The pair of marsquakes — the Martian equivalent of earthquakes — were determined to have magnitudes of 4.2 and 4.1. Both are five times stronger than the previous record-holding tremor detected by NASA on the Red Planet. They are also the first marsquakes that InSight’s seismometer has detected originating from the opposite side of the planet to the lander.
According to seismologist Dr Anna Horleston of the University of Bristol and her colleagues, the findings could help scientists better understand Mars’ interior structure, in particular the nature of the planet’s core-mantle boundary.
The larger of the two marsquakes — dubbed S0976a — struck on August 25 last year and appears to have originated in the Valles Marineris, a 2,500-mile-long canyon system that is four times deeper than the Grand Canyon here on Earth.
The presence of cross-cutting fault lines and landslide debris in this area previously seen in satellite images of the canyon had suggested that Valles Marineris would be geologically active, however, this is the first time that scientists have direct evidence of its seismicity.
The researchers said that they were able to detect so-called “PP” and “SS” waves from the magnitude 4.2 event — that is, respectively compressional and shear waves that have been reflected at least once off of the Martian surface en route to InSight’s seismometer.
Dr Horleston said: “S0976a looks like many of the events we have located to Cerberus Fossae — an area of extensive faulting — that have depths modelled to be around 50 kilometres [31 miles] or more and it is likely that this event has a similar, deep, source mechanism.”
The magnitude 4.1 marsquake, meanwhile, was recorded 24 days later on 18 September — and has the distinction of being the longest recorded on Mars, lasting for 94 minutes.
Analysis of data from the InSight revealed that, alongside PP and SS waves, the lander also picked up so-called Pdiff waves that travelled along Mars’ core-mantle boundary for part of their journey from the quake focus to the seismometer.
This is the first time that Pdiff waves have been detected by InSight.
The researchers said that, like S0976a, this second quake — dubbed S1000a — had its origins on the far side of the planet, although they have been unable to pin down the source location.
Unlike the low-frequency S0976a, however, the waves from S1000a had a very broad frequency spectrum.
The second quake, Dr Horleston said, “is a clear outlier in our catalogue and will be key to our further understanding of Martian seismology.”
S1000a, she added, “has a frequency spectrum much more like a family of events that we observe that have been modelled as shallow, crustal quakes, so this event may have occurred near the surface.”
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The respective locations of the InSight lander and the origin points of the two marsquakes mean that the seismometer readings were taken in what seismologists call the “core shadow” zone.
This is the relative region on Mars’ surface where P and S waves from a given quake cannot directly travel to a seismometer, because the waves are either refracted or blocked by the planet’s liquid iron core.
Paper author and seismologist Dr Savas Ceylan of ETH Zürich said: “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.
“Being within the core shadow, the energy traverses parts of Mars we have never been able to seismologically sample before.”
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This, the team explains, makes the two new far-side quakes special in our current record of Mars’ seismic activity.
Dr Horleston concluded: “Not only are they the largest and most distant events by a considerable margin, [but] S1000a has a spectrum and duration unlike any other event previously observed.
“They truly are remarkable events in the Martian seismic catalogue.”
The full findings of the study were published in the journal The Seismic Record.