EARTH SCIENCES – Mars and the giant tsunami

Once upon a time, massive tidal waves carved the cliffs of the Red Planet, RICHARD A. LOVETT reveals.

Scientists studying a region of Mars known as Arabia Terra think they have identified the source of a tsunami that might have crashed into its shore billions of years ago.

The idea of a Martian ocean dates 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 terrain separating the planet’s northern lowlands from its southern highlands.

Scientists later concluded that if an asteroid crashed into that ocean, a tsunami was a likely result.

Now, in research published 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 of one or perhaps two Martian tidal waves.

Costard and colleagues began by looking for craters that might reveal the locations of tsunami-producing impacts. Then they modelled how such waves would propagate, using what were described as “well-verified terrestrial models”.

Tsunami processes on Mars would not be exactly the same as on Earth. To begin with the planet’s lower gravity should increase the amount of sediment the wave could carry. That means that for any givensize tsunami, the resulting deposits would be relatively thicker.

Also, a tsunami generated by an asteroid impact of the size examined by Costard’s team would have been huge – unlike anything humans have ever witnessed.

According to the modelling, the waves near the impact point would have been 300 metres high. By the time they hit the coast they would still have reached 75 or 80 metres.

The most probable source of the tsunami, the scientists concluded, is a 60-kilometre-wide crater about 1,000 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 research also explains bizarre features on the seaward ends of some of the tsunami deposits. Composed of curving, concentric ridges 10 to 20 metres high, these look just like the ridges in a thumbprint.

The explanation 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.

The water pulses would have crashed together in the centre of the impact zone in a giant splash, rebounding 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 to sea. There it would have met the oncoming second wave, causing sediment to be dropped in patterns exactly like the enigmatic thumbprint terrains.

The primary value of the study, however, 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.

“ACCORDING TO THE MODELLING, THE WAVES NEAR THE IMPACT POINT WOULD HAVE BEEN 300 METRES HIGH.”