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When NASA’s Viking 1 lander made history as the first spacecraft to touch down on Mars on July 20, 1976, it returned images of a landscape no one expected.
These early images taken from the ground 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 suggests 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 catastrophic event likely happened when an asteroid slammed into the shallow Martian ocean – similar to the impact of asteroid Chicxulub that wiped out dinosaurs on Earth 66 million years ago, according to the researchers.
Five years before Viking I landed, NASA’s Mariner 9 spacecraft had orbited Mars, spotting the first landscapes on another planet that suggested evidence of ancient flood channels there.
Interest in the potential for life on the Red Planet prompted scientists to select its northern equatorial region, Chryse Planitia, as the first Mars landing site for Viking I.
“The lander was designed to search for evidence of existing life on the Martian surface, so to select a suitable landing site, engineers and scientists at the time faced the daunting task of using some of the first images acquired of the planet, along with terrestrial images, a radar survey of the planet’s surface,” said the study’s lead author, Alexis Rodriguez, senior scientist at the Planetary Science Institute in Tucson, Arizona. , by email.
“Selection of the landing site had to meet a critical requirement – the presence of ample evidence of ancient surface water. On Earth, life always needs the presence of water to exist.
At first, scientists thought the rocky surface might be a thick layer of debris left over from space rocks crashing into Mars and creating craters or shattered pieces of lava.
But there were not enough nearby craters and lava fragments were rare on the site floor.
“Our investigation provides a new solution – that a megatsunami washed ashore, laying down sediment on which, some 3.4 billion years later, the Viking 1 lander touched down,” Rodriguez said.
Researchers believe the tsunami occurred when an asteroid or comet hit the planet’s northern ocean. But finding a resulting impact crater was difficult.
Rodriguez and his team studied maps of the Martian surface created from different missions and analyzed a newly identified crater that appeared to be the likely point of impact.
The crater measures 68 miles (nearly 110 kilometers) in part of the northern lowlands – an area once likely covered in ocean. The researchers simulated collisions in this region using modeling to determine what impact was needed to create what is called the Pohl crater.
This was possible under two different scenarios, one caused by a 5.6 mile (9 kilometer) asteroid encountering strong ground resistance and releasing 13 million megatons of TNT energy, or a 1.8 mile asteroid (2.9 kilometers) tilling softer ground and releasing 0.5 million megatons of TNT energy.
For perspective, the most powerful nuclear bomb ever tested, Tsar Bomba, created 57 megatons of TNT energy.
During simulations, the two impacts created a Pohl-sized crater – as well as a megatsunami that reached 932 miles (1,500 kilometers) from the impact site.
The 1.8-mile asteroid generated a tsunami that was 820 feet (250 meters) high when it hit earth.
The results were similar to those of the Chicxulub impact on Earth, which created a crater with an initial width of 100 kilometers and triggered a massive tsunami that circled the globe.
The impact likely sent water vapor into the atmosphere, which would have affected the Martian climate and potentially created snow or rain during fallout. Large amounts of shallow ocean water, as well as sediment, would have been displaced, Rodriguez said, although most of the water returned to the ocean shortly after the megatsunami reached its peak. .
“The seismic shaking associated with the impact would have been so intense that it could have dislodged seafloor material within the megatsunami,” said study co-author Darrel Robertson, of the Ames Research Center at the NASA in California’s Silicon Valley, in a statement.
It is also possible that the megatsunami reached the location of the 1997 landing site for the Pathfinder, south of where Viking 1 landed, and even contributed to the formation of an inland sea.
If so, both landers touched down at the site of ancient marine environments.
“The ocean is thought to have been fed by groundwater from aquifers that likely formed much earlier in Martian history – over 3.7 billion years ago – when the planet was “Earth-like” with rivers, lakes, seas and a primordial ocean,” Rodriguez says.
Next, the team wants to investigate Pohl Crater as a potential landing site for a future rover, as the location may contain evidence of ancient life.
“Right after its formation, the crater would have generated submarine hydrothermal systems lasting tens of thousands of years, providing energy- and nutrient-rich environments,” Rodriguez said, referring to the heat generated by the impact. of the asteroid.
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