For reasons unknown, the spin of Mars is speeding up.
NASA’s InSight lander fell silent in December after dust accumulated on its solar panels. But scientists continue to sift through the four years’ worth of data it gathered, learning more about what is deep inside the red planet.
Most of what they have learned about Mars came from InSight’s sensitive seismometer, placed on the surface to measure the minuscule vibrations from small marsquakes and meteor impacts reverberating around and through the planet.
In a paper published in June in the journal Nature, scientists working on the mission reported additional findings gleaned from tiny shifts in the frequencies of radio transmissions between Earth and InSight on Mars.
When InSight’s location on Mars was moving toward Earth, a phenomenon known as the Doppler effect compressed the radio waves slightly, shortening the wavelength and increasing the frequency. When InSight was in motion away from the receiving antenna on Earth, the wavelength lengthened and the frequency decreased. That is the same phenomenon that makes a siren of an approaching ambulance sound higher in pitch when it is approaching and lower as it moves away.
Most of the Doppler effect arises from the relative motions of Mars and Earth as they move around the sun. Interplanetary radio transmissions are also distorted by moisture in Earth’s atmosphere and the solar wind of charged particles streaming from the sun.
Once the scientists accounted for all of that, there were leftover frequency variations. Mars, because it is not a perfectly round sphere, wobbles like a top. “The primary goal is to measure the 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 precise enough, the measurements of the rotation with all of the wobbles place constraints on the structure and composition of the very deep parts of the planet, Dr. Le Maistre said.
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. “This was never done for any planet other than the Earth before,” Dr. Le Maistre said.
From the magnitude of the wobbles, the scientists calculated that the molten core of Mars is about 2,280 miles wide. (Mars as a whole is about 4,200 miles in diameter.)
The size of the core derived from radio wobbles roughly matched what had been observed from seismic waves that passed through the planet. The scientists also saw no sign of a smaller, solid inner core like the one inside Earth. As with the seismic data, the wobbles suggested that the core of Mars is made mostly of molten iron but also has sizable amounts of lighter elements like sulfur, oxygen and carbon.
The higher concentration of light elements in the core “is very difficult to reconcile with the cosmic chemical models that we have of Mars’s formation,” said Bruce Banerdt, the principal investigator for the InSight mission.
The measurements also showed the expected undulations of the planet’s rotation, quickest during winter and summer and slowest during spring and fall, as the distribution of frozen carbon dioxide in the ice caps shifted with the seasons.
When including the Viking mission’s Doppler measurements from 47 years ago, the scientists found a slight speeding up in the spin of Mars, which has led to a shortening of a Martian day by about 1.5 millionths of a 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 did not expect to see that at all.”
That is the opposite of Earth, where the moon is slowly moving farther away and the length of the day is getting longer, the main reason that leap seconds are occasionally added to timekeeping (although that will change in 2035). The melting of glaciers and shifting atmosphere can also change the spin rate.
But on Mars, the cause of the speedup is a mystery. It is known that one of the Martian moons, Phobos, is gradually falling, and that indeed is speeding up the rotation of Mars — like a figure skater pulling in the arms to spin faster — but that is not big enough to explain what was observed.
Dr. Banerdt said the most likely cause was a long-term accumulation 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 down by the weight of glaciers, would then slowly rebound, and the shape of Mars would stretch vertically, speeding the rotation.
Less likely would be asteroid impacts knocking the planet in a direction to spin faster.
“It’s really difficult to torque up a planet and speed it up with torque,” Dr. Banerdt said, “but relatively simple to change the moment of inertia and get it to speed up a little.”