Weathered organic material once dissolved in Enceladus’ subsurface ocean is coating Saturn’s inner moons, explaining surprising variations in their surface colour and UV absorption. This is according to a new analysis that could provide a way to investigate the makeup of our solar system’s most likely watery home to alien life in the post-Cassini era.
This artist’s painting of the south polar region of Enceladus shows massive jets of water ice being blasted into space. Image credit: Karl Kofoed / NASA.
“Saturn’s Mimas, Enceladus, Tethys, Dione and Rhea have presented a puzzle to astronomers for many years,” says Amanda Hendrix, who has been working on Cassini since shortly after its launch, and is co-investigator on its Ultraviolet Imaging Spectrograph (UVIS) instrument team.
Though predominantly water ice worlds, as shown by Earth-based observations from the 1970s, Cassini revealed soon after its arrival these inner bodies were generally much darker spectrally and visually than expected, whilst also displaying surprising variation from moon to moon, and across each moon’s surface. They also absorbed in the ultraviolet-visible wavelength region, a characteristic not typical of water ice.
Tethys has a faint yellowish leading hemisphere (the one that constantly points in the direction of its orbit due to tidal locking with Saturn), whist Dione and Rhea’s display a coral-coloured frost.
The trailing hemispheres of Tethys, Dione and Rhea have a darker, rust-coloured reddish hue.
Enceladus and Mimas by contrast show little variation between trailing and leading hemisphere, with Enceladus instead marked out by blotches of bluish frost.
“Why Saturn’s satellites are coloured like they are is a big problem,” says Hendrix.
So what species is causing this spectral colouration and where does it come from?
Authors like Clark had suggested the colour variation could be attributed to space-weathered meteorite dust. Other suggestions included nanophase iron or salts as both of these get darker and redder with weathering, however their scarcity out in the E-ring (in which all these moons sit) didn’t convince Hendrix who was more interested in the role of organic material.
Earlier this year it was confirmed the jets of water ice and gas from Enceladus’ south pole that originate in the moon’s sub-surface ocean, and produce the E-ring, contained molecular hydrogen and compounds such as methanol.
Hendrix believed further proof of E-ring organics as a source of these ‘colourful moons’ might come from the staining patterns themselves, but thought limiting analysis to just the visible spectrum would unlikely tell the full story.
So she and colleagues from Cassini took data from three of their instruments VIMS, ISS and UVIS analysing the makeup of E-ring debris across the ultraviolet through infrared spectral range, along with Hubble Space Telescope’s analysis of the albedos of the inner icy Saturnian satellites, and combined it with models of E-ring grain deposition.
Their results, published in the journal Icarus, show an inverse correlation between UV absorption and E-ring grain flux.
The moons farthest away from Enceladus, which receive the lowest amount of e-ring grains, were absorbing the most UV. However rather than bad news for the theory of organic material moon colouration, Hendrix argues this is what you might expect when you take in account a third factor.
Saturn, like Earth, is surrounded by plasma, a charged gas-like state of matter created by the splitting of gaseous atoms from the planet itself, and leaky Enceladus, into electrons and ions.
“Lab tests showing many likely hydrocarbon species get redder and darker when weathered by electron bombardment, a process that would increase with exposure time in the E-ring,” says Hendrix.
Moons further away from Enceladus are on longer orbits so the organic component of e-ring grains impacting them will have had far more time to be chemically altered, explaining the darker colouration of Dione and Rhea compared to Enceladus’ next door neighbours Tethys and Mimas.
Also Saturn’s plasma sweeps around the planet faster than its moons’ orbits. When bumping into their backsides (i.e. trailing hemisphere) this would cause further weathering of surface materials via radiolysis, explaining the redness of trailing hemispheres in general across the whole system.
“Whilst silicates and salts also redden with weathering, because organics are present in the E-ring in significantly greater abundance, we suggest here their weathering dominates the colouring of the inner Saturnian moons.”
So what are these moon coating organic species and what can they tell us about the composition of the subsurface ocean within Enceladus from which they come?
“It’s really tricky,” concedes Hendrix. “The organic signature is not exactly distinct. All we can say is they are organics that in general act like this spectrally.”
To try and narrow down on the mystery material the team are playing a numbers game, making their spectral data public so anybody can compare it with lab experiments of different organic species and reinterpret their results however they see fit.
“Now that Cassini is gone, lab work will be a good way of addressing the chemical composition of these organics.”
_____
Amanda R. Hendrix et al. 2018. Icy Saturnian satellites: Disk-integrated UV-IR characteristics and links to exogenic processes. Icarus 300: 103-114; doi: 10.1016/j.icarus.2017.08.037
