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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 ground-based images revealed a surprisingly boulder-strewn surface in the red planet’s northern equatorial region, rather than the smooth plains and flood channels expected from images of the area from space.
The mystery of the Viking landing site has long puzzled scientists who believe an ocean once existed there.
New research suggests the lander landed where a megatsunami from Mars deposited materials 3.4 billion years ago, according to a study published Thursday in the Journal Scientific Reports.
The cataclysmic event likely occurred when an asteroid smashed into the shallow Martian ocean – similar to this one Chicxulub asteroid impact According to researchers, 66 million years ago the dinosaurs wiped out the earth.
Five years before Viking I landed, NASA’s Mariner 9 spacecraft orbited Mars and discovered the first landscapes on another planet, suggesting evidence of ancient flood channels there.
Interest in the potential for life on the Red Planet prompted scientists to choose its northern equatorial region, Chryse Planitia, as the first Martian landing site for Viking I.
“The lander was designed to search for evidence of life on the surface of Mars. To choose a suitable landing site, engineers and scientists of the time faced the arduous task of using some of the earliest recorded images of the planet, accompanied by ground-based radar soundings of the planet’s surface,” said the study’s lead author Alexis Rodriguez, senior scientist at the Planetary Science Institute in Tucson, Arizona, via email.
“The selection of the landing site had to meet one critical requirement – the presence of extensive evidence of past surface water. On Earth, life always requires the presence of water.”
At first, scientists thought the rocky surface might be a thick layer of debris left behind because space rocks slammed into Mars, creating craters or broken pieces of lava.
But there weren’t enough craters nearby, and lava fragments were rare on the ground at the site.
“Our study provides a new solution — that a megatsunami washed ashore and deposited sediment that lander Viking 1 touched down about 3.4 billion years later,” Rodriguez said.
Researchers believe the tsunami was formed when an asteroid or comet struck the planet’s northern ocean. However, finding a resulting impact crater has been difficult.
Rodriguez and his team examined 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 (nearly 110 kilometers) wide and lies in part of the northern lowlands – an area that was probably once ocean-covered. Researchers simulated collisions in this region using models to determine what impact was required to create what is known as Pohl Crater.
It was possible in 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 (2.9-kilometer) one Asteroid plowing into softer ground releasing 0.5 million megatons of TNT energy.
For perspective, the most powerful atomic bomb ever tested, Tsar Bombagenerated 57 megatons of TNT energy.
During simulations, both impacts produced a Pohl-sized crater — and a megatsunami 932 miles (1,500 kilometers) from the impact site.
The 1.8-mile asteroid generated a tsunami that was 250 meters high when it made landfall.
The results were similar to those from the Chicxulub impact on Earth, which created a crater initially 62 miles (100 kilometers) in diameter triggered a massive tsunami that traveled around the world.
The impact likely ejected water vapor into the atmosphere, affecting the Martian climate and possibly creating snow or rain in the fallout. Huge amounts of water from the shallow ocean would have been displaced, as well as sediment, Rodriguez said, although most of the water returned to the ocean shortly after the megatsunami peaked.
“The seismic shock associated with the impact would have been so intense that it could have displaced materials from the seafloor into the megatsunami,” study co-author Darrel 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 site of the 1997 landing site scoutsouth of Viking 1’s landing, and even contributed to the formation of an inland sea.
If so, then the two landers landed on the spot where marine environments used to be.
“The ocean is believed to have been fed by 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.” said Rodriguez.
Next, the team plans to study Pohl Crater as a potential landing site for a future rover, as the site may contain evidence of ancient life.
“Immediately after its formation, the crater would have produced underground hydrothermal systems that last for tens of thousands of years and provide energetic and nutrient-rich environments,” Rodriguez said, referring to the heat generated by the asteroid impact.