[Reading Science] Mars Rotates Like Kim Yuna

Earth's Rotation Speed Increasing Annually
Polar Axis Weight Growing
NASA: "Like a Figure Skater Spinning"

It has been confirmed that the rotation speed of Mars, the planet closest to Earth in the solar system, is gradually increasing every year. Scientists estimate that this is because ice has accumulated at Mars' polar regions, its central axis, increasing its weight. In other words, a phenomenon similar to figure skaters like Kim Yuna pulling in their arms to spin faster is occurring.

Hwaseong. Reference image.

NASA announced on the 7th (local time) that a research team from the Royal Observatory of Belgium recently published a paper containing these research results in the international journal Nature. The team reached this conclusion by using the last observational data transmitted by NASA's 'Mars Geologist' InSight, which ended its mission last December after four years on Mars due to dust accumulation on its solar panels.

The research team used one of the instruments installed on InSight, an antenna and radio transponder called the Rotation and Interior Structure Experiment (RISE), to measure Mars' rotation speed. Through this device, they confirmed that Mars' rotation speed is increasing by 4 milliarcseconds annually. In other words, the length of a day on Mars is shortening by one-thousandth of a second each year. Although the change is slight, the team has not yet identified a definitive cause. They speculate several reasons, such as the accumulation of ice at Mars' poles or the uplifting of ice-covered ground. Regarding this, NASA explained, "The shift in Mars' center of mass can cause an acceleration effect similar to a skater extending their arms while spinning and then pulling them inward to spin faster."

NASA has equipped all Mars landers, including the Viking landers sent in the 1970s and the Pathfinder launched in the 1990s, with RISE antennas to conduct scientific research through radio transmission and reception. In particular, InSight, which landed on Mars in 2018, enabled precise measurements thanks to advancements in radio technology and upgrades to Earth's antennas, such as those in NASA's Deep Space Network. The data obtained was five times more accurate than the radio measurement devices used during the Viking lander missions.

Mars geological exploration rover InSight. Photo by NASA website

Based on the data left by InSight, the research team also determined the exact size of Mars and the size of its internal core. First, using radio measurement results, they measured the magnitude of planetary vibrations caused by the shaking of the liquid core inside Mars as it rotates. Based on this, the calculated radius of Mars' core was approximately 1,835 km. The team then compared this figure with previous measurements taken twice using seismometers. They specifically examined whether seismic waves passing through Mars were reflected by the core or simply passed through it. As a result, the team measured Mars' core radius to be about 1,790 to 1,850 km, and the entire radius of Mars to be approximately 3,390 km, roughly half the size of Earth.

Attilio Rivoldini, a researcher at the Royal Observatory of Belgium, said, "This data suggests that the shape of Mars' core cannot be explained solely by rotation," adding, "Other regions with higher or lower density buried deep within Mars' crust also have an impact."