Decidedly, Toulouse is a real capital of Martian research. Besides the spectrometersspectrometers laser ChemCamChemCam and Supercam, it also produced the very first martian seismometer, with breathtaking precision. Even if his retirement is imminent following the close end of the Insight mission, Seis always surprises us, and today he completes his last scientific objective. It is therefore a total success for the instrument, which has measured more than 1,300 earthquakesearthquakes since early 2019.
The InSight lander recorded the entry of a meteoroid into the tenuous atmosphere of Mars, its explosion and then its impact on the ground. Listen to the recording captured on the surface of Mars. © NASA, JPL-Caltech
A tremendous power of documentation
« We now detect more impacts of meteoritesmeteorites on Mars than on Earth “, tells us Raphaël Garcia, specifying that the only impact on Earth recorded by seismometers took place in Bolivia, creating a crater 10 meters in diameter. As a reminder, the Russian examples of ToungouskaToungouska or Chelyabinsk are explosions that happened in the sky, not impacts.
The only time localization was done using seismometers was during an explosion of gazgaz in Germany. But, here, we are indeed on Mars. And it was with the help of one of the eight probes in orbit around the planet that we were able to complete Seis’s work: the seismometer locates the impacts, the CTX and HiRise cameras Mars Reconnaissance Orbiter confirm them, locate the craters by comparing before and after the impact, and even communicate information about its size. A power of event documentation unmatched in space exploration, and very useful in the future.
Applications for manned missions
Seis’s mission is coming to an end, and already his legacy is foreseen on the LuneLune. Two seismometers, constituting the Farside Seismic Suite – FSS, are to land in the Schrödinger crater, on the far side, near the South Pole, in 2025. FSS will be built under the project management of the JPLJPLin collaboration with Cnes and the French space laboratories involved in the supply of sensorssensors (Institute of PhysiquePhysique of the Paris Globe, ISAE-Supaéro and the Astroparticule laboratory and CosmologyCosmology – APC). With this new method, these seismometers will be able to document the flow of meteorites impacting the lunar South Pole, where the NasaNasa plans to send its astronauts from the Artemis program, and even build a lunar base there. As a reminder, the very first lunar seismometer had been deposited by Buzz AldrinBuzz Aldrin during the Apollo 11 mission.
Interview with Raphaël Garcia, researcher at ISAE in Toulouse, and first author of the study.
Futura: How did you distinguish between meteorite impacts among all the earthquakes recorded by Seis?
Raphael Garcia: First, Seis felt the seismic wavesseismic waves but he also heard the sound of the explosion through the rotations of the ground imposed by the shock wave when it arrives on the sensor. It’s a bit like the explosion of the AZF factory. People felt like there had been two explosions because they first felt the ground vibrate and then heard the sound of the explosion.
Futura: How were you able to locate the craters precisely?
Raphael Garcia: We used this time lag between the arrival of the seismic waves and that of the acoustic wavesacoustic waves to determine the distance between Seis and the place of impact. Then, to find out in which direction the impact was, we scrutinized the deformations of the ground under the effect of the acoustic wave. To have a precise localization, it was supposed that the models ofatmosphereatmosphere of Mars reproduce well the speeds of soundspeeds of sound and ventsvents, which we know well today. So we provided a location, then the CTX imager team [caméra de contexte – Context Imager, du MRO] took pictures there.
They didn’t always find the craters directly, but more the dust that was kicked up on impact, which covers a larger area. Once they found this blast zone, they took precise images at 20 cm from resolutionresolution with the HiRise camera to determine the exact size of the craters.
Futura: Several probes were therefore necessary for this study
Raphael Garcia: What’s nice is to be able to locate with a single instrument capable of recording both seismic waves and acoustic waves, and then to have all this battery of imagers that can take images of a resolution ranging from 6 m to 20 cm. It’s nice to see in this study the complementarity between the missions. Without it, nothing would have been possible.
Futura: Has this method of impact detection already had other antecedents?
Raphael Garcia: Yes, once in Germany. During a gas explosion, seismometers detected the infrasound waves and showed that the polarization pointed in the direction of the explosion.
Futura: So this study is a first for impact detection with Seis?
Raphael Garcia: Indeed. It is believed that other impacts were recorded by Seis, but we were not able to differentiate them from earthquakes. Now, we are taking all the Seis data and looking at the seismic events that result from impacts. Maybe the first recorded earthquake was an impact? We are not sure yet.
Futura: So this is a new scientific goal fulfilled for Seis?
Raphael Garcia: Yeah, that wraps up the success of the Seis mission a bit. This was one of the scientific objectives that remained to be fulfilled, and which is not completely independent of the others because until now we tried to know both the structure of the subsoil and the source of the waves with the same sensor. By separating the source [dans le cas d’un impact, Ndlr]we can better image the crustcrust martian. We have also verified that the current models correctly predicted the arrival of the waves.
Futura: So it is not necessary to know the Martian crust well to do this kind of experiment?
Raphael Garcia: Yes and no, we started on the assumption that we know the relationship between the speeds of the P waves and the speeds of the S waves. We didn’t really need the speed of the P waves because they are more the acoustic zones which constrain the distance since they spread slowly. So yes, we’re going to have better imagery of the internal structure, especially around Insight, and especially the crust. Since the impacts are close, we rather image the first 20 to 30 kilometers [de profondeur].
Futura: Is this also the first time that infrasound has been detected on Mars?
Raphael Garcia: Yes, these are very low sounds frequencyfrequency that propagate through the atmosphere of Mars and that was used in the paper to image the structure of the atmosphere.
Futura: What are the applications for future missions?
Raphael Garcia: We can now quantify the relationships between the impact processes and the seismic waves: for such size of crater, I will have such energyenergy seismic emitted. We will know better how many seismic waves are produced by the impacts and we will be able to better know the impact recording capacities of a future seismometer, like the one there will be on the Moon for example.
Futura: Can a similar process be used on the Moon?
Raphael Garcia: There is no acoustic wave on the Moon because there is no atmosphere, but with the various probes that orbit around it, we will be able to image the craters located by seismometers. There is also the detection of flashflash impact. When an object impacts the surface, it creates a small lightlight very short durationduration which can be observed from the earth or from lunar orbit. It will also be a good way to mobilize astronomersastronomers amateurs, who, by detecting flashes on the Moon, will help the seismometer provide information, such as the exact time of impact.
Futura: So it can be used for the safety of manned missions to the Moon?
Raphael Garcia: For the Moon, we know well the impacts of large meteorites, but not too much of small ones, and this constitutes a significant risk for manned missions. It is especially the micrometeorites whose flux is poorly known since on Earth, they burn immediately in the atmosphere.