A decade ago, we weren't even sure if there was water on the moon. Now scientists are proposing there is an ocean on Pluto.
Wondering why you can't see it in the high-resolution pictures sent back by NASA's New Horizons probe? That's because it's underground, a scientist at Brown University in the US has proposed.
The clue is in photos taken by New Horizons -- massive cracks in the dwarf planet's surface, some as deep as four kilometres. That's more twice as deep as the Grand Canyon.
"What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion," says Noah Hammond, PhD student.
"A subsurface ocean that was slowly freezing over would cause this kind of expansion."
Ice is less dense than water, so a subsurface ocean would push upwards on the crust, causing it to crack.
The evidence also suggests the ocean hasn't completely frozen, and there could be liquid water sloshing around on Pluto, which occupies a far-flung region of the solar system call the Kuiper Belt.
"Because of the low temperatures and high pressure within Pluto, an ocean that had completely frozen over would quickly convert from the normal ice we all know to a different phase called ice II. Ice II has a more compact crystalline structure than standard ice, so an ocean frozen to ice II would occupy a smaller volume and lead to a global contraction on Pluto, rather than an expansion."
The cracks are still there, suggesting no contraction has taken place "and therefore that the ocean hasn't completely frozen".
"The possibility that you could have vast liquid water ocean habitats so far from the sun on Pluto -- and that the same could also be possible on other Kuiper Belt objects as well -- is absolutely incredible."
Finding sources of water is essential for any manned space missions that may take place in the distant future.