Starlight observation of Enceladus' plumes uncovers fresh mystery
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.
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 theMar. 11 sighting, the spacecraft had fixed its gaze on the bright central star of the Orion belt known as Epsilon Orion is. 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 20percent 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.
It had initially beentheorized that the tidal forces were causing a unified variation inactivity over the entirety of the tiger stripes. Instead it appearsthat some vents, such as Baghdad I, are contributing far more thanothers, and that it is this variation is somehow responsible for theinconsistency in the dust particle/water vapor ratio.
Computer modelling bytheorists will be required to explain Enceladus' latest mystery.Whilst it may seem like a step back in terms of our understanding ofthe enigmatic moon, it is actually quite the opposite. Any newphenomena discovered in relation to the jets provides a fresh set ofconstraints that must be taken into account when a new modelexplaining the cryovolcanism is proposed, and could even be used toquestion existing theories.
The true nature ofEnceladus' subsurface ocean will in all likelihood not be known untilwe can directly probe its expanse. The concept for such a mission isbeing given consideration under phase 1 of NASA's NIAC Program.
Known as the icy-mooncryovolcanic explorer (ICE), the concept mission would see a landerset down on the surface of Enceladus and deploy a rover onto itssurface. The rover would then abseil down one of the tiger stripevents and disgorge a series of autonomous underwater vehicles toexplore the subsurface ocean.