Mars is a fascinating planet that has captivated the imagination of many people for centuries. It is the fourth planet from the Sun and the second-smallest in the solar system. It has a thin atmosphere, a reddish surface, and two small moons, Phobos and Deimos. It is also home to the largest volcano and the deepest canyon in the solar system, Olympus Mons and Valles Marineris, respectively.
But how much do we really know about Mars? How does it rotate and orbit around the Sun? How does its core affect its motion? And how has it changed over time?
To answer these questions, NASA launched a mission called InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) in May 2018. Its main objective was to study the interior structure and dynamics of Mars using a lander that touched down on a flat plain called Elysium Planitia in November 2018. The lander carried several instruments, including a seismometer, a heat probe, and a radio transponder and antennas. The latter was called RISE (Rotation and Interior Structure Experiment), and it was the key to measuring the rotation and wobble of Mars.
In this article, we will explain how RISE works, what it found, and what it means for our understanding of Mars.
How RISE Measures the Rotation and Wobble of Mars
RISE is a simple but powerful instrument that uses radio signals to track the spin rate and orientation of Mars. It consists of two antennas on the lander that send and receive signals from Earth-based stations. By measuring the frequency of these signals, RISE can detect tiny changes in the motion of Mars due to the Doppler effect.
The Doppler effect is a phenomenon that occurs when the source or observer of a wave (such as sound or light) is moving relative to each other. The frequency of the wave appears to increase or decrease depending on whether they are approaching or receding from each other. For example, when an ambulance with a siren passes by you, you hear a higher pitch as it approaches and a lower pitch as it moves away.
Similarly, when Mars rotates or wobbles, it causes slight variations in the frequency of the radio signals between RISE and Earth. By analyzing these variations over time, RISE can determine how fast Mars is spinning, how long its day is, how much it is tilted, and how much it is nodding.
However, measuring the rotation and wobble of Mars is not an easy task. There are many factors that can interfere with the accuracy and precision of RISE’s measurements, such as:
- The Martian atmosphere: The radio signals have to pass through the thin but variable atmosphere of Mars, which can cause delays or distortions in their transmission.
- The Earth’s motion: The Earth also rotates and orbits around the Sun, which affects its position relative to Mars. RISE has to account for this motion by using precise timing and positioning data from other sources.
- The solar activity: The Sun emits electromagnetic radiation that can interfere with the radio signals between RISE and Earth. RISE has to avoid transmitting or receiving signals when the Sun is in between them.
- The noise: There are other sources of radio signals in space that can create noise or interference in RISE’s measurements. RISE has to filter out these unwanted signals by using sophisticated algorithms.
Despite these challenges, RISE has been able to collect high-quality data for more than four years, from November 2018 to December 2022, when InSight was retired due to power issues. During this period, RISE made more than 1.5 million observations of Mars’ rotation and wobble.
What RISE Found About the Rotation and Wobble of Mars
The data collected by RISE revealed some surprising and intriguing results about the rotation and wobble of Mars. Some of these results are:
- Mars is spinning faster than previously thought: According to RISE’s measurements, Mars’ spin rate is about 0.24 milliseconds per year faster than previous estimates. This means that Mars’ day (or sol) is about 0.006 seconds shorter than we thought. This may not seem like much, but over millions of years, it adds up to significant changes in Mars’ rotation.
- Mars’ day varies over time: RISE also found that Mars’ day length is not constant, but varies by up to 20 milliseconds over a Martian year. This variation is caused by seasonal changes in the distribution of mass on Mars’ surface, such as ice accumulation on the polar caps or dust storms. These changes affect how fast Mars spins by changing its moment of inertia (a measure of how resistant an object is to changes in its rotation).
- Mars’ axis precesses faster than previously thought: RISE also measured how much Mars’ axis of rotation changes its direction over time, a phenomenon known as precession. RISE found that Mars’ precession rate is about 0.017 degrees per year faster than previous estimates. This means that Mars’ north pole points to different stars over time, similar to how Earth’s north pole points to different stars over a cycle of 26,000 years. However, Mars’ precession cycle is much shorter, about 172,000 years.
- Mars’ axis nods more than previously thought: RISE also measured how much Mars’ axis of rotation tilts back and forth over time, a phenomenon known as nutation. RISE found that Mars’ nutation amplitude is about 0.01 degrees larger than previous estimates. This means that Mars’ axis wobbles more than we thought, similar to how a spinning top wobbles when it is not perfectly balanced. However, Mars’ nutation period is much longer, about 18.6 years.
What These Results Mean for Our Understanding of Mars
The results obtained by RISE have important implications for our understanding of Mars’ interior and evolution. Some of these implications are:
- Mars has a large and liquid core: The main cause of the faster precession and larger nutation of Mars is the “sloshing” of the molten metal inside its core. This sloshing creates a torque that pushes and pulls on the mantle and crust, making them rotate and wobble more. By measuring how much the core affects the motion of the surface, RISE can infer some properties of the core, such as its size, density, and state. RISE estimated that Mars’ core has a radius of about 1,830 kilometers (about half of the planet’s radius), a density of about 6 grams per cubic centimeter (about twice that of water), and a viscosity of less than 10^14 Pascal-seconds (about 10^11 times less viscous than honey). These values indicate that Mars has a large and liquid core, which is consistent with other evidence from InSight’s seismometer and heat probe.
- Mars has a complex and dynamic history: The faster spin rate and variable day length of Mars suggest that the planet has experienced significant changes in its mass distribution over time. These changes could be related to various processes that shaped Mars’ surface and climate, such as volcanic eruptions, tectonic movements, glacial cycles, dust storms, and meteorite impacts. By measuring how these processes affect the rotation of Mars, RISE can provide clues about the history and evolution of the planet.
- Mars is still an active and mysterious planet: The results obtained by RISE show that Mars is not a static and dead planet, but a dynamic and living one. The planet is still spinning faster and wobbling more than we expected, revealing new aspects of its interior and exterior. There are still many questions and mysteries about Mars that remain to be answered, such as what caused the acceleration in its rotation, what is the origin and composition of its core, and what are the sources and effects of its seismic activity. RISE has provided valuable data and insights that can help us address these questions and mysteries in the future.
Conclusion
In this article, we have explained how RISE works, what it found, and what it means for our understanding of Mars. RISE is a simple but powerful instrument that uses radio signals to measure the rotation and wobble of Mars. It revealed that Mars is spinning faster and wobbling more than we thought, due to factors such as ice accumulation on the polar caps, post-glacial rebound, and core-mantle coupling. These results have important implications for our understanding of Mars’ interior structure, dynamics, history, and evolution.
RISE was one of the instruments on board InSight, a mission that aimed to study the interior of Mars using a lander that operated from November 2018 to December 2022. InSight was retired due to power issues after four years of successful operation. During this period, InSight made many scientific discoveries and achievements, such as detecting hundreds of marsquakes, measuring the heat flow from the interior, estimating the size and state of the core, and recording the sounds of wind and dust devils on Mars .
We would like to express our appreciation for InSight and its team for their remarkable contribution to our knowledge and exploration of Mars. We hope that InSight’s legacy will inspire future missions and studies that will continue to unveil the secrets and wonders of the Red Planet.
