Starlight analysis gives new insight into Enceladus gas plume
Viewing the eruptions from Saturn's moon against the backdrop of a star has shown how they change during its orbit. But what's happening beneath the surface remains mysterious. Bill Condie reports.
Scientists have been forced to rethink their modelling on how the gas jets on Saturn's moon Enceladus work after surprising new observations by NASA's Cassini spacecraft.
Cassini recently used the light of a distant star to observe the cryovolcanic eruption on the moon and found it did not behave as anticipated.
It now appears that at least some of the narrow jets are more intense the further the moon is away from Saturn in its orbit, although NASA scientists are not sure what is happening beneath Enceladus' frozen surface.
The region around the moon's south pole is marked by numerous jets that spew gas and dust-sized ice grains hundreds of kilometres into space, in a single plume. Scientists believe this originates in the extensive saltwater sea beneath Enceladus' surface, which could harbour life.
More than 90% of the material in the plume is water vapour, an observation confirmed last year when Cassini passed through the plume to take samples.
At the moon's furthest point in its elliptical orbit round Saturn, the jets spew out three times as much icy dust into space as when closer in. But scientists have not been able to measure the gas output until now.
During a carefully timed operation, Cassini fixed on to the star Epsilon Orionis - the central star in Orion's belt - just as the plume passed between the spacecraft and the star. Cassini's ultraviolet imaging spectrometer (or UVIS) could then measure how water vapour in the plume dimmed the star's ultraviolet light, and so determine how much gas the plume contained.
Scientists had predicted it would increase by a similar amount to the extra dust ejection, but that did not happen. There was only a 20% increase in total gas output.
Looking for an explanation, Cassini scientists focused on one vent known as Baghdad I and saw its overall contribution rise from 2% to 8% of the total jet.
That gave a clue to what was happening, says Larry Esposito, UVIS team lead at the University of Colorado.
"We had thought the amount of water vapor in the overall plume, across the whole south polar area, was being strongly affected by tidal forces from Saturn. Instead we find that the small-scale jets are what's changing," he said.
This increase in the jets' activity is what causes more icy dust grains to be lofted into space, where Cassini's cameras can see them, Esposito said.
That in tuen suggests what may be going on beneath the surface as water moves through cracks and fissues to the surface, but that will take further analysis.
"Since we can only see what's going on above the surface, at the end of the day, it's up to the modelers to take this data and figure out what's going on underground," said Cassini scientist Candy Hansen.