For the first time, researchers have been able to measure how much a planet that is not Earth wobbles. Scientists can now tell that Mars's axis of rotation sways slightly over time, with variations of about ten 10 centimeters (3.94 inches) every 207 days. Comparatively, Earth’s axis rocks up to 6 meters (19.7 feet) within a period of 433 days.
“[The Chandler wobble] is a very small signal, typically,” lead author Dr Alex Konopliv, an aerospace engineer at NASA’s Jet Propulsion Laboratory in Pasadena, told Eos. “You need many years and high-quality data to detect it,”
The fact that we can measure such a tiny wobble on another planet is truly incredible. As reported in Geophysical Research Letters, researchers used radio wave tracking from Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Global Surveyor to properly estimate this value. Radio-tracking suggested that the spacecraft were experiencing some gravitational effect due to this swaying.
The team was also able to establish that the source of the wobble is intrinsic to the planet, and it is not due to surface effects such as the seasonal melting of the Martian polar ice caps. This fact is of crucial importance to our understanding of the Red Planet.
“For the first time for any solar system body other than the Earth, one component of the motion of the Mars spin axis on the surface of Mars, called the Chandler wobble, has been detected.” The authors wrote in the paper abstract. “The detection of the Chandler wobble improves our understanding of energy dissipation in the mantle for time intervals near the wobble period.”
The Chandler wobble should naturally die down on a planet. It is estimated that for our own planet, it would die out in just 68 years. So to keep going, it is necessary that the wobble is constantly re-excited. On Earth, two-thirds of this excitement comes from fluctuating pressure on the seabed due to oceanic currents. The remaining third is from atmospheric fluctuation.
But what about Mars? The planet is dry and with a thin atmosphere, so surface phenomena have been dismissed. The team believes that the cause of the Martian wobble has to be found in the mantle, the layer of the planet below the crust, and the time it takes for the full wobble to take place provides new details on how much the mantle can deform.
Together with the measurements from NASA’s InSight mission, we are learning a lot about what the inside of Mars is like.