For unknown reasons, Mars’ rotation is accelerating.
NASA’s lander InSight went silent in December after dust accumulated on its solar panels. But scientists continue to scrutinize the four years of collected data and learn more about what lies deep within the red planet.
Much of what they learned about Mars came from InSight’s highly sensitive seismometers, which are placed on the surface of the planet and act as small firequakes and meteor strikes that reverberate around Mars and throughout the planet. We measured minute vibrations caused by
In a paper published in June Published in Nature magazinescientists working on the mission reported additional findings gleaned from subtle changes in radio transmission frequencies between InSight on Earth and Mars.
As InSight’s position 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. As InSight moved away from its receiving antenna on Earth, the wavelength lengthened and the frequency decreased. This is similar to the siren of an approaching ambulance, which sounds higher as it approaches and lower as it recedes.
Most of the Doppler effect comes from the relative motion of Mars and Earth as they move around the Sun. Interplanetary radio transmissions are also distorted by moisture in the Earth’s atmosphere and the solar wind of charged particles flowing from the Sun.
When scientists explained all of this, we were left with frequency fluctuations. Mars is not a perfect sphere, so it wobbles like a top. “The main purpose is to measure rotation,” said Sébastien Le Mestre of the Royal Observatory of Belgium, who led the radio science experiment known as the Rotation and Internal Structures Experiment (RISE).
If accurate enough, any wobbled rotation measurement would impose constraints on the structure and composition of the planet’s very deep parts, Dr. Le Mestre said.
Similar measurements were attempted on NASA’s Viking missions in the 1970s and later missions such as Pathfinder in 1997, but they were not sufficiently accurate or long enough. “This has never been done before on any planet other than Earth,” said Dr. Le Mestre.
Based on the size of the wobble, scientists calculated the width of Mars’ molten core to be about 3,280 miles. (The entire diameter of Mars is about 4,200 miles.)
The core sizes obtained from the radio wobbles closely match those observed from seismic waves that have passed through the planet. Scientists also saw no signs of a smaller, solid inner core, such as the core inside the Earth. Like the seismic data, the wobble suggested that the core of Mars is mostly made of molten iron, but also contains large amounts of lighter elements such as sulfur, oxygen and carbon.
The higher the concentration of light elements in the core, the Matching our cosmic chemical model of Mars formation is very difficult,” said Bruce Banart, principal investigator of the InSight mission.
The measurements also show that the expected rotational undulations of the planet are fastest in winter and summer and slowest in spring and autumn because the distribution of frozen carbon dioxide within the ice sheet changes with season.
Including Doppler measurements from the 47-year-old Viking program, scientists found that Mars is rotating slightly faster, shortening a Martian day by about 1.5 millionths of a second per Martian year. (A year on Mars is almost twice as long as a year on Earth.)
“It was a surprise,” Dr. Banert said. “We never expected that to happen.”
This is the opposite of Earth, where the Moon is slowly moving away and the days are getting longer. This is the main reason why leap seconds are occasionally added to timekeeping (although that will change in 2035). Melting glaciers and atmospheric changes can also change the rotation rate.
But on Mars, the cause of the speed increase is a mystery. Phobos, one of Mars’ moons, is slowly falling and is known to actually accelerate the rotation of Mars – like a figure skater pulling on his arm to spin faster – but , which is not large enough to explain what was observed.
Dr. Banert said the most likely cause was the long-term accumulation of ice at the poles. Dr. Le Mestre said another possibility is the melting of polar glaciers. The planet’s crust, no longer compressed by the weight of the glaciers, slowly recoils, stretching the shape of Mars vertically and speeding up its rotation.
An asteroid impact would be less likely to cause a planet to collide in a direction that spins faster.
“It’s very difficult to torque a planet and get it to speed up with the torque,” Dr. Banert said. “But it’s relatively easy to change the moment of inertia to speed it up a bit.