Using data from InSight, scientists discover that Mars is rotating faster

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In December 2022, NASA lost contact with its Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) Mars lander, which had been operating on the Martian surface for just over four years. In the time since InSight’s mission was declared over, scientists have continued to analyze the incredible data the lander collected and have made some exciting discoveries.

One team of scientists, led by Sebastien Le Maistre of the Royal Observatory of Belgium, recently published research that utilized InSight’s Rotation and Interior Structure Experiment (RISE) instrument to show that Mars’ rotational speed is increasing. The measurements are the most precise measurements of Mars’ rotation ever taken, and even show how the planet wobbles due to the movement of the planet’s molten metal core.

As mentioned, Le Maistre et al. used InSight’s RISE instrument, which is a collection of radio transponders and antennas, to make their measurements. They ultimately found that Mars’ rotational speed is accelerating by approximately four milliarcseconds per year squared. The increase in rotational speed corresponds to the length of the Martian day decreased by a fraction of a millisecond per year.

Artist’s depiction of InSight on the surface of Mars. Note the RISE antennas on the lander. (Credit: NASA/JPL-Caltech)

“It’s really cool to be able to get this latest measurement — and so precisely. I’ve been involved in efforts to get a geophysical station like InSight onto Mars for a long time, and results like this make all those decades of work worth it,” said Bruce Banerdt, InSight’s principal investigator of NASA’s Jet Propulsion Laboratory.

While the acceleration is quite small, scientists are still unsure of its cause. One scenario that scientists believe could’ve caused the acceleration is that ice accumulated on the polar ice caps or post-glacial rebound (the rising of landmasses after being buried under ice). By conservation of angular momentum, a significant shift in the mass of a planetary body leads to an acceleration in rotation — similar to how a spinning ice skater speeds up after pulling their arms inward.

Using radio waves to learn more about Mars’ characteristics is not unique to InSight. Previous Mars landers, such as the two Viking landers and Pathfinder, employed the use of radio waves to learn more about Mars’ interior and characteristics. However, InSight’s access to advanced radio technology and upgrades to NASA’s Deep Space Network (DSN) allowed InSight to provide scientists with data that was five times more accurate than that from Pathfinder and the Vikings.

So, how did Le Maistre et al. measure the rotational speed of Mars?

When InSight was still operational, the scientists used the DSN to beam a radio signal at InSight, which operated in the Elysium Planitia region of Mars. When InSight received the signal, RISE reflected the signal back to Earth. When the DSN received the reflected signal, scientists would take a look at the data and look for small changes in the signal’s frequency caused by the Doppler shift. Measuring the shift in frequency allowed the scientists to determine the rotational speed of Mars.

“What we’re looking for are variations that are just a few tens of centimeters over the course of a Martian year. It takes a very long time and a lot of data to accumulate before we can even see these variations,” said Le Maistre.

Given the extremely small variations in the signal frequencies, Le Maistre et al. had to look at frequency data from InSight’s first 900 Martian days, or sols, to notice the variations. Furthermore, the slight variations in frequency meant that eliminating sources of noise, such as moisture from Earth’s atmosphere and solar wind, from the data would be a challenge.

“It’s a historic experiment. We have spent a lot of time and energy preparing for the experiment and anticipating these discoveries. But despite this, we were still surprised along the way — and it’s not over, since RISE still has a lot to reveal about Mars,” Le Maistre said.

As mentioned, the RISE data used by Le Maistre et al. was so precise that it picked up Mars’ nutation — the planet’s wobbling motion caused by the sloshing of Mars’ molten metal core. The RISE data allowed the team to measure the size of the core, which they found to have a radius of approximately 1,835 kilometers. Additionally, the nutation gave scientists further insight into the shape and characteristics of the core.

But Le Maistre et al. didn’t stop there. They continued with their research of the core and compared their measurement of the core’s size to other measurements derived from data collected by InSight’s seismometer. Specifically, the team looked at whether seismic waves within Mars reflected off of the core or if they traveled through the core uninterrupted, which allowed them to estimate the radius of the core. All three measurements showed that the core was anywhere between 1,790 and 1,850 kilometers.

“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 co-author Attilio Rivoldini of the Royal Observatory of Belgium.

Le Maistre et al.’s results are just the beginning. The team will continue to analyze the RISE data in hopes of learning more about Mars’ rotation, core, and other planetary characteristics. Although InSight’s mission may be over, the incredible data it gathered during its four-year mission will continue to allow scientists to make groundbreaking discoveries in planetary science for the months, years, and decades to come.

Le Maistre et al.’s data and results were published in the journal Nature in June 2023.

(Lead image: InSight takes a selfie on April 24, 2022, the 1,211th sol of the mission. Credit: NASA/JPL-Caltech)

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