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What the First Marsquake Means for NASA's Insight

After two months, Insight's seismometer finally picked up the first rumblings in Martian soil

 Seismometer on NASA's Insight lander

This image shows InSight's domed Wind and Thermal Shield, which covers its seismometer.
NASA / JPL-Caltech

NASA's Insight lander installed one of the most sensitive seismometers ever built on Mars earlier this year. If it's not enough to make you go, now it has finally detected its first march.

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. However, they are still analyzing the signal to other sources, such as a meteorite impact. The signal lasted around 10 minutes and had a magnitude of 2 to 2.5, so shaking so little that humans would not have felt it had happened on Earth.

The French-built SEIS started science operations on February 4th, after The instrument was deployed to its current placement and few meters from the lander. Since then, it has recorded three other signals that could have had a seismic origin, but are still working to rule out other possible causes, such as winds. Insight Principal Investigator Bruce Banerdt (NASA's Jet Propulsion Laboratory) announced the April 23rd 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. While researchers originally thought they were picking up one or two seismic movements a month, now they are estimating detections will come at the rate of 6 or 7 per year. That means that the planned two-year nominal mission duration, SEIS might record only 12 to 14 March queries.

Nevertheless, whatever shakes SEIS records will be the first hints we have of Martian activity. While both Viking 1 and 2 missions carried seismometers to Mars, they did not function as expected. One of them failed to work altogether. The other could make reliable measurements because it was installed on the lander platform, from the ground up, 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 we have a liquid metal core, a thick mantle, and a basaltic crust, the sizes of these layers are only known 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 of the opinion of the Earth when you've got thousands of stations to work with, "Better says. "But there are lots of ways that you can use a single station to actually get to 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 by more than 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 was on the landers' platform. Even with all these precautions, Insight's weather station is still crucial to removing spurious signals.

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