A new study shows Mars' speed is different than we thought

Credit image: NASA/JPL-Caltech  NASA’s InSight lander captured this selfie on April 24, 2022, the 1,211th Martian day, or sol, of the mission
Credit image: NASA/JPL-Caltech NASA’s InSight lander captured this selfie on April 24, 2022, the 1,211th Martian day, or sol, of the mission

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Article by: Andacs Robert Eugen, on 07 August 2023, at 09:31 am PDT

According to a recent paper published in the journal Nature, scientists have made a significant breakthrough in measuring the rotation of Mars with high precision. This research has uncovered that the planet has a slight wobbling caused by its molten metal core movement. The data used in this study was collected by NASA's InSight Mars lander, which operated for four years before concluding its extended mission in December 2022.

The study relied on one of InSight's instruments, the Rotation and Interior Structure Experiment (RISE). This instrument consists of a radio transponder and antennas, which revealed that Mars' rotation is experiencing a subtle acceleration of around four milliarcseconds annually. This acceleration has led to a minor reduction in the length of a Martian day by a fraction of a millisecond each year.

Although scientists have postulated several theories, the cause of this delicate acceleration remains a mystery. Some of the suggested ideas include the accumulation of ice on the polar caps and post-glacial rebound, a phenomenon where landmasses rise after being released from the weight of ice.

Analogously, a planet's change in mass can result in acceleration, comparable to an ice skater spinning faster when pulling in their arms.

Bruce Banerdt, the principal investigator of InSight at NASA's Jet Propulsion Laboratory, expressed his excitement, stating, "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."

The RISE project is following in the footsteps of previous Mars landers, such as the Viking and Pathfinder, by using radio waves for scientific research. However, the technology used in InSight is more advanced, with improved antennas and cutting-edge radio technology that is five times more precise than that used by the Viking landers.

Scientists aimed a radio signal from Earth's Deep Space Network at the InSight lander to collect data. RISE reflected the signal, and researchers analyzed the slight changes in frequency caused by the Doppler effect. This effect is similar to the sound of an ambulance siren changing as it approaches and then moving away, allowing scientists to measure Mars' rotational speed accurately.

The lead author of the paper and RISE's principal investigator, Sebastien Le Maistre from the Royal Observatory of Belgium, elaborated, "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."

The research covered data from InSight's first 900 days on Mars, enough time for scientists to identify changes. The scientists faced challenges due to noise sources, such as interference caused by water in Earth's atmosphere and the solar wind-charged particles ejected by the Sun into space.

The RISE data also allowed measurement of Mars' nutation, which is the wobbling attributed to the movement of its liquid core. With this measurement, researchers could estimate the core's radius to be around 1,140 miles (1,835 kilometers).

By comparing this measurement with two previous estimates obtained from the spacecraft's seismometer, the researchers determined that the core's radius was between 1,112 and 1,150 miles (1,790 and 1,850 kilometers). Notably, Mars has an overall radius of 2,106 miles (3,390 kilometers), roughly half Earth's size.

Additionally, insights into Mars' wobbling shed light on the shape of its core. Attilio Rivoldini of the Royal Observatory of Belgium, the paper's second author, explained, "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."

As researchers continue to delve into InSight's data, this study marks the culmination of Bruce Banerdt's role as the mission's principal investigator. After an impressive 46-year tenure at JPL, he retired on August 1, leaving a lasting legacy of exploration and discovery.

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