Evidence of ammonia found on Pluto

Scientists studying data from NASA’s New Horizons space mission have found evidence of ammonia on the surface of Pluto.

It’s an exciting find, says Cristina Dalle Ore, a planetary scientist at the Search for Extraterrestrial Intelligence (SETI) Institute and NASA Ames Research Centre, California, because by geological standards, ammonia can’t survive long on the surface of Pluto without being destroyed by ultraviolet light, cosmic rays, or other radiation.

“It’s really fragile,” she says. “When you have ammonia on the surface, that means that it got there recently.” 

It’s also a strong indication that Pluto not only once had an underground ocean, but still does, because ammonia-containing water spewing from such an ocean is the compound’s most likely source.

More evidence for this comes from the fact that the ammonia was detected along a deep trough in Pluto’s surface, called Virgil Fossae. 

Not only does the trough appear to be a crack in Pluto’s shell that might allow a route for underground liquids to reach the surface, it also appears to be the site of volcanic eruptions, where some kind of liquid sprayed out, then promptly froze.

On Earth, places where such liquids spew out are volcanoes, in the form of molten rock. But Pluto is cold: so cold that its surface temperatures hover around 40 degrees Kelvin (minus-233 degrees Celsius), and water ice has the consistency of bedrock. At these temperatures, even the coldest water escaping from an underground ocean would make a dandy substitute for lava. 

The discovery of ammonia, Dalle Ore says, makes it more likely that such an underground ocean exists. That’s because ammonia is a great antifreeze, allowing ammonia-water mixes to remain liquid at temperatures down to minus-90 degrees Celsius — temperatures that could be more easily generated by radioactive heat slowly escaping from Pluto’s core.

It’s not the only evidence that such an ocean might exist. {%recommended 7776%}

The giant basin of Sputnik Planitia — the western lobe of Pluto’s famous “heart” — lies not far from Virgil Fossae, and it is associated with a gravity anomaly that can best be explained by underlying water. 

Recently, scientists calculated that conditions at the base of Pluto’s icy crust are suitable for the formation of an insulating layer of an exotic type of ice called gas hydrates, which Dalle Ore compares to a down comforter or winter jacket. 

“Between that, which keeps it warmer, and the ammonia that allows the water to be liquid at cold temperatures, [we have] more evidence that we have an ocean,” she says.

 Ammonia is also exciting, she adds, because it an important precursor to prebiotic chemistry, including the formation of amino acids and a cascade of other biologically interesting chemicals. 

“We’re not talking aliens,” she adds. “We’re not talking life. But we are talking the possibility of an environment that could foster it. That is one of the main reasons that ammonia is so interesting.”

Francis Nimmo, a planetary scientist at the University of California, Santa Cruz, US, who was not part of Dalle Ore’s team, says the biggest caveat about the study is that he’s not 100% convinced the study actually found ammonia.

“It’s a pretty subtle spectral feature, so it’s not entirely clear to me that it’s really there,” he says.

But if the find is correct, he says, the geology of Virgil Fossae makes it particularly interesting. 

“[Virgil Fossae] has sharp edges and cuts across craters, so it might be only a few hundred million years old,” he says. “That young age is consistent with the detection of ammonia, which gets destroyed fairly rapidly at the surface.”

The find is also consistent with how planetary scientists think Pluto is evolving, with a subsurface ocean that is slowly freezing, he says. That would put the ocean water under pressure as the ice expands, possibly causing pressurised water to spurt to the surface via cracks. 

He, too, thinks that the existence of an ocean on Pluto, if it is really there, is exciting.

“If Pluto does have an ocean, as seems likely,” he says, “then that means that even the outermost fringes of the solar system are potential habitable regions.” 

And if they do indeed have cryovolcanoes, he says, that means that these eruptions may have brought samples of that ocean to the surface, where, in principle, they could be studied by future rover missions, comparable to the ones that have long been exploring Mars.

The study appears in the journal Science Advances.