The rotation rate of Mars is accelerating, and as a consequence, its days are getting shorter, even if this reduction is only by a fraction of a millisecond per year. There is still uncertainty about the cause of this change in the speed of the red planet, but for the moment, two are considered: the first is related to the nature of the core of this planet –which, contrary to what was believed, is not solid– and the second, with the ice on its surface.
The finding, published in the journal Nature, has been possible thanks to the data obtained by the NASA InSight module, which has managed to break with the biggest problem that scientists faced in the study of the internal structure of the planet: the interior of Mars is not directly accessible. For this reason, until the arrival of this mission "most of the geophysical data provided global information, in which the core, the mantle and the crust could not be separated," the article alleges.
More specifically, it was one of their instruments, a radio transponder and antennae collectively known as RISE (Interior Structure and Rotation Experiment), that allowed the red planet's rate of spin to be tracked. Thus, they identified and quantified its acceleration, which is approximately 4 milliarcseconds per year, which translates into a cut of a fraction of a millisecond for each turn it makes around the sun.
As a whole, the InSight module represents the most precise technology that has been used, to date, for a Martian exploration. So much so, that already during the landing it detected, for the first time, that the core of Mars was moving, which led to the argument that it is liquid (cast iron), and not solid, as was believed. It has also been possible to measure its size, which is estimated to be between 1,790 and 1,850 kilometers in radius.
That it is liquid and not solid, would mean a loss in the internal homogeneity of the planet. For this reason, one of the causes considered in this study is that the acceleration of the rotation rate of Mars "could be the result of a long-term trend in the internal dynamics of the planet."
The other cause that arises is a change in the mass of the planet, caused by an accumulation (extremely slow, according to the assessment of scientists) of ice in the polar caps of Mars. Because although there is no liquid water on this planet, it is possible to find it in the form of ice in these caps or in the form of vapor in its atmosphere.
Although the temperature of Mars is adequate for liquid water to exist (as on Earth), its atmosphere is too thin (about a hundred times less dense than our planet's atmosphere) and consequently, it is not capable of supporting liquid water.
Also in relation to the polar caps, postglacial rebound (through which landmasses rise after being buried by ice) is considered as a possible factor influencing the planet's velocity change. In essence, all of them are related to the distribution of mass, internal or superficial, of Mars. "These changes in the mass of a planet can cause it to speed up a bit like an ice skater spinning with its arms outstretched and then retracting them," explains NASA.
The key is in the "angular momentum" or amount of spin, a concept that is present in all spin movements, from the rotation of the planets to that skater. The angular momentum remains constant and for this, you need to play (in a crude way) with two of its determining factors: the distribution of mass and speed.
Thus, when the skater extends his arms (expands his mass), the speed of the turn decreases, in order to conserve his angular momentum. If the skater retracts them (contracts their mass), the speed will increase for the same reason. And since this law -the conservation of angular momentum- is universal, it also applies to the red planet, even if it is not dressed in sequins or has limbs that need to change position.