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Leigh Fletcher

The Plumes of Europa

Posted by Leigh Fletcher

12-12-2013 12:01 CST

Topics: Jupiter's moons, Hubble Space Telescope, Europa, explaining science

Editor's note: This post originally appeared on Leigh Fletcher's personal blog and was reposted here with his permission. --ESL


2013 has been a rather exciting year for Europa scientists, even without a spacecraft anywhere near the jovian system. We've seen evidence of the salty composition of Europa's oceans; models explaining the deep ocean flow and influence on surface features; evidence of surface phyllosilicates from a cometary or asteroidal impact, and today's exciting news: the Hubble Space Telescope discovery of water vapor plumes from the south pole of this icy moon.

Artist's concept of a water vapor plume from Europa

NASA / ESA / K. Retherford / SWRI

Artist's concept of a water vapor plume from Europa

Hubble's Plumes

The ultraviolet observations by the Hubble Space Telescope 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 vapor plumes being dissociated by electron bombardment into their constituent atoms. The atoms reveal themselves to Hubble through ultraviolet hydrogen emission (Lyman alpha, at a wavelength of 121.6 nanometers) and oxygen emission (130.4 and 135.6 nanometers). The excess emission rises 200 kilometers 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. [Enceladus' plumes are taller, about 500 kilometers; Enceladus' surface gravity is significantly less than Europa's. They also reported that an estimated 7000 kilograms per second of material are coming out of these plumes, compared with only 200 kilograms per second for Enceladus. --ESL]

But it's important to note that we've only seen this once, in December 2012, when Europa was at apocenter (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.

Europa's variable plumes

Roth et al. 2013

Europa's variable plumes
The Hubble Space Telescope has spotted a plume of water 200 kilometers above Europa's south polar region, but only at one position along its orbit, when the moon was at apoapsis. The bottom diagrams show a model for stresses across cracks in Europa's southern ice shell at different positions in its orbit. As the moon goes in its orbit around Jupiter, it gets squeezed pole-to-pole when close to Jupiter and returns to a more spherical shape when farther from Jupiter. Cracks near the moon's south pole are squeezed together when Europa is close to Jupiter and pulled apart when Europa is far from Jupiter.

P42A-01. HST Observations of Europa's UV Aurora Morphology
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 3128 kilometers 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 and 100 kilometers thick, depending on the model you consider. The ocean is kept 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 some sort of 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 more 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.

Europa in color: trailing hemisphere

NASA / JPL / Ted Stryk

Europa in color: trailing hemisphere
Galileo captured this global view of Europa on its 10th orbit of Jupiter, on September 19, 1997.

But how might we probe this potentially-habitable sub-surface ocean? Cryobots that melt their way down to the dark oceans remain in the realms of science fiction for now, so we're left with whatever observations can be done remotely. 2013 has seen plenty of new evidence come to light that we can probe the interior by careful observation of the surface and external environment. The first was a paper in the Astronomical Journal in April by Mike Brown and Kevin Hand that used the Keck Observatory to detect magnesium sulphates (potentially epsomite) on Europa's trailing side. They hypothesised that salty ocean brines containing sodium, potassium and magnesium chlorides are somehow delivered to the icy surface (by some geologically active process), where they are bombarded from behind by sulfur being emitted by its neighbour Io to form sulfates (the sulfur being whacked into the trailing hemisphere of Europa by the rotation of the magnetosphere). Most of the sodium and potassium are sputtered (i.e., knocked off the surface) to create a thin atmosphere (e.g., Brown 2001), leaving behind the magnesium sulfates as the product of radiolysis occurring on the ocean brines. So that means we can get a good idea of what the interior ocean is like by looking at the chemistry of the surface materials.

Europa from the Keck telescope and the Galileo spacecraft

Keck image: Mike Brown; Galileo image: NASA / JPL / Ted Stryk

Europa from the Keck telescope and the Galileo spacecraft
The brighter parts of the Keck image shows where there is less water ice. The Galileo image is of a slightly different face than the Keck image; the non-water ice spot that shows up so clearly in the Keck image is the paleish reddish yellowish terrain in the bottom left quadrant of the Galileo image. The composition of that material has been a mystery since the time of Voyager.

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 serves 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.

But both of these results rely on indirect remote observations - either 'reading' the geology, or conducting infrared spectroscopy to measure the composition. They still don't allow us to directly probe that deep ocean. Until now. The Europa plumes revealed by Lorenz Roth's paper ("Transient Water Vapor at Europa's South Pole") in Science offer the tantalising prospect of directly sampling the chemical composition of material spewed out of the global ocean by some future mass spectrometer. Now, we don't know for sure that these plumes have a water source in the sub-surface ocean, and Hubble only has one plume detection so far, in December 2012. The plumes appear highly variable, and were not spotted when Europa was closer to pericenter in November 2012 (i.e., at the closest point to Jupiter), which suggests that changing stresses along the cracks can open fissures when the moon is 'unsquished' at apocentre, as it was in December 2012 for the full seven hours of Roth's Hubble observations. This variability mirrors the processes governing Enceladus' plumes. Galileo didn't really cover the poles during its 11 passes of Europa, so it doesn't really help us here. But it looks like we have two great examples of icy moon plumes in our solar system (Europa and Enceladus), but we're basing all this on one observation, and we really must go back for more!

A Tantalising Prospect for JUICE

The plume discovery makes the ultraviolet observations from ESA's Jupiter Icy Moons Explorer (JUICE) even more tantalizing. 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 kilometers 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 kilometers 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

Flyby of Jupiter's moon Europa by the JUICE spacecraft in 2031
Movie showing the European Space Agency's JUICE (Jupiter Icy Moon Explorer) spacecraft flying past Jupiter's icy moon Europa in 2013. The design of the spacecraft has not been finalized so this is just a mock-up featuring some of the parts of the spacecraft.
Beneath the icy crust, these global oceans will be perpetual darkness. Life would presumably be as simple as it comes, but that doesn't matter - if we can one day reveal that life developed somewhere other than Earth, no matter its complexity, or whether it's on Mars, Europa or Titan, it'll be the most profound discovery we'll ever make.

More Reading:

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)
 
See other posts from December 2013

 

Or read more blog entries about: Jupiter's moons, Hubble Space Telescope, Europa, explaining science

Comments:

Enzo: 12/12/2013 03:46 CST

It would seem counter-intuitive that the ice is thicker at the poles where the plumes are. Anyway, this makes a sample return mission much much easier with a probe that flies through the plume and captures particles with aereogel or something similar and returns to Earth. Something similar was proposed for Enceladus, but Europa is a lot closer.

Tim Reyes: 12/12/2013 04:45 CST

@Enzo. It may not be that the ice is thicker at the poles, certainly not for the reasons it is on Mars or the Earth. Tidal forces acting on Europa might make the low latitude crust thicker. Even if the ice is all the same thickness, pole to equator, the reason for geysers at the poles of Europa and Enceladus seems to be due to changes in transverse (tidal) forces opening and closing fissures. Fletcher and others do not jump to the conclusion that polar geysers imply that the Europa crust is likely thinner than presently estimated. A 2012 SETI talk by Dr. Britney Schmidt presented a model for chaos terrains that includes water at shallow depths (~3 km) but it will take more modeling and probably more observational data to infer that these apparent Europan water geysers are indicative of a thinner crust closer to 10 km than 100 km. But it would seem so, doesn't it.

Messy: 12/12/2013 05:06 CST

This is NOT a new discovery. There were reports of this sort of thing during the Voyager flyby back in 1979. But it seems that the photos weren't that great, and unlike Io they weren't spectacular, and of course Galileo was broken.... Someone who has access to the literature should go back to the old magazines from 1979 and '80 and find the articles.

Anonymous: 12/12/2013 06:48 CST

@ Tim Reyes I was not implying that the ice would be thicker at the poles for the same reason it is here on Earth but because the ocean circulation model "The new models suggest that turbulent convective motions within the global ocean serves to focus Europa's internal heat at lower latitudes, making the ice thinner there." I should have been clearer. @Messy, Any sources for this ? I never heard this for Europa. For Enceladus, it was suspected long before Cassini. There was even a drawing of Cassini passing by Enceladus plumes before 2004 on its web page.

Paul Fieseler: 12/12/2013 08:05 CST

Oh my. I think that I may have seen a plume in Galileo data from one of the later Europa flybys, and I didn't realize what I was seeing until now....

Paul McCarthy: 12/13/2013 01:28 CST

I agree with Enzo. The plumes should be confirmed, and then Science (ie, ALL of it) should massively and unitedly push for direct development of a sample-return mission in the SHORTEST possible time frame. The existence of these features would provide an amazingly serendipitous short cut to the biggest question there is, potentially "the most profound discovery we'll ever make" (Leigh's words), and what we all want to know IN OUR OWN LIFETIMES. (And for the celestial body long considered most promising). It will make little sense to go through several 30-year mission development and execution cycles simply aimed at gradually accumulating ever-finer geochemical and geophysical data to buttress the scenario! Confirm, then Go,Go,Go!

Max80: 12/13/2013 04:19 CST

I just watched "Europa Report", so this is really cool news!

Olaf: 12/13/2013 12:10 CST

What puzzles me, if Europa is venting material like Enceladus, could there be a structure similar to Saturn's E-Ring at Jupiter? And, if yes, should that be visible in Galileo or other imagery?

Emily Lakdawalla: 12/13/2013 12:34 CST

No, this was not observed in either Galileo or Voyager images.

Torbj??rn Larsson: 12/14/2013 04:10 CST

Personally, this changes everything. What Enceladus and perhaps Titan (since its methane should have replenishment) had going for themselves were easy access to pristine subsurface material. Europa isn't only closer, it is on the assured side of the solar panel/RTG divide. "Life would presumably be as simple as it comes, but that doesn't matter". It doesn't matter, and it is unlikely anything more complex would evolve without access to concentrated energy flows. But to show what is possible in biomes without oxygen involved anywhere in the metabolism Loricifera is our current best example. Living in anoxic sulfuric mud with sulfur metabolizing bacteria, they have evolved hydrogenosomes from our stem mitochondria. Despite that Loricifera are sexual, multicellular, movable, enervated with a central nervous system, having a larvae stage, developing a (pseudo-)coelome and some grow to 1 mm in length. [ http://en.wikipedia.org/wiki/Loricifera ] Unfortunately the oxygenation of the atmosphere seems to have lost us most earlier lineages. And while archaea and bacteria split before that, their ecological specializations - low energy specialists vs medium energy generalists, with later mitochondriated eukarya as high energy specialists - likely derives after ubiquitous oxygen enabled them. E.g. cyanobacteria multicellularity evolved after. We don't really know the complexity their ancestors were capable of.

Gregory Katz: 12/18/2013 08:36 CST

Anybody have any information as to the level of certainty on the plume detection? Has anyone knowledgeable expressed an opposing view or pointed to weaknesses in the detection?

Emily Lakdawalla: 12/19/2013 10:36 CST

I asked that at the press briefing. This is the best and only explanation, they said. Many other scientists have told me that they have no specific reason to doubt this work but that they will feel substantially better about it if it is detected a second time.

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