Salt glaciers on Mercury could harbor habitable niches

Glaciers on Mercury: Planet with mottling of various colors on its cratered surface, with black background.
View larger. | This enhanced-color image of Mercury comes from NASA’s MESSENGER spacecraft. The colors bring out the chemical, mineralogical and physical differences between the rocks that make up Mercury’s surface. Scientists know that water ice can exist in permanently shadowed craters near the planet’s poles. Now there is new evidence for salt glaciers on Mercury as well. Image via NASA/ Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington.

Mercury – the closest planet to the sun – is probably one of the last places you would expect to find glaciers. Surprisingly, however, there is evidence for water ice in permanently shadowed craters at its poles, where temperatures are always extremely cold. But glaciers? Scientists at the Planetary Science Institute in Tucson, Arizona, said on November 17, 2023, that they have found evidence of glacier-like formations on Mercury. Unlike the ice deposits, however, they are likely made of salt. And even more intriguing is that the salt glaciers on Mercury may be similar to the ones on Earth, which can provide habitable conditions for microbes in extreme environments. The findings are reshaping scientists’ perceptions of Mercury’s geological history.

The researchers published their intriguing peer-reviewed results in The Planetary Science Journal on November 17.

Salt glaciers on Mercury?

Salt glaciers exist on Earth, too, but people are generally more familiar with our planet’s icy glaciers. And Mars has ice glaciers, also. Both types of glaciers are composed of volatiles, substances that can vaporize easily. There are other kinds of glaciers, as well. Pluto, for example, has glaciers of frozen nitrogen. Finding evidence for glaciers on Mercury is rather unexpected, however. The new findings suggest that glaciers of various types are rather common in our solar system. As lead author Alexis Rodriguez at the Planetary Science Institute, explained:

Our finding complements other recent research showing that Pluto has nitrogen glaciers, implying that the glaciation phenomenon extends from the hottest to the coldest confines within our solar system. These locations are of pivotal importance because they identify volatile-rich exposures throughout the vastness of multiple planetary landscapes.

Co-author Bryan Travis at the Planetary Science Institute added:

These Mercurian glaciers, distinct from Earth’s, originate from deeply buried Volatile Rich Layers (VRLs) exposed by asteroid impacts. Our models strongly affirm that salt flow likely produced these glaciers and that after their emplacement they retained volatiles for over one billion years.

The researchers say the Volatile Rich Layers – the salt glaciers – are several miles deep.

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Glaciers are composed of hollows

Interestingly, the salt glaciers on Mercury aren’t solid. Rather, they contain many hollows that form pits in the glaciers. The glaciers may also help explain the origin of the hollows, which scientists have seen before inside impact craters. As co-author Deborah Domingue at the Planetary Science Institute explained:

The glaciers on Mercury are marked by a complex configuration of hollows that form widespread (and very young) sublimation pits. These hollows exhibit depths that account for a significant portion of the overall glacier thickness, indicating their bulk retention of a volatile-rich composition. These hollows are conspicuously absent from surrounding crater floors and walls. This observation provides a coherent solution to a previously unexplained phenomenon: the correlation between hollows and crater interiors. The proposed solution hypothesizes that clusters of hollows within impact craters may originate from zones of [Volatile Rich Layer] VRL exposures induced by impacts, thereby elucidating a connection that has long baffled planetary scientists.

Gray-brown terrain seen from orbit with grid lines added, irregular red patch, and two small black ovals.
This map shows chaotic terrain called Borealis Chaos near Mercury’s north pole. It is in this region that scientists have found evidence for possible salt glaciers. Image via NASA/ Planetary Science Institute.

How did the glaciers on Mercury form?

Apart from the associated hollows and pits, how did the glaciers themselves originally form? The new findings suggest they formed on top of the already solidified surface of Mercury early in its history, instead of from material coming from within the planet. Rodriguez said:

A central mystery concerning Mercury revolves around the genesis of its glaciers and chaotic terrains. What mechanism was responsible for the formation of [][Volatile Rich Layers] VRLs? In our research, we introduce a model that integrates recent observational data to address this question. Notably, we examine the Borealis Chaos, located in Mercury’s north polar region. This area is characterized by intricate patterns of disintegration, significant enough to have obliterated entire populations of craters, some dating back approximately 4 billion years.

Beneath this collapsed layer lies an even more ancient, cratered paleo-surface, previously identified through gravity studies. The juxtaposition of the fragmented upper crust, now forming chaotic terrain, over this gravity-revealed ancient surface, suggests that the VRLs were emplaced atop an already solidified landscape.

These findings challenge prevailing theories of VRL formation that traditionally centered on mantle differentiation processes, where minerals separate into different layers within the planet’s interior. Instead, the evidence suggests a grand-scale structure, possibly stemming from the collapse of a fleeting, hot primordial atmosphere early in Mercury’s history. This atmospheric collapse might have occurred mostly during the extended nighttime periods when the planet’s surface was not exposed to the sun’s intense heat.

An underwater origin?

The researchers even suggest that the salt glaciers may have formed underwater. The water would have been in pools or shallow seas, released from volcanoes when Mercury was still young. Co-author Jeffrey Kargel at the Planetary Science Institute said:

Underwater deposition could have significantly contributed to the emplacement of a salt-dominated Mercurian VRL, marking a significant departure from previous theories about the planet’s early geological history. In this scenario, water released through volcanic degassing may have temporarily created pools or shallow seas of liquid or supercritical water (like a dense, highly salty steam), allowing salt deposits to settle. Subsequent rapid loss of water into space and trapping of water in hydrated minerals in the crust would have left behind a salt- and clay mineral-dominated layer, which progressively built up into thick deposits.

Lumpy brownish terrain with many small irregular pits filled with pale blue, with white edges.
View larger. | Some of the strange bright hollows on Mercury in the Raditladi impact basin. The pits can be up to about 1 mile (1,600 meters) across and 120 feet (37 meters) deep. The bright material may be salts in the glaciers exposed by impacts. Image via NASA/ Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution for Science.

Astrobiological implications

Salty glaciers are interesting on their own, but they also have other possible implications. On Earth, some salts can create habitable environments for microorganisms in extreme environments, such as deserts, called extremophiles. Could such a thing even be possible on Mercury? Rodriguez said:

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. This line of thinking leads us to ponder the possibility of subsurface areas on Mercury that might be more hospitable than its harsh surface. These areas could potentially act as depth-dependent Goldilocks zones, analogous to the region around a star where the existence of liquid water on a planet might enable life as we know it, but in this case, the focus is on the right depth below the planet’s surface rather than the right distance from a star.

This groundbreaking discovery of Mercurian glaciers extends our comprehension of the environmental parameters that could sustain life, adding a vital dimension to our exploration of astrobiology also relevant to the potential habitability of Mercury-like exoplanets.

Bottom line: Researchers have found evidence of glaciers on Mercury. They are composed of salt, not ice, and could potentially create habitable niches for microorganisms.

Source: Mercury’s Hidden Past: Revealing a Volatile-dominated Layer through Glacier-like Features and Chaotic Terrains

Via Planetary Science Institute

Read more: Images from the 3rd Mercury flyby of BepiColombo!

November 22, 2023

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