NASA InSight Study Finds Mars Is Spinning Faster

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Aug. 7, 2023


NASA’s InSight lander captured this selfie on April 24, 2022, the 1,211th Martian day, or sol, of the mission. Dust on its solar panels caused the lander to lose power in December of that year, but data recorded by InSight’s instruments is still leading to new science. Full image details
Credit: NASA/JPL-Caltech

Data sent by the spacecraft before it retired last December has provided new details about how fast the planet rotates and how much it wobbles.

Editor’s note: This story was updated on Aug. 7, 2023, to reflect that Mars’ rotation rate is accelerating by about 4 milliarcseconds per year².

Scientists have made the most precise measurements ever of Mars’ rotation, for the first time detecting how the planet wobbles due to the “sloshing” of its molten metal core. The findings, detailed in a recent Nature paper, rely on data from NASA’s InSight Mars lander, which operated for four years before running out of power during its extended mission in December 2022.

To track the planet’s spin rate, the study’s authors relied on one of InSight’s instruments: a radio transponder and antennas collectively called the Rotation and Interior Structure Experiment, or RISE. They found the planet’s rotation is accelerating by about 4 milliarcseconds per year² – corresponding to a shortening of the length of the Martian day by a fraction of a millisecond per year.

It’s a subtle acceleration, and scientists aren’t entirely sure of the cause. But they have a few ideas, including ice accumulating on the polar caps or post-glacial rebound, where landmasses rise after being buried by ice. The shift in a planet’s mass can cause it to accelerate a bit like an ice skater spinning with their arms stretched out, then pulling their arms in.

“It’s really cool to be able to get this latest measurement – and so precisely,” said InSight’s principal investigator, Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Southern California. “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.”

How RISE Works​

RISE is part of a long tradition of Mars landers using radio waves for science, including the twin Viking landers in the 1970s and the Pathfinder lander in the late ’90s. But none of those missions had the advantage of InSight’s advanced radio technology and upgrades to the antennas within NASA’s Deep Space Network on Earth. Together, these enhancements provided data about five times more accurate than what was available for the Viking landers.

See video URL:

View: https://youtu.be/NTo5-YsjWlI


This video explains how NASA’s Deep Space Network – made up of multiple giant antenna dishes arrayed at three roughly equidistant ground stations on Earth – helps conduct radio science around planets, moons, and other planetary bodies. Credit: NASA/JPL-Caltech

In the case of InSight, scientists would beam a radio signal to the lander using the Deep Space Network. RISE would then reflect the signal back. When scientists received the reflected signal, they would look for tiny changes in frequency caused by the Doppler shift (the same effect that causes an ambulance siren to change pitch as it gets closer and farther away). Measuring the shift enabled researchers to determine how fast the planet rotates.

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

The paper examined data from InSight’s first 900 Martian days – enough time to look for such variations. Scientists had their work cut out for them to eliminate sources of noise: Water slows radio signals, so moisture in the Earth’s atmosphere can distort the signal coming back from Mars. So can the solar wind, the electrons and protons flung into deep space from the Sun.
“It’s a historic experiment,” said Le Maistre. “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​

RISE data was also used by the study authors to measure Mars’ wobble – called its nutation – due to sloshing in its liquid core. The measurement allows scientists to determine the size of the core: Based on RISE data, the core has a radius of roughly 1,140 miles (1,835 kilometers).
The authors then compared that figure with two previous measurements of the core derived the from spacecraft’s seismometer. Specifically, they looked at how seismic waves traveled through the planet’s interior – whether they reflected off the core or passed through it unimpeded.
Taking all three measurements into account, they estimate the core’s radius to be between 1,112 and 1,150 miles (1,790 and 1,850 kilometers). Mars as a whole has a radius of 2,106 miles (3,390 kilometers) – about half the size of Earth’s.
Measuring Mars wobble also provided details about the shape of the core.
“RISE’s data indicate the core’s shape cannot be explained by its rotation alone,” said the paper’s second author, Attilio Rivoldini of the Royal Observatory of Belgium. “That shape requires regions of slightly higher or lower density buried deep within the mantle.”
While scientists will be mining InSight data for years to come, this study marks the final chapter for Banerdt’s role as the mission’s principal investigator. After 46 years with JPL, he retired on Aug. 1.

More About the Mission​

JPL managed InSight for NASA’s Science Mission Directorate. InSight was part of NASA’s Discovery Program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight spacecraft, including its cruise stage and lander, and supports spacecraft operations for the mission.

A number of European partners, including France’s Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR), are supporting the InSight mission. CNES provided the Seismic Experiment for Interior Structure (SEIS) instrument to NASA, with the principal investigator at IPGP (Institut de Physique du Globe de Paris). Significant contributions for SEIS came from IPGP; the Max Planck Institute for Solar System Research (MPS) in Germany; the Swiss Federal Institute of Technology (ETH Zurich) in Switzerland; Imperial College London and Oxford University in the United Kingdom; and JPL. DLR provided the Heat Flow and Physical Properties Package (HP3) instrument, with significant contributions from the Space Research Center (CBK) of the Polish Academy of Sciences and Astronika in Poland. Spain’s Centro de Astrobiología (CAB) supplied the temperature and wind sensors.

News Media Contact​

Andrew Good
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-2433
andrew.c.good@jpl.nasa.gov

Karen Fox / Alana Johnson
NASA Headquarters, Washington
301-286-6284 / 202-358-1501
karen.c.fox@nasa.gov / alana.r.johnson@nasa.gov

See: https://www.jpl.nasa.gov/news/nasa-..._campaign=nasajpl&utm_content=daily20230807-3

Mars InSight Mission

InSight’s Rotation and Interior Structure Experiment, RISE, precisely tracked the location of the lander to determine just how much Mars' north pole wobbles as it orbits the Sun. These observations provide detailed information on the size, composition, and characteristics of Mars' iron-rich core.

Tech Specs​

Main Job	RISE tracked the wobble of Mars' north pole as the Sun pushes and pulls it in its orbit. This helps scientists determine the size and composition of Mars' core.
Location	Two Medium-Gain ‘horn’ Antennas (MGAs) on the lander deck, and an X-band radio transponder and transmitter in the lander's equipment bay, where electronics can be shielded from the harsh, cold conditions of space.
Mass	About 16 pounds (7.3 kilograms).
Antennas	3 pounds (1.4 kilograms).
Transponder and Transmitter	9.6 pounds (4.4 kilograms).
Power	78 Watts (operated up to one hour per day).
Volume	0.7 cubic feet (19.8 liters) total.
Electronics	0.13 cubic feet (3.8 liters).
Medium Gain Antennas	0.57 cubic feet (16 liters).

• RISE Tracks InSight
  RISE tracks the location of the InSight lander every day and knows its location to within a few inches (centimeters).
  
  
  
  
  
• A Fast-Moving Siren
  RISE uses the same principle that causes the sound from a fast-moving siren to change as it moves away, to study Mars' core.
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  Hard-Boiled or Raw?
  RISE helps determine whether Mars has a solid metal core, like a hard-boiled egg, or a molten, liquid metal core, like soft-centered, raw egg.
  
  
  
• RISE is Like a Mirror
  RISE simply sends back the signal sent to the lander from Earth, via the Deep Space Network, revealing the lander's location.
  
  
  
  
  
• Measuring Mars Days
  As the seasons change, carbon dioxide sublimates and condenses at the poles, causing tiny changes in the rotation rate of Mars, affecting the length of the Martian day. RISE will be able to measure these changes!

How It Worked​

RISE worked like a mirror: it reflected a signal sent to the lander from Earth, and revealed the exact location of the InSight lander –and thereby, Mars – in space. In doing so, it measured changes in the frequency of the signal, known as the "Doppler shift,” as the signal approached and departed. Scientists can use this information, collected over the course of InSight's mission, to understand just how much Mars wobbles in its orbit. This is key to determining the size of Mars' core, and whether it is liquid or solid.

The Planets are Like Moving Sirens​

As a siren approaches, it sounds higher in pitch than when it is moving away. This change in the sound is because of the “Doppler shift” in frequency due to the motion of the vehicle. If you know how the signal changes, you can determine how the vehicle is moving. Scientists can use this phenomenon to study the planets. They measure the difference in the original signal sent to RISE from Earth, and the signal that arrives at Earth from RISE. This information tells them how the lander is moving in space, and hence, how Mars wobbles in its orbit.

Tracking Mars' Wobble​

We know that Earth wobbles every 18 years as it is pushed and pulled by the Moon. And with measurements from the Viking landers and from Mars Pathfinder, we know that Mars wobbles over one Mars year. But we don't yet know by how much. RISE helps scientists determine exactly how much Mars wobbles.

RISE Tells Us About Mars' Core​

How much a planet wobbles depends on what's inside it. A hard-boiled egg spins faster than a raw egg. In the same way, a planet that is liquid at its core will wobble more as it spins than one that’s solid at its core. Understanding how much Mars wobbles will reveal what its core is like.

Scientists know that Mars only has a weak magnetic field frozen into the crust, but no field coming from its core today. What we learn about the core from RISE will help them understand why Mars' core isn’t generating a magnetic field like Earth's.

Sebastien Le Maistre​

Principal Investigator​

Royal Observatory of Belgium
Brussels, Belgium
Meet the RISE Team ›

Did You Know?​

Earth completes a wobble every 18 Earth years as it is pushed and pulled by the Moon. Mars completes a wobble every one Mars year (two Earth years). RISE will tell us exactly how much Mars wobbles, using the same technology that your smartphone uses to tell you your location.

See: https://mars.nasa.gov/insight/spacecraft/instruments/rise/

The NASA InSight mission provided high-quality seismic and lander radio science data. The InSight’s radio science data is used to determine fundamental properties of the core, mantle and atmosphere of Mars. By precisely measuring the rotation of the planet, a resonance with a normal mode was detected that allowed the characterization of the core and mantle separately. The 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, in its atmosphere or in its ice caps. The Rotation and Interior Structure Experiment (RISE) provides information on the rotation of Mars by measuring the Doppler shift of radio transmissions from InSight to NASA’s Deep Space Radio Telescope Network (DSN) on Earth. Through the combination of one year of Doppler data from InSight with previous data from the Viking-1 lander, Mars Pathfinder, and Opportunity missions, data spanning 43 years from 1976 to 2019 are used to estimate the Mars precession rate as −7605 ± 3 milliarcseconds per year.
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