New Horizons’ capture of Pluto’s haze and icy mountains, 15 minutes after its closest approach on July 14. Image credit: NASA/JHUAPL/SwRI
Ice volcanoes and spinning moons are just some of the discoveries being discussed by NASA’s New Horizons science team as the mission continues to yield more secrets about Pluto and its lunar companions.
The team is discussing the latest data this week at the 47th Annual Meeting of the American Astronomical Society’s Division for Planetary Sciences in National Harbor, Maryland.
One discovery saw the team create 3D maps of Pluto’s surface that showed two of its mountains could in fact be ice volcanoes – called cryovolcanoes – that were active in the recent past.
The two mountains in question measure tens of kilometers across and several kilometers high.
These ice volcanoes are thought to spew out a mixture of substances like water ice, nitrogen, ammonia or methane.
Such a discovery, if confirmed, would provide new information regarding Pluto’s geologic and atmospheric evolution.
“These are big mountains with a large hole in their summit, and on Earth that generally means one thing – a volcano,” says Oliver White, New Horizons postdoctoral researcher at NASA’s Ames Research Center.
“If they are volcanic, then the summit depression would likely have formed via collapse as material is erupted from underneath.
The strange hummocky texture of the mountain flanks may represent volcanic flows of some sort that have traveled down from the summit region and onto the plains beyond, but why they are hummocky, and what they are made of, we don’t yet know.”
By studying Pluto’s impact craters, the New Horizons team have also been able to infer that the dwarf planet’s surface ranges in age.
Crater counts indicate Pluto has surface regions dating as far back as four billion years, just after the formation of planets in our Solar System.
But the area called ‘Sputnik Planum’, on the left side of Pluto’s heart-shaped region, is completely crater free, meaning it may have formed within the last 10 million years.
Other data shows some areas of Pluto are of intermediate age, indicating the dwarf planet has been geologically active throughout its life-span.
“We’ve mapped more than a thousand craters on Pluto, which vary greatly in size and appearance,” says Kelsi Singer of the Southwest Research Institute (SwRI) in Boulder, Colorado.
“Among other things, I expect cratering studies like these to give us important new insights into how this part of the solar system formed.”
Another discovery made by the New Horizons team is the unusual behaviour of Pluto’s moons.
While almost all other moons in the Solar System keep one face towards their planet as they rotate, Pluto’s four smaller moons – Styx, Nix, Kerberos and Hydra – are spinning much faster.
Pluto’s most distant moon Hydra, for example, rotates 89 times during a lap around the dwarf planet, while Nix is tilted on its axis and spinning backwards.
“These are four of the strangest moons in the Solar System,” says Mark Showalter, co-investigator on New Horizons.
“If Hydra were spinning much faster, material would fly off its surface due to the centrifugal force.”
The NASA team believe this behaviour could be a result of the pull from Pluto’s largest moon Charon.
The moons have been described as behaving like ‘spinning tops’.
“There’s clearly something fundamental about the dynamics of the system that we do not understand,” Showalter says.
“We expected chaos, but this is pandemonium.”
Images also indicate some of Pluto’s moons could be the result of a merger of two or more moons.
“The New Horizons mission has taken what we thought we knew about Pluto and turned it upside down,” says Jim Green, director of planetary science at NASA Headquarters in Washington.
“It’s why we explore: to satisfy our innate curiosity and answer deeper questions about how we got here and what lies beyond the next horizon.”