A team of scientists has presented a new gravity map of Mars at the Europlanet Science Congress 2024. The map shows the existence of dense, large-scale structures beneath Mars’ long-lost ocean and that mantle processes are affecting Mount Olympus, the world’s largest volcano in the solar system.

The new map and analysis incorporate data from a variety of missions, including NASA’s InSIGHT (interior exploration using seismic, geodesy, and heat transfer) mission. They also use data from small deflections of satellites in Mars orbit.

The paper, “Mars’ Global Gravitational Field Reveals an Active Interior,” will appear in the upcoming issue of JGR: Planets. The lead author is Bart Ruth of Delft University of Technology. Some of the results contradict an important concept in geology.

Geologists work with a concept called fold isostasy. It describes how the planet’s outer, solid layer responds to large-scale loading and unloading. The layer is called the lithosphere and consists of the crust and the upper part of the mantle.

When something heavy is loaded into the lithosphere, it responds by sinking. A good example of this on Earth is Greenland, where a huge ice sheet is pressing down on it. As the ice sheets melt due to global warming, Greenland will rise.

This downward curvature, although small in effect, generally causes uplift of the surrounding areas. The greater the load, the more pronounced the downward curvature, but this also depends on the strength and elasticity of the lithosphere. Fold isostasy is an important concept for understanding glacial rebound, mountain building, and sedimentary basin formation.

The authors of the new paper say scientists need to rethink how floor isostasy works on Mars. That’s thanks to Mount Olympus, the largest volcano in the solar system, and an entire volcanic region called the Pharsis Rise, or Pharsis Mountains. Mount Pharsis is a massive volcanic region that includes three other large shield volcanoes: Mount Arsia, Mount Pavonis, and Mount Askrei.

Fold isostasy states that this huge region should cause the planet’s surface to sink. But the opposite is true. Tharsis Montes is much higher than the rest of the Martian surface. NASA’s InSIGHT lander also told scientists a lot about Mars’ gravity, and it’s forcing researchers to rethink how everything works on Mars.

“This means we need to rethink how we understand the support of a large volcano and its environment,” the authors write. “The gravity signal from its surface is in good agreement with a model that views the planet as a thin crust.”

The research suggests that active processes in the Martian mantle are lifting Tharsis Montes upwards. “There appears to be a large mass (of something light) deep within the Martian crust, possibly rising from the mantle,” the authors write. “This suggests that the Martian interior may still be active, creating new volcanic formations on the surface.”

The researchers found a subsurface mass about 1,750 kilometers across and 1,100 kilometers deep. They suspect it is a mantle plume rising beneath Mount Pharsis and strong enough to counteract the downward pressure from the entire mass.

“This suggests that the plume head is currently flowing upwards into the lithosphere to generate active volcanism in the geological future,” the authors write in their paper.

There is debate about how volcanically active Mars is. Although there are no active volcanic features on the planet, studies suggest that the Pharsida region has resurfaced within the last few tens of millions of years in the recent geologic past.

If there is a mantle plume beneath Pharsis Montes, could it eventually reach the surface? This is purely speculative and further research is needed to confirm these findings.

Researchers have also discovered other gravitational anomalies. They found mysterious dense structures beneath Mars’ northern polar plains, likely buried beneath a thick, smooth layer of sediment that accumulated on the ancient seabed.

Approximately 300–400 kg/m2 anomalies³ are denser than their surroundings. Earth’s Moon has gravitational anomalies associated with giant impact basins. Scientists believe that the impactors that formed the basins were denser than the Moon and that their mass became part of the Moon.

Non-conformist basins on Mars also exhibit gravity anomalies, but the anomalies in the northern hemisphere of Mars do not show any traces on the surface.

“These dense structures could be volcanic in origin or could be compressed material from ancient impacts. There are about 20 objects of various sizes that we have identified in the area around the north polar cap, one of which resembles the shape of a dog,” Dr Ruth said.

“There doesn’t seem to be any trace of them on the surface. But thanks to the gravity data, we have a promising glimpse into the ancient history of the northern hemisphere of Mars.”

The only way to understand these mysterious structures, and Mars’ gravity in general, is to have more data. Ruth and her colleagues are advocates of a mission that can collect the necessary data.

It’s called the Mars Quantum Gravity (MaQuls) mission. MaQuls will be based on the same technology used by the GRAIL (Gravity Recovery and Interior Laboratory) and GRACE (Gravity Recovery and Climate Experiment) missions, which are mapping the gravity of the Moon and Earth respectively. MaQuls will have two satellites that move behind each other and are connected by an optical link.

“Observations with MaQuIs will allow us to better explore the interior of Mars. This will help us learn more about these mysterious hidden features and study the ongoing convection of the mantle, as well as understand dynamic processes on the surface, such as atmospheric seasonal changes and the detection of soil water reservoirs,” said Dr. Lisa Werner of DLR, who gave a talk about the MaQuIs mission at EPSC2024 this week.