About 3.4 billion years ago, an enormous megatsunami swept over the face of Mars after an asteroid slammed into one of the planet’s oceans. Now, researchers think they have found the crater where the megatsunami began. The size of the crater hints that the impact was similar to that of the Chicxulub asteroid on Earth, which is thought to have killed off the dinosaurs.
In fact, the first images we have of the Martian landscape – taken by the Viking 1 lander in the 1970s – may have contained evidence of this megatsunami. We just didn’t know it yet. Observations of the surface of Mars have previously suggested that a megatsunami happened on the planet, but scientists had not yet found the impact site of the asteroid that caused it. Alexis Rodriguez at the Planetary Science Institute in Arizona and his colleagues combined data from several Mars orbiters to undertake a search.
They found a crater 110 kilometres wide called Pohl in the northern lowlands of Mars that seems just right. It sits atop channels that likely formed as the area first flooded, creating a huge ocean, but there are deposits thought to have come from a later tsunami on top of it. That means that it almost definitely formed in the right time period, before Mars dried out.
Based on the dimensions of the crater and a series of simulations, the researchers found that the asteroid which caused it was either about 9 kilometres across or 3 kilometres across, depending on the properties of the ground it hit. Either way, it probably generated a megatsunami with 250-metre-tall waves reaching as far as 1500 kilometres from the impact site.
“When we think of a tsunami, we think of a wave, a wall of water approaching the shoreline and overrunning it. This would have been very different,” says Rodriguez. “You would have seen this massive wall of turbulent, reddish water, with some of it flying upwards and falling back into the wave along with rocks and soil.” Because Mars has lower gravity than Earth, the water and debris would fall more slowly than it does on Earth.
The impact would have also generated a seismic wave propagating hundreds of kilometres around the crater, throwing dirt and rocks into the air and creating a catastrophic flow of debris along with the wave. “Very terrifying, definitely nothing to surf on,” says Rodriguez. “But if you have a debris flow, you have a lot of soil spread around. So if you actually landed there, you have a chance to sample the ancient marine sediments.”
We actually have landed in the area. The Viking 1 lander, the first craft to ever land on Mars, touched down in the northern lowlands in 1976, within the area the tsunami would have probably reached. The strange boulders in the first pictures we ever saw from the surface of Mars were probably tossed there by a megatsunami, and strange channels on that landscape may have been caused by the water sloshing back into the ocean afterwards.