After two months, Insight’s seismometer has finally picked up the first rumblings in Martian soil.
NASA’s Insight lander installed one of the most sensitive seismometers ever built on Mars earlier this year. If that isn’t enough to make you go “wow!”, now it has finally detected its first marsquake.
After two months of science operations, Insight’s Seismic Experiment for Interior Structure (SEIS) picked-up a clear seismic signal. Researchers think that the trembling originated inside the planet, as opposed to causes above the surface, such as wind. Marsquakes are expected from two sources: loss of heat from the Martian interior, which wrinkles and breaks the crust on the planet's surface, or more prosaically, from meteorite impacts. The team is still working to figure out which of these sources generated the marsquake. The signal lasted around 10 minutes and had a magnitude of 2 to 2.5, a shaking so slight that humans wouldn’t have felt it if it had happened on Earth.
The French-built SEIS started science operations on February 4th, after a two-month period where the instrument was deployed to its current placement a few meters from the lander. Since then, it has recorded three other signals that could have had a seismic origin, but researchers are still working to rule out other possible causes, such as winds. Insight Principal Investigator Bruce Banerdt (NASA’s Jet Propulsion Laboratory) announced the detections on April 23rd at the Seismological Society of America’s Annual Meeting in Seattle.
While this first detection is too weak for scientists to learn much about the Martian interior, it already shows that seismic activity is less frequent than expected. Researchers originally thought they’d pick up one or two seismic movements a month. Now, depending on whether further analysis confirms that SEIS has observed four signals or just one, estimates range from 5 to 18 marsquakes per year.
Nevertheless, whatever shakes SEIS records will be the first hints we have of Martian activity. While both the Viking 1 and 2 missions carried seismometers to Mars, they didn’t function as expected. One of them failed to work altogether. The other couldn’t make reliable measurements because it was installed on the lander’s platform, far from the ground, and the lander’s legs would have dampened any seismic vibrations. “This is the opening round for Mars seismology,” Banerdt said.
Probing the Martian Depths
One of Insight’s primary goals is to understand the basic structure of Mars by watching for variations of seismic waves as they travel through materials with different compositions. While we already know Mars has a liquid metal core, a thick mantle, and a basaltic crust, the sizes of these layers are only known to within hundreds of kilometers.
Unlike Earth, though, Mars doesn’t have plate tectonics, which means the planet experiences fewer quakes. Scientists need tremors of a certain magnitude in order to probe the planet’s depths. Because Mars is slightly more than half Earth’s diameter, so a quake of magnitude 4 — which would be felt on Earth but only cause minor damage — becomes a global event on Mars. In fact, that’s the minimum magnitude required to acquire a full picture of the planet’s interior using a single seismometer. The more events the instrument records, the more accurate the picture that emerges.
However, there are other ways seismometers can reveal details about the planet’s inner structure. For instance, the Martian moon Phobos passes over the instrument every seven hours, and the seismometer can feel its slightly gravitational pull. Analyzing Phobos’s tidal pull on Mars can reveal details about the Martian interior, such as the size of its liquid core.
“Single-station techniques are kind of out of vogue on the Earth when you've got thousands of stations to work with,” Banerdt says. “But there are lots of ways that you can use a single station to actually get at planetary structure.”
Extraordinary Measures for an Extraordinary Instrument
The SEIS instrument is extremely sensitive — so much so that it has to be isolated from the elements as much as possible. Temperature can vary as much as 80°C from Martian day to night, and SEIS can pick up the ensuing noises from the lander and the surrounding environment as they adjust to the thermal changes. Even the thick tether cable that connects the seismometer to the lander cracks every now and then, and researchers must carefully remove this noise from their recordings before analyzing them. Wind can also create vibrations.
SEIS is shielded in a titanium vacuum-sealed enclosure designed to reduce thermal strain, and covered by a thermal insulator. This copper-colored box with honeycombed walls traps Martian air for thermal insulation. Over that, an aerodynamic dome reduces shaking from the wind. A chain-mail skirt and thermal blankets underneath the device prevents any wind to slip under the device. All of these mechanisms allow for a sensitivity hundreds of times better than when the Viking instrument instrument was on the landers’ platform. Even with all these precautions, Insight’s weather station is still crucial to removing spurious signals.
— SEIS (@InSight_IPGP) February 2, 2019
First Finds from Insight
Insight’s instrument suite has already revealed interesting details about the landing site. For example, there’s a cycle to the winds, which pick up around noon and weaken in the evenings. Meanwhile, the magnetometer — the first such instrument to be deployed to the surface of another planet — has found a surface magnetic field at least 10 times stronger than orbital observations had suggested. Before being deposited into the ground, the seismometer was also able to record the noise of the wind as it blew over the lander platform and between the solar panels. Listen to the winds blowing here: