Mars’ Faster Spin Explained by Underground Rock Plume, Study Suggests

The rotation of Mars is subtly, yet measurably, increasing in speed, and scientists are beginning to pinpoint a surprising cause: activity deep beneath the surface of the planet. A new study, published February 18 in the Journal of Geophysical Research: Planets, suggests a massive plume of buoyant rock within the Martian mantle is responsible for the shift, and may likewise explain how the planet retains internal heat for far longer than previously thought.

For years, researchers have known that Mars’ day is getting incrementally shorter – shrinking by roughly 70 microseconds each year. This acceleration, while small, has been a puzzle. The new research offers a compelling explanation rooted in the planet’s internal structure, specifically within the vast Tharsis volcanic region. This region, home to Olympus Mons – the largest volcano and highest known mountain in our solar system – is not simply a collection of ancient, inactive volcanoes. It appears to be a site of ongoing geological activity.

Looking Under the Surface

Mars differs significantly from Earth in its geological history. Unlike our planet, Mars lacks active plate tectonics, the process where the Earth’s crust is divided into shifting plates. On Earth, this process drives much of our volcanic activity. Instead, Martian volcanoes tend to remain stationary, building up massive structures over billions of years as lava accumulates in one place. This has resulted in the formation of the Tharsis volcanic province, a sprawling region stretching 3,700 miles (6,000 kilometers) across the planet’s surface.

To better understand the Martian interior, NASA’s InSight lander, which operated on Mars from 2018 until late 2022, provided invaluable data. The lander directly measured the thickness of the Martian crust and offered insights into the planet’s internal structure. Analyzing this data, researchers led by Bart Root of the Delft University of Technology in the Netherlands, ran computer simulations to determine what kind of subsurface structures could account for the concentration of volcanic activity in the Tharsis region.

Their models pointed to a “negative mass anomaly” – an area of less dense material – within the Martian mantle beneath Tharsis. This plume of buoyant rock is thought to be rising slowly, exerting pressure on the crust above. According to Root, “The negative or light mass anomaly will move upwards and hit the lithosphere of Mars, introducing melt pockets that have the potential to penetrate the crust and erupt as volcanoes.” (The lithosphere, the rigid outer layer of the planet, is approximately 310 miles or 500 kilometers thick.)

How Internal Shifts Affect Planetary Spin

The connection between this subsurface plume and Mars’ increasing spin rate is a matter of mass distribution. The researchers theorize that the rising plume causes a redistribution of mass within the planet. As the less-dense material rises and the denser material sinks, it subtly shifts the planet’s moment of inertia – a measure of how tough it is to change an object’s rotation. This is analogous to a figure skater pulling their arms inward to spin faster.

“With some simple back-on-the-envelope calculations, One can explain the order of magnitude of the observed speed up,” Root explained. “Of course more complicated modeling will be needed to actually link this better.” This isn’t simply about a faster spin, however. The ongoing activity suggested by the plume also challenges existing assumptions about how quickly small, rocky planets like Mars cool and lose their internal heat. If Mars still possesses enough internal energy to drive this kind of mantle convection, it suggests that these planets may remain geologically active for longer periods than previously believed.

Understanding the internal dynamics of Mars isn’t just about unraveling the mysteries of the Red Planet itself. It provides valuable insights into the evolution of planetary systems as a whole. As Root notes, “Understanding Mars will facilitate in understanding our solar system, as its history is laid out on the red soil.”

Implications for Planetary Science

The findings have implications for how scientists view the thermal evolution of rocky planets. Traditionally, smaller planets were thought to cool relatively quickly, becoming geologically inactive after a few billion years. The evidence from Mars suggests that internal processes, like mantle plumes, can sustain geological activity for much longer. This could mean that other seemingly inactive planets or moons in our solar system – and beyond – may harbor hidden reservoirs of internal heat and potentially even ongoing geological processes.

Further research will be needed to confirm the existence and behavior of the mantle plume beneath Tharsis. Future missions to Mars, equipped with advanced geophysical instruments, could provide more detailed data on the planet’s internal structure and dynamics. The study also highlights the importance of long-term monitoring of planetary rotation rates, as subtle changes can reveal valuable information about internal processes.

The study’s findings also underscore the power of combining data from different sources – in this case, surface observations from Viking and InSight, and sophisticated computer modeling – to unravel complex planetary phenomena. This integrated approach is likely to be crucial for advancing our understanding of the solar system and the worlds beyond.

You can find more information about the study, including the full research paper, at the Journal of Geophysical Research: Planets.