|Illustration of icy Europan plumes |
The ultraviolet observations by HST are reported in Science (Roth L., J. Saur, K. D. Retherford, D. F. Strobel, P. D. Feldman, M. A. McGrath, F. Nimmo, "Transient Water Vapor at Europa's South Pole," Science, 12 Dec 2013), and suggest water vapour plumes being dissociated by electron bombardment into their constituent atoms, revealing themselves to Hubble as ultraviolet hydrogen emission (Lyman alpha at 121.6 nm) and oxygen emission (130.4 nm and 135.6 nm). The excess emission rises 200 km from Europa's south pole, reminiscent of the icy geysers of Enceladus in the Saturn system, with incredible implications for our ability to probe the potentially-habitable conditions on this small satellite. But it's important to note that we've only seen this once, in December 2012, when Europa was at apocentre (its furthest point from Jupiter), so it'll be extremely important to follow this up with future observations. Thankfully, the team is led by researchers at the Southwest Research Institute (SwRI) in San Antonio, Texas, who happen to be the brains behind the Ultraviolet Spectrograph (UVS, on which I'm very lucky to be a science co-investigator) on ESA's Jupiter Icy Moons Explorer (JUICE). The results are being presented at the AGU meeting in San Francisco on Thursday (Roth et al.) and Friday (Retherford et al.), and were subject of a press conference earlier today.
|Artist impression of the Europa south polar plume. |
Credit: NASA, ESA, and L. Roth (Southwest Research
Institute and University of Cologne, Germany)
Lorenz Roth; Joachim Saur; Kurt D. Retherford; Darrell F. Strobel; Paul D. Feldman; Melissa A. McGrath; Francis Nimmo
P53A-1838. Discovery of Europa's Water Vapor Plumes: Europa's Atmosphere and Aurora: Recent Advances from HST-STIS and Plans for Plume Searches with JUICE-UVS
Kurt D. Retherford; Randy Gladstone; Lorenz Roth; Melissa A. McGrath; Joachim Saur; Paul D. Feldman; Andrew J. Steffl; Darrell F. Strobel; Thomas K. Greathouse; John R. Spencer; Fran Bagenal; Leigh N. Fletcher; John S. Eterno
Europa's Sub-Surface Ocean
But first back to enigmatic Europa, the smallest and smoothest of the four Galilean satellites (Io, Europa, Ganymede and Callisto) at 1940 miles across, so roughly a quarter of the size of the Earth. What makes Europa so intriguing is the suggestion of a global sub-surface ocean, beneath an icy crust somewhere between 10-100 km thick depending on the model you consider. The ocean is kept as a liquid by the energy released by powerful tidal forces raised by Europa's 3.5-day orbit around Jupiter. What's more, that ocean is thought to be in direct contact with the rocky silicate mantle, and with the surface ices, meaning that all the necessary ingredients for habitability (a source of energy, water as a chemical solvent, as well as a source of elements and minerals) come together in this fascinating environment. Europa's icy surface is fractured, cracked and in some places 'geologically-young', meaning that there are few craters because of the resurfacing processes at work. As Europa is tidally-locked, with one side continually facing Jupiter, it exhibits stark differences between the leading (forward-orbit-facing) and trailing hemispheres, with the latter being bombarded by the materials being swept around by Jupiter's powerful magnetic field. And the Galileo spacecraft discovered a weak 'induced' magnetic field, caused by the interaction of Jupiter's magnetosphere with a highly-conductive layer beneath the crust, most likely the liquid ocean. For all these reasons and more, Europa has long been the top destination for a future mission to the outer solar system.
|Dark striations across Europa's cracked surface |
(NASA/JPL/University of Arizona/University of Colorado)
The second result came from theoretical modelling of Krista Soderlund and colleagues in early December ("Ocean-driven heating of Europa’s icy shell at low latitudes") in Nature Geoscience. These authors used ocean dynamics simulations to try to understand the chaotic terrain that covers approximately 40% of Europa's surface and is more common at the equator than at the poles. The jumbled, criss-cross patterns could be caused by thinner regions melting and refreezing, or by solid-state convection within the ice shell. The new models suggest that turbulent convective motions within the global ocean serve to focus Europa's internal heat at lower latitudes, making the ice thinner there. The oceanic model suggests three zonal jets and two Hadley-like circulation cells. Once again, the properties of the sub-surface ocean can be inferred by 'reading' the surface features, and I particularly like how this paper bridged the gap between oceanic circulation models and the features of the ice shell.
|Artist impression of Europa's plumes|
(Image: NASA/ESA/K. Retherford, SwRI)
|Ice rafts in Conemara Chaos, a region targeted by |
JUICE in 2031 (Credit: NASA/JPL/Univ. Arizona)
A Tantalising Prospect for JUICE
The plume discovery makes the UV observations from ESA's Jupiter Icy Moons Explorer (JUICE) even more tantalising. The JUICE mission is currently in the planning and definition phase, but it is envisaged that it will make two close flybys over Europa's chaos terrains in February 2031, reaching within 1000 km of the surface. These chaos terrains will be targeted as the potentially-active and thinner crust offers our best opportunities to map the ice-ocean interface. JUICE will be using radar to sound through the ice, laser altimetry to map the topography, magnetic field measurements to measure the ocean conduction, and a range of remote sensing to understand the composition, chemistry and physical properties of the icy surface. The UVS observations envisaged by the SwRI team responsible for the plume discovery now take on a great deal more importance: UVS will conduct detailed plume searches via stellar occultations and far-UV imaging scans of auroral emission (driven by interactions of Europa's plasma with the magnetosphere). Limb imaging will be performed within the several hours of the closest approach to Europa (less than 1000 km above the icy surface), supplemented by stellar occultations at relatively large distances from the moon. A movie of the proposed flyby is shown below, and although it's still 18 years away, these data will be worth the wait! [PS. That also means that the graduates who'll be working on these data are probably in nursery today...].
Video courtesy of C. Arridge, UCL, created for the Royal Society Ice Worlds Exhibit 2013
Flow of an alien ocean, Jason Goodman, Nature Geoscience (2013)
Ocean-driven heating of Europa’s icy shell at low latitudes, K. M. Soderlund et al., Nature Geoscience (2013)
Tilting at Europa, Emily Lakdawalla, Nature Geoscience 6, 899 (2013)
Jupiter's Icy Moon: Window Into Europa's Ocean Lies Right at the Surface, Science Daily
Salts and Radiation Products on the Surface of Europa, Brown and Hand, 2013.
Europa’s Underground Ocean Surfaces, Phil Plait (Bad Astronomy)
Jupiter Icy Moons Explorer (JUICE): An ESA Mission to Orbit Ganymede and to Characterise the Jupiter System, Grasset et al., Planetary and Space Science
Transient Water Vapor at Europa's South Pole, Science, (2013)