The NASA InSight Mars lander might not be as well known as its rover cousins, Perseverance and Curiosity, but it is doing important work in understanding more about the interior of Mars and how the planet is shaken by marsquakes. Now, researchers have identified two of the largest marsquakes seen to date.
The research, published in the journal The Seismic Record, describes how two marsquakes were detected from InSight data. The first occurred on August 25, 2021, and the second shortly after on September 18, 2021. These two events were significant for a number of reasons: Firstly, they were the largest marsquakes detected to date, and secondly, they occurred on the far side of Mars from InSight, while most detected marsquakes have originated nearer to the lander.
The August quake, called S0976a, was a magnitude 4.2 event, while the September quake, called S1000a, was a magnitude of 4.1. That makes them five times stronger than previously detected marsquakes. The first quake lasted a typical period of a few seconds, but the second event lasted a long time, clocking in at a total of 94 minutes, making it the longest event recorded so far. It also had unusually broad frequency, meaning its energy spread across frequencies from 0.1Hz to 5Hz.
“Not only are they the largest and most distant events by a considerable margin, S1000a has a spectrum and duration unlike any other event previously observed,” said lead researcher Anna Horleston in a statement. “They truly are remarkable events in the Martian seismic catalog.”
The first quake is particularly interesting as it was found to originate in the Valles Marineris canyon network. Researchers had previously predicted there would be seismic activity in this region, but this is the first time they have actually found it there. Instead, most marsquakes detected to date originate in the Cerberus Fossae region.
These quakes originated on the far side of the planet from the InSight lander, in an area called the core shadow zone. This is the region from which seismic waves (called P and S waves) cannot travel directly to the lander, because the core of the planet gets in the way. To detect the origin of the quake, researchers have to look at the reflections of these waves instead (called PP and SS waves).
The ability to detect marsquakes coming from this zone is a major achievement in seismology on Mars. “Recording events within the core shadow zone is a real steppingstone for our understanding of Mars. Prior to these two events, the majority of the seismicity was within about 40 degrees distance of InSight,” said Savas Ceylan, a co-author of the paper from the ETH Zürich institution. “Being within the core shadow, the energy traverses parts of Mars we have never been able to seismologically sample before.”