The question in the wake of a paper published in June in the journal Nature is: Why is the rotation of Mars speeding up? NASA’s InSight lander fell silent in December after dust accumulated on its solar panels but new data have revealed surprises.
To track the planet’s spin rate, the study’s authors relied on one of InSight’s instruments: a radio transponder and antennas collectively called the Rotation and Interior Structure Experiment, or RISE. The technique for measurement entailed beaming a radio signal to the lander using the Deep Space Network. RISE then reflected the signal back. When scientists received the reflected signal, they then looked for tiny changes (variations) in frequency caused by the Doppler shift (the same effect that causes an ambulance siren to change pitch as it gets closer and farther away). Measuring the shift enabled researchers to determine how fast the planet rotates.
The researchers found the planet’s rotation is accelerating by about 4 milliarcseconds per year² – corresponding to a shortening of the length of the Martian day by a fraction of a millisecond per year. I.e. a more rapid rotation rate. This is a subtle acceleration, and planetary scientists aren’t entirely sure of the cause. But they have a few ideas, including ice accumulating on the polar caps or post-glacial rebound, where landmasses rise after being buried by ice. The shift in a planet’s mass can cause it to accelerate a bit like an ice skater spinning with their arms stretched out, then pulling their arms in.
The paper examined data from InSight’s first 900 Martian days – enough time to look for such variations. Scientists had their work cut out for them to eliminate sources of noise that could produce similar variations: Water slows radio signals, so moisture in the Earth’s atmosphere can distort the signal coming back from Mars. So can the solar wind, the electrons and protons flung into deep space from the Sun. So can the relative positions of Earth and Mars in their respective orbits.
Once planetary scientists accounted for these ancillary factors there remained leftover frequency variations, suggesting an enhanced rotation. In particular, the additional findings gleaned from tiny shifts in the frequencies of radio transmissions between Earth and InSight on Mars.
RISE is part of a long tradition of Mars landers using radio waves for science, including the twin Viking landers in the 1970s and the Pathfinder lander in the late ’90s. But none of those missions had the advantage of InSight’s advanced radio technology and upgrades to the antennas within NASA’s Deep Space Network on Earth. Together, these enhancements provided data about five times more accurate than what was available for the Viking landers.
According to the paper’s lead author and RISE’s principal investigator, Sebastien Le Maistre at the Royal Observatory of Belgium:
"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.”
Adding:
"Mars, because it is not a perfectly round sphere, wobbles like a top. but the primary goal was to measure the rotation,”
An additional related bonus is that precise measurements of the rotation (with all of the wobbles) places constraints on the structure and composition of the very deep parts of the planet, according to Dr. Le Maistre.
Similar measurements had been attempted during NASA’s Viking missions in the 1970s and also during later missions like Pathfinder in 1997, but those were not precise enough or long enough. To show how unique, Le Maistre emphasized:
“This was never done for any planet other than the Earth before,”
From the magnitude of the wobbles, the Le Maistre's team has calculated that the molten core of Mars is about 2,280 miles wide. (Mars as a whole is about 4,200 miles in diameter.)
See Also:
Mars is
spinning faster, here's what it could mean | This World - YouTube
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