Cassini Probe, courtesy: nasa.gov
Astronomers suggest that tidal tugs from Saturn likely explain the mysterious geysers blasting off of one of the planet's small moons.
The moon, Enceladus, was discovered to be releasing plumes of water vapor from its south pole in 2005. Observations made in a fly-by made by the international Cassini spacecraft revealed that the plumes emerged from four "tiger stripe" fissures on the south pole of the moon.
Debate ensued over whether the small moon, only about 310 miles wide, hosts a hidden ocean under its icy crust and where the energy originated that powered the plumes. One argument focused on whether gravitational tidal forces alone (or else radioactivity in the moon's core) could explain the plumes.
In the journal Nature, a planetary science team led by Cornell's Matthew Hedman reports the plumes appear several times brighter, and thus bigger, than normal when Enceladus is farthest from Saturn on its elongated orbit around the planet. That seems to confirm that the tidal tugs of Saturn, the second-largest planet in the solar system, power the plumes. "More material therefore seems to be escaping from beneath Enceladus' surface at times when geophysical models predict its fissures should be under tension and therefore may be wider open," they conclude.
You might suppose that Saturn's gravitational pull on Enceladus would be at its weakest when it is farthest away from the planet, but that is exactly the point in time where the tidal stress on the icy moon are greatest, the study says.
Essentially, at that far point in the moon's orbit, the ringed world's gravitational pull reels Enceladus back onto its closer orbital track around the planet, raising a tidal effect on the icy crust of Enceladus that might crease open crevices.
The evidence all points to a southern sea or lake under the crust of the moon, which vents from the tiger stripes when Saturn's gravity pulls them wider open. Astronomers are particularly interested in the plumes because they contain traces of complex organic, or carbon, chemistry thought to be one of the necessary ingredients for life on other worlds.
"Enceladus is thus one of the few places beyond Earth where we can watch geology happen in real time," says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colo., in a commentary accompanying the study. "The likely presence of liquid water and complex organic chemistry makes Enceladus especially intriguing as a potential habitat for extraterrestrial life, providing additional motivation for investigating its interior."
Dan Vergano @dvergano, USA TODAY