A double earthquake in 2021 sent seismic waves deep into the Red Planet’s core, providing scientists with the best data ever on the planet’s size and composition. Data from its seismometer will be studied for decades as NASA retires the InSight Mars lander in December. By looking at the seismic waves the device recorded during the two shocks in 2021, scientists were able to conclude that Mars’ liquid iron core is smaller and denser than previously thought.

The findings, the first direct observations of another planet’s core, are detailed in a paper published April 24 in the journal Proceedings of the National Academy of Sciences. These two earthquakes, which occurred on August 25 and September 18, 2021, were the first earthquakes detected by the InSight team to occur on the opposite side of the planet, called long-range earthquakes. Distance has proven to be crucial: the farther an earthquake is from InSight, the deeper its seismic waves can travel to the planet before being detected.

“We needed both luck and skill to find and then use these earthquakes,” said lead author Jessica Irving, an Earth scientist at the University of Bristol in England. “External earthquakes are more difficult to detect because a lot of energy is lost or deflected as seismic waves travel across the planet.”

Irving noted that the two earthquakes occurred after the mission had been on the Red Planet for more than a full Martian year (about two Earth years), meaning that the Earthquake Service – the scientists who first studied seismographs – had already refined their skills. It also helped that the meteor strike caused one of the two earthquakes; shocks provide accurate location and more accurate data for the seismologist to work with. (Since Mars has no tectonic plates, most earthquakes are caused by heat and stress faults or rock cracks in the planet’s crust.) The size of the earthquakes was also a factor in detection.

This artist’s concept shows a cross section of Mars with seismic wave trajectories from two separate earthquakes in 2021. Discovered by NASA’s InSight mission, these seismic waves were the first in history to penetrate the core of another planet.

“These two long-range earthquakes were among the largest ever heard by InSight,” said Bruce Banerdt, InSight principal investigator at NASA’s Jet Propulsion Laboratory in Southern California. “If they weren’t this big, we wouldn’t be able to detect them.”

One of the challenges of detecting these particular earthquakes was that they were in the “shadow zone”—the part of the planet where seismic waves tend to break through InSight, making it difficult for earthquake echoes to reach the lander unless they were very large. . It is extremely difficult to detect seismic waves passing through the shadow region; It’s even more impressive that the InSight team did this using a single seismometer they had on Mars. (By contrast, there are many seismometers spread across the Earth.)

“Extracting the signals from the complex seismograms recorded by the lander required extensive seismological expertise from the entire InSight team,” said Irving.

A previous paper that offered a first look at the planet’s core relied on seismic waves bouncing off its outer boundary and provided less precise data. Detecting seismic waves actually passing through the core allows scientists to refine their models of what the core looks like. About one-fifth of the core is made up of elements such as sulfur, oxygen, carbon and hydrogen, according to the findings documented in the new paper.

“Determining the abundance of these elements in the planetary core is important for understanding the conditions in our solar system when planets form and how these conditions affect the planets that form,” said Doyon Kim, ETH Zurich. Said. .

This has always been the main goal of the InSight mission: to explore the deep interior of Mars and help scientists understand how all rocky worlds, including Earth and its moon, were formed.