We know Mars once had an active geological past, filled with water, volcanic activity, and dynamic geological changes. Now, the planet seems more or less inert — but there may be more to the story. Recent studies are uncovering more than just surface details. New insights from gravity data are revealing hidden structures beneath the Martian crust, providing evidence of ancient oceans and potentially ongoing volcanic activity.

Gravity on Mars

Like all bodies in the universe, the Earth has a gravitational pull. But this pull isn’t constant across our planet’s surface. For starters, the Earth isn’t a perfect sphere. It bulges at the equator and is a bit flattened around the poles. So, at the equator you’re slightly farther from the planet’s center and thus, the gravitational pull is slightly lower. This difference isn’t enough for you to feel when falling, but it’s significant enough that special equipment can detect it.

But it’s not only about where you are on the globe; there can also be more local variations.

Let’s say you’re on top of a large iron ore deposit. The iron is probably denser than the soil and rock around it — this can create a detectable gravity difference. This type of gravity survey is routinely used on Earth to detect large-scale ore deposits and geological structures. Even your smartphone has a simple gravity meter built inside of it.

Doing this type of survey on another planet is, of course, much more difficult. Researchers use a slightly different method, relying on variations in satellite orbits. But the idea is the same: you highlight areas with higher density that create a higher or lower gravitational pull.

Bart Root, an assistant professor at TU Delft in Denmark, carried out such a study on Mars. He used gravity variations measured through satellite data to unearth dense, large-scale structures buried beneath Mars’ northern polar plains. His findings provide tantalizing evidence that Mars may still be geologically active, particularly in the region surrounding Olympus Mons, the largest volcano in the Solar System.

Subsurface anomalies

The measurements focused on northern plains of Mars which are believed to be the bed of an ancient ocean. These smooth plains are covered by thick layers of sediment, obscuring any visible clues about the structures lurking beneath. The gravity data shows several mysterious features hidden below the surface. These features, scattered across the plains, are dense enough to alter Mars’ gravitational field even as they remain invisible from the surface.

These anomalies point to features 300 to 400 kg/m³ denser than the surrounding material, suggesting that they could either be volcanic in origin or the result of ancient impact events.

It’s hard to explain what these anomalies could mean. However, the proximity to Olympus Mons may not be a coincidence. Standing at 22 kilometers high, Olympus Mons dwarfs any volcanic structure on Earth, but scientists have long debated whether Mars’ volcanic activity is a thing of the past or an ongoing process. The new gravity data, combined with models of Mars’ mantle, suggest that the story of Olympus Mons is far from over.

Is the Solar System’s Giant Volcano Still on the Rise?

The Tharsis region in which the volcano lies is itself is an anomaly. It sits high above the rest of Mars’ surface, yet is ringed by a region of lower-than-expected gravity. While volcanoes are typically dense and create strong gravitational signals, the surrounding area of Tharsis does not follow this pattern. This gravitational “low” has puzzled scientists for years, but Root’s team may have found an explanation.

Their study suggests that a large, light mass beneath the surface — roughly 1,750 kilometers wide and buried at a depth of 1,100 kilometers — could be providing a “boost” to the entire region. This mass could be a massive plume of lava rising from deep within Mars’ mantle, pushing upwards and supporting the elevation of Olympus Mons and the surrounding volcanic area.

If this is true, it implies that Mars still has active processes in its mantle, slowly feeding volcanic activity on the surface. It would make Olympus Mons, and possibly other Martian volcanoes, far more dynamic than previously thought.

This new knowledge challenges the long-standing assumption that Mars is geologically dead. Instead, it suggests that Mars may still experience internal movements, including mantle convection, where heat from the planet’s core drives the slow movement of molten rock toward the surface. These processes are similar to those that drive volcanic activity on Earth, indicating that Mars might still have the capacity for eruptions at some point.

More Mars research

The gravitational data in this study was complemented by seismic data gathered by NASA’s InSight mission. Launched in 2018, InSight has been studying Mars’ seismic activity and crustal thickness. This mission has provided crucial data that helped Root’s team refine their models of Mars’ interior.

By analyzing how seismic waves travel through the planet, InSight has revealed details about the planet’s crust and mantle that were previously unknown. The gravity study used this information to improve its understanding of how Mars’ deep interior could be influencing surface features. In particular, InSight’s data on the thickness and flexibility of the Martian crust has helped scientists better understand the gravitational anomalies seen in the Tharsis region.

However, there’s still plenty of uncertainty. For starters, it’s not clear that the gravitational anomalies are linked to magma flow or volcanic activity. They could be, perhaps, linked to meteorite impact, which Mars has a lot of. They could also be some other geological phenomenon. The volcanic interpretation seems the likeliest explanation at the moment, but it’s still far from proven.

A New Mission?

To further explore these gravitational anomalies and the hidden features beneath Mars’ surface, a new mission has been proposed: the Martian Quantum Gravity (MaQuIs) mission. This mission, led by a team of researchers including Dr. Root and Dr. Lisa Wörner from the German Aerospace Center (DLR), aims to map Mars’ gravity field in unprecedented detail.

Using technology similar to that employed by the GRAIL (Gravity Recovery and Interior Laboratory) mission to the Moon and the GRACE (Gravity Recovery and Climate Experiment) mission to Earth, MaQuIs would provide a comprehensive view of Mars’ subsurface. By measuring even finer gravitational variations, scientists hope to unlock the mysteries of the hidden structures in Mars’ northern hemisphere and gain a better understanding of the ongoing processes beneath Olympus Mons.

The research team presented details of the MaQuIs mission at European Planetary Society Congress 2024 (EPSC), highlighting its potential to not only study mantle convection and volcanic activity but also to detect groundwater reservoirs and monitor seasonal changes in Mars’ atmosphere. This mission would mark a significant leap forward in our understanding of Mars’ geological and atmospheric processes, providing critical information that could shape future missions and our search for life on the Red Planet.

The research was presented at the EPSC.