Gravity study provides insight into features hidden beneath Mars’ lost ocean and rising Olympus Mons

Studies of gravity variations on Mars have revealed dense, large-scale structures hidden beneath layers of sediment from a vanished ocean. The analysis, which combines models and data from multiple missions, also shows that active processes in the Martian mantle could be driving the largest volcano in the solar system, Olympus Mons. The results were presented this week at the Europlanet Science Congress (EPSC) in Berlin by Bart Root of Delft University of Technology (TU Delft).

Mars has many hidden structures, such as ice deposits, but the features discovered in the northern polar plains remain a mystery because they are covered by a thick, smooth layer of sediment thought to have been deposited on the ancient seafloor.

“These dense structures could be volcanic in origin or formed from compacted material from ancient impacts. We have identified about 20 structures of various sizes scattered around the northern polar cap, one of which resembles the shape of a dog,” said Dr Root. “There appears to be no evidence of these structures on the surface. However, thanks to gravity data, we have a fascinating insight into the ancient history of the northern hemisphere of Mars.”

Dr Root and colleagues at Delft University of Technology and Utrecht University used tiny deviations in the satellites’ orbits to study Mars’ gravity field and find clues about the planet’s internal mass distribution. These data were fed into models that use new observations from NASA’s Insight mission of the thickness and flexibility of the Martian crust, as well as the dynamics of the planet’s mantle and deep interior, to create a global map of Mars’ density.

The density map shows that the North Polar features are about 300 to 400 kg/m³ Volcanoes are denser than their surroundings. However, the study also revealed new information about the structures underlying the immense Tharsis Rise volcanic region, which includes the colossal Olympus Mons volcano.

Although the volcanoes are very dense, the Tharsis region is much higher than the average surface of Mars and is surrounded by a region of relatively low gravity. This gravitational anomaly is difficult to explain by looking solely at differences between the Martian crust and the upper mantle. Dr. Root and his team’s study suggests that a light mass about 1,750 kilometers across and at a depth of 1,100 kilometers is giving the entire Tharsis region an upward boost. This could be explained by a huge plume of lava, deep in the Martian interior, moving toward the surface.

“NASA’s InSight mission has given us vital new information about the hard outer layer of Mars. This means we need to rethink our understanding of what supports the Olympus Mons volcano and its surroundings,” Dr Root said. “This shows that Mars may still be experiencing active movements, affecting and perhaps creating new volcanic features on the surface.”

Dr Root is part of the team proposing the Martian Quantum Gravity Mission (MaQuls), which aims to use technology developed for missions such as GRAIL and GRACE to the Moon and Earth respectively to map the gravity field of Mars in detail.

“Observations with MaQuIs would allow us to better explore the Martian subsurface. This would help us learn more about these mysterious hidden features and study ongoing mantle convection, as well as understand dynamic surface processes such as atmospheric seasonal changes and the detection of underground water reservoirs,” said Dr. Lisa Wörner from DLR, who presented the MaQuIs mission at EPSC2024 this week.