How NASA uses gravity and radio waves to study planets and moons
Mars’ rotation is slowly but inexorably increasing — shortening the length of the planet’s day by a fraction of a millisecond each year.
This is the conclusion of researchers who studied data collected by NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander, which spent four years studying the Red Planet before running out of power last December.
By sending a radio signal from Earth to InSight’s radio apparatus and back, the team were able to determine exactly how fast the planet is rotating and how this changes with time.
Alongside revealing Mars’ rotation to be accelerating by four milliarcseconds per year, the data also revealed how the planet is wobbling thanks to “sloshing” molten metal in its core.
The cause of the acceleration is unclear — although the researchers think it may be related to ice accumulation at the poles, or landmasses rising slowly after being buried by ice.
These shifting mass distributions could cause the planet’s rotation to speed up much like how ice skaters can spin faster by drawing their arms inwards.
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The study was undertaken by planetary geophysicist Dr Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Southern California and his colleagues.
Dr Banerdt said: “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.”
He added: “Results like this make all those decades of work well worth it.”
The study relied on radio data from one of InSight’s instruments known as the “Rotation and Interior Structure Experiment”, or “RISE” for short.
RISE — which incorporates both radio antennas and a transponder — is part of a long tradition of NASA’s Mars landers using radio waves for science, including both the twin Viking landers of the 1970s and Pathfinder in the 1990s.
InSight has advantages over its predecessors, however, in the form of more advanced radio technology and upgrades to the antennas that make up NASA’s Deep Space Network here on Earth — giving its data five times the accuracy of those from the Viking Landers.
The researchers explained that they are able to calculate Mars’ rotation from the signal bounced back from InSight thanks to the Doppler effect.
Likely familiar from high school physics, this is the same phenomenon that causes the sirens of passing ambulances to change pitch as they grow closer and then pull further away.
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Planetary astronomer Dr Sébastien Le Maistre, also of the Jet Propulsion Laboratory, is RISE’s principal investigator.
He said: “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.”
In fact, the team explained, they examined 900 days worth of data collected by InSight.
Part of the time-consuming analytic process involved eliminating sources of noise that would otherwise distort off the data — such as variations in the moisture in Earth’s atmosphere (through which the radio signals must pass) and similar fluxes in the solar wind.
Dr Le Maistre concluded: “It’s a historic experiment. We have spend a lot of time and energy preparing 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.”
The full findings of the study were published in the journal Nature.
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