Scientists have made the most precise measurements of Mars rotation to date, revealing for the first time how the planet wobbles as a result of the “sizzling” of its molten metal core. The findings, detailed in a recent Nature paper, are based on data from NASA’s InSight Mars lander, which operated for four years before running out of steam during its extended mission in December 2022.
To monitor the planet’s rotation rate, the study authors relied on one of InSight’s instruments: a radio transponder and antennas collectively called the Rotation and Interior Structure Experiment, or RISE. They found that the planet’s rotation is speeding up by about 4 milliseconds per year² equivalent to shortening the length of a Martian day by a fraction of a millisecond per year.
It’s a subtle speedup, and scientists aren’t entirely sure what’s causing it. But they have several ideas, including ice accumulating on the polar caps or post-glacial rebound, where landmasses rise after being buried by ice. The shift in the planet’s weight can cause it to speed up a bit, like a skater spinning with arms outstretched and then pulling them back.
How RISE works
RISE is part of a long tradition of Mars landers using radio waves for science, including two Viking landers in the 1970s and the Pathfinder lander in the late 1990s. However, none of these missions had the benefit of InSight’s advanced radio technology and antenna enhancements within NASA’s Deep Space Network on Earth. Together, these improvements provided data about five times more accurate than that available for the Viking landers.
In the case of InSight, scientists would transmit a radio signal to the lander using the Deep Space Network. RISE would then reflect the signal back. When the researchers received the reflected signal, they looked for tiny changes in frequency caused by Doppler shift (the same effect that causes an ambulance siren to change tone as it approaches and recedes). Measuring the displacement allowed the researchers to determine how fast the planet was spinning.
What we’re looking for are variations that are only a few tens of centimeters over the course of a Martian year,” said lead author of the paper and RISE principal investigator Sebastien Le Maistre of the Royal Observatory in Belgium. “It takes a very long time and a lot of data to accumulate before we even see these variations.”
The paper examined data from InSight’s first 900 Martian days — plenty of time to look for such variations. Scientists had to eliminate sources of noise: Water slows down radio signals, so moisture in the Earth’s atmosphere can distort the signal coming from Mars. So is the solar wind, electrons and protons ejected into deep space from the Sun.
“It’s a historic experiment,” Le Maistre said. “We spent a lot of time and energy preparing for the experiment and anticipating these discoveries. But even so, we were still surprised along the way – and it’s not over, because RISE still has a lot to reveal about Mars.”
Measuring the core of Mars
Data from RISE was also used by the study authors to measure the wobble of Mars called its nutation due to splashing in its liquid core. The measurement allows scientists to determine the size of the core: Based on the RISE data, the core has a radius of roughly 1,140 miles (1,835 kilometers).
The authors then compared this data with two previous measurements of the core derived from the spacecraft’s seismometer. Specifically, they looked at how seismic waves traveled through the planet’s interior whether they bounced off the core or passed through it unhindered.
Taking all three measurements into account, they estimate the core radius to be 1,112 to 1,150 miles (1,790 to 1,850 kilometers). Mars as a whole has a radius of 2,106 miles (3,390 kilometers), about half the size of Earth.
Measurements of Mars’ wobbles also provided details about the shape of the core. “The RISE data suggest that the shape of the core cannot be explained by its rotation alone,” said the paper’s second author Attilio Rivoldini of the Royal Observatory in Belgium. “This shape requires regions of slightly higher or lower density buried deep in the mantle.”
While scientists will mine InSight data for years to come, this study represents the final chapter in Banerdt’s role as the mission’s principal investigator. He retired on August 1 after 46 years at JPL.
More about the mission
JPL managed InSight for NASA’s Science Mission Directorate. InSight was part of NASA’s Discovery program, which is managed by the 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.
The InSight mission is supported by a number of European partners, including France’s Center National d’Études Spatiales (CNES) and the German Aerospace Center (DLR). CNES provided NASA’s Seismic Experiment for Interior Structure (SEIS) instrument with principal investigator IPGP (Institut de Physique du Globe de Paris).