The landing of NASA’s Perseverance Mars rover next month will make serious waves, some of which may help scientists better understand the Red Planet’s structure.
Perseverance, the centerpiece of NASA’s $2.7 billion life-hunting, sample-caching Mars 2020 mission, is scheduled to touch down inside the 28-mile-wide (45 km) Jezero Crater on Feb. 18. The epic landing will generate seismic signals that one of the rover’s cousins, NASA’s InSight Mars lander, will try to detect from more than 2,000 miles (3,200 kilometers) away, a new study reports.
If that happens, it will be a spaceflight first: No spacecraft has ever “heard” such a landing on another planet in this way, InSight team members said.
InSight’s supersensitive seismometer suite has picked up hundreds of marsquakes since the lander touched down in November 2018 on a Martian plain known as Elysium Planitia. InSight team members are using these measurements to map out the Red Planet’s interior in unprecedented detail, the key goal of the mission.
Such interpretive work can be tricky, however.
“Unlike on Earth, where you can independently figure out when and where a [seismic] source happened, and of course how big it was, on Mars, we’ve got a single station, and we’re both trying to identify the mechanics of the source and the structure of the planet that the waves propagated through,” study lead author and InSight team member Ben Fernando, a Ph.D. student at the University of Oxford in England, told Space.com.
“Disentangling those two from each other is not necessarily trivial,” he added. “In the simplest explanation, if you were in a room and you couldn’t see, it’s hard to tell if someone’s speaking loudly far away or quietly close to you. And in addition to that, if you didn’t know what the shape of the room was, it’d be even harder.”
Perseverance’s landing therefore represents a great opportunity for InSight scientists — a chance to collect seismic data generated by an impact, the details of which are known in advance, Fernando and his colleagues wrote in the new study.A decent chance
Mars 2020 will employ the same entry, descent and landing (EDL) strategy that got its predecessor, the Curiosity Mars rover, down safely in August 2012.
Mars 2020 will hit the thin Martian atmosphere hard, be slowed considerably by friction and then deploy a supersonic parachute to decelerate further. About seven minutes after atmospheric entry, a rocket-powered sky crane will lower Perseverance to Jezero’s floor gently on cables, then fly off to intentionally crash-land a safe distance away.
That final step won’t generate seismic waves of appreciable strength. But two other points during the EDL sequence are likely to produce relatively powerful signals, according to Fernando and his team.
One such signal will be spawned by a sonic boom, which will occur after Mars 2020 gets within about 60 miles (100 km) of the Martian surface, an altitude at which the atmosphere is dense enough “for substantial compression to occur,” the researchers wrote in the new study.
Some of the energy from this boom — which will peter out when the spacecraft goes subsonic, about three minutes before touchdown — will hit the Martian surface and be converted into seismic waves. But this signal won’t be strong enough to be picked up by InSight, which sits about 2,145 miles (3,452 km) from Perseverance’s landing site, Fernando and his team calculated, citing the dissipating effect of Martian winds as a key factor.
The other signal will come via an actual surface impact — twin impacts, in fact. Shortly after Mars 2020 hits the atmosphere, the spacecraft will eject two “Cruise Mass Balance Devices” (CMBDs) to alter its center of mass. The CMBDs, each of which weighs 170 lbs. (77 kilograms), will be dropped from very high up, at an altitude of about 900 miles (1,450 km), and will hit the ground at an estimated velocity of 8,700 mph (14,000 kph), Fernando said.
It’s unclear how strong the seismic waves from the CMBD impacts will be; InSight has not yet detected any confirmed impacts on Mars, so predictions are difficult. But Fernando and his team generated estimates based on data gathered here on Earth and on the moon, and those numbers suggest that InSight has a decent chance of measuring the waves.
“In the realistic best-case (and assuming identical weather and noise spectra to the same period one Martian year earlier), the requisite signal-to-noise ratio would be sufficient for a positive detection 40% of the time,” the researchers wrote in the new study, which has been submitted to (but not yet accepted by) the journal Earth and Space Sciences. You can read a free preprint of it here.
There’s some luck involved in this relatively rosy figure: The CMBD-generated waves will arrive at InSight’s location in early evening Elysium Planitia time, the quietest part of the day, Fernando said.
A detection would be a pretty big deal for InSight team members. They would know precisely how far, and how fast, the seismic waves traveled.
“If you know how fast they went, you can start to work out what the structures they propagated through were,” Fernando said.
Perseverance will aim to document its own landing in an unprecedented fashion, by the way. Mars 2020 carries two microphones, one of which will attempt to capture the dramatic sounds of EDL on Feb. 18. (The other is part of Perseverance’s rock-zapping SuperCam system.) No Mars spacecraft has ever successfully recorded the raw sounds of the Red Planet before.