NASA’s InSight mission has finally peered inside Mars — and discovered that the planet’s crust might be made of three layers. This is the first time scientists have directly probed the inside of a planet other than Earth, and will help researchers to unravel how Mars formed and evolved over time.
Before this mission, researchers had measured only the interior structures of Earth and the Moon. “This information was missing, until now, from Mars,” said Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne in Germany, in a prerecorded talk played at the virtual American Geophysical Union meeting on 15 December. She declined an interview with Nature, saying that the work is under consideration for publication in a peer-reviewed journal.
It is a major finding for InSight, which landed on Mars in November 2018 with a goal of working out the planet’s internal structure1. The InSight lander squats near the Martian equator, on a smooth plain known as Elysium Planitia, and uses an exquisitely sensitive seismometer to listen to geological energy thrumming through the planet2. So far, the mission has detected more than 480 ‘marsquakes’, says Bruce Banerdt, the mission’s principal investigator and a scientist at the Jet Propulsion Laboratory in Pasadena, California. Mars is less seismically active than Earth, but more so than the Moon.
Just as they do with earthquakes on Earth, seismologists are using marsquakes to map the red planet’s interior structure. Seismic energy travels through the ground in two types of wave; by measuring the differences in how those waves move, researchers can calculate where the planet’s core, mantle and crust begin and end, and the general make-up of each one. Those fundamental geological layers reveal how the planet cooled and formed billions of years ago at the fiery birth of the Solar System. Now, “we have enough data to start answering some of these big questions”, says Banerdt.
Earth’s continental crust is generally divided into sublayers of different types of rock. Researchers had suspected, but didn't know for sure, that the Martian crust was also layered, says Justin Filiberto, a planetary geologist at the Lunar and Planetary Institute in Houston, Texas. Now, InSight’s data show that it is made up of either two or three layers.
A three-layered crust would fit best with geochemical models3 and studies of Martian meteorites, says Julia Semprich, a planetary scientist at the Open University in Milton Keynes, UK.
Depending on whether the crust actually has two or three layers, it is either 20 or 37 kilometres thick, Knapmeyer-Endrun said during her talk. That thickness probably varies at different locations around the planet, but is likely to be no greater than 70 kilometres on average, she added. On Earth, the crustal thickness varies from around 5 to 10 kilometres beneath the oceans, to around 40 to 50 kilometres beneath the continents.
In the coming months, InSight scientists plan to report measurements taken even deeper in Mars, says Banerdt — ultimately revealing information about the planet’s core and mantle.
Along with listening to marsquakes, InSight’s other big scientific goal is to measure heat flow through the Martian ground using a probe dubbed the mole. It was meant to bury itself deep in the soil, but has struggled to do so — at one point even popping out of the ground altogether. The mole has finally managed to get itself several centimetres deep, says Banerdt, and will try digging one final time in the coming weeks before giving up. “We’re at what we consider to be the end game now,” he says.
Nature 589, 13 (2021)
Knapmeyer-Endrun, B. & Kawamura, T. Nature Commun. 11, 1451 (2020).
Banerdt, W. B. et al. Nature Geosci. 13, 183-189 (2020).
Semprich, J. & Filiberto, J. Meteorit. Planet. Sci. 55, 1600-1614 (2020).