On April 6, 2019, the Seismic Experiment for Interior Structure (SEIS) instrument on NASA’s Mars Insight Lander recorded a phenomenon never observed, even since the agency’s Viking spacecraft began exploring the Red Planet 45 years ago: a trembling of the ground emanating, it appeared, from deep inside Mars.
Since then, more than 480 such seismic events have been recorded. That first “marsquake” was estimated to have a magnitude between 1 to 2, a level barely detectable on seismometers on Earth. In the words of Bruce Banerdt, Insight Principal Investigator of NASA’s Jet Propulsion Laboratory, Pasadena, California, it marked “the beginning of a new field: martian seismology.”
Hitting the Ground Listening
Developed by the French space agency Centre National d’Études Spatiales, Insight’s SEIS instrument is designed to sense vibrations caused by marsquakes and Martian weather systems such as dust storms. It’s also the first seismometer to be deployed directly on the Martian surface.
The earlier Viking 1 and 2 seismometers were mounted on top of their lander’s equipment bay, which made them vulnerable to noise caused by the wind, reducing their sensitivity by three orders of magnitude. SEIS’s innovative technology includes several insulating barriers, including a dome-shaped wind and thermal shield, to protect it from Mars’ extreme temperatures and high winds.
Insight’s Marsquake Service Team, which is led by the Swiss Federal Institute of Technology (ETH Zürich), reviews and analyzes SEIS data about twice per day to learn more about Mars’ geology.
According to an ETH Zürich news report, the Martian surface is very quiet seismically. But during the day, strong winds shake the Lander and its instrumentation, which creates vibration and a high level of ambient noise. This noise effectively drowns out faint seismic signals from marsquakes. Then, during evening hours after the winds have died down, the innovative technology of the SEIS instrument — ETH Zürich calls it “arguably the most sensitive seismometer ever operated” — allows it to detect and record Mars’ faintest seismic signals.
Unraveling the Waves
On Earth, quakes occur on fault lines created by the movement of adjacent tectonic plates. The Red Planet, however, does not have tectonic plates. Scientists believe that marsquakes are caused primarily by the cooling and contraction of Mars, which creates stress, as ETH Zürich reports. Over time, this stress builds up until it’s strong enough to break the crust, resulting in marsquakes.
The SEIS scientists point out that marsquakes, like earthquakes, generate two types of seismic waves: fast-moving P- (primary) waves and slower-moving S- (secondary) waves. When P-waves travel through a planet and encounter a discontinuity in materials (think interface between a solid mantle and a liquid core), they can be reflected back to their source, change direction or even change into a slower-moving S-wave.
By analyzing the arrival times of different types of seismic waves at the Insight lander’s location, seismologists can gain valuable insight into Mars’ geology, including the location, depth, thickness and composition of its internal layers.
Following the Science
To date, as Nature magazine reports, the SEIS instrument has measured two types of marsquakes. The most common type shakes the ground at high frequencies and is linked to fractures of the shallow Martian crust. The less common type is characterized by low-frequency vibrations and appears to emanate from sources within Mars’ mantle.
The February 2020 analysis by seismologists at ETH Zürich revealed that of 174 registered marsquakes, 24 could be considered low-frequency events with magnitudes between 3 and 4, with seismic waves propagating through the Martian mantle. The two largest of these appeared to emanate from the tectonically active zone of Mars known as Cerburus Fossae. The other 150 quakes had even smaller magnitudes and high-frequency waves trapped in the Martian crust.
As Nature magazine highlights, the analysis of these seismic waves have led the ETH Zürich seismologists to conclude that the upper 8 to 11 kilometers of the Martian crust beneath the Insight lander is fractured or highly altered. They are still investigating composition of the mantle.
The ETH Zürich team has also observed that marsquakes are much different from earthquakes in their duration. Instead of the 10 to 30 seconds typical for earthquakes, marsquakes appear to last for 10 to 20 minutes. The scientists are still investigating the reasons behind this dramatic difference.
To date, according to NASA, the Insight mission has “produced exceptional science.” As a result, the agency has extended the Lander’s mission for another two years, through December 2022. This extension will give the Lander’s instruments more time to monitor and measure marsquakes and weather phenomena on the Martian surface.
According to NASA, between now and the summer of 2021, the Insight science team will be operating the Lander’s key instruments in a power conservation mode. This strategy reflects the onset of the Martian winter, when the already faint sunlight on the Red Planet’s surface will become even fainter and the dust accumulation on Insight’s solar panels — which has reduced their output power to just 27% of their “dust-free” capacity — will increase. NASA mission planners had hoped that small dust storms near the Lander would periodically clean off its solar panels, but to date, that has not happened. The Insight team plans to resume full science operations in July when Mars begins approaching the Sun again.
Weather and astronomical issues notwithstanding, NASA scientists are looking forward to continuing Insight’s very productive science mission on Mars, making new discoveries and even adding momentum to human dreams to one day explore and colonize the Red Planet.
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