The Viking 1 and 2 Mars probes looked for evidence of quakes on Mars, but failed to find anything definitive. Now a reanalysis suggests Viking 2 found marsquakes after all.
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| Viking 2 on the surface of Mars NASA/JPL-Caltech |
NASA’s Viking 2 lander probably discovered quakes on Mars almost 50 years ago, but this has only just been confirmed thanks to data from the more recent InSight probe.
In 1975, NASA launched two missions to Mars, Viking 1 and 2, to study the planet’s surface and search for life. Both probes were also equipped with a seismometer to hunt for marsquakes, tremors on the Martian surface similar to earthquakes, to see if they existed.
Viking 1’s seismometer failed to operate, but Viking 2’s worked for two years. While rudimentary, the instrument did pick up at least two signals that were thought to be quakes, one of which had a magnitude of 3.5, while the other was of unknown strength. But mission scientists couldn’t be sure if they really were quakes or simply wind passing over the lander.
“We couldn’t prove those two events,” says Andrew Lazarewicz, who is now retired but was a graduate student on the Viking team. “It was very frustrating.”
In 2018, another NASA lander called InSight was sent to Mars with a much more advanced seismometer – “200,000 times more sensitive than ours,” says Lazarewicz. The lander picked up 1319 marsquakes, the largest being a magnitude-5 “monster quake”, proving their existence. It is thought marsquakes are mostly caused by the planet cooling and its crust cracking, or by meteorite impacts, as it is thought Mars today doesn’t have the tectonic plates that cause quakes on Earth.
Using these findings, Lazarewicz went back to the Viking 2 data. He found that the events seen by Viking 2 matched some of the quakes detected by InSight, with a similar signal that pointed to seismic activity and not just wind passing over the lander, all but confirming that Viking 2 did, in fact, detect the first marsquakes in the 1970s.
“The results are really fascinating,” says Bruce Banerdt at NASA’s Jet Propulsion Laboratory in California, the lead on InSight, which ran out of power in 2022. “It was a little puzzling that [Viking 2] hadn’t seen anything.”
There is still some uncertainty, however. “It’s a very exciting, yet tentative, interpretation,” says Simon Stähler at Swiss Federal Institute of Technology Zurich in Switzerland. “There are some more tests one could run that are necessary to put a high confidence in the result,” he says, such as understanding the direction of the seismic waves recorded by the lander.
Aside from the historic interest, the findings could prove scientifically valuable. Viking 2’s landing site was relatively close to InSight, just 3000 kilometres away. They are both in a region called Utopia Planitia. The source of Viking 2’s marsquakes may have been Cerberus Fossae, a suspected site of former volcanic activity and the source of some of InSight’s marsquakes, potentially hinting at tectonic or remnant volcanic activity in this region.
“If Cerberus Fossae has been active since the 1970s, that is an interesting observation and tells us something about the source of the activity, whether it’s volcanic or tectonic,” says Banerdt.
Viking 2’s results could help scientists narrow down the thickness of the Martian crust too, particularly at Utopia Planitia. “We got 53 kilometres for the crustal thickness,” says Lazarewicz, in line with estimates from InSight.
Benjamin Fernando at the University of Oxford says the findings could also tell us something about the seasonality of marsquakes. “I’ll be really interested to see whether the Viking data falls at the time of year we would expect an event at InSight’s location.”
Lazarewicz hopes the reanalysis of Viking 2’s findings will bring some closure to the team. “When we ended with inconclusive results, everybody felt disappointed,” he says. “This is my way to repay the team. We did get marsquakes, and it was successful.”
Journal reference:
JGR PlanetsDOI: 10.1029/2022JE007660
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