NASA's InSight Martian lander has added meteorite impacts to the activity it has detected on Mars, having previously detected quakes thought to be caused by the cooling of the Martian interior and volcanic building. Although the space rocks landing nearby have been small, InSight is so sensitive it picked up the seismic waves from collisions as much as 290 kilometers (180 miles) away, and now NASA has released the sound of meteorites hitting Mars.
The Earth’s atmosphere experiences a regular bombardment of items ranging from the size of sand grains up to great boulders. These can make for spectacular displays in the sky, but only the larger ones hit the ground rather than burning up on the way down. The much thinner atmosphere on Mars lets far more objects through. Knowing this is happening, however, and actually detecting it, are different things. Despite decades of Martian landers and rovers, none of them had felt the seismic waves caused by a space rock hitting the planet.
InSight has been on Mars since 2018, but the first time scientists noticed seismic waves from an impact was after an event on September 5, 2021, now reported in a new paper. It was worth the wait, however. Even the thin Martian atmosphere created enough friction to cause the meteoroid to explode. NASA’s Mars Reconnaissance Orbiter detected not one, but three darkened spots produced by pieces of the object after it broke up.
"It was super exciting," Dr Ingrid Daubar of Brown University said in a statement. "My favorite images are the ones of the craters themselves. After three years of waiting for an impact, those craters looked beautiful."
Having determined InSight is capable of detecting the seismic waves from meteorite impacts, Daubar and co-authors looked further. Upon re-examining InSight’s previous data they found three smaller events from 2020 and earlier in 2021. Each created seismic waves smaller than a magnitude 2.0 Marsquake.
In three cases, InSight also picked up the acoustic wave from the object’s passage through the atmosphere. Presumably coincidentally, one detection was only five days before the impact that first caught their attention.
The craters are of more than aesthetic interest to planetary scientists. “Having a really precise location for the source of the impacts calibrates all other data for the mission,” Daubar said. “This validates the estimates we’ve made” for the location and size of the impacts. It will also allow scientists working with InSight’s data to locate future collisions more precisely.
Daubar and colleagues were surprised impacting space rocks hadn’t been detected before. Being closer to the asteroid belt than Earth, Mars should encounter more such objects, allowing for its smaller size. Previous Mars landers may not have been sensitive enough to record such collisions, but InSight has already detected 1,300 Marsquakes, despite the fact the Red Planet is less seismically active than our own.
The authors suspect InSight has in fact picked up the seismic waves from other meteorite strikes before, but that these were misinterpreted because the team didn’t recognize the distinctive shape of such waves. Now, with four confirmed events as yardsticks, they hope to find more.
Measuring the frequency of crater-forming events allows us to calculate the age of the Martian landscape, telling us how long craters like this last before being buried by sand or other processes. “Impacts are the clocks of the Solar System," said the study’s lead author Dr Raphael Garcia of Institut Supérieur de l'Aéronautique et de l'Espace in France.
Ironically, the InSight/Reconnaissance Orbiter collaboration is actually beating all our abundant Earthly seismic devices and satellites. Only one crater on Earth’s formation has been matched to equivalent seismic disturbances and infrasound detections from the meteorite’s passage through the atmosphere. The seismic network installed by Apollo astronauts has picked up the vibrations from many impacts, but none have been matched to newly formed craters.
The study was published in Nature Geoscience.