Underground Oceans Can't Last Forever

If Pluto and Enceladus have oceans under the crust, why are they still liquid after billions of years?

You can’t see them, but they must be there. Oceans under the crusts of planets and icy moons, inferred from orbital mechanics and geology, serve as working hypotheses to explain anomalies found by space probes. They were not predicted before the spacecraft sent back photos of weird surfaces. Enceladus and Pluto are recent candidates for oceanic worlds.

Scientists on the New Horizons team have started considering an underground ocean at Pluto. In “Present-day subsurface ocean on Pluto?” Science Daily infers hidden sloshing seas from surface features that would have been different without a global ocean under the crust. Conor Gearin writes on New Scientist, “Pluto must have liquid ocean or it’d look like an overripe peach.” Telltale wrinkles of a shrunken, dry crust are not apparent. According to Astrobiology Magazine, this was not exactly a ho-hum expectation of planetary science:

“That’s amazing to me,” Hammond said. “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.”

Science Daily doesn’t answer how such an ocean could survive for billions of years, but New Scientist takes a stab at that problem, saying that Francis Nimmo pondered the possibility back in 2011. He “argued that the dwarf planet’s icy shell could insulate an ocean,” and now feels vindicated by Pluto’s findings. Astrobiology Magazine suggests that “exotic ices” might be good insulators, provided the crust is 260 km thick or more.

Planetary scientists have been toying with subsurface oceans at Enceladus for the last decade since Cassini found geysers erupting (see Space.com review of Saturn’s 2nd moon). “An ocean lies a few kilometers beneath Saturn’s moon Enceladus’s icy surface,” Science Daily announced from indirect indications and physical models. The new estimate is much thinner (5 km) than initially modeled (30-60 km). Ideas about whatever Enceladus has under its crust are evolving faster than the crust itself.

Back at Pluto another astonishing discovery has been refined: a super-canyon on the moon Charon that is longer and deeper than the Grand Canyon. It’s five times as deep as Arizona’s, Astrobiology Magazine says, and 150 miles longer – remarkable for a world much smaller than Earth. Another record may be set: “There appear to be locations along the canyon’s length where sheer cliffs reaching several miles high occur, and which could potentially rival Verona Rupes on Uranus’ moon Miranda (which is at least 3 miles, or 5 kilometers, high) for the title of tallest known cliff face in the solar system.”

Geologists and mineralogists should take a look at Space.com‘s article about graphite. Apparently several solar system worlds have generated this pencil-lead form of carbon, including Mercury, asteroid Ceres, and Pluto’s moon Charon. “Graphitized carbon forms when carbon is heated to high temperatures in the absence of oxygen,” the article explains. Therein lies a puzzle: carbon was found on Charon, but not on Pluto. Cassini scientist Amanda Hendrix can’t explain it; it blows away her theory about how graphite forms from the solar wind.

Hendrix called these results surprising. Radiation from the solar wind should be significantly weaker at Charon than it is at Ceres, because Charon lies, on average, about 10 times farther from the sun than Ceres does. If the moon’s surface is covered with graphite, she said, “it likely formed a different way.“

With all the talk of water, water everywhere, hydrobioscopy can’t be far behind. New Scientist remarks, “Pluto probably has a rocky seabed, which could provide the chemicals needed for life.” And Science Daily, less concerned about keeping water liquid for billions of years, comments, “This suggests that there is a strong heat source in the interior of Enceladus, an additional factor supporting the possible emergence of life in its ocean.” The focus should be on what kind of heat source could last for billions of years.

These findings should represent severe problems for old-agers. Oceans on tiny icy worlds or dwarf planets should not last for billions of years, to say nothing of geysers. Nor should deep, sharp-sided canyons. So instead of looking clueless, planetary scientists change the subject to talk about how life might have ’emerged’ out there. This is like a fast-talking athlete distracting attention by committing a foul while he tells a funny joke to the referee.