On Thursday, NASA announced that the InSight lander was steadily losing power after its solar panels became covered in dust. The agency reckons it is likely to lose contact with the lander within the next two months. But it goes out in style, as its onboard seismometer captured the largest impacts we’ve observed since we put a high-resolution camera in orbit around the red planet.
In addition to telling us a lot about the structure of Martian crust, the seismic data has validated a technique used to extract positional information from a single seismometer. This technique shows that about half of the seismic energy InSight has detected comes from a single location on Mars.
Mars Reconnaissance Orbiter (MRO) cameras have been observing Mars for 16 years. Before 2021, they had not observed any impacts forming a crater over 130 meters in diameter. In 2021, it discovered two. One of them wasn’t particularly useful. MRO imaging did not capture exactly when the impact occurred, and it was far enough from the InSight lander’s location for direct seismic waves to penetrate the planet’s core, meaning only indirect seismic energy reached the instruments on InSight .
The impact itself took place over somewhat complicated terrain, with the meteorite hitting a cliff. This made interpreting the details of the impact difficult.
None of this applied to 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 a low-resolution weather camera on the spacecraft captured the region about 24 hours apart. Additionally, the impact crater and debris were obvious enough that even this relatively limited camera could tell it happened on December 24, 2021.
This narrow time window unequivocally associates it with a seismic event detected by InSight’s seismometer. The impact was also close enough for seismic waves to reach the lander directly.
The impact itself is interesting, with a central crater over 130 meters in diameter and large jets of debris extending from it. The walls of the crater indicate that the impactor arrived at a significant angle. The unusual number of smaller craters in the immediate vicinity suggests that a small burst of air occurred prior to the impact, generating some of the seismic energy recorded by InSight. There is 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 place on the equator where we have discovered such ice deposits.
These were the first seismic events large enough for surface waves to appear in InSight’s data. By measuring how these waves propagated during their journey, the researchers were able to infer the properties of the Martian crust along their direction of propagation. And this indicated that much of the travel took place through crust, which was denser than that at the lander’s site. If this type of local variation in Martian crust becomes widespread, it will have significant implications for the geological evolution of Mars.
Where was that?
On Earth, we can usually determine where seismic events are occurring by using multiple seismographs to triangulate the source. On Mars there is only one seismograph for the entire planet. Researchers have developed methods to estimate location based only on InSight data, relying on differences in arrival times for different wave classes. But without further clues as to where the event took place, there was no way to validate those 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 pretty good. One event was estimated to be 3,530 ± 360 km away, and it turned out to be 3,460 km from the lander, a difference of only 70 km. The second at 7,591 ± 1,240 km distance, and this estimate was only 130 km off. In both cases the actual error was much smaller than the estimated error.
These measurements give us additional confidence in another piece of data released today, based on the location information of other seismic events. Previous work suggested that a class of seismic events detected by InSight originated in a region called 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 Martian surface and also originated from Cerberus Fossae.
This is somewhat surprising considering there are other features that suggest recent surface activity nearby. However, the researchers argue that the low-frequency marsquakes may indicate a warm pool of material that may be left over from younger magma, below the area where high-frequency events occur. All in all, the team estimates that the two classes of events together account for about half of the seismic energy released across the planet.
near the end
There is no doubt that InSight’s data will keep researchers busy for years. But InSight is nearing the end of its life. Bruce Banerdt, an InSight lead at Jet Propulsion Lab, said the lander’s solar panels have accumulated a lot of dust, causing them to drop from 400 watt-hours per sunny day on Mars to 300 Wh per sol. At this level, the batteries are constantly being drained, leaving only enough energy to run the seismograph one day in four.
The power loss is expected to disrupt communications within the next two months. And that will be the end of InSight. Although some of its hardware development efforts will live on, there are already plans for future landers with seismographs.
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