NASA’s InSight lander has detected seismic waves from four space rocks that crashed on Mars in 2020 and 2021. Not only do these represent the first impacts detected by the spacecraft’s seismometer since InSight touched down on the Red Planet in 2018, it also marks the first time seismic and acoustic waves from an impact have been detected on Mars.

A new paper published Monday in Nature Geoscience details the impacts, which ranged between 53 and 180 miles (85 and 290 kilometers) from InSight’s location, a region of Mars called Elysium Planitia.

The first of the four confirmed meteoroids – the term used for space rocks before they hit the ground – made the most dramatic entrance: It entered Mars’ atmosphere on Sept. 5, 2021, exploding into at least three shards that each left a crater behind.

Then, NASA’s Mars Reconnaissance Orbiter flew over the estimated impact site to confirm the location. The orbiter used its black-and-white Context Camera to reveal three darkened spots on the surface. After locating these spots, the orbiter’s team used the High-Resolution Imaging Science Experiment camera, or HiRISE, to get a color close-up of the craters (the meteoroid could have left additional craters in the surface, but they would be too small to see in HiRISE’s images).

“After three years of InSight waiting to detect an impact, those craters looked beautiful,” said Ingrid Daubar of Brown University, a co-author of the paper and a specialist in Mars impacts.

After combing through earlier data, scientists confirmed three other impacts had occurred on May 27, 2020; Feb. 18, 2021; and Aug. 31, 2021.

Researchers have puzzled over why they haven’t detected more meteoroid impacts on Mars. The Red Planet is next to the solar system’s main asteroid belt, which provides an ample supply of space rocks to scar the planet’s surface. Because Mars’ atmosphere is just 1% as thick as Earth’s, more meteoroids pass through it without disintegrating.

InSight’s seismometer has detected over 1,300 marsquakes. Provided by France’s space agency, the Centre National d’Études Spatiales, the instrument is so sensitive that it can detect seismic waves from thousands of miles away. But the Sept. 5, 2021, event marks the first time an impact was confirmed as the cause of such waves.

InSight’s team suspects that other impacts may have been obscured by noise from wind or by seasonal changes in the atmosphere. But now that the distinctive seismic signature of an impact on Mars has been discovered, scientists expect to find more hiding within InSight’s nearly four years of data.

Science Behind the Strikes

Seismic data offer various clues that will help researchers better understand the Red Planet. Most marsquakes are caused by subsurface rocks cracking from heat and pressure. Studying how the resulting seismic waves change as they move through different material provides scientists a way to study Mars’ crust, mantle, and core.

The four meteoroid impacts confirmed so far produced small quakes with a magnitude of no more than 2.0. Those smaller quakes provide scientists with only a glimpse into the Martian crust, while seismic signals from larger quakes, like the magnitude 5 event that occurred in May 2022, can also reveal details about the planet’s mantle and core.

But the impacts will be critical to refining Mars’ timeline. “Impacts are the clocks of the solar system,” said the paper’s lead author, Raphael Garcia of Institut Supérieur de l’Aéronautique et de l’Espace in Toulouse, France. “We need to know the impact rate today to estimate the age of different surfaces.”

Scientists can approximate the age of a planet’s surface by counting its impact craters: The more they see, the older the surface. By calibrating their statistical models based on how often they see impacts occurring now, scientists can then estimate how many more impacts happened earlier in the solar system’s history.

InSight’s data, in combination with orbital images, can be used to rebuild a meteoroid’s trajectory and the size of its shock wave. Every meteoroid creates a shock wave as it hits the atmosphere and an explosion as it hits the ground. These events send sound waves through the atmosphere. The bigger the explosion, the more this sound wave tilts the ground when it reaches InSight. The lander’s seismometer is sensitive enough to measure how much the ground tilts from such an event and in what direction.

“We’re learning more about the impact process itself,” Garcia said. “We can match different sizes of craters to specific seismic and acoustic waves now.”

The lander still has time to study Mars. Dust buildup on the lander’s solar panels is reducing its power and will eventually lead to the spacecraft shutting down. Predicting precisely when is difficult, but based on the latest power readings, engineers now believe the lander could shut down between October of this year and January 2023.

More About the Mission

NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages InSight for the agency’s Science Mission Directorate in Washington. InSight is part of NASA’s Discovery Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight spacecraft, including its cruise stage and lander, and supports spacecraft operations for the mission.

A number of European partners, including France’s Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR provided the Heat Flow and Physical Properties Package (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronika in Poland. Spain’s Centro de Astrobiología (CAB) supplied the temperature and wind sensors.