On May 4, 2022, NASA’s InSight lander detected a 4.7 magnitude quake on Mars from 2,000km away. “[It] almost saturated our seismometer,” said scientist John Clinton.
Still, 4.7 is not titanic by Earth standards. “On Earth, a quake of this size would probably break windows [and] shake things off shelves, but would not bring the house down,” said one researcher.
But considering that Mars does not have colliding tectonic plates, the six-hour event was both impressive and intriguing. This was the biggest quake ever recorded on the Red Planet, and any other planet besides Earth. What caused it?
At first, scientists thought that it was a meteorite crash. Though larger than other marsquakes, it was not dissimilar to eight previous smaller ones caused by meteorites. S1222a, as they called the event, created surface waves. Only two other quakes have done this, and meteorite impacts caused both of these. Both left craters 150m in diameter.
The spectrogram of marsquake S1222a. Image: NASA/JPL-Caltech/ETH Zurich
No new crater
This quake was so much stronger than the others that researchers predicted that a meteor strike would have left a 300m ditch. Scientists scoured the data from a number of different satellites. NASA, the European Space Agency, the United Arab Emirates Space Agency, the Indian Space Research Organization, and the China National Space Administration all supplied information to try to puzzle out this mystery.
After searching 142 million square kilometers on Mars, they did not find any fresh craters or other signs of impact. They also looked for blast zones and dust clouds, but nothing.
Eventually, they came up with a new theory to explain the quake. Instead of being caused by tectonic plates, it was caused by tectonic forces.
What’s the difference? While the Earth’s crust is composed of many little pieces (the plates), Mars’ crust is one piece but is still subject to geologic forces within it.
Stress relief
“This event was likely caused by the release of stress within Mars’ crust,” said planetary physicist and lead author Benjamin Fernando. “These stresses…include the cooling and shrinking of different parts of the planet at different rates.”
They don’t fully understand why the stresses occur in certain places. But S1222a has “advanced our understanding of Martian seismic activity and takes us one step closer to better unraveling the planet’s tectonic processes,” co-author Constantinos Charalambous told Reuters.
As NASA focuses on getting humans to Mars, understanding where these stresses occur will help decide the location of potential bases.
Engineers at Lockheed Martin Space, Denver, Colorado, test the solar arrays on NASA’s InSight lander months before launch. Photo: NASA/JPL-Caltech/Lockheed Martin Space
InSight landed on Mars in November 2018 and spent over four years collecting data, including about the Martian crust, mantle, and core. Its on-board seismometer picked up 1,300 marsquakes before eventually running out of power. S1222a was one of the last events it recorded.
Before the mission, the InSight team predicted that Mars could have quakes up to a magnitude of 5. “This is a nice confirmation that the estimates weren’t wildly wrong,” Fernando says.
