Scientists have discovered that salt glaciers may exist on Mercury, the planet closest to the Sun and the smallest world in the solar system. The discovery could show that even the most volatile conditions deep within the solar system can occasionally echo conditions found on Earth.
The team’s findings complement recent discoveries that revealed that Pluto has nitrogen glaciers. Because Pluto exists on the other side of the solar system, the two findings imply that glaciation extends from the warmest regions of the solar system, close to the Sun, to its icy outer reaches.
Even more exciting, scientists at the Planetary Science Institute (PSI) believe these salt glaciers could create the right conditions for life, similar to some of the extreme environments on Earth where microbial life flourishes. “Specific salt compounds on Earth create habitable niches even in some of the harshest environments where they occur, such as the arid Atacama Desert in Chile,” said lead author of the research and PSI scientist Alexis Rodriguez. said in a statement. “This line of thinking leads us to consider the possibility that there are subterranean areas on Mercury that might be more hospitable than its harsh surface.”
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Sites like those highlighted by the team are of fundamental importance because they identify volatile-rich exposures across the vastness of multiple planetary landscapes. They also suggest that the solar system could contain so-called “depth-dependent Goldilocks zones,” regions on planets and other bodies where life might be able to survive not on the surface, but at specific depths that have the right conditions.
“This groundbreaking discovery of Mercurian glaciers expands our understanding of the environmental parameters that could support life, adding a vital dimension to our exploration of astrobiology, also relevant to the potential habitability of Mercury-like exoplanets,” said Rodriguez.
Mercury may be richer in volatiles than we thought
This research challenges the idea that Mercury is devoid of volatiles, chemical elements and compounds that can be easily vaporized and that were vital to the emergence of life on Earth.
This indicates that volatiles may be buried beneath the tiny planet’s surface in Volatile Rich Layers (VRLs). The team also has an idea of how these VRLs were exposed to Mercury’s surface.
“These Mercurian glaciers, distinct from those on Earth, originate from deeply buried VRLs exposed to asteroid impacts,” said research co-author and Planetary Science Institute (PSI) scientist Bryan Travis. “Our models strongly argue that salt flow likely produced these glaciers and that, after their emplacement, they retained volatile substances for more than a billion years.”
The team thinks that Mercury’s glaciers are arranged in a complex configuration with cavities that form young “sublimation pits” – sublimation being the process by which a solid is instantly transformed into a gas, skipping the liquid phase.
“These cavities exhibit depths that represent a significant portion of the glacier’s overall thickness, indicating its mass retention of a volatile-rich composition,” said PSI scientist and team member Deborah Domingue. “These cavities are conspicuously absent in the floors and walls of the surrounding craters.”
Domingue added that this observation, by showing that asteroid impacts revealed VRLs, provides a coherent solution to a previously unexplained phenomenon – the apparent correlation between cavities and crater interiors. The team’s research suggests that clusters of cavities within impact craters may originate from zones of VRL exposure caused by space rock impacts; As impacts expose volatiles, they sublime into gases, leaving cavities behind.
Salty chaos on Mercury
Rodriguez and colleagues examined Borealis Chaos to determine the connection between Mercury’s glaciers and its chaotic terrain and deduce what might be responsible for the formation of VRLs.
This area is located in Mercury’s north polar region and is marked by intricate disintegration patterns that appear significantly large enough to have wiped out entire populations of craters, some dating back around 4 billion years. Beneath this collapsed layer in the Borealis Chaos lies an even older cratered surface that was previously identified through gravity studies.
“The juxtaposition of the fragmented upper crust, now forming a chaotic terrain, on this ancient gravity-revealed surface suggests that the VRLs were placed on top of an already solidified landscape,” Rodriguez said. “These findings challenge prevailing theories of VRL formation that have traditionally focused on mantle differentiation processes, where minerals separate into different layers in the planet’s interior. Instead, the evidence suggests a large-scale structure, possibly arising from the collapse of a hot, fleeting primordial atmosphere early in Mercury’s history.”
The PSI team believes this atmospheric collapse may have occurred primarily during long nighttime periods on Mercury, when the planet’s surface was not exposed to the Sun’s intense heat, causing temperatures to drop from about 800 degrees Fahrenheit (430 degrees Celsius) – hot enough to melt lead – down to minus 290 degrees Fahrenheit (minus 180 degrees Celsius).
Salt-dominated VRLs on Mercury may also have grown extensively due to underwater deposition, an idea that also represents a significant departure from previous theories about the early geology of the planet closest to the Sun.
“In this scenario, water released through volcanic outgassing may have temporarily created pools or shallow seas of liquid or supercritical water as a dense, highly salty vapor, allowing salt deposits to settle,” said Jeffrey S. Kargel, a member of the PSI team and researcher. “The subsequent rapid loss of water to space and the trapping of water in hydrated minerals in the crust would have left behind a layer dominated by salts and clay minerals, which progressively accumulated into thick deposits.”
The team’s research is published in Planetary Science Magazine.