Hidden sea on a Saturn moon? New evidence says yes.
Satellite Cassini flew through geyser plumes spouting from Enceladus and gathered ice crystals. These crystals are salt-rich, suggesting that a hidden salty sea lurks beneath Enceladus's icy cap.
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If this explanation is correct, it suggests that the moon provides an environment suitable for at least simple forms of life to emerge. Liquid water would imply an energy source, while water itself is seen as a necessary ingredient for organic life.Skip to next paragraph
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Questions of life aside, Enceladus is one of only four objects in the solar system currently displaying active volcanism, or in the case of chilly objects, cryovolcanism. The others are Earth, Jupiter's moon Io, and Neptune's moon Triton.
On Enceladus, southern geysers erupt along ice fissures in a region scientists have dubbed the "tiger stripes."
Over the course of Cassini's many flybys, the geysers' output has remained essentially constant over time, according to Kempf – an observation he calls "really stunning" and also at odds with what one would expect from the "dry" explanation for the plumes.
It takes a large, steady delivery of energy to maintain that constancy, he explains. And it takes a large amount of energy to maintain a subsurface liquid reservoir.
Indeed, the amount of energy needed exceeds estimates of the heat available from two oft-cited sources: radioactive decay of rocks in the core, and heating by friction from the gravitational tugs of Saturn and its other moons.
At the moment, no one seems to have a reasonable working hypothesis for the mechanism providing the energy, although scientists are working on it, he adds. The challenge is two-fold: A plausible explanation not only has to explain the current source, but also how it evolved over the moon's history.
The plumes themselves were discovered in 2005, when NASA's Cassini spacecraft, currently orbiting Saturn and its moons, made several close fly-bys of Enceladus.
The plumes feed Saturn's E-ring, its outermost ring, which was discovered in 1967.
Analysis of the ring's crystals revealed a surprise. Although the ring is dominated by salt-poor ice, some ice is heavily salted. This fueled speculation that the geysers rise from subsurface water in contact with the moon's rocky core. That core would have been the source of the salts.
This new study involves the analysis of particles taken directly from the moon's plumes during additional fly-bys. The observations, along with modeling studies the team performed, indicate that the ring holds more salt-poor crystals than the plumes themselves because the salt-rich crystals are heavier, and fall back to Enceladus's surface.
To test this model, scientists would like to look for a salt-enriched surface near the geysers, but unfortunately, Cassini's instruments aren't sensitive enough to detect that level of surface enrichment, Kempf says.