When the Perseverance rover lands on Mars on Thursday, another NASA spacecraft already there will be listening to the blow that will occur when the newcomer arrives.
The hope is that these blows will produce enough tremors to be detected by InSight, a stationary NASA probe that arrived in 2018 to use an extremely sensitive seismometer to look for marsquakes. The InSight lander is located more than 2,000 miles east of where Perseverance is scheduled to land.
“We have a reasonable chance of seeing it,” said Benjamin Fernando, a PhD student at Oxford University in England and a member of the InSight science team.
Unless something goes catastrophically wrong, the seismic signals InSight may hear will not come from the rover itself. Endurance must be brought to the surface by a hovering crane that gently tumbles to the ground slower than 2 miles per hour.
Rather, the scientists will search InSight’s seismic data for evidence of the effects of two 170-pound blocks of tungsten metal that helped keep Endurance in a stable, balanced spin during its 300-million-mile journey from Earth . At an altitude of 900 miles above Mars, they will be dumped as trash, and with no parachutes or retrorockets to slow them down, they will then slam against the surface at a speed of 9,000 miles per hour
“That tremendous speed means they’re forming a pretty big crater,” Fernando said. In 2012, similar tungsten blocks from the Curiosity Rover, which has almost the same design as Perseverance, left scars from orbit.
If the blocks hit at a shallow 10 degree angle, the impact will be to the east, which should create a splash of seismic energy towards the InSight, which increases the likelihood that the vibrations will be detected.
Detecting the impact waves is not just a technical feat. The data could help shed light on the structure of the Martian crust.
The main purpose of the seismometer on InSight is to record marsquakes, and the spacecraft has recorded more than 400 such tremors to date. Scientists also expected InSight to detect tremors from space rocks that occasionally crash into Mars.
So far, however, the number of recorded meteorite impacts is zero. At least there is no wobbling motion that the scientists could safely conclude that it was caused by such collisions. The lack of obvious signals suggests that the crust of Mars is more similar to that of Earth’s moon than that of Earth.
Seismic waves travel further through solid rock than a pile of loose material like sand. On Earth, the constant swirling of plate tectonics creates new solid rocks on the surface. There are no more lava eruptions on the moon, and for billions of years the bombardment of meteors has broken the old lunar crust into tiny pieces. The result is a loose topcoat that explains why the astronauts left so many boot prints on their visits.
“Mars is probably somewhere between the moon and the earth,” said Fernando.
However, with Perseverance, the exact time and location of the landing will be known, and hence InSight scientists know where to look in the seismic data and get a tiny signal that is usually overlooked.
This is similar to the way scientists were able to calibrate the seismometers decades ago that NASA’s Apollo astronauts left on the moon when rocket pieces and lunar landers hit the moon.
With that knowledge, they could then sift through previous data and look for similar patterns that could be meteorite impacts.
Fernando and the other InSight scientists also took into account other signals that the seismometer might pick up. Perhaps air pressure waves from the sonic boom of incoming persistence would be enough. Or the sonic boom would rock the ground and create a wave that would travel to InSight.
But their calculations showed that these rumblings were too small to be recognizable.
They also considered looking for larger parts of the spacecraft, like the heat shield, which is also hitting the ground. However, these are thrown off at lower altitudes and do not move as quickly, creating small seismic waves.
The weather could be another complication. If the winds on Mars are too strong on Thursday, they could sniff InSight’s seismometer and make noise that could also obscure the signal of Perseverance’s arrival.
What lies below the surface of Mars remains largely a mystery. In fact, the innards of the planet thwarted InSight’s other primary goal of deploying a thermal probe, nicknamed the Mole, that hammered its way into the Martian soil about 16 feet. But the probe kept jumping up.
The sand around the mole exhibited an unexpected clumping property that prevented sufficient friction for the device to move more than 14 inches below the surface.
In January, NASA announced that it would give up the mole. Even so, the InSight mission was extended to December 2022 to collect more seismic data.
Now InSight has to survive the Martian winter. The dust-covered solar collectors only generate 27 percent as much electricity as they did when they were new and clean. None of the hundreds of dust devils – essentially tiny whirlwinds of tornadoes – in the neighborhood have come close enough to blow the dust away. So the mission’s managers are figuring out how to run the spaceship with less energy, including turning off some scientific instruments. That should be enough to keep it from freezing to death. Such was the fate of NASA’s Opportunity Rover in 2018 after being enveloped in a global dust storm.