Mega-tsunamis Flooded Martian Shorelines

The geologic shape of what were once shorelines through Mars’ northern plains convinces scientists that two large meteorites – hitting the planet millions of years apart – triggered a pair of mega-tsunamis. These gigantic waves forever scarred the Martian landscape and yielded evidence of cold, salty oceans conducive to sustaining life.

The northern hemisphere of Mars hosts a vast, smooth basin that around 3.4 billion years ago, could have held an ocean that covered a greater portion of the planet’s surface than the Atlantic Ocean does on Earth. Already in 1976, features shown by the Viking orbiters revealed two possible ancient shorelines near the pole, Arabia and Deuteronilus, each thousands of kilometers long. Further evidence came in March 2015, when a six-year study was published that used ESO’s Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infrared Telescope Facility, to monitor the chemical properties of the water in different parts of the planet's atmosphere.

Now a team of researchers has given extraordinary new scrutiny to the landforms of Mars' northern regions. As a result, the scientists have identified huge deposits of rocks and sediments, backwash gullies, and other topographic features on the edges of the basin. They report that these features were likely produced when tsunamis raced through the basin’s ocean roughly 3.4 billion years ago.

The giant waves, thought to have reached up to 120 metres in height, went as far as 650 kilometers inland and were powerful enough to shape much of the ancient shorelines on Mars, said J. Alexis Palmero Rodriguez, of the Planetary Science Institute in Tucson, Arizona, who led the study. His tsunami hypothesis helps explain why the shorelines of Mars' ancient ocean can’t be seen everywhere they should be.

The tsunamis could have been triggered by two large meteorites big enough to have left behind craters roughly 30 kilometers in diameter. The scale of impact took place with about the expected frequency over a period of several million years, after the period of the so-called Late Heavy Bombardment.

Similar dramatic events could have occurred elsewhere on Mars: “Although we have only identified evidence for two tsunami events in our study area, other regions in the northern plains likely experienced similar tsunami-related coastal resurfacing, perhaps associated with other impacts, huge landslides or large Marsquakes,” the authors of the study write.

Rodriguez began thinking about Martian tsunamis after visiting the eastern coast of Japan in 2011, which had been devastated by a tsunami generated by a magnitude-9 earthquake. His team grew to include some top Martian geology experts from the US, China and Germany. The group zeroed in on a region on Mars where the highlands known as Arabia Terra bump up against the lowlands of Chryse Planitia—a place where the waters of an ancient ocean might have lapped at the shoreline.

Using imagery from the Mars Reconnaissance Orbiter and the Mars Global Surveyor, Rodriguez’s group identified two particular geological formations that they say formed during two different tsunamis. The first, older formation looks as if an enormous wave had rushed up onto the edge of the highlands, dropping boulders as big as 10 metres across. The water then drained back down into the ocean, leaving channels cut through the freshly deposited debris., and left behind fields of sediment and boulders, and large backwash channels cut into the Martian surface.

During the millions of years between this event and the next, Mars underwent serious global cooling, during which the ocean partially froze and much of it evaporated, causing its sea level to drop by about 300 meters and the shorelines to retreat. Because the climate was so much colder, the second tsunami moved over the landscape like an icy slurry. It froze before it had a chance to wash back into the ocean, leaving dense lobes of frozen debris on the ground.

Parts of the deposits left by this younger tsunami do not appear to have been disrupted over time, suggesting they could yet contain clues about the ancient ocean, such as whether it could have supported life. What’s more, the Curiosity mission’s presence in Gale crater just a couple of hundred kilometers away from the lobes has already demonstrated the feasibility of a future rover reaching the tsunami site.
(May 2016)

Ancient Mars