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Thought to once have been Earthlike, Mars apparently still has some things in common with Earth — like seismic events that make its sun-blasted surface tremble.
NASA’s InSight lander is also shook. The Marsquakes it picked up on might not have been quite as intense as the (literally) Earth-shattering 9.9 quake that almost ended California in Earthquake, but two Mars-shattering quakes have been found to be the most powerful ever recorded on the Red Planet. InSight picked up on seismic waves from magnitude 4.1 and 4.2 events S1000a and S0976a, on the far side of Mars relative to the lander. They are now the most enormous and most distant Marsquakes ever known to have rocked the planet.
This is kind of a huge deal because the strongest quake, S0976a, which started rumbling in the Valles Mariners region, comes in at 5 times stronger than the previous record holder. InSight also recognized a type of seismic wave never observed on Mars until now, Pdiff (core-diffracted) waves. Researcher Anna Horleston of the University of Bristol and InSight’s Marsquake Service (MQS) and her team found out exactly how massive these quakes were from data beamed back by InSight. She led a study recently published in The Seismic Record.
“Pdiff arrivals come from seismic waves that have traveled down to the core-mantle boundary and then been diffracted so that they travel along the boundary before returning to the surface,” she told SYFY WIRE. “Energy at the boundary is spread out and often lost.”
There were other types of seismic waves that had been observed on Mars before. P waves, or compressional waves, result in shaking that goes back and forth in both the same and opposite direction that the wave is moving in. S Waves are shear waves. Wherever these waves go, the ground perpendicular to that direction will shudder. Pdiff waves are basically P waves that are diffracted, or scattered, as they make their way along the edge of mantle that is right outside the core. It is because of diffraction that the waves can lose energy on the way to the observer.
Besides the interference from diffraction, another thing that makes signals from Pdiff waves difficult for InSight pick up is how waves of various frequencies reach the lander’s seismometer at different times. S0976a and S1000a sent out a faint signal because they were so far away. It doesn’t help that all the background noise on Mars gets in the way of signals like this. Though it has detected over a thousand Marsquakes, meaning its seismometer is obviously hypersensitive, these would have been almost impossible to detect if they hadn’t been so enormous. Each of the quakes had particular features that stood out to Horleston.
“S1000a has a broader spectrum of energy than any other Marsquake seen before, and at the time of recording, was the longest Marsquake ever seen,” she said. “S0976a doesn’t have the high-frequency energy of shallow Marsquakes, and likely has a relatively deep source.”
Most Marsquakes are one of two types. They are either shorter events, usually thought to originate in the crust, which occur at high frequencies and have smaller magnitudes, or longer events, which probably originate in the mantle, occurring at lower frequencies. S1000a appeared to be a hybrid of these. It not only had monster magnitude but lasted for a staggering 94 minutes, with both high and low-frequency energy. While S0976a didn’t have that kind of frequency, it was close to previous quakes that originated in the bowels of Mars.
The fact that S0976a emerged near the vast canyon of Valles Marineris — a behemoth that is easily one of the hugest in the entire solar system — probably means that it was caused by a stressed mantle and crust. It was not unlike some of the deeper quakes that have occurred at Cerberus Fossae. If those were around 30 miles deep, you can only imagine.
“These Marsquakes will allow us to test and refine our models for the interior structure of Mars,” said Horleston. “They are unique in our catalogue and will be instrumental in furthering our understanding of the Red Planet.”