Pluto has always been something of an outcast. It orbits the sun at a weird angle, not even close to those of the planets.
It's also way, way out there — some six billion kilometres from the sun. And for a variety of reasons it was kicked out of our planetary family in 2006, demoted by astronomers to a mere "dwarf planet."
Until recently, Pluto was thought to be little more than a frozen rock; a dull footnote at the end of the solar system.
A close-up of Pluto's equatorial region revealed a big surprise: an icy mountain range with peaks as high as 3,500 metres, comparable in size to some of the smaller Rocky Mountains.
NASA scientists believe the mountains may still be forming, which suggests the region might still be geologically active, throwing up some combination of water and gases from a heated core.
"Who would've supposed there were ice mountains? It's just blowing my mind," project scientist Hal Weaver said at a news conference.
Elsewhere in the solar system, surface features are formed on icy moons by heat caused by gravitational interactions with heavyweight planets like Jupiter and Saturn. But there's nothing that big anywhere near Pluto.
NASA says "some other process" must be making those mountains.
"This may cause us to rethink what powers geological activity on many other icy worlds," said John Spencer of the Southwest Research Institute.
Another surprise is that Pluto and at least one of its moons lack the many craters one would expect to find on an object that's been around for some 4.5 billion years, taking hits from meteorites and the like.
Earth's moon is about the same age, and looks it.
But Pluto has "one of the youngest surfaces we've ever seen in the solar system," according to Jeff Moore, leader of the geology and geophysics team at NASA's Ames Research Centre.
NASA is suggesting that recent geologic activity may have given the region "a facelift, erasing those pockmarks." Scientists will now go looking for geysers and so-called "ice volcanoes."
