NASA's InSight provides new details about Mars' rotation and core

Researchers have made the most precise measurements to date of Mars' rotation, uncovering the intriguing wobble of the Red Planet caused by the movement of its molten metal core. The findings are based on the data collected from NASA's InSight Mars lander, which concluded its extended mission in December 2022.

The pivotal instrument behind this discovery is the Rotation and Interior Structure Experiment (RISE). The researchers found a subtle yet significant acceleration in Mars' rotation, at a rate of approximately 4 milliarcseconds per year². This translates to a minuscule shortening of the Martian day by a fraction of a millisecond annually.

The exact driving factors behind this acceleration remain a mystery. However, several ideas have been proposed, including the accumulation of ice on polar caps and post-glacial rebound. Similar to an ice skater spinning faster upon retracting their outstretched arms, the shift in a planet's mass can cause it to accelerate a bit, the researchers said.

To elucidate Mars' rotation, scientists utilized a novel method: a radio signal beamed from Earth to InSight via NASA's Deep Space Network, which was then reflected back by RISE. By scrutinizing subtle frequency shifts caused by the Doppler effect, similar to how an approaching or receding ambulance siren alters pitch, researchers were able to measure Mars' rotational speed.

Using data from @NASAMars' InSight lander, scientists have made the most precise measurements ever of the Red Planet’s rotation. https://t.co/cL0Ic6cflfHow? Giant radio antennas on Earth! 🌏📡🔴 Specifically, @NASASCaN's Deep Space Network. pic.twitter.com/FgP88rfdcw

— NASA Solar System (@NASASolarSystem) August 7, 2023

For this study, the researchers examined data from InSight's initial 900 Martian days. Scientists had their work cut out for them to eliminate noise sources including signal distortion due to atmospheric moisture and interference from solar wind, the stream of charged particles emitted by the Sun into space.

Furthermore, scientists also leverage RISE data to calculate Mars' wobble, called its nutation, stemming from the sloshing of its liquid core. This facilitated the determination of the core's size, approximating a radius of about 1,140 miles (1,835 kilometers).

In the next step, they compared these measurements with data obtained from InSight's seismometer and estimated a core radius range of 1,112 to 1,150 miles (1,790 to 1,850 kilometers), subsequently leading to an estimate of Mars' core size. Notably, Mars' overall radius is 2,106 miles (3,390 kilometers), roughly half the size of Earth.

"RISE's data indicate the core’s shape cannot be explained by its rotation alone. That shape requires regions of slightly higher or lower density buried deep within the mantle," said the paper's second author, Attilio Rivoldini of the Royal Observatory of Belgium.