NASA finds Mars days shrinking, cites increase in spin speed

Measuring Mars' rotation using RISE

The latest results were derived from the radio science instrument called the Rotation and Interior Structure Experiment (RISE), comprising a radio transponder and antennas. 

The RISE transponder possesses the capability to initiate communication with the extensive antennas of NASA's Deep Space Network (DSN) located in proximity to Canberra, Australia.

Scientists would send a radio signal to the lander through DSN, and RISE would then relay radio signals back to Earth.

So, how do scientists determine the rotating speed of the planet? 

They precisely observe the slightest frequency variations resulting from the Doppler shift in the reflected radio signal, which is the same occurrence responsible for the alteration in pitch of an ambulance siren as it moves nearer or farther away.

By detecting this tiny shift, scientists could ascertain the changes in the planet’s rotational speed. 

“What we’re looking for are variations that are just a few tens of centimeters over the course of a Martian year. It takes a very long time and a lot of data to accumulate before we can even see these variations,” said Sebastien Le Maistre at the Royal Observatory of Belgium, and the paper’s lead author and RISE’s principal investigator, 

The research analyzed data spanning the initial 900 sols (Martian days) of the InSight mission, providing a significant amount of information to assess these fluctuations.

The radius of Mars' core  

RISE was also specially designed to monitor Mars' wobble (nutation) as it spins on its axis. Simply put, the wobble causes the planet to sway slightly on its axis. 

Scientists track this wobble in order to identify the size and composition of the planet's core.

Furthermore, the study authors used RISE data to calculate Mars' wobble caused by sloshing in its liquid core. According to the data, the core has a radius of approximately 1,140 miles (1,835 kilometers).

While Mars has a total radius of 2,106 miles (3,390 kilometers), which is roughly half the size of Earth.

However, the underlying cause of this slight increase in the planet's rotation remains uncertain, leaving scientists unsure of the precise explanation.

However, they have put forth a plausible explanation, indicating that the accumulation of ice on the polar caps of Mars, or the phenomenon of post-glacial rebound where landmasses elevate after being covered by ice for an extended period, could potentially lead to alterations in the planet's mass. Consequently, this might contribute to a slight acceleration in the planet's rotation.

The research was recently published in the journal Nature.

Study Abstract:

Knowledge of the interior structure and atmosphere of Mars is essential to understanding how the planet has formed and evolved. A major obstacle to investigations of planetary interiors, however, is that they are not directly accessible. Most of the geophysical data provide global information that cannot be separated into contributions from the core, the mantle and the crust. The NASA InSight mission changed this situation by providing high-quality seismic and lander radio science data. Here we use the InSight’s radio science data to determine fundamental properties of the core, mantle and atmosphere of Mars. By precisely measuring the rotation of the planet, we detected a resonance with a normal mode that allowed us to characterize the core and mantle separately. For an entirely solid mantle, we found that the liquid core has a radius of 1,835 ± 55 km and a mean density of 5,955–6,290 kg m−3, and that the increase in density at the core–mantle boundary is 1,690–2,110 kg m−3. Our analysis of InSight’s radio tracking data argues against the existence of a solid inner core and reveals the shape of the core, indicating that there are internal mass anomalies deep within the mantle. We also find evidence of a slow acceleration in the Martian rotation rate, which could be the result of a long-term trend either in the internal dynamics of Mars or in its atmosphere and ice caps.