Shaking up what we know about Mars

Three years after InSight’s safe arrival on Mars, the lander picked up several large seismic events on the Red Planet.
 

By Davitia James, Temblor Earthquake News Extern (@davitiaa)
 

Citation: James, D., 2022, Shaking up what we know about Mars, Temblor, http://doi.org/10.32858/temblor.262
 

In May, the largest earthquake ever recorded on another planet — a “marsquake,” technically, not an “earthquake” as it did not occur on Earth — struck the Red Planet. The magnitude-5.0 event was recorded by InSight, the lander that’s been taking notes on Martian seismicity, weather and orbit since 2019. This marsquake, which occurred on May 4, 2022, is the strongest event ever detected off Earth and near the maximum of what scientists expect to see. The quake struck just after scientists started releasing details about the previous two largest marsquakes, which occurred in 2021. It was as if Mars decided to really give us something to talk about — and tease new findings to look forward to.

The magnitude-4.2 and -4.1 marsquakes that occurred in September and October 2021, respectively, are discussed in a new research paper in The Seismic Record. These marsquakes offer important clues to geologic activity on Mars.
 

Taking a look inside Mars with InSight

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission is a lander serving as a geophysical research station near the equator of our neighboring planet. InSight is equipped with two cameras, a weather station, seismometer, heat probe and communication equipment. This mission is the first of its kind — we have never placed seismic equipment on another planet before. The goal is to learn more about Mars to help us understand how Earth and the other rocky inner solar system planets formed and shed some light on the origins of similar planets beyond our neighborhood.

Between February 2019, when InSight’s seismometer came online after operators carefully positioned it on the surface of Mars, and Oct. 1, 2021, a total of 951 marsquakes were recorded by the InSight instruments. Scientists are using these quakes to improve estimates of the size and nature of the core, mantle and crust of Mars. Models describing how quickly seismic waves travel through each layer are constantly being improved as the lander sends new data.

Scientists determine the magnitude of a seismic event on Mars using the scales developed by Bose et al (2021) and each quake is assigned a Mars moment magnitude based on characteristics of the seismic waves.
 

 

Sorting the signals

Martian seismic signals are complicated to sort through. The equipment is settled on softer, wind-altered material, resulting in noisier signals than would be recorded here on Earth. There is also only a single station on Mars and strong wind gusts add to the background noise.

The first step to deciphering marsquakes is looking at the levels and patterns of different energy frequencies to separate wind signals from potential marsquakes, says Anna Horleston, a planetary seismologist at the University of Bristol, co-leader of the InSight Marsquake service team and lead author of the paper.

Wind signals are nearly continuous during the day and the sharper seismic signals can be picked out from weather activity. Marsquakes are classified by the frequencies registered during each event. 2.4 hertz is the distinguishing frequency because it is easily affected by seismic activity. Events in the frequency range below 2.4 hertz are classified as low-frequency events, and those with signals at 2.4 and above can be considered high or very high frequency. Generally, researchers relate frequency to how quickly the ground ruptured during a marsquake. Slower ruptures are assumed to give off lower frequencies, and faster ruptures are linked to higher frequencies. A marsquake that includes a range of low- and high-frequency signals is called a broadband event — the seismic origins of these signals are still being studied. After evaluating the frequencies, the team applies velocity models to estimate the origin location.

The quakes are named after the day of the mission (sol) when they occurred and assigned a letter if there are multiple events on one day. For InSight’s first 975 days on Mars, there were no events greater than a Mars magnitude-3.8 — similar to a magnitude-3.8 earthquake. Then on Sol976a (Aug. 25, 2021), a low-frequency magnitude-4.2 event shook the Red Planet. The seismic energy from this quake continued to move through the planet — similar to earthquake attenuation — for over an hour, making it the longest to date.

Sol1000a, the second marsquake described in the new study, was a magnitude-4.1 broadband event on Sept. 8, 2021, with a span of frequencies that have never been recorded during a single marsquake. This makes pinpointing the source of the September event harder to unravel. InSight data suggest the primary wave — the first signal to arrive at a seismic station — traveled to the boundary of the Martian core and mantle, another novel observation. On Earth, when seismic waves reach the liquid outer core, the change in material causes primary waves to bounce back at a new angle and blocks secondary waves from continuing at all. As the first marsquake to reach this zone, Sol1000a could provide data on the distance to the core, its size, shape and phase(s).
 

Telling tales of Mars

Sol1000a is a unique event, says Horleston. “The fact that we see all that high frequency suggests it had a fast rupture,” but she says the cause and location remain a source of debate.

Earlier marsquakes identified by InSight were all located around Cerberus Fossae — a fault system formed 10 million years ago that lies roughly 930 miles (1,500 kilometers) to the east of InSight. These recent quakes are different. Sol976a is linked to Valles Marineris just south of the equator and much farther away. At four miles (seven kilometers) deep and 2,500 miles (4,000 kilometers) long, Valles Marineris is one of the largest canyons in our solar system. According to the paper, other researchers anticipated marsquakes in this area based on faults and slides visible at the surface. Sol976a is being studied to understand the processes that may occur in the Valles Marineris region. The origin of Sol1000a is not as clear but can be linked to the same hemisphere of Mars.

To better constrain the locations of the quakes, Horleston and her colleagues are applying a new technique. They are using the polarization of the signals to look at the angles of each phase of the seismic waves they can identify. With better constrained origin estimates, they will use the geology of that area to find clues to what caused the quake.

“Images from orbit may be able to see traces of the marsquakes as boulder trails and impacts,” which will help narrow down the location, says Robert Myhill. Myhill is a planetary scientist at the University of Bristol in England who was not involved in this research.
 

 

As to the cause, the marsquakes could be a result of any combination of processes, says Myhill. Mars lost a lot of its internal heat early in its history, and as the planet cools, it shrinks and cracks — like a cake. Additionally, as the immense pressures that formed the giant fault systems relax, the energy has to go somewhere. Or, perhaps magmatic processes continue far belowground, says Myhill. Contraction from cooling, slowly releasing tension, or volcanic and hydrothermal activity below the surface, are some potential causes of marsquakes, says Myhill.
 

 

Another record-setting quake

As exciting as these quakes are, given that they were at the time the largest Martian quakes, they are deeply puzzling as well, Horleston says. The InSight mission is still going strong beyond its original length of one Martian year (687 Earth days). As seismic data continue to pour in, like from the massive May 4 quake, the team is working on integrating the data from these major quakes to improve the velocity models, along with estimates of the radius of the core and distance to the mantle, says Horleston. The record-breaking May 4 event will also tell us more about the Martian core shadow zone, and how seismic waves travel below the surface of the planet. The location and cause of this newest quake — and any subsequent ones — will take the mission team some time to unravel and will build on the analysis of Sol976a and Sol1000a.
 

References

Horleston et al., 2022. “The Far Side of Mars: Two Distant Marsquakes Detected by InSight,” The Seismic Record, 2 (2): 88–99. doi: https://doi.org/10.1785/0320220007
 

Further Reading

Böse et al., 2021. “Magnitude scales for marsquakes calibrated from insight data,” Bull. Seismol. Soc. Am., 111, no. 6, 3003–3015. DOI: 10.1785/0120210045

Clinton et al., 2021. “The Marsquake catalogue from InSight, sols 0–478,” Phys. Earth Planet. In. 310, 106595. DOI: 10.1016/j.pepi.2020.106595

Giardini et al., 2020. “The seismicity of Mars,” Nature Geosci. 13, no. 3, 205–212. DOI: 10.1038/s41561-020-0539-8

Stähler et al., 2021. “Seismic detection of the Martian core.” Science, 373, 6553, 443-448. DOI: 10.1126/science.abi7730