InSight’s domed Wind and Thermal Shield

InSight’s domed Wind and Thermal Shield, which covers its seismometer. The image was taken on the 110th Martian day, or sol, of the mission. The seismometer is called Seismic Experiment for Interior Structure, or SEIS.

The Mars InSight lander’s seismometer, Seismic Experiment for Interior Structure (SEIS), has created the first-ever recording of a “marsquake.” On April 6, InSight’s 128th Martian day, a faint seismic signal was detected.

This first recorded marsquake appears to have come from inside the planet; it was not caused by forces above the surface, such as wind. The science team is still examining the data to determine the exact cause of the event.

The surface of Mars is very seismically quiet, allowing SEIS to detect extremely faint rumblings. The InSight team hopes to gain knowledge of Mars’ interior from the data collected by SEIS, but this seismic event was too small to provide any helpful data.

If this event had occurred in Southern California, it would have been lost among the dozens of tiny crackles that occur every day. Because of the weather and the movements of the oceans, Earth’s surface is moving constantly. This creates a seismic noise in which the Mars event would have been lost.

There were some other seismic events on Mars that were detected by SEIS’ more sensitive Very Broad Band sensors on March 14, April 10 and April 11, but they were even smaller than the April 6 event and more ambiguous in origin. The team will study these events to try to determine their cause, but the cause of the April 6 event was definitely below the surface, making it the first recorded marsquake.

“We’ve been waiting months for a signal like this, says Philippe Lognonne, SEIS team leader. “It’s so exciting to finally have proof that Mars is still seismically active. We’re looking forward to sharing detailed results once we’ve had a chance to analyze them.”

It is not easy to detect these tiny quakes. It requires a lot of high-level engineering. On Earth, seismometers are often sealed in underground vaults that isolate them from changes in weather, but this is not possible on Mars.

InSight’s instrument is protected by several ingenious insulating barriers, including a cover built by JPL called the Wind and Thermal Shield, to protect it from Mars’ extreme temperature changes and high winds.

Most of the quakes on Earth occur on faults between the tectonic plates. Earth’s surface is made up of several ridged surface plates that slowly move in different directions. The faults are the places where these plates meet. Because the plates are moving in different directions, frictional stress can build up along the faults, and when this stress is released quickly, we have an earthquake.

But as far as we know, Mars and the Moon do not have tectonic plates, yet they still have quakes. One possible cause could be the continual process of cooling, which causes contraction that would create stress. This stress would build over time until it was strong enough to break the crust, causing a quake.

During the Apollo program astronauts installed five seismometers on the Moon. While operating between 1969 and 1977, these instruments measured thousands of quakes. Astronauts also exploded mortar rounds to create seismic vibrations, which gave us a peek at the structure of the first 328 feet under the surface. The information thus gained helped us learn about the structure of the interior of the Moon.

Different layers of materials can change the speed and/or direction of seismic waves. The boundaries between layers can even reflect the waves.

These changes in the path of the seismic wave can be used to map the internal structure of a planet or moon.

Using the seismic data from the moon, scientists identified four classifications of moonquakes: deep moonquakes, thermal moonquakes, meteoroid impacts, and shallow moonquakes.

The study of Mars’ seismic activity is already giving insights about Mars.

The April 6 event fit the profile of moonquakes detected on the lunar surface better than that of quakes on Earth.

The size and long duration of the marsquake resembled a moonquake rather than an earthquake.

InSight placed the seismometer on the surface of Mars on Dec. 19, 2018, and we have detected a marsquake by April of 2019, so this is only the beginning.

By studying the deep interior of Mars, we hope to learn how other rocky worlds, including Earth and the Moon, formed.

The measurements that SEIS will make in the years to come will help answer many of the questions we still have today.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at  Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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