On May 8th, 2016 we learned about

The ice jets of Enceladus increase according to the moon’s orbit

Saturn’s moon Enceladus has a few leaks. The icy outer shell of the moon covers a layer of liquid water, which the Cassini spacecraft has been able to observe because that water also sprays out into space through fissures in the surface. These plumes, many of which are clusters along the moon’s south pole, are composed of water, water vapor and dust. They reach hundreds of miles into space, making for some beautiful imagery when Enceladus is properly back-lit. With continued observation, NASA scientists noticed that the blasts aren’t always the equal- what’s more, they seemed to wax and wane according to how close Enceladus was to Saturn in its orbit.

The initial assumption was that this variation was tied to the composition of the plumes themselves. If a blast had more water vapor in it than normal, that gas would propel more dust than normal when it erupted. From the right angle, that extra dust catches sunlight and is visible as a brighter, more pronounced plume. Any gas in an eruption isn’t more visible in the visible spectrum, so NASA had Cassini watch the plumes with an ultraviolet imaging spectrometer, which would be able to detect the increased amounts of vapor mixed in with the dust. It would, that is, if the extra vapor had been there.

Changes in the surface, not the substance

The plumes, while clearly showing more activity at certain points in the moon’s orbit, didn’t seem to have any changes in their composition. Looking closely at particular jets, scientists realized that this wasn’t a question of quality, but of quantity. Small-scale jets were more active than usual, and the spike in their activity lead to the more dramatic plumes. The leading hypothesis at this point is that these small jets are growing because the fissures they burst from grow when the moon is further from Saturn in its orbit. With less gravity from the planet squeezing Enceladus, these openings may be opening up a bit more, allowing more water vapor to vent. They close again when the moon moves closer to Saturn, where the gravitational influence compresses the icy surface. More analysis is needed at this point, but it may turn out that the surface isn’t as frozen in place as first assumed.

My first grader asked: Do the plumes maybe just get bigger when the moon is in the far part of its orbit because Saturn’s gravity is compressing them less too? Maybe they can just spread out more and look bigger?

This makes sense, and probably happens to an extent, but NASA wasn’t only looking at the size of the plumes. The more critical measurement was the brightness of the plumes, which would speak to the concentration of dust being kicked up. If less material was just being dispersed further, it would end up being dimmer, not brighter, as observed.

Source: Enceladus Jets: Surprises in Starlight by Preston Dyches, NASA News

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