Viking 1 may have landed at the site of an ancient Martian megatsunami

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When NASA’s Viking 1 lander made history as the first spacecraft to land on Mars on July 20, 1976, it sent back images of a landscape no one expected.

These first images taken from the ground there showed a surprisingly boulder-strewn surface in the red planet’s northern equatorial region, rather than the smooth plains and flood channels expected based on images of the area taken from space.

The mystery of the Viking landing site has long puzzled scientists, who believe an ocean once existed there.

Now, new research shows that the lander touched down where a Martian megatsunami deposited material 3.4 billion years ago, according to a study published Thursday in the journal Scientific reports.

The cataclysmic event likely occurred when an asteroid struck the shallow Martian ocean—similar to Chicxulub asteroid impact which wiped out the dinosaurs on Earth 66 million years ago, according to researchers.

Five years before Viking I landed, NASA’s Mariner 9 spacecraft orbited Mars, spotting the first landscapes on another planet that suggest evidence of ancient flood channels there.

Interest in the red planet’s potential for life prompted scientists to select its northern equatorial region, Chryse Planitia, as the first Martian landing site for Viking I.

“The lander was designed to look for evidence of existing life on the surface of Mars, so in order to choose a suitable landing site, engineers and scientists at the time faced the difficult task of using some of the earliest images obtained of the planet, accompanied from Earth-based radar surveys of the planet’s surface,” said study lead author Alexis Rodriguez, a senior scientist at the Planetary Science Institute in Tucson, Arizona, via email.

“The selection of a landing site had to fulfill a critical requirement – the presence of extensive evidence of former surface water. On Earth, life always requires the presence of water to exist.

At first, scientists thought the rocky surface might be a thick layer of debris left behind by space rocks slamming into Mars and creating craters, or broken pieces of lava.

But there weren’t enough craters nearby, and lava fragments turned out to be rare on the ground at the site.

“Our investigation provides a new solution — that a megatsunami washed ashore, depositing sediments that the Viking 1 lander touched down about 3.4 billion years later,” Rodriguez said.

Researchers believe the tsunami was triggered when an asteroid or comet struck the planet’s northern ocean. But finding a resulting impact crater is difficult.

Rodriguez and his team studied maps of the Martian surface produced by various missions and analyzed a newly identified crater that appeared to be the likely point of impact.

The crater is 68 miles (almost 110 kilometers) across part of the northern lowland, an area that was once likely covered by an ocean. The researchers simulated collisions in this region using modeling to determine what impact was needed to create what is known as Paul Crater.

This was possible under two different scenarios, one caused by a 5.6-mile (9-kilometer) asteroid encountering strong Earth drag and releasing 13 million megatons of TNT energy, or a 1.8-mile (2.9-kilometer) asteroid , which crashes into softer ground and releases 0.5 million megatons of TNT energy.

For perspective, the most powerful nuclear bomb ever tested, King Bombacreated 57 megatons of TNT energy.

During the simulations, both impacts created a Paul-sized crater as well as a megatsunami that reached 932 miles (1,500 kilometers) from the impact site.

The 1.8-mile-long asteroid generated an 820-foot (250-meter) tsunami after it hit land.

The results were similar to those of the Chicxulub impact on Earth, which created a crater that was initially 62 miles (100 kilometers) in diameter and caused a huge tsunami that went around the world.

The impact likely sent water vapor into the atmosphere, which would have affected the Martian climate and potentially created snow or rain in precipitation. Huge amounts of water from the shallow ocean, as well as sediment, would have been displaced, Rodriguez said, although most of the water returned to the ocean soon after the megatsunami reached its peak.

“The seismic shaking associated with the impact would have been so intense that it may have dislodged materials from the seafloor in the megatsunami,” study co-author Darrell Robertson of NASA’s Ames Research Center in California’s Silicon Valley said in a statement.

It is also possible that the megatsunami reached the landfall site in 1997 Pathfindersouth of where Viking 1 docked, and even contributed to the formation of an inland sea.

If so, then the two landers landed on the site of an ancient marine environment.

“The ocean is thought to have been fed by groundwater from aquifers that probably formed much earlier in Mars’ history – more than 3.7 billion years ago – when the planet was ‘Earth-like’ with rivers , lakes, seas and primary ocean,” Rodriguez said.

Next, the team wants to study Paul Crater as a potential landing site for a future rover, as the location may contain evidence of ancient life.

“Immediately after its formation, the crater would have generated underwater hydrothermal systems lasting tens of thousands of years, providing an environment rich in energy and nutrients,” Rodriguez said, referring to the heat generated by the asteroid impact.