Mars seems to have more going on under its ruddy crust than we thought, with a new study unveiling nearly 50 previously unknown marsquakes.
A pair of geophysicists found signs of 47 quakes in a year's worth of data collected by NASA's InSight Mars Lander, which has been recording the planet's internal gurgles and grumbles since 2019.
The study was published in the journal Nature Communications.
Study co-author Hrvoje Tkalčić, from the Australian National University (ANU), says if the new-found quakes are verified, this kind of work may help the mostly manual current process of sifting through data for signs of marsquakes, and give planetary scientists clues as to their origins.
"Their repetitive nature is indicative of a volcanic process that has to do with magma movement, or, as we speculate, some sort of deep depressurisation that is known to happen on Earth as well," he said.
Curtin University planetary scientist Katarina Miljkovic, who is involved with the InSight mission but not the new study, says it is "fantastic" to see the scientific community's interest in marsquake data collected by the lander.
"Different methods of signal processing may yield different results and makes it a great place to expand on our scientific understanding of the Martian subsurface."
How to take the pulse of a planet
The InSight mission was designed to pick up faint vibrations generated by the red planet's shifting innards, or by meteorites slamming into the surface.
These vibrations or waves change as they travel through each layer — outer solid crust, hot liquid mantle and the innermost core — giving scientists a way to "see" the planet's internal structure.
InSight's marsquake sensor, or seismometer, is like a stethoscope of sorts, listening to Mars's (granted, irregular) geological pulse.
InSight started collecting data in 2019, and has since picked up a few decent-sized marsquakes, including three magnitude 4.1 and 4.2 quakes over August and September last year.
(To compare, the earthquake that struck near Melbourne in September was magnitude 5.9.)
These were just a few of the more than 700 confirmed marsquakes so far found by InSight, most of which were minuscule, and far smaller than could be felt on Earth.
But we know they occurred because the seismometer is incredibly sensitive — so much so that it can pick up the jiggling of air molecules around it.
To get some quiet, it sits under a heavy protective dome.
The idea is if you minimise unwanted interference or "noise" as much as possible, the minute shakes of quakes are more easily identified in the data.
And while the dome makes for a pretty good shield, it's not perfect.
A major source of noise comes from gusty winds, which pick up when the Sun warms the atmosphere and die down again about an hour before the Sun sets.
For this reason, most marsquakes have been detected during the quieter night and pre-sunset time, when the minute waveforms generated by small quakes aren't lost in the noise.
InSight's data is beamed twice daily back to Earth, where seismologists in the Marsquake Service manually pore over it as it comes in, searching for telltale quake signatures, says John Clinton, who leads operations at the Marsquake Service at ETH Zurich and was not involved in the study.
"We learnt very early on that Mars data is exceedingly complex and noisy — far more noisy and variable than data we would record on Earth, and full of large spikes.
Any marsquakes verified by the Marsquake Service are added to the online catalogue.
Finding a quake in a noisy haystack
Last year, Professor Tkalčić and Weijia Sun, a geophysicist at the Chinese Academy of Sciences, wondered if they could tease out marsquake waveforms hidden in the noisy daytime data.
Their idea was to use confirmed marsquakes, and the specific wiggly wave patterns they generated, as templates to make "cookie cutters" of sorts, Professor Tkalčić said.
"A match means you've found a new marsquake in generally the same location as your template and with generally the same physical mechanism, which cause the wiggles to be the same."
In a year of InSight data, the pair detected 47 new marsquakes this way.
Almost all had wiggly waveforms that matched with two major confirmed marsquakes, magnitude 3.1 and 3.3, detected in March 2019.
These quakes were thought to have originated in an area known as Cerberus Fossae, roughly 1,700 kilometres east of the InSight lander.
The Cerberus Fossae system sports a range of features that point to recent geological action.
For instance, there's a pair of parallel fissures — the "fossae" — running almost 1,000 km across the geologically young 10-million-year-old volcanic plains.
And there are what look like trails left behind by boulders that were shaken free by marsquakes and bounced down cliffs onto the sediment below.
Dr Clinton says the Marsquake Service already uses the same cookie-cutter technique to search for quakes that are likely generated by the thermal expansion and contraction of rocks.
They have not explored using it for longer-reverberating marsquakes, though, because those waveforms are so susceptible to interference by, say, the wind.
"If this technique [was] integrated at [the Marsquake Service], we would need to integrate a check that there are no coincident bursts of broadband energy from wind before confirming any of these detections as new events," Dr Clinton said.
Take the pressure down
The repetitive nature of the newly discovered marsquakes not only suggests they're replicas of the two major "parent" marsquakes, Professor Tkalčić says, but also gives clues to their origin.
"It indicates that most likely their cause is volcanic. And that's what's interesting — because when you have active volcanism, it implies that the Martian mantle is mobile."
This type of volcanism probably isn't the kind we mostly see on Earth, where red hot magma oozes or spurts out and rolls along the surface as lava.
Quakes generated by volcanism on Mars are likely produced as magma shuffles around, or when gas dissolved in the hot magma is liberated, a process called degassing.
But it's trapped under a thick later of rocky crust, and the gas can't escape to the atmosphere.
"Degassing will increase pressure [under the crust], and when the pressure is high enough, rocks can break or open fractures, and then you get a quake," Professor Tkalčić said.
This phenomenon can also be seen on Earth, where dormant or "quiescent" volcanoes such as Hawai'i's Mauna Kea occasionally rumble as their plumbing is reshaped.
