Mercury’s Mountains
by Amanda Rossillo | American Scientist Magazine | April 20, 2022
Mercury’s mountains are globally distributed but form in clusters (indicated by white arrows), suggesting that their formation is not random. Thomas Watters of the National Air and Space Museum and his colleagues used images and mathematical models to understand how crustal thickness may be tied to mountain formation in the absence of plate tectonics. In this false-color photograph, lower elevations are depicted in shades of blue and higher elevations in shades of red. Courtesy of NASA/Johns Hopkins University/Carnegie Institution of Washington/Smithsonian Institution.
The Solar System’s innermost planet may be hiding big surprises beneath its small, battered surface. One of Mercury’s most distinctive features is its long, linear mountain chains, called lobate scarps. For years, most scientists interpreted the scarps as wrinkles on a cooling, shrinking, slowly dying world. But a new analysis suggests that lobate scarps may actually be a sign of a hot, churning interior and a surface that remains geologically active to this day.
Earth’s surface is broken into moving sections, or plates, that create mountains when they collide. However, Mercury’s crust is a single continuous shell. Because there are no plates that could crash into one another and form mountains, it was long thought that Mercury’s widespread lobate scarps had formed as a result of the planet’s interior cooling over time, causing the crust to shrink and randomly wrinkle.
In a paper published in August 2021 in Geophysical Research Letters, Thomas Watters and Michelle Selvans of the Smithsonian’s National Air and Space Museum and Peter James of Baylor University discovered a pattern in the seemingly random distribution of these mountain chains: Lobate scarps are concentrated in areas of thicker crust, particularly in the southern hemisphere. They also found that these areas of thick crust had been pushed together more forcefully than areas of thinner crust. Although the global distribution of lobate scarps supports the idea that Mercury’s surface is indeed wrinkling, their concentration in regions underlain by strained, thick crust suggests that additional factors have influenced their formation. “Something is helping to organize the forces that are acting to produce these faults,” said Watters.
The team developed models of Mercury’s crustal thickness based on topographic and gravitational data collected by NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission, which was launched in 2004 and ended in 2015. The models suggest that Mercury’s crust was pushed together in specific locations because of geological activity in the mantle, which sits between a planet’s crust and core. “There’s this phenomenon called downward mantle flow where the material in the mantle is descending toward the core,” said Watters. “As it does, it pulls crustal material together, thickens it, and compresses it.” This thickening and compression causes the crust to crack and shift, producing lobate scarps.
Watters believes that Mercury may still be geologically active today because it has a magnetic field, which NASA’s Mariner 10 mission discovered in the 1970s. “There’s still a hot outer core on Mercury that’s liquid, and possibly still moving and convecting to generate that magnetic field,” he said. “There’s no reason to believe that Mercury has stopped contracting.” This idea is supported by photographs that MESSENGER captured during the last 18 months of its mission. Very-high-resolution images revealed small, relatively young lobate scarps, suggesting that newly formed faults are actively modifying Mercury’s ancient surface.
Very-high-resolution images revealed small, relatively young lobate scarps, suggesting that newly formed faults are still actively modifying Mercury’s ancient surface.
Studying Mercury up close is not an easy task. Spacecraft require a lot of fuel to stay in orbit because the Sun’s gravitational pull is incredibly strong. Once there, spacecraft must then withstand the Sun’s scorching heat and intense radiation. Although there have been 48 missions to Mars, including failed attempts, only two missions had been sent to Mercury as of 2017: MESSENGER in the 2000s and Mariner 10 in the 1970s.
In spite of these hurdles, the next voyage to Mercury is already underway. Planetary scientists are eagerly waiting for the BepiColombo spacecraft, jointly developed by the European Space Agency and the Japan Aerospace Exploration Agency, to enter orbit around Mercury in 2025 following its 2018 launch. The spacecraft is named after Italian engineer Giuseppe “Bepi” Colombo, who discovered that Mariner 10 could use Venus’s gravity as a slingshot to fly by Mercury multiple times, ultimately allowing it to photograph nearly half of the planet’s surface. The BepiColombo spacecraft used this same maneuver to make its first Mercury flyby in October 2021.
Mariner 10 and MESSENGER revealed surprising insights about Mercury’s turbulent interior, and BepiColombo is expected to do the same through detailed analyses of the planet’s core, chemical composition, and surface. Ultimately, learning more about this enigmatic planet will help inform efforts to understand distant planets outside of our Solar System. “We still have a lot to learn in our own Solar System about how these rocky bodies evolve as they’re losing their interior heat,” said Watters. “That’s going to give us important insight into what we may be finding when we can examine the variety of Earth-like exoplanets that are out there.”