4.12.2022

It left a crater 69 miles wide.

A view from NASA's Mars Reconnaissance Orbiter of a crater on Mars that formed in 2012. (Image credit: NASA/JPL-Caltech/Univ. of Arizona)

A Martian megatsunami — a giant killer wave that may have reached more than 80 stories tall — may have raced across the Red Planet after a cosmic impact similar to the one that likely ended Earth's age of dinosaurs, a new study finds.

Although the surface of Mars is now cold and dry, a great deal of evidence suggests that an ocean's worth of water covered the Red Planet billions of years ago. Previous research found signs that two meteor strikes might have triggered a pair of megatsunamis(opens in new tab) about 3.4 billion years ago.The older tsunami inundated about 309,000 square miles (800,000 square kilometers), while the more recent one drowned a region of about 386,000 square miles (1 million sq. km).

A 2019 study found what may have been ground zero for the younger megatsunami — Lomonosov Crater, a 75-mile-wide (120 km) hole in the ground in the icy plains of the Martian Arctic. Its large size suggests the cosmic impact that dug the hole itself was big, similar in scale to the one from a 6-mile-wide (10 km) asteroid that struck near what is now the town of Chicxulub in Mexico 66 million years ago, triggering a mass extinction that killed off 75% of Earth's species, including all dinosaurs except birds.

Now the new study finds what may be the origin point of the older megatsunami — 69-mile-wide (111 km) Pohl Crater, which the International Astronomical Union named after science-fiction grandmaster Frederik Pohl in August. 

The scientists focused on the landing site of NASA's Viking 1, the first spacecraft to operate successfully on the Martian surface. Viking 1 touched down in 1976 in Chryse Planitia, a smooth circular plain in the northern equatorial region of Mars. The probe landed near the endpoint of a giant channel, Maja Valles, carved out by an ancient catastrophic flood, the first time scientists identified an extraterrestrial landscape carved by a river.

Unexpectedly, instead of discovering the kind of flood-related features scientists had expected of the site, such as streamlined islands worn smooth by flowing water, they found a boulder-strewn plain. Now the researchers suggest these boulders may be debris from a megatsunami, the giant wave carrying pulverized rock away from the site of the cosmic impact.

"The marine floor would have been tossed up in the air, feeding the wave with sediments and probably aiding the development of a catastrophic debris flow front," study lead author Alexis Rodriguez, a planetary scientist at the Planetary Science Institute in Arizona, told Space.com

The scientists analyzed maps of the Martian surface, created by combining images from previous missions to the planet. This helped them identify Pohl, which is located about 560 miles (900 km) from Viking 1's landing site, within a region of the Martian northern lowlands.

"The northern plains of Mars comprise an enormous basin where about 3.4 billion years ago, an ocean formed and subsequently froze," Rodriguez said. "The ocean is considered to have formed due to catastrophic floods released from aquifers. So my initial approach to looking for a megatsunami-triggering impact was to look for a crater beneath the ocean's frozen residue and above the channels that discharged the ocean-forming floods." Pohl was the only crater the scientists found that met this criterion, he noted.

The researchers simulated cosmic impacts on this region to see what type of impact might have created Pohl. Their findings suggest that Viking 1's landing site is "part of a megatsunami deposit emplaced about 3.4 billion years ago," Rodriguez said.

Then, the scientists used simulations to understand how a crater with similar dimensions to Pohl might have originated. If an asteroid encountered strong ground resistance, it would have needed to be about 5.6 miles (9 km), with the impact unleashing energy equivalent to 13 million megatons of TNT; if the asteroid met weak ground resistance, it might have been only 1.8 miles (3 km) across, releasing the energy of 500,000 megatons of TNT. (In comparison, the most powerful nuclear bomb ever tested, Russia's Tsar Bomba, had the strength of only 57 megatons of TNT.)