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  Solar system lousy with subsurface oceansMar 17, 2015 10:32 PM PDT | url
Added 1 new A* page:You'll recall that in episode 23, Selenis traveled to a planetoid where a fairly thin layer of ice covered a subsurface ocean. This was inspired by evidence pointing to the possible existence of oceans under thick ice layers on a number of the moons in our solar system!
Saturn's moon Enceladus is famous for its spectacular geysers shooting water ice crystals into space, photographed as far back as 2005 by the Cassini probe:
Cassini found that the ejected ice particles contain salt, pointing specifically to a salty subsurface ocean.
Last April, scientists reported an anomaly in the effect of Enceladus' gravity on Cassini's motion in the passes it has made near the moon, specifically past the south polar region, where geysers emerge from four "tiger stripe" gashes in the icy crust; a large depression covers the south polar region, which should weaken the gravitational effect felt by the probe as it flies past, but the effect was not as pronounced as expected, and the scale of the difference from the expected reading could be accounted for by the existence of something under the ice, denser than ice, though not as dense as rock—and water fits that bill: calculations "suggest a large, possibly regional, ocean about 6 miles (10 kilometers) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick." Cassini is due to make more passes over Enceladus later this year.
NASA has also been allocating funds to study the possibility of a subsurface ocean beneath the crust of Jupiter's moon Europa. "The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons)." Photos of the moon taken by the Voyager and Galileo probes showed terrain features that suggested upheavals from beneath the surface, where new ice formed in surface areas perhaps torn or melted apart, which could be consistent with tidal movements of an ocean beneath the ice. Mathematical models based on the theoretical heating and observed surface features "predicted that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km (60 mi) deep"; "this leads to a volume of Europa's oceans of 3 × 10^18 cubic meters, slightly more than two times the volume of Earth's oceans." Furthermore, Galileo, which orbited Jupiter from 1995 to 2003, found that Europa responded less than expected to the force of Jupiter's powerful magnetic field, which could be accounted for by "a highly electrically conductive material in Europa's interior" that would intercept Jupiter's field and conduct it around rather than through the rest of the interior; "the most plausible candidate for this role is a large subsurface ocean of liquid saltwater."
In December 2013, the Hubble Space Telescope detected water vapor above Europa's south polar region, strongly suggesting the presence of water geysers roughly equivalent to those on Enceladus. Then, last September, scientists studying Europa's surface features announced that they'd discovered evidence for subduction—that is, for shelves of ice being shoved down below the surface by collisions with nearby ice plates, similar to plate tectonics on Earth. This kind of movement could be facilitated by a subsurface ocean of the type suggested by the earlier evidence.
Jupiter's largest moon, Ganymede, has been considered a candidate for a subsurface ocean ever since such a feature was suggested by scientific models of the moon's composition calculated in the 1970s. And the Galileo spacecraft, as it had found with Europa, noticed a weaker than expected response to Jupiter's magnetic influence, again suggesting a conducting subsurface ocean (Galileo had a similar finding for another of Jupiter's moons, Callisto).
And just a few days ago, news emerged of Hubble data providing the "best evidence" yet for the existence of a subsurface ocean on Ganymede: aurorae at Ganymede's poles—ultraviolet light emitted by high energy particles caught in the interplay of magnetic fields between the moon and Jupiter—would normally be rocking back and forth by six degrees as the moon orbits the planet, but Hubble, watching them, observed a mere two degrees of movement in Ganymede's aurorae, and what could account for that difference would be a lunar subsurface ocean no deeper than 330 km: "The data are consistent with an ocean of a 100km thickness with a certain salt content of about 5g per one litre of water. But it could equally well be an ocean of only 10km but with 10 times more salt."
Those are the moons with the best evidence for subsurface oceans—but there's more! That last news article lists more suspects in what could be a very "soggy" solar system:
- Estimates of the dwarf planet Pluto's density based on Hubble observations suggest a large water ice content, and the "decay of radioactive elements" below the surface could melt that ice, "creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary."
- In 2014, the Herschel Space Telescope observed water ice around the dwarf planet Ceres; models of the dwarf based its observed size and orbit again suggest a significant water ice density presence; "also, some characteristics of its surface and history (such as its distance from the Sun, which weakened solar radiation enough to allow some fairly low-freezing-point components to be incorporated during its formation), point to the presence of volatile materials in the interior of Ceres"; "it has been suggested that a remnant layer of liquid water may have survived to the present under a layer of ice." The Dawn spacecraft, just arrived around Ceres, has observed "soft" surface features, suggesting a possible water ice composition.
- Based on density calculations, Saturn's largest moon, Titan, is thought to be 50% water ice. If its interior is still warm, a water/ammonia "magma" could remain liquid even at very low temperatures. The Cassini spacecraft has detected "natural extremely-low-frequency radio waves in Titan's atmosphere"; "Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean." And gravity and visual observations by Cassini suggest that Titan's crust and core are moving independently, which would require a liquid layer of some sort separating the two.
- One explanation for the larger than expected wobble, reported this past October, observed by Cassini in Saturnian moon Mimas' orbit is a subsurface ocean, 15 to 20 miles beneath the surface. Mimas has been measured with a very low density suggesting a primarily water ice composition, but it is so small that while some theories say a more elongated ancient orbit could have produced enough tidal heating to melt the ice and form a subsurface ocean, others say that any such ocean would have refrozen long ago.
- Chemical and density calculations suggest a 30-45% water ice composition for Neptune's moon, Triton; water ice covers about 15-35% of its surface. "There is enough rock in Triton's interior for radioactive decay to power convection in the mantle. The heat may even be sufficient to maintain a subterranean ocean similar to that which is hypothesized to exist underneath the surface of Europa."
Whoa, that turned out to be way more subsurface oceans than I'd bargained for when I started writing this! : o
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