Big impacts picked up by seismograph on Mars

On Thursday, NASA announced that the InSight lander was continually losing power after dust coated its solar panels. The agency expects that it will probably lose contact with the lander within the next two months. But it is going out in style, as its onboard seismometer picked up the largest impacts we've observed since we put a high-resolution camera in orbit around the red planet.

Not only does the seismic data tell us a lot about the structure of Mars' crust, but it has validated a technique used to extract positional information from a single seismometer. That technique indicates that roughly half the seismic energy that InSight has picked up comes from a single location on Mars.

Impactful events

The cameras on the Mars Reconnaissance Orbiter (MRO) have been observing Mars for 16 years. Before 2021, they had not observed any impacts that formed a crater over 130 meters across. In 2021, it spotted two. One of them was not especially useful. MRO imaging didn't capture exactly when the impact occurred, and it was far enough from the site of the InSight lander that direct seismic waves ran into the planet's core, which meant that only indirect seismic energy reached the instruments on InSight.

The impact itself took place on a bit of complicated terrain, with the meteorite hitting a slope. This made the details of the impact difficult to interpret.

None of that was true for the impact called S1094b, which took place on a flat plane. Liliya Posiolova, who works for Malin Space Science Systems and helps manage the MRO, said that a low-resolution weather camera on the spacecraft imaged the region about 24 hours apart. Also, the impact crater and debris were obvious enough that even this relatively limited camera could identify it as having happened on December 24, 2021.

This narrow time window clearly associates it with a seismic event picked up by InSight's seismometer. The impact was also close enough that seismic waves could travel directly to the lander.

The impact itself is interesting, with a central crater over 130 meters across, and large rays of debris extending away from it. The walls of the crater indicate that the impactor arrived at a considerable angle. The unusual number of smaller craters in the immediate area suggests there was a bit of an air-burst before impact, which generated some of the seismic energy picked up by InSight. There's also a lot of bright material scattered by the impact, which Ingrid Daubar of Brown University described as "boulder-sized chunks of ice." This is the closest to the equator that we've detected ice deposits like this.

These were the first seismic events that were large enough for surface waves to show up in the data from InSight. By measuring how these waves dispersed as they traveled, it was possible for researchers to infer the properties of the Martian crust along their direction of travel. And this indicated that much of the travel took place through the crust that was more dense than that at the site of the lander. If this sort of local difference is widespread in the Martian crust, it will have significant implications for the geological evolution of Mars.

Where was that?

On Earth, we can usually pinpoint where seismic events take place by using multiple seismographs to triangulate the source. On Mars, there's only a single seismograph for the entire planet. Researchers have developed ways of estimating location just based on InSight data, relying on differences between the arrival time of different classes of waves. But, without any other indication of where the event took place, there was no way to validate these estimates.

Knowing that these two impacts generated events allowed for a direct comparison between the estimates and the impact location. And it turns out the estimates are quite good. One event was estimated at 3,530 ± 360 km away, and it turned out to be 3,460 km from the lander, a difference of just 70 km. The second at 7,591 ± 1,240 km away, and that estimate was off by only 130 km. In both cases, the actual error was far smaller than the estimated error.

Those measurements give us added confidence in yet another bit of data released today that relies on the positional information from other seismic events. Earlier work indicated that one class of seismic events detected by InSight had originated in a region named Cerberus Fossae. The new work suggests that the rest of the events, known as high-frequency marsquakes, are the product of seismic activity near the surface of Mars and originate in Cerberus Fossae as well.

That's somewhat surprising, given that there are other features that suggest recent surface activity nearby. But the researchers argue that the low-frequency marsquakes could be indicative of a warm pool of material, possibly left over from recent magma, below the area where high-frequency events occur. All told, the team estimates that the two classes of events combined account for about half the seismic energy released on the entire planet.

Near the end

There's little doubt that data from InSight will keep researchers busy for years. But InSight is nearing its end of life. Bruce Banerdt, an InSight lead at the Jet Propulsion Lab, said that the lander's solar panels accumulated a lot of dust, and that's recently gotten worse, causing them to drop from providing 400 watt-hours per solar day on Mars down to 300 Wh per sol. At that level, there's a steady depletion of the batteries, and only enough energy to run the seismograph one day out of four.

Expectations are that the loss of energy will cut communications within the next two months. And that will be the end of InSight. Although some of its hardware development efforts will live on, and there are already plans for future landers with seismographs.

Science, 2022. DOI: 10.1126/science.add8574 and papers linked there.
Nature Astronomy, 2022. DOI: 10.1038/s41550-022-01803-y  (About DOIs).