Starlight observation of Enceladus' plumes uncovers fresh mystery

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Spectactural cryovolcanic plumes leap from Enceladus' southern polar region in this image captured by Cassini in 2009(Credit: NASA/JPL/Space Science Institute)

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NASA's Cassini spacecraft has used the light of a faraway star to observe the vast cryovolcanic eruption taking place on Saturn's moon Enceladus. Captured at a distant point in its orbit relative to the gas giant, Enceladus' tiger stripe vents behaved in a way that will force astronomers to rethink their current models regarding the enigmatic moon's internal plumbing.

The plumes of Enceladus are a priority observational target for the Cassini spacecraft, as they are believed to be powered by the tidal forces of Saturn as they interact with the moon's subsurface ocean. This ocean is thought to be one of the most likely dwelling places for life beyond planet Earth.

In October 2015, Cassini was able to dive through the jets of Enceladus during one of its final close encounters with the moon, capturing a number of particles with its Cosmic Dust Analyzer (CDA) instrument in the process. Occasions such as this, alongside observations carried out by the probe's other instruments, have allowed scientists to determine the composition of the material thrown out in the plumes.

A mosaic created from data captured by Cassini'simaging science subsystem in 2006 displaying how the plumes on Enceladus become brighter the further out the moon is from Saturn(Credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI)

Around 90 percent of the particles expelled by the geysers are made up of water vapor. This vapor picks up and carries out icy dust particles that reflect passing starlight, and become visible to the probe's cameras.

Cassini's latest observations were not the product of a chance alignment. Prior to the Mar. 11 sighting, the spacecraft had fixed its gaze on the bright central star of the Orion belt known as Epsilon Orionis. As the moon passed in front of the distant star, Cassini's ultraviolet imaging spectrometer (UVIS) was able to read the spectra of light from the star and deduce the composition of Enceladus' jets.

A previous observation of Enceladus taken while the moon was in a distant stage of its orbit relative to Saturn had noted a threefold increase in the number of dust particles expelled by the tiger stripes. Astronomers reasoned that, upon further observation of this orbital stage, there would be a commensurate increase in the levels of water vapor.

The results of the UVIS analysis failed to support the theory, instead displaying only a 20 percent increase in the volume of water vapor in the plumes. Data from the instrument also displayed an alteration in the amount of material contributed to the jet from the individual vents. One vent in particular known as Baghdad I saw its overall contribution rise from 2 percent to 8 percent of the total jet.

Mosaic of Enceladus "tiger stripe" vents as captured by Cassini in 2009(Credit: NASA/JPL/Space Science Institute)

It had initially been theorized that the tidal forces were causing a unified variation in activity over the entirety of the tiger stripes. Instead it appears that some vents, such as Baghdad I, are contributing far more than others, and that it is this variation is somehow responsible for the inconsistency in the dust particle/water vapor ratio.

Computer modelling by theorists will be required to explain Enceladus' latest mystery. Whilst it may seem like a step back in terms of our understanding of the enigmatic moon, it is actually quite the opposite. Any new phenomena discovered in relation to the jets provides a fresh set of constraints that must be taken into account when a new model explaining the cryovolcanism is proposed, and could even be used to question existing theories.

The true nature of Enceladus' subsurface ocean will in all likelihood not be known until we can directly probe its expanse. The concept for such a mission is being given consideration under phase 1 of NASA's NIAC Program.

Known as the icy-moon cryovolcanic explorer (ICE), the concept mission would see a lander set down on the surface of Enceladus and deploy a rover onto its surface. The rover would then abseil down one of the tiger stripe vents and disgorge a series of autonomous underwater vehicles to explore the subsurface ocean.

Source: NASA

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