For the first time in history, the landing of a spacecraft on Mars might be “heard” in an unusual way – by the seismic waves it produces as it punches the surface of the planet.
The ears in this scenario are built into NASA’s InSight lander, which touched down in a region called Elysium Planitia on Mars in 2018. Since then, the stationary lander has been using instruments to study the planet’s geology, including a seismometer to detect seismic waves, and has found hundreds of seismic events known as marsquakes.
The lander will soon be joined on Mars by NASA’s Perseverance rover, which is scheduled to touch down on 18 February 2021. Benjamin Fernando at the University of Oxford and his colleagues have found that InSight might be able to detect the seismic waves from the landing, a first in planetary exploration.
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“It’s never been done before because there hasn’t been a seismometer on the surface of another planet before,” he says, although seismometers deployed on the moon by some of the Apollo missions did catch the impacts of rocket boosters from later missions that struck the surface. Fernando and his team’s research was presented at a meeting of the American Geophysical Union on 10 December.
Perseverance’s entry capsule will hit Mars’s atmosphere at about 19,000 kilometres per hour, then take around 7 minutes to reach the surface. This will create an enormous sonic boom that InSight could detect, says Fernando, but he stresses that the lander’s seismometer is unlikely to be sensitive enough to do so.
What is more plausible is that seismic waves may be detected when Perseverance drops two stabilising dummy weights called the cruise mass balance devices during the landing process, which are blocks of tungsten weighing 77 kilograms each. These will strike the surface with a force comparable to a small car making impact at thousands of kilometres per hour, sending so-called body waves through the interior of Mars, says Fernando.
Despite InSight being located about 3500 kilometres away from Perseverance’s landing site at Jezero crater, these impacts could be seismically loud enough for InSight to hear them as long as there isn’t too much background noise from the local Martian weather.
If conditions are just right, both a non-detection or detection would be useful. The former would provide a limit for the seismometer’s sensitivity, while the latter would give the precise date, time and magnitude of an impact of known size. This could help identify meteorite impacts in InSight’s seismic data, which were expected but haven’t been found yet – even when a small new crater formed near the lander in 2019.
“We can look at some of our previous detections and see if they have some of the same characteristics,” says Bruce Banerdt at NASA’s Jet Propulsion Laboratory in California, the InSight mission’s lead.
The landing of China’s Tianwen-1 rover, which is expected by April 2021, could also be heard by InSight, but more information about how and when it will touch down is needed. “If anyone who does have the right information about Tianwen-1 is happy to collaborate, that would be really great,” says Fernando.
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