First seismograph for the far side of the Moon built with spare parts

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Enlarge / One of NASA’s Commercial Lunar Delivery Service designs.

On May 5, 2022, the seismometer aboard the InSight lander recorded a magnitude 4.7 earthquake on the Martian surface, while the epicenter was 2,250 km from the lander. It was one of the largest earthquakes recorded on Mars and the largest recorded by the Insight mission. In September, during the very first measurement of this type, the instrument recorded an earthquake generated by the impact of a meteorite on Mars.

InSight’s seismometer is called the Seismic Experiment for Internal Structure (or SEIS), and it recorded these earthquakes and 20 others. Now an instrument based on the same design will measure ground vibrations on the far side of the Moon, the first seismographs on our neighbor since the Apollo era.

Up to SEIS

Developed by the Institut de Physique du Globe de Paris (IPGP) and the French space agency (CNES), the SEIS Very Broad Band (VBB) seismometer now on Mars can detect the slightest movement, up to 10 picometers, which is much smaller than the size of an atom. Comprised of three pendulums placed 120 degrees apart, SEIS measures both the vertical and horizontal vibrations of the Martian surface.

During the development of InSight, an aftermarket model of SEIS was built. Now, the VBB in this spare will be part of the Farside Seismic Suite that will be deployed to the Moon in 2025 as part of NASA’s Commercial Lunar Payload Services. program. It’s one of two seismometers that will operate on the far side of the Moon in an impact crater called Schrödinger’s Basin. The other seismometer will be a short-period sensor.

According to Gabriel Pont, Seismic continuation of the far side project manager at CNES, the instrument on the Moon will have only one broadband pendulum that will measure the vertical vibrations of the ground. The short period sensor will process measurements in the other directions.

The new environment required minimal changes. “We used an aftermarket model of the SEIS instrument. The Seismic continuation of the far side the seismometer will be set for lunar gravity. It will be placed in a protective vacuum box called a seismobox,” Pont said.

Philippe Lognonné of IPGP and Université Paris Cité, who is the principal investigator of SEIS on Mars and the co-principal investigator of the broadband sensor on the Seismic continuation of the far side, said the single vertical axis sensor will be used with little modification. “Depending on the frequency, this seismometer will either be comparable to or up to 10 times better than the Apollo seismometers,” Lognonné noted.

Many firsts

The Seismic continuation of the far side will mark the first time a seismometer has been placed on the lunar surface since the Apollo missions. It will also be the first time that a seismometer will work on the far side of the Moon.

“The originality of Seismic continuation of the far side is that it will be independent of the lander. Indeed, it must survive several lunar days and nights, which is not the case of the lander. The Seismic continuation of the far side will have its own solar panels, antennas to communicate with the orbiters, and its own thermal control devices,” Pont said.

According to Pont, one of the main objectives of Seismic continuation of the far side is to determine the seismic activity and impact rate of micro-meteorites in the region where they land. “It can also be useful for future exploration missions, whether they’re manned or deploying a telescope on the far side of the Moon,” Pont said.

“Over a long period, the VBB will be able to detect the interaction of seismic waves with a possible deep fusion zone of the Moon. This is crucial to understanding how the Moon has evolved since its formation,” Lognonné added.

The success of SEIS on Mars and its selection for the next lunar mission comes after years of research and development. Lognonné remembers that the first broadband seismometer proposal to the IPGP was accepted in 1993. “Since the mid-1990s, we have started its development and worked tirelessly to make it ready to fly. It took us 15 years before it was selected in early 2010 for the InSight mission,” said Lognonné, who has worked on the project since its inception.

Dhananjay Khadilkar is a journalist based in Paris.

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