Earth has earthquakes. Mars has Martian earthquakes. There is only one difference. Mars earthquakes are most often caused by meteorite impacts because there are no plates on Earth that move parts of the Earth’s crust. So what caused the most violent earthquake ever, despite no evidence of an impact?
The vibrations from the magnitude 4.7 earthquake shook the Martian crust for six hours (if not longer) and were captured by NASA’s InSight lander in May 2022. This Mars earthquake, also known as S1222a, was thought to have been caused by a meteorite impact. An international research team immediately began looking for evidence of a new crater. The problem was that none existed. That’s when a team led by planetary geophysicist Benjamin Fernando began to wonder if something was potentially going on underground.
“We conducted a comprehensive search of the area where the large earthquake occurred,” Fernando and his team said in a recently published study. Geophysical Research Letters. “No new craters were identified in this area, suggesting that this earthquake was likely caused by geological processes.”
invisible culprit
If the hypothetical crater left in S1222a’s wake had actually existed, there would have been little way to miss it. Researchers estimate that it must have been more than 300 meters (about 1,000 feet) in diameter.
InSight has so far identified eight major earthquakes caused by the impact, the two largest each measuring 150 meters (500 feet) in diameter. There were similarities between these and S1222a, as these were the only three of his events where seismic waves were seen at the surface. The waves were long-lasting, lasting up to 10.5 h in S1222a. Another thing all three earthquakes had in common was energy that spanned a wider range of frequencies than the others. These seemed to be precursors to another earthquake, but wait a minute.
Despite the similarities that seem to indicate a meteorite faceplant on Mars, there were clear differences that the research team couldn’t ignore. S1222a’s magnitude far exceeded her two other similar earthquakes, and it produced more diverse seismic waves than the others.
Still, Fernando and his colleagues decided to look for impact craters. Craters from both previous events were surrounded by a darker blast zone, which can also be seen in low-resolution images from Mars Reconnaissance Orbiter’s MARCI (Mars Color Imager) instrument. There is no doubt that the crater from S1222a would have required a larger blast zone. However, no craters or blast zones were observed.
If S1222a was caused by an impact and the crater formed, scientists concluded that one of two things must be true. The crater may be too small to image it or its blast zone with current equipment. Or they may have formed on parts of the surface that are hard to see in particularly strange terrain or don’t have a lot of dust. However, Mars is a dusty planet, and S1222a is localized in a region covered in reddish dust, which may rule out the dust-poor hypothesis. Medium- and high-resolution images from various spacecraft also showed no craters or explosion zones consistent with those produced by S1222a.
beneath the surface
If the culprit wasn’t a space rock, what could have caused the largest known earthquake? A huge crater, a blast belt, potentially rivaling the size of S1222a? Since there was no dust cloud, the researchers finally concluded that underground forces must be behind the earthquake.
“The only explanation consistent with current observations is the origin of underground structures,” they said in the same paper. study.
But how can Mars have geological sources without tectonic plates? Tectonic forces have a major impact on the planet’s crust, as well as sliding plates that cause phenomena such as earthquakes and volcanoes on Earth. It can be caused by anything. Fernando suggests that S1222a may be the result of enormous stress on the Martian crust over billions of years as it cools and contracts.
All these processes do not occur evenly across the globe. Changes can occur at different times in different regions, and why some regions of Mars are more stressed than others is a mystery that scientists continue to study.
Tectonic forces on alien planets can be very different from those on Earth, but at least the main suspects thought to have caused S1222a have now been ruled out. Future spacecraft with even more powerful seismic-wave detection capabilities than InSight could gradually tell us more about what’s going on beneath the red, rocky, sunlit surface.
Geophysical Research Letters, 2023. DOI: 10.1029/2023GL103619.
