After succeeding in a bold attempt to brush dangerous amounts of dust from its solar panels, NASA’s InSight Mars lander has hit two major milestones: 1,000 sols (Mars days) on the surface, and the detection of the three largest marsquakes ever recorded.
Earlier this year, says the mission’s principal investigator, Bruce Banerdt of NASA’s Jet Propulsion Laboratory, in Pasadena, California, dust had reduced the capacity of the lander’s solar panels by nearly 80%. There was no indication that dust devils or other strong breezes might come by to clean them, the way they had periodically rejuvenated the Spirit and Opportunity rovers in the 2000s. And without a strong-enough wind, Banerdt says, “dust clings pretty well.”
But there’s another way to get rid of it: use the materials at hand to create a whisk broom.
And what material does Mars have a lot of that might do this? Sand.
The idea, Banerdt says, came from geologists on the team, who noted that not only does it take less wind to set sand grains in motion than to blow off clinging dust, but that tumbling sand often leaves dust-free streaks on the Martian surface. So why not use the lander’s robotic arm to put sand on the upwind side of one of the solar panels and let the wind whisk it across the surface, knocking some of the dust free in the process?
The idea, Banerdt admits, sounded so preposterous that when the scientists first suggested it, they pretty much got laughed out of the room. “The last thing you want to do is put more dirt on the panels when they’re getting dirty,” he says.
But still, everyone had seen images of how blowing sand had removed dust on the Martian surface. So, with no better option, they tried it.
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And it worked. It didn’t sweep away all of the dust, but it cleaned away enough that the lander is again very much a going concern.
Had nobody thought of doing this, Banerdt says, the lander’s all-important seismometer would probably have had to be shut down, or maybe reduced to operating every third or fourth day.
Instead, it was up and functional when the Martian crust rumbled. “I feel very strongly that we would not have seen these quakes if we had not been able to clean these panels the way we did,” Banerdt says.
Marsquakes were above magnitude 4.2
The quakes came in fairly rapid succession. Two were on 25 August, with magnitudes of 4.2 and 4.1. The third was on 18 September and clocked in at magnitude 4.2.
By earthly standards, none of these was huge, but Mars isn’t as tectonically active as Earth. Before these three, the record-holding Martian temblor measured in at magnitude 3.7—which, on the logarithmic scale used for measuring earthquake strength, corresponds to only one-fifth the power.
But these quakes weren’t just big. One rattled the lander’s seismometer for an hour and a half.
On Earth, such a thing would be unthinkable. Yes, Banerdt says, some giant earthquakes can make the entire Earth ring like a bell for a day or longer, but only at very low frequencies, on the order of 15–20 minutes. For the more rapid type of shaking normally detected by seismometers, he says, “I don’t think that has ever happened for more than 20 minutes.”
What caused it, he thinks, is that the marsquake set off seismic waves that initially went off in all directions. Some headed quickly toward InSight and its seismometer. Others went in different directions, then hit discontinuities in the Martian interior that reflected them back toward InSight sometime later. Some may have followed even more circuitous paths. “So, you have waves that have traveled this drunken walk to get to your seismometer,” Banerdt says. “That’s our theory.”
At the moment, he says, the sources of these quakes haven’t been localised, though it does appear that one came from 8,500 kilometres away, vastly farther than the region 1,500km away that has been the source of almost all prior events.
More sophisticated analysis, he says, may allow scientists to pinpoint the precise source, something that can be done by studying how the seismic waves are polarized (meaning that their vibrations are mostly in one plane of motion). “That’s tough,” he says, “but we can do it with some events.”
“We’re still working on it,” he says.
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