have been huge – unlike anything humans have ever witnessed.
According to the team’s modeling, the waves near the point where the asteroid struck would have been approximately 300 metres tall. When they hit the coast they wouldn’t be quite that big, but would still have reached 75 or 80 metres, Costard says.
The most probable source of the tsunami, the scientists concluded, is a 60-kilometre impact crater located about 1000 kilometres off the putative coast. But it is also possible that the deposits could have been produced by the combined results of two independent impacts, represented by smaller craters closer to the shore.
The new research also explains bizarre features known as thumbprint terrain, on the seaward ends of some of the tsunami deposits. Composed of curving, concentric ridges 10 to 20 metres high, these look for all the world like the ridges in a fingerprint.
The explanation, Clifford says, starts with the fact that the tsunami would have come in two pulses. The first would have been produced when the asteroid hit, shoving tremendous amounts of water out of its way. The second would have occurred when water rushed back into the resulting depression from all sides.
The onrushing water would have crashed together in the centre of the impact depression in a giant splash, then rebounded outward in a second tsunami, even larger than the first.
And that’s just the beginning of the story. When the first wave, a few minutes ahead of the second, hit the shoreline, part of it would have been reflected back out to sea. There, it would have met the oncoming second wave, where the turbulence would have caused sediment to be dropped in patterns exactly like the enigmatic thumbprint terrains.
But the primary value of the study may not lie so much in localising the source of the tsunami as in confirming the theory that Mars may once have had an ocean capable of producing one.
The idea of an ocean on Mars has been controversial for many years, says Jeff Andrews-Hanna, a planetary scientist from the Lunar and Planetary Laboratory at the University of Arizona, Tucson, who was not associated with the study. And while it may be that no single piece of research will resolve the issue, he says, “this study provides one more piece of evidence”.
This article appeared in Cosmos 75 – Winter 2017 under the headline “Mars and the giant tsunami”
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