Author: 一毫秒的永恒

In the early hours of April 24, 019, a magnitude 6.3 earthquake struck Medog County in Nyingchi, Tibet. Tremors were widely felt in the surrounding area, but fortunately the region within 50 kilometers of the epicenter was extremely sparsely populated, and no casualties were reported.

Earth has earthquakes, but do other worlds in the Solar System “quake” too? Of course they do. Consider Earth’s satellite, the Moon. In 1969, American scientists first set foot on the Moon aboard Apollo spacecraft and installed 5 seismometers there. These continuously transmitted records of moonquakes to Earth, beginning humanity’s observation and study of lunar seismic activity. In 2019, signs of quakes were also found on our neighbor Mars.

Marsquake signal detected by SEIS

The Seismic Experiment for Interior Structure (SEIS), carried by the InSight lander, detected and recorded the mysterious marsquake. Developed by France’s National Centre for Space Studies (CNES), the instrument is sensitive enough to “feel” movements of the Martian surface on the scale of a hydrogen atom’s radius. It records seismic waves produced by marsquakes, Martian winds, and meteorite impacts.

On April 6, 2019, InSight’s 128th Martian day on the surface, SEIS may have captured a marsquake signal for the first time. It was the first vibration from inside the Red Planet that InSight had “heard,” but the signal was so weak that scientists could not confirm it at the time. On April 23, 2019, CNES held a press conference and formally announced that InSight had detected one weak Martian earthquake, along with three other possible marsquakes.

“We have been waiting months for a signal like this,” Philippe Lognonné, principal investigator for SEIS at the Institut de Physique du Globe de Paris, said in a CNES statement. “It is so exciting finally to have proof that Mars is still seismically active.”

Scientists believe marsquakes will be less frequent than earthquakes because Mars has no tectonic plates, whose interactions cause many earthquakes on Earth. The continued cooling and contraction of the Martian interior nevertheless create stress. Once enough stress accumulates, or when a meteorite strikes Mars, cracks can form in the crust and trigger a marsquake. Receiving such exceptionally precise signals requires an instrument designed with great care and protected from wind and dust. This is why SEIS is covered by a white dome: a shield that lets it concentrate on studying vibrations on Mars.

The team had now obtained its first result. On April 6, 2019—the lander’s 128th Martian day, or sol, on the surface—the extremely sensitive detector captured a tiny vibration from inside Mars, an event NASA also called Sol 128. This provisionally confirmed signal may have been related to similar seismic events measured on March 14, April 10, and April 11, 2019, but those signals were weaker than Sol 128 and their origins more uncertain. Although the scientists were delighted by the detection, it could not help them achieve InSight’s ultimate objective of analyzing the Martian interior: the quake was not strong enough to produce the data that analysis required.

Illustration of InSight operating after landing on Mars | Credits: NASA/JPL-Caltech

The InSight team had other ways to gather data as well. The Heat Flow and Physical Properties Package (HP3), nicknamed the “mole,” would take Mars’s “temperature,” while the Rotation and Interior Structure Experiment (RISE) would study the planet’s motion and the size of its core. The roughly 40-centimeter-long mole, however, ran into trouble. Its self-hammering spike was designed to penetrate about 5 meters below the surface and obtain important scientific information about the Martian core, but in practice it encountered a serious problem after digging less than 50 centimeters. Scientists initially suspected that it had struck a large rock; later analysis showed that insufficient soil friction and related factors were the main difficulty. They built a replica on Earth and tried to diagnose and solve the problem, but did not succeed.

InSight was able to place SEIS directly on the Martian surface, making it a successor to the moonquake instruments installed by Apollo astronauts. The year 2019 marked 50 years since the United States placed seismometers on the Moon through the Apollo program.

“The activity from the Sol 128 signal is very exciting,” said Lori Glaze, director of NASA Headquarters’ Planetary Science Division. “Its size and duration match the profile of moonquakes detected on the lunar surface during the Apollo missions.” Bruce Banerdt, a geophysicist at NASA’s Jet Propulsion Laboratory and principal investigator of the InSight mission, said, “InSight has carried the Apollo program’s scientific investigations forward to Mars for the first time. Until now, we have been collecting background information about Mars. This event officially opens a new field: Martian seismology.”

Humanity Detects a Marsquake for the First Time

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