For unknown reasons, the rotation of Mars is accelerating.
NASA’s InSight lander fell silent in December after dust accumulated on its solar panels. But scientists continue to sift through the four years of data collected, learning more about what lies deep within the Red Planet.
Most of what they’ve learned about Mars comes from InSight’s sensitive seismometer, placed on the surface to measure tiny vibrations from small Mars tremors and meteor impacts that reverberate around and through the planet.
In a paper published in June in the journal Nature, scientists working on the mission reported additional findings drawn from tiny changes in the frequencies of radio transmissions between Earth and InSight on Mars.
As InSight’s Mars location moved toward Earth, a phenomenon known as the Doppler effect compressed radio waves slightly, shortening the wavelength and increasing the frequency. As InSight moved away from the receiving antenna on Earth, the wavelength lengthened and the frequency decreased. This is the same phenomenon that causes the siren of an approaching ambulance to sound higher as it approaches and lower as it moves away.
Most of the Doppler effect comes from the relative motions of Mars and Earth as they move around the sun. Interplanetary radio transmissions are also distorted by the humidity of the Earth’s atmosphere and the solar wind of charged particles from the sun.
Once scientists took all of this into account, there were still variations in frequency. Mars, because it is not a perfectly round sphere, wobbles like a top. “The main goal is to measure rotation,” said Sébastien Le Maistre of the Royal Observatory of Belgium, who led the radio science experiment, known as the Rotation and Interior Structure Experiment, or RISE.
If accurate enough, measurements of the rotation with all the oscillations impose constraints on the structure and composition of very deep parts of the planet, Dr Le Maistre said.
Similar measurements had been attempted on NASA’s Viking missions in the 1970s and also on later missions like Pathfinder in 1997, but these were neither precise enough nor long enough. “This has never been done for a planet other than Earth before,” said Dr Le Maistre.
Based on the magnitude of the oscillations, scientists have calculated that Mars’ molten core is about 2,280 miles wide. (Mars as a whole is about 4,200 miles in diameter.)
The size of the core derived from the radio oscillations roughly matched what had been observed from the seismic waves that passed through the planet. Scientists also saw no signs of a smaller, more solid inner core like the one inside Earth. Similar to seismic data, the oscillations suggest that Mars’ core is made mostly of molten iron, but also contains significant amounts of lighter elements like sulfur, oxygen and carbon.
The higher concentration of light elements in the nucleus” is very difficult to reconcile with the cosmic chemical models we have of the formation of Mars,” said Bruce Banerdt, the InSight mission’s principal investigator.
The measurements also showed the ripples expected from the planet’s rotation, fastest in winter and summer and slowest in spring and fall, as the distribution of frozen carbon dioxide in the ice sheets changed with the seasons.
Including Doppler measurements from the Viking mission 47 years ago, scientists found a slight acceleration in the rotation of Mars, which led to a shortening of a Martian day by about 1.5 millionths of second per Martian year. (A Martian year is almost twice as long as a year on Earth.)
“It was a surprise,” Dr. Banerdt said. “We didn’t expect to see this at all.”
This is the opposite of Earth, where the moon slowly recedes and the length of the day lengthens, the main reason why leap seconds are sometimes added to timing (although this will change in 2035). The melting of glaciers and the displacement of the atmosphere can also modify the speed of rotation.
But on Mars, the cause of the acceleration is a mystery. One of the Martian moons, Phobos, is known to gradually fall, and indeed to speed up Mars’ rotation – like a figure skater pulling on his arms to spin faster – but that’s not big enough to explain. what was observed.
Dr Banerdt said the most likely cause was a long-term buildup of ice at the poles. Dr Le Maistre said another possibility was the melting of glaciers in the polar regions. The planetary crust, no longer pressed by the weight of the glaciers, would then slowly rebound, and the shape of Mars would stretch vertically, accelerating the rotation.
Asteroid impacts are less likely to hit the planet in one direction to spin faster.
“It’s really hard to couple a planet and speed it up with a torque,” Dr. Banerdt said, “but relatively simple to change the moment of inertia and speed it up a bit.”
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