The Role of Salt in Shaping Mercury's Terrains, Including Glacier-like Attributes

The surface of Mercury could be more volatile over geological timeframes than previously believed, a new study suggests. According to research published in the November Planetary Science Journal, the closest planet to the sun is likely shaped by volatiles - transient compounds that can freeze, flow, or evaporate into the atmosphere over time, much like water does on Earth. Primarily, it appears that salt, which is Mercury's main volatile, has modified the planet's terrain over billions of years and could even be slowly flowing in forms that are comparable to glaciers on Earth. The authors even proposed that viable habitats could form deep underground where the volatiles are located.

Alexis Rodriguez, a space research scientist at NASA’s Marshall Space Flight Center, suggests that volcanic debris from deep inside the planet is not solely responsible for Mercury’s unique landscape. Instead, Rodriguez claims that "volatile-driven resurfacing on Mercury has been one of the major engines in the evolution of the landscape." This theory is contrary to previous beliefs that Mercury's close proximity to the sun would have removed or baked off volatile compounds. However, these volatiles were detected by NASA’s Messenger spacecraft that orbited the planet in the early 2010s.

According to the new study, Mercury's landscape, which consists of a jumble of hills, plateaus and grooves, might have been shaped by the volatile compounds that have been stored abundantly within the planet's crust, possibly in a planet-wide reservoir. These volatiles could have shaped two of Mercury’s geologic features: chaotic terrains and glacier-like flows.

If earlier volcanic activity was responsible for the chaotic terrains, it would have selectively erased certain former geologic features like smaller impact craters. However, Rodriguez and his team highlighted that chaotic terrains that contain remnant craters exist, suggesting that the terrain was formed by volatiles leaking from the ground into space, causing the land to lose structural integrity and collapse. Further analysis of features at Mercury's north pole indicates that salts have also shaped the terrain.

The study also suggests that basins of asteroid craters, which look like salt "glaciers", give further evidence of volatile sculpting. These formations would have formed after asteroids hit the planet’s surface and exposed hidden volatiles which moved towards lower ground due to the impact’s heat, pooling like a thick syrup.

Similar to Earth's glaciers, these forms also carved the land as they moved. The scientists theorize that these salt glaciers, which have divots indicating loss of volatiles into Mercury’s thin atmosphere, could disappear after a billion years. Interestingly, the prevalence of surface volatiles suggests that even more might be found underground, potentially creating pockets of habitable space that could protect organisms from Mercury's extreme surface temperatures.

Whilst it remains uncertain whether life could exist in these hypothetical places, the mere fact that "glaciers" could exist on Mercury is surprising. If Mercury's geologic features are identified as glaciers, they would be present throughout the solar system, from Mercury to far-reaching Pluto.

Despite the exciting findings, some researchers challenge the use of the term "glaciers", arguing that the structures might contain more rocky materials than volatiles. However, the process proposed by the study for the formation of these structures is considered plausible.

The new ideas are radical, notes David Rothery, a planetary scientist at the Open University in Milton Keynes, England. “But it all fits the pattern: Mercury is surprisingly rich in volatiles, and we have yet to understand the limits of what the volatiles are able to do to Mercury’s landscape.” A revisit with more advanced instruments than Messenger’s is very much in order, Rothery says.

Fortunately, BepiColombo, a joint endeavor between the European and Japanese space agencies, is on the way (SN: 1/15/21). Launched in 2018, the spacecraft will enter Mercury’s orbit in December 2025. “BepiColombo, as well as answering some of those questions, is going to give us some more surprises,” says Rothery, who is involved with the mission. “I’d be very surprised if we are not surprised.”