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NASA鈥檚 New Horizons spacecraft spotted the signatures of ammonia in ice on the surface of Pluto, which might be responsible for turning parts of its surface red.

In space, ammonia doesn鈥檛 last long 鈥� it is easily broken up by ultraviolet light and charged particles from the sun, as well as cosmic rays from elsewhere in the galaxy.

鈥淎mmonia is a fragile molecule in a space environment, so the fact that we see it exposed on the surface means that it was put there recently,鈥� says New Horizons team member Dale Cruikshank at NASA鈥檚 Ames Research Center in California. 鈥淚 don鈥檛 mean last Thursday, but maybe 100 million years ago.鈥�

The researchers spotted the ammonia in an region called Virgil Fossae, spread over an area about 200 kilometres wide. Virgil Fossae contains deep troughs聽and water ice, which could be oozing up from a possible subsurface ocean. Ammonia lowers the melting point of water, so it may be helping to keep that ocean from freezing.

Because the ammonia is spread over such a large area, it probably emerged in spurting fountains of ice particles as well as by oozing, Cruikshank says. He and his colleagues calculated that this activity must have taken place at most one billion years ago for the ammonia to still be detectable, though it聽may be more recent.

Ammonia is known for its strong odour, but on Pluto it is likely in the form of ice or salts, so wouldn鈥檛 make the surface smell. 鈥淚t isn鈥檛 as if it鈥檚 pure ammonia lying on the ground waiting for someone to take a sniff,鈥� Cruikshank says.

The compound could be an important part of the complex molecules thought to litter Pluto鈥檚 surface, turning it red in places. Shining ultraviolet light on Pluto-like ices in the lab has even produced nucleobases, the compounds that form DNA. Finding ammonia on the surface could be a hint at more complex organic chemistry on Pluto, although it鈥檚 so cold that there is almost definitely no life there.