Scientists track shock waves on Mars and find a meteorite impact

Special for Infobae of New York Times.

On Christmas Eve last year, Mars shook.

Thanks to its exquisite sensitivity, the seismometer on NASA’s InSight lander accurately recorded a burst of seismic vibrations and sent the data back to Earth the next day: a gift from science.

InSight scientists were busy celebrating the holidays. In early January, when they studied the tremor in detail, it looked different from the more than a thousand marsquakes the stationary spacecraft had recorded during its mission to study the bowels of the red planet.

“It was definitely a seismic event and a big one,” said Mark Panning, a project scientist on the InSight mission. “And he immediately moved us.”

In scientific papers published Thursday, scientists using data from two NASA spacecraft revealed that the seismic event was not rocks that were cracking apart from the Red Planet’s internal pressures. Rather, they were shock waves emanating from the impact of a space rock on Mars. The discovery will help scientists better understand what’s inside Mars and serves as a reminder that, like Earth, Mars also gets hit by meteorites.

Mars doesn’t have tectonic plates, the sliding pieces of crust that shape Earth’s surface. Earthquakes do occur, however, because of other tectonic pressures such as the red planet’s crust shrinking and cracking as it cools. The largest marsquakes are modest by Earth standards.

The December shake was one of the most powerful on record, with a magnitude of 4. However, it did not occur in the region of tectonic activity where most of the strongest earthquakes had been observed.

Most crucially, the Christmas Eve seismic event was the first time such surface waves had been detected: vibrations that travel along the outer crust of rocks on the surface of Mars. For all other marsquakes, InSight’s seismometer had only observed what are known as body waves, vibrations that travel through the planet’s interior.

The event became more mysterious because the epicenter occurred far away: more than 3,200 kilometers from InSight. This suggested that it was not just a large earthquake, but one with shallow depth.

“It was difficult to determine why there were surface waves,” said Philippe Lognonné, a professor at the University of Paris who serves as the seismometer’s principal investigator.

This remained a mystery until two months later, when scientists on another NASA spacecraft—the Mars Reconnaissance Orbiter—discovered that the seismic event was not a marsquake after all.

Rather it was the hit of a space rock on Mars.

Nor was it a tiny space rock, which is estimated to have been 15 to 40 feet in diameter, said Liliya Posiolova, director of orbital science operations at Malin Space Science Systems in San Diego, which built and operates two of the Mars Reconnaissance Orbiter cameras.

The impact released the energy equivalent of an explosion of between 2.5 and 10 kilotons of TNT, Posiolova noted (the atomic bomb that was dropped on Hiroshima at the end of World War II was the equivalent of 15 kilotons of trinitrotoluene). The meteor left a crater wider than a football field.

During a NASA news conference Thursday, Ingrid Daubar, a planetary scientist at Brown University who heads the impact science working group on InSight, said a meteor this size enters Earth’s atmosphere about least once a year.

“We see them pretty regularly,” Daubar said. “But because Earth has a thicker atmosphere, asteroids that size burn up and are usually pretty harmless.”

The scientists, including Panning, Lognonné, Posiolova and Daubar, reported the findings in two papers published Thursday in the journal Science.

When InSight — an abbreviation for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport — landed in November 2018, scientists expected to observe not only marsquakes, but also a few meteor strikes a year. Instead, for more than three years, they didn’t see any meteorite impacts in the seismic data.

This indicated a lack in computer models that simulated the expected seismic signals and in more knowledge about the Martian crust.

Last month, scientists reported identifying four small meteorite impacts within a couple of hundred kilometers of InSight based on the trilling sounds the rocks made as they entered the Martian atmosphere.

Now, they also know of larger meteorite impacts at a greater distance.

In early February, Posiolova and other scientists were working to take a three-dimensional stereo image of part of Mars. They had already taken an image of the region a few years ago and were now taking a second one from a slightly different angle.

However, the second image had a large smear, an impact zone of churning dust spreading outward for more than 10 miles that hadn’t shown up in the first image.

It was so large that it was visible in daily global climate images taken from another camera on the orbiter. “At that point, we essentially started going backwards from that February image,” said Posiolova, the first author of one of the papers published in Science.

The stain was present on December 25. But not on December 24.

Posiolova mentioned looking back and remembering that InSight had recorded one of its largest seismic events on Christmas Eve. “I thought, ‘Could this be it?’” she wondered.

And yes it was.

Higher-resolution images show how the meteor carved a crater about 150 meters wide in the center of the impact zone and even raised frozen water from below the surface. It is the area closest to the Martian equator where ice has been detected.

Once there was no doubt that they had identified the seismic signatures of a meteorite impact, InSight scientists reviewed their data to see if any of the previous marsquakes had actually been the same type of impact.

Indeed, the shaking of a magnitude 4.2 seismic event three months earlier, on September 18, seemed similar. The orbiter’s cameras then looked around that epicenter, located about 7,400 kilometers from InSight, and detected a crater about 130 meters in diameter.

Posiolova noted that they were by far the two largest new craters the orbiter had detected during its sixteen years of studying Mars. According to Panning, the two impacts were unlikely to be related; the fact that they had only been a few months apart was a random and lucky fluke.

One of the possible surprises is that the surface waves seem to have traveled at almost the same speed over the crust of the northern hemisphere as over the southern hemisphere.

The topography of the northern half of Mars—an area that may have been covered by an ocean—is much lower than the southern highlands. However, the velocity data suggest that the crustal rocks in both hemispheres are similar in density. On Earth, the crust under the oceans is denser than the crust of the continents.

“We are just beginning to unravel the mystery of this dichotomy,” said Doyeon Kim, a planetary scientist at the Federal Polytechnic Institute in Zurich, Switzerland, and the first author of the Science paper describing the InSight findings.

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