Big rocks slam into Mars, gouge craters, reveal subsurface features


Mars has a lot of craters. Now it has two new ones — gouged by rocks that slammed into the planet late last year.

Mars will be fine. This is not a natural disaster. Nor an omen that Earth is newly imperiled by asteroids. The solar system is full of debris. Mars has a feeble atmosphere, and when a meteoroid comes flying in from space, it’s unlikely to burn up before smacking into the surface.

But what has scientists buzzing — to the point that NASA scheduled a news conference Thursday to highlight the discovery, detailed in two papers published in the journal Science — is that the crater-making impacts were documented by two NASA spacecraft, an orbiter and a lander. This was a nifty demonstration of combining scientific resources, one providing an eye on the impact events while the other provided an ear.

The result is an unusual trove of data about the Martian interior, a topic of great interest to planetary scientists who want to understand why this rocky world that was probably warmer and wetter 4 billion years ago became a frigid desert with no obvious sign of life.

And this was also an event for the record books: the biggest crater-forming impact on one of the rocky inner planets of the solar system ever documented in real time, according to Philippe Lognonné, lead author of one of the newly published papers.

The larger of the new craters is about 150 meters across and about 21 meters deep, and was formed so violently that it hurled rocks 40 kilometers (nearly 25 miles) from the impact, according to Liliya Posiolova, a senior scientist at Malin Space Science Systems, which operates two cameras on NASA’s Mars Reconnaissance Orbiter.

The orbiter often sees the results of small impacts, leaving a feature at most a few meters in diameter. But what the scientists on Posiolova’s team saw in February was by far the biggest crater detected by the orbiter. In fact, it was so big, it almost escaped notice.

“It’s a huge, huge feature. You’re trained to see small features. With your eye, you’re looking for smudges,” Posiolova said.

The crater was spotted Feb. 11, but the scientists knew they had other images of the Martian surface obtained on a daily basis, and they went back in time looking for when the crater first appeared.

Posiolova recalled that another spacecraft on Mars, NASA’s InSight lander, which has been parked on the surface for four years to monitor seismic activity, had detected a major jolt on Christmas Eve. Suddenly everything lined up. The crater’s first appearance in images taken from the orbiter coincided with the seismic signal registered by the instrument of the surface.

The seismic data could then be analyzed in the context of the distance to the impact. That has helped refine existing models of the interior of Mars, Lognonné said.

The larger of the two craters was probably caused by an object between five and 12 meters in diameter, Posiolova said. Such an object would probably burn up in Earth’s atmosphere were it to hit our planet, she said.

The origin of the meteoroids is unknown, but they probably came from the asteroid belt between Mars and Jupiter, she said.

“These impacts are very large, but we can continue to sleep well on Earth,” Lognonné said. “Our atmosphere is protecting us.”