The word glacier conjures swaths of brilliant ice surveyed by bighorn sheep or holiday skiers. Planetary exploration has expanded our view, taking in the rock glaciers of Mars and even the vast nitrogen glacier of Pluto’s heart-shaped Tombaugh Regio. They’ve all got one thing in common: they’re cold.
But now, researchers at Tucson’s Planetary Science Institute (PSI) have presented evidence of a different kind of glacier on the closest planet to the Sun. It seems that Mercury hosts glaciers made of salt, perhaps the remnants of a violent atmospheric collapse sometime in the planet’s past. And adding to the intrigue, planetary scientists suggest that Mercury’s glaciers may constitute buried environments that could sustain life.
Jeff Kargel, Senior Research Scientist at the University of Arizona and coauthor of the new PSI paper on the subject, is particularly taken by the parallels between Mercury’s flows and the glacial forms on Mars, forms he has studied extensively. The makeup of Mercury’s glaciers is unique, consisting of salt and debris. But do these alien glaciers have anything in common with their icy counterparts on Mars and Earth? “They do!” Kargel declares. “If you view them at similar resolutions, they’re almost dead ringers for lobate debris aprons on Mars.”
Glaciers from beneath
Mercury’s salty glaciers appear to originate from a deep layer of volatiles entombed beneath the surface. This Volatile Rich Layer (VLR) of various gases and minerals was emplaced, long ago, on top of ancient buried terrain. The volatiles became exposed—or were forced upward—toward the vacuum of space when asteroids or meteorites impacted the planet’s surface. The resulting glaciers of surface material were driven primarily by salt flows.
Outgassing and sublimation of volatiles (solid materials turning into vapor) result in a variety of strange geological features. Among the most prominent on Mercury are the famous “hollows,” sunken pits that may be related to volcanism or other outgassing. For years, researchers have been baffled about the location of hollows. They form as clusters on the interior walls of craters, and along impact rings and central peaks, but not on the surrounding terrain. The PSI team hypothesizes that material from the deep Volatile Rich Layers is uplifted and exposed by impacts, triggering formation of the hollows as volatiles leak out into the vacuum.
Glacier central on Mercury is a region called Borealis Chaos, a vast area of intricate canyons and collapsed landscapes. The disrupted landscape has obliterated most of the oldest craters. But beneath the fractured realm of Borealis Chaos lies a paleo-surface of ancient cratered terrain. This older layer—hiding beneath the glacial formations—was first seen in gravity maps returned by the MESSENGER spacecraft. The region’s shattered surface layer (consisting of materials including basalts and graphite) overlays the ancient surface, suggesting that the volatile enriched layers were deposited over the solid older landscape. The PSI team suggests that as chaotic terrain is distributed globally across Mercury, localized events cannot explain the geology as well as a widespread, planetary event such as the collapse of a temporary atmosphere.
The idea has stirred some controversy in the planetary science community. Ron Vervack, planetary atmospheres expert at the Johns Hopkins University’s Applied Physics Laboratory, says that Mercury is far more volatile-rich than we used to think. “That is one of the major discoveries from the MESSENGER mission, which found sodium, potassium, chlorine, and sulfur abundances higher than expected,” he states. “[The paper’s authors] aren’t claiming that Mercury is volatile-rich so much as trying to explain why it is.” Mercury’s volatiles were originally thought to originate from the interior of the planet as it differentiated, with heavy material settling toward the core and lighter, volatile-rich material rising to the crust.
The new model suggests a more global process, perhaps generated by the collapse of a transient, hot, primordial atmosphere. In the new volatile-rich scenario, impacts or volcanic activity could have vented water vapor to the surface, where it pooled just long enough to emplace the salt deposits before drifting off into the vacuum of space. The surface terrain above the unstable salt layer began to fragment, and those salty blankets began to flow, resulting in the glacial terrain seen today. As Kargel explains, “We think the lobate debris aprons are connected with impact processes, with the impact excavating a volatile-rich layer and providing heat and exposing the buried material on steep slopes. These things flow out some around 10 to 15 kilometers from the peak ring. Its very mobile material. Thermal modeling suggests that it would flow on the order of centuries to a few millennia.”
Life on a cooked world
From an astrobiological standpoint, Mercury’s glaciers present intriguing possibilities. Even in the most extreme desert conditions on Earth (for example, the Atacama Desert in Chile or the McMurdo Dry Valleys of Antarctica), salt-rich compounds create localized habitable environments. If Mercury has retained enough subsurface water at the just the right depth, briny environments may contain depth-dependent habitable—or “Goldilocks”—zones, the researchers say. But Kargel warns that “Earth had a huge amount of liquid water. Mercury never did.” The question is: how can a desiccated, parched world like Mercury retain so much subsurface water? The answer may lie in minerals and compounds like calcium sulfide and hydroxyls. The heat of impacts can free water from them, and water may also be added by an incoming asteroid or comet. The resulting underground zone may be capable of supporting the kinds of extreme microbial life seen in some of Earth’s hottest, driest climes.
Life or no life, the salty glaciers of Mercury offer entirely new geologic formations for comparison to other planets and moons. Vervack, who was not involved in the study, says “this is a bold hypothesis that attempts to assimilate a variety of observations with martian and terrestrial analogs to explain some of the outstanding questions about Mercury.”