The red planet, along with Earth, are the only two worlds in which this strange movement has been detected, whose origin is unknown.
Like a spinning top, Mars wobbles as it spins, so that its poles are constantly moving closer to and away from its axis of rotation. This has been confirmed by a study just published in Geophysical Research Letters in which, however, the researchers admit they have no idea why.
This is the first time that, apart from on Earth, this curious swaying has been detected in a body in the Solar System. Known as “Chandler wobble” (in honor of the astronomer Carlo Chandler, who discovered the phenomenon more than a century ago) it is an effect that arises when a rotating body is not a perfect sphere. The result is a “wiggle” similar to that of a spinning top that sways as it slows down. Nothing to do, therefore, with the smooth turn of a perfectly balanced balloon.
The data obtained over almost two decades by the numerous probes that have visited Mars reveal, in effect, that the planet’s poles deviate up to ten centimeters from its axis of rotation, in a cycle that repeats approximately every 207 days.
Here on Earth, the other planet in our system where the wobble has been observed, the wobble is much more pronounced. The poles of our world, in effect, deviate approximately 9 meters from the Earth’s axis of rotation, in a circular pattern that repeats every 433 days.
Led by Alex Kanopliv, an aerospace engineer at NASA’s Jet Propulsion Lab, the team of researchers contributes with this work new knowledge about the interior of the red planet. In fact, the amount of time it takes for a pole to complete an oscillation cycle is a true reflection of how much the mantle of Mars can deform, which gives new clues about its properties and its thermal state.
“In general,” Konopliv explains, “the Chandler wobble signal is very small. It takes many years and high-quality data to detect it.” Previous studies on the same topic, in effect, had failed to reach any conclusion.
In their study, however, Konopliv and his colleagues were able to confirm that strange motion of Mars by calculating the gravitational effects the wobble had on the orbits of two NASA spacecraft found there: the Mars Odyssey and the Mars Reconnaissance Orbiter. The huge amount of data collected over 18 years, and not available in previous studies, made it clear that the reason for the oscillation was in the interior of the planet, and not in external factors such as the seasonal melting of the polar ice caps.
It is believed that both the wobble of Mars and the Earth should disappear with time. Some scientists think that, in the case of our planet, this oscillation should not last more than a hundred years, counted from the moment of its origin. However, the Earth’s wobble has lasted much longer, and its intensity shows no sign of abating. Something, it was said in a 2001 study, seems to be continually rekindling the wobble, although the exact mechanism is unknown.
The truth is that, both on Earth and on Mars, scientists do not know what may be causing this mysterious oscillation. It has been suggested that, at least in the terrestrial case, it could be a combination of pressure changes in the atmosphere and in the oceans. But Mars has no oceans, so the cause must, by force, lie elsewhere. But where? New research may, at some point, shed light on this true planetary enigma.