Glaciers on Mercury and Eris force scientists to rethink the habitable zone

Glaciers are one of Earth’s iconic features, and most of us have known from childhood what they are made of and where they can be found: snow and ice, in the Arctic, Antarctic, or on the slopes of big mountains.

But glaciers hold mysteries and maybe even the answers to some of the questions about how a planet might become habitable.

Scientists are increasingly finding unexpected types of glaciers in surprising places. This month, in fact, a pair of studies found them at opposite poles of our Solar System: Mercury – whose surface temperature rises high enough to melt lead – and Eris, a dwarf planet currently three times farther out from the Sun than Pluto, whose surface is only a couple dozen degrees Celsius above absolute zero.

What they found has led to a re-evaluation of how some planets become conducive to life. It’s a complex story.

Not that these glaciers are identical to what you’d find in New Zealand or the Antarctic. Mercury’s are made of salt, and Eris’s are below its surface in a mantle of “squishy” ice. But both have much in common with traditional glaciers.

Salt glaciers exist on Earth, most famously in the Zagros Mountains of Iran. “These are glaciers in the sense of their morphological features and the processes that form them, even though the material is different from what we typically associate with a glacier,” says glaciologist Ulyana Horodyskyj Pena, founder of Science in the Wild in Boulder, Colorado.

These are glaciers in the sense of their morphological features and the processes that form them, even though the material is different from what we typically associate with a glacier.

Ulyana Horodyskyj Pena, founder of Science in the Wild in Boulder, Colorado

On Earth, they are produced when tectonic forces squeeze underlying salt deposits into plumes that rise to the surface, where the salt then slowly flows downhill. On Mercury, they are formed when impacts expose underlying salt deposits on crater walls or in the crater’s central peak, says Alexis Rodriguez, a planetary scientist at NASA’s Marshall Space Flight Center and lead author of a paper in The Planetary Science Journal. The salt then flows away from the newly formed cliffs, producing glaciers that can be seen in images from NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015.

But where did the salt on Mercury come from? Salts are volatile – meaning that once exposed on the surface, they gradually evaporate (a process that Rodriguez’s team could see happening via small “hollows” that pockmark the glaciers’ surfaces). Scientists had long thought that super-hot, relatively low-gravity Mercury had lost all of its volatiles to space very early in its history. The presence of salt glaciers proves this wrong.

The answer, Rodriquez says, is still being investigated, but one possibility is that some of primordial Mercury’s volatiles escaping from its interior got trapped in its megaregolith (a layer of broken rock created by repeated impacts) when the megaregolith was capped by a layer of fine-grained material, effectively sealing them in.

Then, some major cataclysm – possibly massive volcanism – broke the cap and released the gasses all at once. “So, you have a gradual process of accumulation, then a very sudden release,” Rodriguez says. “That might have produced a proto-atmosphere that then froze out during the planet’s 87-day night. These deposits were then capped by later volcanism before they could evaporate off into space. “That’s one possibility,” Rodriguez says.

Another possibility, he says, is that Mercury had a close encounter with a large icy body from the outer Solar System. But instead of hitting Mercury directly (a giant impact that would have left a still-detectable effect on Mercury’s gravitational field), that body was shredded into tiny bits that rained down on the surface – carrying enough volatiles to form the proto-atmosphere that way.

Meanwhile, glaciers on Eris…

Eris is one of the largest known dwarf planets in the solar system. It’s about the same size as Pluto but spends most of its orbit at least 1.7 times farther from the Sun

Eris is one of the largest known dwarf planets in the solar system. It’s about the same size as Pluto but spends most of its orbit at least 1.7 times farther from the Sun.

Eris’s glaciers are very different. They are entirely subsurface and instead of flowing downhill in response to gravity, they rise and fall through its interior carrying heat from its core, much like mantle plumes do on Earth.

They were discovered by modeling the orbital dynamics of Eris’s 615-kilometer-wide moon, Dysnomia, says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, and coauthor of a paper in Science Advances.

That paper, Nimmo says, was based on two discoveries. One is that Eris and Dysnomia are tide-locked, meaning that each always turns the same face toward the other – something that will eventually happen to the Earth and the Moon, but not for a very long time.

The second discovery was a measurement of Dysnomia’s mass, which Nimmo’s coauthor Michael Brown, of California Institute of Technology, Pasadena, obtained from super-precise measures of Eris’s own motion in response to Dysnomia’s gravity, taken with Chile’s giant ALMA radio telescope.

Based on that, Nimmo and Brown were able to model what Eris’s interior had to be like in order to allow it to become tide-locked to its much smaller companion within the history of the Solar System (Eris’s diameter is 2,326 kilometers).

They found that for this to occur Eris had to be “a lot squishier than you would expect,” Nimmo says. That meant that Eris had gotten warm enough early in its history for rock to fall into its center to form a core, while the ice rose to form a mantle.

They found that for this to occur Eris had to be ‘a lot squishier than you would expect’

That mantle then would have started convecting heat away from the core, in the process flowing much like glacial ice. “The only difference is that with a glacier, the thing that’s acting on it is gravity pulling it downhill, whereas what’s happening in Eris is that the ice gets warmed up and becomes buoyant,” Nimmo says. “But the motion of the ice is the same in both cases.”

All of this is fascinating in and of itself because it means that glaciers are a good deal more diverse and common than people once imagined. “I expect that someday in the far future, we’ll find even more range when we can explore the planets around other stars in detail,” says Alan Stern, principal investigator for NASA’s New Horizons spacecraft, which had previously discovered nitrogen glaciers on Pluto. 

“It’s almost like nature will find a way [to produce] a fluid that is mobile,” adds Nimmo. “The fluid changes as you move around the Solar System, [but] it’s almost as if wherever you go, there’s going to be some substance that flows on geologically interesting timescales. I have speculated that [under the right conditions] the argon in the atmosphere would freeze out and then you’d probably have argon glaciers.”

But there are also ramifications for astrobiology, because the presence of these glaciers means that both frigid Eris and sunbaked Mercury might actually once have been habitable to life as we know it.

Historically, astrobiologists thought of a planet’s habitability in terms of the “Goldilocks Zone” determined by its distance from its star. Too close, and surface water boils away. Too far out, and it freezes. Only in the middle was liquid water believed to be possible, and without liquid water, life as we know it cannot exist.

But that concept is too narrow-minded, Rodriguez says, because there’s also a vertical Goldilocks zone. Anyone who’s ever gone on a cave tour understands the gist. Whatever the temperature on the surface, the temperature underground is more moderate – until you go too deep and encounter heat rising from the core.

It’s almost like nature will find a way [to produce] a fluid that is mobile

Francis Nimmo, a planetary scientist at the University of California, Santa Cruz

That means that on many worlds, there is a domain somewhere between the surface and the core, where temperatures are conducive to life as we know it. “You have to think in [in terms] of habitable niches,” Rodriguez says.

What’s needed, of course, is a source of liquid water. In the case of Mercury, that may come in the form of water trapped in salt crystals. On Eris, it is possible that the squishy ice contains (or once contained) pockets of water. “There’s certainly enough energy available to melt some of the ice,” Nimmo says. Not that this means these places actually harbor life. “We’re not saying that there is life on Mercury or that there is a possibility that there is,” says Rodriguez. “We’re talking about habitability.”