New observations from NASA's Saturn-orbiting Cassini spacecraft have revealed at least 101 individual geysers erupting from Enceladus' crust and, through careful analysis, planetary scientists have uncovered their origin.
Over a period of seven years, Cassini has been gradually building up a map of the small icy world. One of Enceladus' most famous features - its south polar "tiger stripes" - have been of special interest.
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From the cracked ice in this region, fissures blast out water vapor mixed with organic compounds as huge geysers. Associated with these geysers are surface "hotspots," but until now, there has been some ambiguity as to whether the hotspots are creating the geysers or whether the geysers are creating the hotspots.
In two new publications published in the Astrophysical Journal today (July 29), this Saturnian "chicken or the egg" conundrum has now been solved.
Enceladus' tiger stripes were first spotted in 2005 during a Cassini flyby. During each consecutive flyby, astronomers have been carefully studying these 80-mile-long, 1.5-mile-wide depressions in the ice. Associated with each linear stripe is a heat signature, from which the geysers are actively ejecting vapor into space.
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One theory suggests that the heat signature is caused by large fissures along the stripes rubbing against one another through tidal interactions with Saturn. This frictional heating causes the surface ice to heat up and vent into space.
However, there was always a more compelling alternative: what if frictional heating isn't causing the geysers? Perhaps the geysers are actually carrying the heat from a sub-surface ocean.
Through the combination of survey data with high-resolution hotspot data from Cassini's head-sensing instruments, individual geysers coincided with small-scale hotspots, according to the researchers. The resolution of the observations is so fine that 101 individual geyser sources were counted. But the hotspots were only a few dozen feet long, corresponding to very small fractures. This proves that the features are too small to create enough frictional heating to produce geysers - the opposite is actually true. The geysers are transporting the heat from deep inside the moon, creating the heated fissures.
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"Once we had these results in hand, we knew right away heat was not causing the geysers, but vice versa," said Carolyn Porco, leader of the Cassini imaging team from the Space Science Institute in Boulder, Colo., and lead author of one of the research papers. "It also told us the geysers are not a near-surface phenomenon, but have much deeper roots."
So where are the geysers' roots? Like Jupiter's moon Europa, Enceladus appears to be hiding an ocean that's trapped deep inside a thick crust of ice.
A second paper published in the Astrophysical Journal also reports on observations of Enceladus' geysers, supporting the idea that a heated ocean is trapped inside the ice. During the moon's orbit around Saturn, the brightness of the water vapor plume created by the geysers appear to wax and wane.
According to the researchers, this brightness change corresponds to increased and decreased water vapor output through fissures in the ice - fissures that may be opened and closed through tidal flexing of the icy crust.
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So a detailed picture is emerging:
A subsurface ocean supplies liquid water through narrow pathways in the ice.
Eventually this water is pushed through the surface, rapidly venting as a vapor.
The supply of water to the surface may be modulated by tidal flexing during Enceladus' orbit around Saturn.
Enceladus' subsurface ocean is a tantalizing place for astrobiology studies. Now we know a large body of water exists, protected from the harsh space environment by a thick icy shell, a habitable environment may have formed. What's more, the moon is venting water vapor from this ocean that can be directly sampled by spacecraft flying through it, just as Cassini has done in the past.
So, like Europa, Enceladus has firmly staked its claim as one of the premier places for us to be looking for life beyond Earth.