19.10.2023
Images from each and every spacecraft now orbiting Mars have ruled out a meteorite strike as the cause of a 4.7-magnitude marsquake, the strongest temblor ever detected beyond Earth
NASA's InSight lander deployed its Wind and Thermal Shield on February 2 (Sol 66). Credit: NASA/JPL-Caltech
Bone-dry, bitterly cold and bathed in cosmic radiation, the surface of Mars may well be dead, with not so much as a single microbe breaking its state of barrenness. But just below its frozen exterior, the planet itself is alive with the sound of thunder. There is still warmth deep within, leftover from the world’s formation eons ago, and as that heat slowly escapes to space, the planet’s crust cools, contracts and quivers. Last year a NASA mission sent to listen to such seismic rumbles heard its loudest one. This “marsquake” was far mightier than any other extraterrestrial tremor ever detected. Researchers have now ruled out a meteorite impact as the cause of this huge event, boosting the case that—seismically speaking—reports of the Red Planet’s death have been greatly exaggerated.
NASA’s stationary InSight lander launched to Mars in May 2018, touching down six months later in November in a plain called Elysium Planitia just north of the Martian equator. Of its handful of instruments, particularly notable was the seismometer it placed delicately on the ground. NASA’s hope was to pick up marsquakes, whether they were caused by crustal cooling, space-rock strikes or even volcanic activity. The instrument was wildly successful: it detected more than 1,300 temblors before InSight ran out of power in December 2022.
Almost as a swan song, the lander had recorded its biggest catch earlier that year—a 4.7-magnitude whopper dubbed S1222a, which was detected on May 4, 2022. This monster marsquake was as large as all the others that InSight detected combined—so strong, in fact, that scientists struggled to explain its origin. “When we first saw it, we were very uncertain,” says Mark Panning of NASA’s Jet Propulsion Laboratory (JPL), project scientist on InSight. The quake didn’t appear to be coming from a nearby region of suspected volcanic activity called Cerberus Fossae, which had been pinpointed by InSight as the source for most of its recorded seismic events—and scientists could find no other surface feature suitable for sparking a spasm of this size. The leading idea was that a meteorite had struck the surface of Mars—something that InSight had detected twice before, albeit on smaller scales. “One member of the team made a bet that if it were not an impact, he would do karaoke at a team meeting,” Panning says.
It might be time to warm up the karaoke machine. Fresh analysis led by Benjamin Fernando of the University of Oxford, published today in Geophysical Research Letters, has scoured the Martian surface for a new impact crater linked to this quake. In an ambitious international effort combining imagery from every spacecraft circling Mars—involving orbiters from the U.S., Europe, India, the United Arab Emirates and even China—Fernando and his team examined an area of tens of thousands of square kilometers around InSight and looked at imagery both before and after the monster marsquake. The result? “We didn’t find a crater,” Fernando says, “which strongly suggests this event was tectonic.”
The quake’s mysterious source, Fernando and his co-authors posit, lies perhaps 20 kilometers below the surface, stemming from faults and folds that form in the planet’s slowly shrinking crust. “The [Martian] ground has cracks everywhere,” says Bruce Banerdt of JPL, principal investigator of InSight. “If they slide past each other, that’s called a fault, and the motion on a fault causes a quake.” This activity can form wrinkle ridges on the surface—protruding ridges hundreds of kilometers long that are associated with fairly shallow crustal activity. No wrinkle ridge on Mars has been linked to one of InSight’s quakes before, however, and it’s unclear why this biggest quake of all would be the only one to be caused by such a feature. “We just don’t know at the moment,” says Simon Stähler, a seismologist at the Swiss Federal Institute of Technology in Zurich (ETH Zurich). Other than its sheer magnitude, “this quake has no features that are remarkable in any way.”
If tectonic activity is the cause, that would mean Mars is releasing “closer to the amount of seismic energy that we expected before the mission,” Panning says. Scientists had predicted Mars would exhibit quakes up to a magnitude of about 5, but this forecast was only borne out by the single monster quake in InSight’s final months of operations. “This is a nice confirmation that the estimates weren’t wildly wrong,” Fernando says. “Mars really does support these pretty hefty marsquakes. 4.7 on Earth wouldn’t bring your house down, but you’d certainly notice it.”
Had the quake been caused by an impact, the incoming meteorite would likely have formed a crater hundreds of meters wide, with debris strewn for kilometers across the surrounding landscape. InSight detected meteorite impacts on at least two other occasions: one in September 2021 and another later that year on December 24. Scientists know these were impact events because they were eventually traced back to specific new-formed surface craters, which appeared as bluish-black, smudges in satellite images. A similar feature from the May 2022 marsquake “would have been easily recognized,” says co-author Daniela Tirsch of the German Aerospace Center (DLR). “We’re very confident” that it was not an impact, Tirsch says. Alternatively, a landslide could have conceivably spawned the supersized tremor, but none were found to have occurred sufficiently close enough to InSight.
That leaves tectonic activity as the most plausible explanation. “I’m satisfied beyond reasonable doubt that this was not an impact,” Banerdt says. Mars’s crust today comprises just one tectonic plate, unlike Earth’s crust with its multiple jostling plates. Yet this single global plate is still thought to experience flexing and accumulate stresses from the remnant heat bubbling up from the planet’s slowly cooling, partially molten core. “Mars still has heat, and that heat is still trying to get out,” Panning says. “That’s going to cause stresses to build up that lead to marsquakes.”
Researchers have linked most of InSight’s marsquakes to Cerberus Fossae, some 1,700 kilometers eastward of the lander, a site striped with parallel fissures thought to have formed from volcanic activity several tens of thousands of years ago. The region’s quakes may be due to deformation from magma intruding tens of kilometers underground. The arrival times of different waves from S1222a, however—pressure waves propagating through the planet’s interior arrived first, followed by slower “surface” waves—allowed for a crude localization of its origin to the southeast, far away from Cerberus Fossae. That makes the quake’s source particularly confusing because there are no obvious surface features indicating active tectonic processes to account for it. One possibility might be that the southern part of Mars has a more fractured and less dense crust than the north, and seismic waves “cannot propagate as cleanly,” Stähler says. “It could be that quakes from the south just always look weird. But because S1222a was the sole southern marsquake InSight detected, we just cannot say.” This strongly fractured crust might also harbor tectonic faults that are just not visible on the surface.
Even so, future analysis of this lone event could still yield important revelations, Fernando says. “Clearly there’s a massive piece of the tectonic and seismic puzzle that we haven’t yet unraveled,” he says. For example, any future human explorers on Mars “would want to know where this sort of thing was localizing” to beef up any vulnerable infrastructure to withstand strong ground shaking. More fundamentally, discerning the monster marsquake’s true origins could greatly improve both our understanding of Martian history and the broader possibility of life on other rocky worlds. “If Mars was ever habitable, did that change when the large-scale geological activity stopped on the planet as well?” Fernando asks. “The extinction of life on Mars and the extinction of its plate tectonics are very open-ended questions.”
Quelle: SCIENTIFIC AMERICAN