Despite being so far from the sun, tiny Pluto, which is smaller than Earth's moon, has had an active geologic life from the start, one that continues to present day, research published on Thursday shows.
The evidence is all over Pluto's face, which was observed close-up for the first time by NASA's New Horizons spacecraft on July 14, 2015.
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With most of the high-resolution images from the flyby now back on Earth, scientists say Pluto's mountains, glacial flows, rotated ice blocks, volcano-like mounds and other features rival the geology found on much larger, warmer planets like Mars.
The physical and chemical conditions on Pluto, located about 40 times farther away from the sun than Earth, have played out in unusual and largely unforeseen ways. Highly volatile cryogenic ices, such as nitrogen, carbon monoxide, methane, vaporize into Pluto's hazy and surprisingly compact atmosphere. Internal heating, fueled by the natural decay of radioactive elements in Pluto's rocks and other sources, likely keeps an ocean of ammonia-rich water liquid beneath the dwarf planet's frozen surface.
"We now have half a dozen worlds, like (Saturn's moon) Enceladus, (Jupiter's moons) Europa and Ganymede, and now Pluto, that seem to have oceans in their interiors," New Horizons' lead scientist Alan Stern, with the Southwest Research Institute told Discovery News.
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"It's interesting that only Earth wears its ocean on the outside," he said. "From the surface, we don't see them. Who know that oceans would turn out to be fairly common?"
As far as the prospect for life on Pluto, Stern said, "Anytime you have liquid water, the astrobiologists get interested in that place. That's as far as I'm willing to go."
For Earth-like life to exist, a planet would need water, an energy source, the right chemical elements and time, added planetary scientist William McKinnon, with Washington University in St. Louis.
"All we can say is that we think that Pluto has an ocean and we think that this ocean has survived to the present day. It's the kind of ocean that is deep inside the interior of Pluto, in total darkness. But, it would lie between a floating water ice shell and the rocky interior, so it would be in contact with rock. There would be a modest amount of heat leaking out. You certainly couldn't rule it out, but anything about life on Pluto is simply speculation," McKinnon told Discovery News.
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New Horizons scientists also discovered that Pluto's primary moon Charon was once active as well, but has been geologically dead for about 2 billion years, with its one-time liquid ocean now frozen solid.
A related study shows that Charon and Pluto's four small moons -- Nix, Hydra, Kerberos and Styx -- formed from debris blasted into space after a Pluto-sized body and Pluto smashed into each other some 4 billion years ago.
The research is published in five papers in this week's Science.