The Mars of the past was very different from the one NASA’s Viking 1 lander found when it made history as the first lander on the Red Planet in 1976. A new study suggests that Viking 1 landed at the margins of a megatsunami deposit from the planet’s earlier, wetter phase. Researchers say they have found the remains of a crater left 3.4 billion years ago when an asteroid impacted. It was this, they say, that produced a 250-meter wall of water that washed over the landscape and altered Viking 1’s landing zone.
When planning the Viking 1 landing, NASA chose a landing zone that scientists believed was an ancient outflow channel. However, the probe discovered a boulder-strewn surface lacking in any of the formations associated with flowing water. Scientists speculated that Viking 1 had landed atop debris ejected by nearby impacts, but the relative lack of craters and lava fragments was a problem. The research, led by Planetary Science Institute Senior Scientist Alexis Rodriguez, proposes an alternative explanation. A megatsunami resulting from the asteroid impact could have deposited the sediments on which Viking 1 landed 3.4 billion years later.
The study cites a newly discovered crater as evidence of these claims. The possible marine impact crater, dubbed Pohl by the team, is in the northern Martian lowlands, which was a shallow sea billions of years ago. The crater is 68 miles (110 kilometers) in diameter, indicating a 1.8-mile (3-kilometer) impactor. Simulations show the resulting wave may have been as much as 820 feet (250 meters) tall. The seismic shaking from the event may have been so intense that the ocean floor dislodged and contributed to the debris seen around the Viking 1 site.
In a previous study, Rodriguez proposed two ancient megatsunamis on Mars. The study posits that the Pohl crater caused the first of them as it shows evidence of ocean-generated formations — that means it formed during the time when the Martian lowlands were covered in water. This was a time when Mars was cooling rapidly, and the ocean may have been giving way to glaciers that eroded the crater rim. That could explain why it has gone unstudied all this time. The crater was spotted using data from the Mars Reconnaissance Orbiter’s HiRISE camera, which can capture images at 25cm per pixel.
The team calls for a more detailed examination of Pohl crater and the surrounding area. The most recent exploration of this region was Pathfinder in the late 1990s, but NASA does not currently have plans to send another rover to Mars. The next Martian operation is the joint ESA operation to collect samples from the Perseverance rove and return them to Earth. Those missions are expected to launch at the tail end of this decade.