The word glacier conjures swathes 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,” Tombaugh Regio.
They’ve all got one thing in common: cold. But now, researchers at Tucson’s Planetary Science Institute (PSI) have presented evidence, published in November 2023 in The Planetary Science Journal, for a different kind of glacier found on the closest planet to the Sun. Mercury appears to host glaciers made of salt, perhaps the remnant of a violent atmospheric collapse in the planet’s past. And adding to the intrigue is the suggestion that such glaciers may constitute buried environments that could sustain life.
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!” says Jeff Kargel, Senior Research Scientist at the University of Arizona and study co-author, who has extensively studied glacial forms on Mars. “If you view them at similar resolutions, they’re [Mercury’s glaciers] almost dead ringers for lobate debris aprons on Mars,” he says. Lobate debris aprons are features on the Red Planet features that show the hallmarks of creeping, debris-filled ice.
Glaciers from beneath
The glaciers of Mercury appear to originate from a deep layer of volatiles entombed beneath the surface. (A volatile is simply any substance that evaporates at ambient temperatures — think water or carbon dioxide, here on Earth.) This volatile-rich layer (VRL)of various gases and minerals was emplaced long ago on top of ancient, now-buried terrain. Its volatiles were later exposed — or were forced upward — to the vacuum of space when asteroids or meteors impacted the surface. The resulting glaciers of surface material were driven primarily by salt, which is Mercury’s most abundant volatile.
Outgassing and sublimation (when a solid material turns directly into vapor) of volatiles can cause a variety of strange geological features. Among the most prominent on Mercury are its famous hollows, sunken pits that may be related to volcanism or other outgassing. Yet 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 as material from the deep VRLs is uplifted and exposed by impacts, it triggers formation of hollows as volatiles leak out into space.
Glacier central on Mercury is a region called Borealis Chaos, a vast area of intricate canyons and collapsed landscapes that have obliterated most of the oldest craters there. But beneath this fractured realm lies ancient, cratered terrain. This older layer, or paleo-surface, hiding beneath the glacial formations was first seen in gravity maps returned by the MESSENGER Mercury orbiter. The region’s current-day 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. And the PSI team suggests that because this chaotic terrain is distributed globally across Mercury, localized events cannot explain it as well as a widespread planetary event.
Ron Vervack, a planetary atmospheres expert at the Johns Hopkins University’s Applied Physics Laboratory who was not involved in the study, 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. [The paper’s authors] aren’t claiming that Mercury is volatile-rich so much as trying to explain why it is,” he says. Mercury’s volatiles were originally thought to issue 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, such as the collapse of a transient, hot primordial atmosphere. In the new scenario, impacts or volcanic activity could have vented water vapor to the surface, where it pooled just long enough to emplace 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.” That exposed material is “very mobile,” he adds. “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 certainly 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 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 much subsurface water? The answer may lie in minerals like calcium sulfide and hydroxyls. The heat of impacts can free water from these, and water may also be added by the incoming asteroid or comet. The resulting underground zone may be capable of supporting the kinds of extreme microbial life seen in some of the Earth’s hottest, driest climes.
Life-friendly or not, the salt glaciers of Mercury offer entirely new geologic formations for comparison to other planets and moons. Vervack says that the paper presents “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.”
