NASA’s InSight Lander Finds Evidence of Increased Rotation Speed on Mars
Scientists have made the most precise measurements to date of Mars’ rotation, detecting for the first time how the planet wobbles due to the “sloshing” of its molten metal core. Data from NASA’s InSight Mars lander, which operated for four years until its extended mission ended in December 2022 and was documented in a recent paper published in Nature.
To monitor the planet’s rotation rate, the study’s authors utilized one of InSight’s instruments: the Rotation and Interior Structure Experiment, or RISE, a radio transponder and antennas. They discovered that the planet’s rotation is accelerating by approximately 4 milliarcseconds per year, which corresponds to a fraction of a millisecond per year reduction in the length of the Martian day.
It is a subtle acceleration, and scientists are uncertain as to its cause. However, they have a few theories, such as ice accumulation on the polar caps and post-glacial rebound, in which land masses rise after being submerged by ice. The change in a planet’s mass can cause it to accelerate in a manner similar to an ice skater twirling with their arms extended and then drawing them in.
InSight’s principal investigator, Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Southern California, remarked, “It’s really cool to be able to get this latest measurement—and so precisely.” “I’ve been involved in efforts to get a geophysical station like InSight onto Mars for a long time, and results like this make all those decades of work worth it.”
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How RISE Works?
RISE is part of a long line of Mars landers that have utilized radio waves for scientific purposes, including the twin Viking landers from the 1970s and the Pathfinder lander from the late 1990s. None of these missions, however, had the benefit of InSight’s sophisticated radio technology and enhancements to NASA’s Deep Space Network antennas on Earth. These enhancements provided data that was approximately five times more precise than what the Viking settlers had access to.
In the case of InSight, scientists would use the Deep Space Network to transmit a radio signal to the spacecraft. The signal would then be reflected back by RISE. When scientists received the reflected signal, they would search for minute frequency changes caused by Doppler shift (the same effect that causes an ambulance siren’s pitch to change as it approaches or recedes). By measuring the displacement, scientists were able to ascertain the planet’s rotational speed.
The paper analyzed data from InSight’s first 900 Martian days, a sufficient amount of time to detect such variations. Scientists had their work set out for them in order to eliminate pollution sources: Radio signals are slowed by water, so precipitation in the Earth’s atmosphere can distort the signal returning from Mars. Also, the solar wind, which consists of electrons and protons ejected from the sun into interstellar space, can be a source of radiation.
“It’s a historic experiment,” Le Maistre explained. “We have spent a lot of time and energy preparing for the experiment and anticipating these discoveries. But despite this, we were still surprised along the way—and it’s not over, since RISE still has a lot to reveal about Mars.”
Martian Core Measurements
The authors of the study also used RISE data to measure Mars’ oscillation, known as its nutation, caused by the movement of its liquid core. The measurement enables scientists to ascertain the core’s size: According to RISE data, the radius of the core is approximately 1,140 miles (1,830 kilometers).
The authors then compared this value to two previous measurements of the core derived from the seismometer aboard the spacecraft. Specifically, they examined whether seismic waves reflected off the planet’s core or passed through it unimpeded as they traveled through its interior.
Using all three measurements, the radius of the core is estimated to be between 1,112 and 1,150 kilometers (1,780 and 1,850 miles). Mars’ total radius is approximately half that of Earth, at 2,106 miles (3,390 kilometers).
Measuring Mars’ oscillation revealed information about the core’s shape.
Although scientists will continue to analyze InSight data for years to come, this study signals the end of Banerdt’s tenure as the mission’s principal investigator. He retired on August 1 after 46 years with JPL.
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Source: Phys.org
