The dwarf planet has short-lived ammonia on its icy surface
Red ice found on Pluto suggests the dwarf planet recently spewed fountains of water into space. And it hints at complex — and possibly organic — chemistry in Pluto’s salty subsurface sea, researchers report May 29 in Science Advances.
“This was a huge surprise to all of us about Pluto,” says planetary scientist Dale Cruikshank of the NASA Ames Research Center in Moffett Field, Calif. “It means there are lots of surprises waiting to be uncovered in that part of the solar system.”
Cruikshank and colleagues analyzed the wavelengths of light, which can act as signatures of chemical compounds, in images of Pluto’s surface taken when the New Horizons spacecraft flew past the dwarf planet in July 2015 (SN: 12/26/15, p. 16). Those images previously had revealed a variety of ices overlaying a bedrock of water ice, which at Pluto’s frigid temperatures is so hard that it can sculpt mountains.
In the new analysis, the researchers found signs of ammonia where that water ice was exposed. Ammonia is a fragile molecule that can be broken down by ultraviolet sunlight and cosmic rays in just 400,000 to a billion years. “If you find it at all, it suggests that it has been put there fairly recently,” Cruikshank says. “There is really no limit [to how recently], as far as I can see in the geology.”
The ammonia-rich water ice is clustered around a large crack in the dwarf planet’s surface called Virgil Fossa, west of Pluto’s large heart-shaped feature. That crack is probably a fissure, from which liquid water once erupted in a cryovolcano, the team reports.
The jagged red crack called Virgil Fossa, shown in this 2015 image (left) cutting across the surface of Pluto, could represent a volcano that violently sprayed ammonia and organic-rich water recently. In the colored image (right), the red, yellow, orange and purple pixels correspond to higher concentrations of ammonia in water ice.
Previous observations had suggested Pluto could hide a watery ocean beneath its icy crust (SN Online: 9/23/16). But “there’s a big difference between seeing evidence of liquid ocean from various surface features, and seeing parts of the ocean, the liquid, actually come out onto the surface,” says planetary scientist Steven Desch of Arizona State University in Tempe, who was not involved in the new discovery.
Seeing evidence for a cryovolcano on Pluto is vindicating, Desch says. “We have thought for a long time that these [small, icy] planets, Pluto included, should have cryovolcanic activity,” he says. “It’s kind of a mystery why we don’t see more indications of it.”
Ammonia is an excellent antifreeze that can lower water’s freezing point by 100 degrees Celsius, Desch says. If Pluto’s subsurface ocean contains ammonia, that could explain how the water remains liquid so far from the sun.
Some scientists are still waiting for more evidence of cryovolcanism, though. "I’d love to believe this,” says Marc Neveu, an astrobiologist at NASA Goddard Space Flight Center in Greenbelt, Md., who was not involved in the study. “But the images are still a little bit too blurry to really make this a slam dunk." Barring getting better pictures, scientists could use a combination of New Horizons data and laboratory studies to analyze the ammonia and organics in the mix to figure out how those molecules might have formed in a space environment.
Evidence suggests Pluto’s eruption, or eruptions — the researchers can’t tell if it was a one-time event or not — spread ammoniated ice as far as 200 kilometers from the fissure, Cruikshank says. That means the eruption would have shot water at about 300 meters per second up into the atmosphere, where it froze midair before falling back to the surface across a wide swath.
“It would have been a violent thing,” Cruikshank says, not unlike his experience standing 50 meters from an erupting geyser in Yellowstone National Park one winter. “I was completely coated with frozen ice.”
The ice observations on Pluto revealed another surprise: a red material that Cruikshank’s team thinks represents complex organic material. The team had seen organic molecules on Pluto before and thought the molecules were formed on the surface or in the atmosphere. This is the first suggestion that the subsurface ocean could contain organic molecules, too.
Irradiating ice rich in ammonia and organics in the lab can create molecules that are important to life, including the nucleobases that make up DNA and RNA, Cruikshank and colleagues noted in the journal Astrobiology in March. That doesn’t necessarily mean life got a start on Pluto, but it could mean that the chemical precursors to life can happen in surprisingly inhospitable environments.
"It’s telling you pretty much that you don’t need to be around a star for interesting things to happen,” the NASA astrobiologist Neveu says. “If it happens deep inside Pluto, in the dark, with a very cold surface ... this prebiotic, prelife chemistry can happen anywhere."
C. M. Dalle Ore et al. Detection of ammonia on Pluto’s surface in a region of geologically recent tectonism. Science Advances. Published online May 29, 2019. doi:10.1126/sciadv.aav5731.
D. P. Cruikshank et al. Prebiotic chemistry of Pluto. Astrobiology. Published online March 23, 2019. Doi:10.1089/ast.2018.1927.
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