Liquid nitrogen may once have flowed on the surface of Pluto – and could exist in pockets beneath the ice we see today. That’s the latest finding from the New Horizons probe, which is now nearly 300 million kilometres past the dwarf planet, but still beaming data back to Earth.
The discovery comes from using data from New Horizons to model how Pluto’s climate and atmospheric pressure have changed over time. These models point to periods in Pluto’s history where the temperature and pressure are high enough for frozen nitrogen ice on the surface to melt.
If correct, this could explain features seen on the surface, including extended networks of terrain that could have been carved by rivers and bodies of still liquid. “We see what for all the world looks to a lot of our team like a former lake,” said Stern.
“It’s very smooth, as if a liquid has frozen across one height,” he told New Scientist. “It’s hard to come up with an alternate model that would explain that morphology.”
The team came to this conclusions after combining the detailed topographic map of Pluto’s surface gathered by New Horizons with models of its climate zones.
On Earth, climate zones are the result of the 23 degree tilt of our planet’s axis relative to the sun. The tropics is the warm region where the sun always passes directly overheard, and is confined near the equator. But Pluto, never one to conform, has a 120 degree tilt, spreading its tropics wide.
“Most of Pluto is tropical,” said New Horizons team member Richard Binzel of the Massachusetts Institute of Technology.
But that doesn’t mean it isn’t chilly. On Earth, the arctic zones are the smaller regions at each pole that experience prolonged periods of light and darkness.
On oddball Pluto, these arctic zones extend much further down, actually crossing into the tropics – meaning some regions are both tropical and arctic. “There is no analogue to that here on Earth,” said Binzel.
The team studied how these zones have changed over time, and how they might evolve in the future, as a result ofPluto’s tilt changing. For example, 800,000 years ago the dwarf planet would have experienced a climate extreme as its axis reached 103 degrees.
The tropics extended to near the poles, while the arctic reached down near the equator. This could explain the dark region seen around Pluto’s equator – it is the only region that is always tropical, and never arctic, meaning ice doesn’t build up there.
At this point in the past, the atmospheric pressure could have been just a tenth of Earth’s at sea level – higher than that seen on Mars. “This really changes your view of this little planet and how it operates,” said Stern. The increased pressure would allow liquid nitrogen to pool on the surface, meaning a New Horizons mission launched around that time would have seen a much wetter Pluto.
But New Horizons could still see liquid on Pluto today, if indirectly. The team is still developing models of the nitrogen ice glacial flows the probe spotted, but early indications suggest the crushing weight of ice could create the pressure conditions necessary for a liquid layer.
“You could have a liquid state at the base,” says Orkan Umurhan of NASA’s Ames Research Center in California. In the past, this could have been 100 metres below the surface, but in the present day it would have to be 1 kilometre deep.
Our limited understanding of the extreme behaviour of nitrogen is actually hindering the team’s analysis, says Umurhan. “Those experiments haven’t been done yet in the laboratory.”