NASA’s Curiosity rover has been investigating Mars since touching down inside Gale Crater in 2012. During its mission, the rover has uncovered evidence of ancient lake environments, identified organic compounds and provided scientists with a much clearer picture of Mars’s geological history. Yet one major question remains unresolved: how did the enormous mountain at the center of Gale Crater actually form?

Curiosity is currently climbing the mountain known as Aeolis Mons, or Mount Sharp. The massive formation rises above the surrounding crater rim, indicating that it is unlikely to be simply the central peak produced by the original impact that created Gale Crater.

One previous explanation suggested that sediment accumulated inside the crater long after its lakes and rivers disappeared. According to this idea, wind gradually transported enough material into Gale Crater to fill it almost completely, potentially creating a deposit roughly three miles high. Later, powerful Martian winds would have stripped much of that material away, leaving the mountain standing in the crater's center.

A new study published in Science on February 1 challenges that scenario. Researchers found an unexpected way to use Curiosity’s existing navigation equipment to investigate the geology beneath the rover, even though the spacecraft was never equipped specifically for this type of scientific measurement.

Turning Curiosity’s Navigation Equipment Into a Scientific Instrument

To investigate how Mount Sharp formed, Kevin Lewis of Johns Hopkins University and his research team needed information about the rocks beneath Curiosity. A specialized gravimetric instrument would normally be required to measure subtle variations in gravitational attraction and use them to estimate the density of underground material.

Curiosity does not carry such an instrument. Instead, Lewis and his colleagues found a way to use equipment already installed for navigation.

Modern smartphones, tablets and computers often contain accelerometers that detect movement, orientation and changes in acceleration. Curiosity has comparable technology as part of its navigation system. Known as the Rover Inertial Measurement Unit, or RIMU, the instrument records the rover’s movements and orientation.

The researchers analyzed RIMU data collected over nearly five years. During that period, Curiosity traveled approximately 11 miles, or 18 kilometers, across the floor of Gale Crater and onto the lower slopes of Mount Sharp. The route included an elevation gain of about 1,150 feet, or 350 meters, as the rover began climbing the mountain in 2014.

Measuring Gravity While Climbing Mount Sharp

Gravity becomes slightly weaker as an object moves farther from the center of Mars. However, Curiosity was traveling across solid ground rather than simply rising through the atmosphere. The rocks beneath its wheels also contribute to the gravitational force experienced by the rover.

By analyzing extremely small changes in gravitational acceleration as Curiosity moved upward, the researchers were able to estimate the density of the rocks forming Mount Sharp.

The results were surprising. The mountain’s rocks appeared to have a significantly lower density than researchers had expected.

Ray Arvidson of Washington University in St. Louis, a member of the Curiosity science team who was not involved in the research, noted that the material is considerably less dense than typical sedimentary rocks found on Earth. Such unusually low density would be difficult to explain if the rocks had once been buried beneath enormous quantities of sediment.

What the Low-Density Rocks Reveal

If Gale Crater had once been completely filled with miles of sediment, the immense weight of those layers would have compressed the material beneath them. Similar to how a heavy vehicle compacts soil beneath its wheels, enormous amounts of overlying sediment would have squeezed out much of the empty space within the buried rock.

Instead, Curiosity’s measurements indicate that the rocks beneath it remain surprisingly porous. This suggests that they could never have been buried beneath the enormous thickness of material required by the earlier theory.

Based on the gravitational measurements, Lewis and his team estimate that sediment may have filled Gale Crater to less than half of its eventual depth.

That finding points toward a different explanation for Mount Sharp. Rather than being the remnant of a giant pile of sediment that was later carved down by wind, the mountain may have developed progressively through several different stages of deposition and erosion.

How Mount Sharp May Have Developed

One possible reconstruction begins with a much wetter ancient Mars. Heavy rainfall and snowfall could have carried sand and gravel from the walls and rim of Gale Crater toward its interior.

Over time, shallow lakes may have formed and deposited additional layers of mud containing sulfate minerals on top of the earlier material.

As Mars gradually became colder and drier, water activity declined. Wind then became increasingly important in shaping the landscape, slowly removing exposed sediments.

Additional material, including windblown sand, dust and volcanic ash, could subsequently have accumulated on top of the existing layers. Over long periods, these processes would have produced the complex, layered structure of Mount Sharp that Curiosity is exploring today.

The New Theory Changes How Scientists Read Mount Sharp

The new findings do not completely overturn the broader geological picture of Mount Sharp. Arvidson explains that the overall interpretation of the mountain as a kind of geological record remains largely intact as Curiosity moves upward through its layers.

However, understanding exactly how the mountain formed is important because it influences how scientists interpret the evidence Curiosity encounters.

The origin and history of each layer can affect expectations about the minerals found there, the environmental conditions under which they formed and the way ancient water moved through Gale Crater.

Consequently, the new gravity measurements could help scientists refine their understanding of the environmental history recorded in Mount Sharp.

Curiosity Continues Its Journey

Curiosity has already moved away from Vera Rubin Ridge and is now exploring a lower section of Mount Sharp containing clay-rich deposits.

The rover’s future route could eventually take it toward much higher elevations. About 6,500 feet away are sulfate-rich layers that scientists believe may have formed when shallow lakes occupied Gale Crater.

Beyond those layers, roughly another 2,600 feet farther uphill, lies a sharp transition toward deposits that may have been produced primarily by wind.

These regions could provide important evidence about how Mars transitioned from a wetter world with lakes and flowing water to the colder and drier planet seen today.

How Far Will Curiosity Go?

Whether Curiosity will actually reach all of these scientifically interesting locations remains uncertain. The mission has already received multiple extensions, and future plans will depend heavily on what the rover discovers along the way.

Scientists must balance traveling toward distant targets with stopping frequently to investigate particularly valuable rocks and geological formations.

As Arvidson puts it, the rover’s progress depends on how interesting the rocks prove to be and whether the team chooses to cover more ground or pause to investigate the discoveries along the route.

For now, Curiosity continues its slow climb up Mount Sharp, using not only its dedicated scientific instruments but also equipment originally designed for navigation to uncover new clues about the ancient history of Mars.