Tuesday, 8 April 2014

Carbon on the Ice Giants

The bulk composition of a giant planet provides insights into the source reservoirs from which it formed, and a window onto the epoch of planetary formation.  For the gas giants Jupiter and Saturn, Galileo and Cassini observations are starting to pin down the abundances of key elements and isotopes (with lots of caveats, especially regarding oxygen).  But for the ice giants Uranus and Neptune, the story is murkier and incomplete.  Until recently, the data seemed to suggest that both the bulk abundances of deuterium and carbon were increasing with distance from the Sun, such that Uranus and Neptune were distinct in terms of their compositional make up.  But more recent observations, from sources like Hubble and Herschel, have shown that this is untrue, and that distinguishing between Uranus and Neptune on the basis of composition is rather more challenging!

On Uranus, the best estimate of the methane mole fraction has been bouncing around a lot over the past few years.  Lindal et al. (1987) measured a 2.3% mole fraction from Voyager radio occultations.  But this was just one of a suite of possible solutions, up to a maximum of 4%.  Baines et al. (1995) inferred a much smaller deep value of 1.6{-0.5,+0.7}% using ground-based spectroscopic observations in the near infrared.  In 2011, Larry Sromovsky combined the Lindal radio refractivity profiles with cloud fitting to HST/STIS data, and found good matches to the data with 3.2-4.5%. Their ‘best compromise’ was 4.0±0.5% at low latitudes on Uranus.

But the problem is that methane appears to be depleted at high latitudes (Karkoschka and Tomasko, 2009; Sromovsky et al., 2011) due to atmospheric subsidence, so the equatorial values have to be taken as indicative of the bulk abundance but could be being redistrubuted by the circulation of Uranus' troposphere.  So there’s a lot of uncertainty out there.  The Baines et al. (1995) value of 1.6% is on the low side, whereas Sromovsky and colleagues seem to favour higher values of around 4% tropospheric methane.

And what about Neptune?  If we look at the study by Karkoschka and Tomasko (2011, Icarus 211 p780-797) using Hubble STIS data, they suggest 4±1% for methane mixing ratio for depths below 3.3 bar, with meridional variations by a factor of three at shallower depths.    So it seems hard to distinguish between Uranus and Neptune in terms of their bulk composition, specifically the abundance of carbon.  Although the values are highly uncertain, both worlds appear to be enriched over protosolar values by a factor of around 90 (Guillot and Gautier, 2014, Treatise on Geophysics).   The same is also true for HD/H2, which has been shown by Herschel (and others) to be the same on both ice giants, indicating that they both formed from similar icy source reservoirs in the distant past.

Tuesday, 11 February 2014

Oxford Researchers on BBC Sky at Night

From an Oxford new release:  AOPP researchers feature strongly as contributors to the first edition of the new series (season two!) of the BBC's Sky at Night programme on BBC4 presented by Dr. Maggie Aderin-Pocock and Oxford Astrophysics's Chris Lintott.

Prof. Peter Read demonstrated a laboratory experiment with guest presenter Helen Czerski that may help to explain the origin of Jupiter's Great Red Spot and other giant, long-lived storms that dominate the weather on Jupiter. In contrast to the Earth, where large-scale temperature contrasts are primarily between the equator and poles, on Jupiter the contrasts are strongest between the bright and dark bands.  The temperature changes considerably from the warm dark belts to the colder, brighter zones.  Peter's experiments show that the main instability of such a flow leads to the formation of compact, recirculating vortices - which look very similar to those seen on Jupiter.

The formation of Jupiter's zonal bands may also result from the effects of its very rapid rotation (once every ten hours) and the curvature of the planet. This was graphically illustrated in Helen Czerski's presentation of some numerical model simulations by AOPP graduate student Yixiong Wang, which show that an Earth-like planetary atmosphere would spontaneously break up into multiple zonal bands if, like Jupiter, the planet were much bigger than Earth and/or it rotated much more rapidly.

The show also ventured out into the Jupiter system to explore some of the processes at work on its diverse collection of moons.  Dr. Leigh Fletcher explained the significance of recent discoveries about the moon Europa, in conversation with Chris Lintott.  Europa has long been regarded as a potentially-habitable environment (i.e., it may have all the ingredients necessary to support life), and as a tantalising destination for future exploration.  Chaotic terrain at Europa's low latitudes has the appearance of ice bergs locked into a frozen sea, allowing us to understand the properties of the global, sub-surface ocean by reading the geology and chemistry on Europa's icy surface.
Artist impression of ultraviolet emission from plumes of
water emanating from Europa's south polar region.
Credit:  NASA, ESA, and M. Kornmesser.

However, new findings from the Hubble Space Telescope suggest another way of probing this deep, hidden ocean - geysers of water appear to be venting 200 km high over Europa's south pole, similar to those found previously on Saturn's icy moon Enceladus.  Leigh described the implications of this discovery for the exploration of the jovian system by ESA's Jupiter Icy Moons Explorer (JUICE), expected to launch in the 2020s.  In particular, Oxford is a co-investigator on the US-led ultraviolet spectrograph (UVS) which will be conducting sensitive searches for this type of spectacular geologic  activity on Jupiter's moons.

The programme is available for another week on BBC i-Player (see http://www.bbc.co.uk/programmes/b03vg99x)

Further details of the Europa plume discovery and JUICE mission can be found here:

…and a discussion of what gives Jupiter its colourful stripes:

Monday, 27 January 2014

Jupiter's Realm: An Introduction to the Galilean Satellites

A comparison of the surface features on the Galilean satellites 

from the NASA photojournal (Credit: NASA/JPL/DLR)
The Jupiter system consists of four large satellites (Io, Europa, Ganymede and Callisto, all over 3000 km in diameter); a ring system; four small satellites (Metis, Adrastea, Amalthea and Thebe, 10-100 km diameters) interior to Io's orbit; and a group of at least 55 irregular satellites out beyond Callisto.  Europa is the smallest of the four moons at 3120 km across (just slightly smaller than our own Moon's 3774 km diameter); Ganymede is the largest at 5258 km across, making it the largest moon in the solar system.  We sometimes refer to these worlds as those of "fire and ice", due to the combination of volcanism and icy oceans that shapes their appearance.  The amount of ice in the moons increases with distance from Jupiter - Io is primarily silicate rock with a molten iron or iron sulphide core, whereas Ganymede and Callisto are composed or roughly equal amounts of ice and rock.

Tides and Geology
The internal structure, surface properties and geologic activity of these moons are shaped by their distance from Jupiter - from the materials available where they formed, to the strength of tidal forces warming their interiors and the strength of the radiation field sterilising their surfaces (a handicap for potential surface habitability).  Tidal interactions due to Io, Europa and Ganymede being trapped in the Laplace resonance are responsible for Io's spectacular volcanism and the presence of subsurface oceans on the other moons.  For example, Io's and Europa's orbits are not perfect circles, and the gravitational forces are stronger when the moon is at its closest to Jupiter.  That means that the degree of tidal flexing changes during the orbit, kneading the interior and providing a source of energy.  Geological activity appears to decrease as we move outward, arriving at the 'dead world' of Callisto that appears the least differentiated of all the satellites.  Callisto has the least severe radiation environment as it orbits further outside of Jupiter's radiation belts, and does not participate in the resonance of the other three Galilean satellites.

Hidden Oceans
Artist impression of a lake beneath Europan 

chaos terrain, above a global subsurface ocean.  

Credit:  University of Texas at Austin
The Galileo spacecraft discovered evidence for subsurface oceans hidden beneath the icy crusts of Europa, Ganymede and Callisto.  Estimates of the ice thickness range up to 100 km for Europa and thicker for the other satellites, depending on the degree of convective activity.   The salty oceans are electrically conducting, so Jupiter's magnetic field is able to induce secondary magnetic fields into these oceans that were then detectable by Galileo which, in addition to ocean-related surface characteristics and models of the thermal evolution of these moons, advocate for the presence of liquid water oceans.   These oceans are kept liquid by a combination of tidal energy dissipation (most important for Io and Europa) and radiogenic heating (most important for Ganymede).  Of the three, Europa is unique in that the ocean may be in direct contact with the rocky silicates of the interior, possibly providing salts and other essential elements to the ocean.  This 'sea floor' may be geologically active like the biologically-rich environment on Earth (e.g., the black smokers).   Conversely, the oceans of Ganymede and Callisto may be sandwiched between layers of thick ice, making it harder to exchange energy and chemicals between the rock and the ocean.  These moons are larger than Europa, and internal pressures may be sufficient to form high-pressure ice phases in the deep interior.

Materials from these internal oceans may be able to rise to the surfaces of Europa and Ganymede, where they are then altered by interaction with Jupiter's radiation and plasma environment, which is stronger at Europa than at Ganymede.  Ice fractures and cryo-magmatic processes (e.g., volcanism, only with ice as the magma) could transport volatiles, organics and minerals upwards to be detected on the satellite surfaces. There are substantial amounts of non-water ice material on the surfaces of all of these satellites, but their origin and relation to subsurface oceans is unclear.

Magnetic Fields
Ganymede is one of only three solid state bodies in the solar system to generate a magnetic field of their own, next to Earth and Mercury, within their liquid iron-rich cores.  This internal magnetic field creates a miniature Ganymede magnetosphere embedded in, and interacting with, the much larger magnetosphere of Jupiter.

Locked Orbits and Asymmetry
The Galilean moons are locked in a stable 1:1 spin orbit resonance, which means that one face is always pointed directly at Jupiter (i.e., it would always be directly overhead if one stood at the sub-jovian point). That also means that the length of the day is the same as the orbital period:  Io takes 1.8 days to orbit Jupiter, Europa takes 3.6 days, Ganymede takes 7.2 days and Callisto takes 16.7 days. On most of the satellites it is possible to distinguish differences in brightness between the leading (forward-facing) and trailing (back-facing) hemispheres, as magnetospheric plasma are continually blasting the trailing hemisphere due to the rapid rotation of the magnetosphere.

Tenuous Atmospheres
All of the moons possess exospheres (a thin atmosphere) and ionospheres, and Ganymede is known to exhibit auroral emissions due to the interaction of its magnetic field with the jovian magnetosphere.  Io has an aurora-like glow that is brightest near the equator (because Io lacks an intrinsic magnetic field).  The exospheres are the result of materials either sublimating or being sputtered (i.e., knocked off by particle bombardment) from the surface.  Europa's atmosphere is extremely thin and composed mostly of O2 released from the surface due to magnetospheric particle bombardment.  Some of the escapes hydrogen and oxygen forms a neutral cloud (a torus) around Jupiter, adding plasma into the magnetosphere just like Io.  Ganymede also possesses a thin oxygen exosphere, whereas Callisto's thin atmosphere may be made up of CO2.

The surface geology of the satellites is shaped by a variety of competing internal and external processes, including marine, convective, cryovolcanic, tectonic, surface degradation and impact phenomena.  We'll now look at each of the satellites in turn, giving an overview of some of the key features on these worlds.

Surface of Io
Map of Io assembled by the British Astronomical Society based on a basemap by the USGS.

  • Io has over 400 active volcanoes, driven from frictional heating as Io is tidally stressed by Jupiter and the other Galilean satellites.  Up to 20% of Io's mantle may be molten, with higher melt fractions near regions of high temperature volcanism.
  • Sulphurous umbrella-shaped plumes can reach hundreds of kilometres above the surface, coating the surrounding planes with colourful sulphur and SO2 frost, contributing to the thin SO2 atmosphere, and suppling material to the Io plasma torus surrounding Jupiter.  
  • Lava flows up to 500 km long cover the surface, between mountains, plateaus, layered terrains and shield volcanoes.  
  • Pele is one of Io's active volcanoes, associated with a lava lake and encircled by a large red sulphurous ring.  It has a patera 30km by 20km in size, and has produced plumes 300 km tall associated with peak temperatures of 1250 degrees celsius or more.
  • Categories of geological features include paterae (volcanic depressions resembling calderas), fluctus (lava flows), vallis (lava channels) and eruption centres.  The largest patera is Loki, with a diameter of 202 km.
  • Io has up to 150 mountains, some larger than Mount Everest.  The largest is South Boosaule Montes at 17.5 km tall.  These mountains are tectonic structures, due to compression at the base of the lithosphere, rather than being due to volcanism.  Some mountains show signs of large landslides on their flanks. Mountains dominate the areas with fewer volcanoes, and vice versa.
  • The first Io plume was spotted after the Voyager 1 encounter in 1979, one of nine plumes spotted in that flyby.  Four months later, when Voyager 2 flew past, 7 of the 9 plumes were still active and the morphology of the surface terrain had been altered, with the volcano Pele shutting down in between the two encounters.  Galileo measured more plumes and the thermal emission from the cooling surface magma.
  • Cassini spotted a plume at Tvashtar Paterae during the December 2000 flyby en route to the Saturn system.  New Horizons monitored further eruptions in 2007, including Girru Patera in the early stages of an eruption.
  • The young and active surface is almost completely lacking in impact craters.  

Surface of Europa

Annotated map of Europa, using the combination of Bjorn Jonsson's basemap, the projection available from the British Astronomical Association, and details from the USGS.

  • Europa's surface is one of the smoothest in the solar system, and is divided into bright plains with numerous dark parallel linear ridges (lineae), and a darker more mottled terrain known as regio.  
  • The most common types of ridges appear in pairs with a trough in between possibly originating from tectonism, cryovolcanism or other processes requiring liquid water in the subsurface or mobile ice.  
  • The bright planes are separated by these dark lineae, possibly related to crustal spreading as plates move on top of the more mobile sub-surface, rather like the crustal movement seen in Earth's ocean ridges.  
  • Chaos terrains (such as Conamara Chaos, just south of the 'X' made by two crossing Lineae, Asterius Linea and Agave Linea) are regions where the pre-existing planes are broken up, rotated and tilted, resulting in a 'hummocky' terrain akin to ice rafts floating on a frozen sea.  Research in 2011 suggested that these chaos terrains could be sat on lakes of liquid water embedded in the ice shell, and distinct from the deeper liquid ocean.   
  • Craters are rare on Europa, indicating a young surface age, but one of the most distinct is the young and bright Pwyll crater, 26 km in diameter with a 600 metre high central peak, surrounding by bright rays of debris.  
  • Europa's equatorial region may feature 10-m tall icy spikes (penitentes) resulting from melting.  
  • Numerous circular or elliptical domes, pits and spots can be found, known as lenticulae and potentially caused by warm ice moving upwards through the colder icy crust.  
  • Most recently, it appears that Europa has periodic plume eruptions from somewhere near its south pole (see my post on "The Plumes of Europa").  
  • Dark regions or spots (maculae), such as Thera and Thrace, could be relatively young enriched regions of non-water ices, possibly due to liquid emplacement.  
  • The dark regiones are named after locations in Celtic mythology, dark spots (maculae) from Greek mythology, and crater names come from Celtic myths and folklore.

Surface of Ganymede
Map of Ganymede assembled by the British Astronomical Society based on a basemap by Bjorn Jonsson.

  • Ganymede's surface is divided into dark and densely-cratered ancient terrain covering about a third of the surface, and bright, younger and grooved terrain showing evidence of tectonic disruption.  
  • Furrows seen in the ancient terrain (a hemisphere-scale set of concentric troughs) may be remnants of vast multi-ring impact basins.  
  • Galileo Regio is a significant dark plane containing a series of concentric furrows on the anti-jovian hemisphere.  It is separated from the dark Marius Regio by the bright young band of Uruk sulcus.
  • Although the surface is largely made of ice, the darker ice is due to non-water contaminants concentrated on the surface.  The darker terrain is divided by brighter regions called sulci.  The brighter, fresher terrain is likely to be the product of tectonic resurfacing over the ancient terrain.  
  • There are indications of caldera-like depressions (called paterae) that could be old volcanic vents, and suggestions of ridged deposits nearby that could have resulted from cryovolcanic flows.  
  • Ganymede's impact features are more diverse than on any other planetary surface, from vast multi-ring structures, to low relief ancient scars, craters with pits and domes, bright ray craters and dark floor craters.  Craters are seen on both the light and dark terrain, and some overlap the grooved systems indicating the extreme age of the grooves.
  • Osiris crater is a bright ray crater with fresh ice ejection to the south of Galileo Regio.
  • Palimpsests are ancient craters whose relief has largely disappeared.
  • Grooved terrain is brighter and more ice-rich than the darker terrain.  Salts, CO2, SO2 and other compounds have been inferred on the surface.  
  • Ganymede's surface is brighter on the leading hemisphere.
  • Ganymede features polar caps of water frost extending to 40 degree latitude, possibly related to plasma bombardment from the magnetic field (the polar regions are unprotected from such bombardment, enhancing sputtering there).

Surface of Callisto
Map of Callisto assembled by the British Astronomical Society based on a basemap by Bjorn Jonsson.

  • Callisto's surface is the most ancient of all, a heavily cratered world bearing witness to the full bombardment history of our solar system.  There are no large mountains, volcanoes, or tectonic structures.  Tectonism is less wide spread than on the other satellites, although some furrows and lineaments are seen.  
  • The surface is dominated by multi-ring structures, crater chains (catenae, named after rivers in Norse mythology) and associated scarps, ridges and deposits.
  • Small craters have bowl shapes, moderate craters have central peaks, some larger ones have central puts, whereas the largest (over 60 km diameter) have central domes due to tectonic uplift following an impact (e.g., Doh and Har craters).
  • Bright frost deposits are seen in elevated regions (crater rims, scarps and ridges), surrounding by a blanket of darker materials in the lowlands.  The surface is predominantly water ice, contaminated by various non-ice materials like silicates, CO2, SO2 and possibly ammonia.  The trailing hemisphere is darker and enriched in CO2 ice, whereas the leading hemisphere is brighter and has more SO2.  
  • Burr and Lofn are bright and relatively  fresh impact craters that shows an enhancement in CO2. 
  • Multi-ring basins are the largest features on Callisto's surface, resulting from concentric fracturing of the lithosphere lying on top of a softer layer (possibly an ocean).  Valhalla is a multi-ring impact structure 3800 km in diameter. Asgard is the second largest. These enormous features are named after the homes of the gods in Norse mythology.

Friday, 17 January 2014

What Gives Jupiter its Colourful Stripes?

Despite four decades of planetary explorers reaching across the solar system to Jupiter, we still don't have the answer to a very basic question - why is Jupiter's Great Red Spot.... red?  And that's not the only conundrum - Jupiter's stripes, storms, vortices and waves can take on a bewildering range of colourful hues, from dusky white to yellows and ruddy browns, and even hints of green and blue tints if you look at the right moments.  And yet the atmospheric chemistry responsible for the jovian rainbow has remained elusive.  This post was inspired by interactions with the BBC research team for some of their upcoming science shows.

What are the clouds made of?
The colourful clouds of Jupiter seen by Cassini
(Credit NASA/JPL/University of Arizona)

When you look at Jupiter through a telescope, you're seeing photons of light being reflected back from the cloud tops in Jupiter's atmosphere, at pressure levels not dissimilar to the atmospheric pressure at the Earth's surface.  These clouds reside in the atmospheric region known as the troposphere, the location of the swirling, churning weather activity we're all familiar with.  Unlike on Earth, where water condenses and/or freezes to form the white clouds in our atmosphere, theory suggests that the ice clouds on the giant planets should be layered.  Ammonia condenses to ammonia ice at the coldest temperatures, so form the uppermost cloud decks.   That means that the bulk of the gaseous ammonia is locked away in a reservoir beneath the clouds.  Above the clouds, there's only a little ammonia left to interact with UV light to form photochemical hazes.

Moving deeper into the planet, the ammonia can react with hydrogen sulphide to form a cloud of NH4SH, the next thick cloud deck.  And even deeper, water condenses to form a thick aqueous cloud layer which may be the source of much of the convective activity that we see.  It's important to note, however, that it's really hard to peer down through the topmost ammonia cloud, so all these deeper clouds are predictions based on chemistry.  We don't know, for example, exactly how much water is locked away within Jupiter, but that determines the height and extent of any water cloud that would be there [that's one of the key goals of NASA's Juno spacecraft, two years away from arrival at Jupiter].  So our understanding of what's going on beneath the clouds you can see through a telescope is really limited, and largely constrained by theory rather than direct observation.

The cloud decks of Jupiter.
Unfortunately, the picture is even murkier than that. Ammonia ice has a well-understood spectral fingerprint, with features that we could potentially identify in spectra of Jupiter's reflected light.  Try as we might, we only ever see these signatures in small regions of the planet.  Working with Galileo spacecraft observations in the near-infrared (NIMS), ammonia clouds were only detected in very localised regions associated with powerful convective dynamics covering less than 1% of the planet, almost like the ammonia ice could only be detected when it was fresh (i.e., newly transported up to the cloud decks from below).  The most prominent signature was found in turbulent wake to the northwest of the Great Red Spot.  Generally speaking, the Galileo observations (in tandem with ground-based studies) showed that these pure, fresh ammonia ice signatures were absent across much of the planet.  In essence, although we suspect that we're seeing clouds of ammonia, we cannot confirm it using data from cutting-edge spacecraft.

In the past five years attention has focussed on anomalous spectral features near 3 microns, observed by both the Infrared Space Observatory (ISO) in the 1990s and the Cassini near-infrared instrument (VIMS) on its 2000/2001 flyby of Jupiter.  This signature seems to be present everywhere, from light zones to dark belts, but the actual cloud composition responsible for the signature is unclear - maybe it is a combination of the ammonia and NH4SH clouds, with one species coating a seed of the other one, masking the signatures we'd expect.  It certainly wasn't the signature of pure ammonia ice.  Possible water ice signatures were even harder to identify, but were tentatively seen in Voyager data.  To summarise - those top clouds certainly have nitrogen-based species in them, but how much, and how it's mixed/coated with other species, remains unclear.

But what about the colours?

The basic problem in all this is that ammonia ice is white (and water ice, for that matter).  Nice, clean, white.  So if that's not the source of the array of colours we see, there must be something else.  Focussing specifically on the red colourants, we're looking for some cloud or haze that absorbs all the blue light from the Sun, and therefore only reflects back the red.  The only thing we know for sure is that it has to be a blue-light absorber that's present in some places (reddish belts and giant vortices) but absent elsewhere (white belts and whitish small ovals).  What might it be?

Here's the crux of the matter - there are lots of candidates that could fit this bill.  The most likely explanation lies in the 'haze' above the main cloud decks.  If we define clouds as those volatile gas species that condense, then the haze is the thinner, more ubiquitous aerosol particles that sits over the top of the main nitrogen-based clouds.  Light being reflected from Jupiter's clouds is being filtered by that haze, so maybe the blue-light absorber is present there.   There are lots of ideas, but observations don't help distinguish between things.  Maybe material dredged upwards from the deeper troposphere by powerful convective updrafts then reddens in the presence of UV light, which might work for phosphorus (e.g., red triclinic phosphorus, P4) or sulphur based species.  Or photochemical reactions of ammonia and phosphine could form hydrazine or diphosphene, respectively, whose spectral properties are poorly understood but could be key compounds in the hazes. Or some sort of poorly-understood ultraviolet tanning of aerosols that hang around for a long period of time.  Maybe the products of methane-based photochemistry in the stratosphere could be raining down into the troposphere like a smog, coating the seed aerosols that are already there.  In that case, we'd be seeing long-chain hydrocarbons, polymers, maybe an organic sludge known as tholins.  It's really tough to rule any of these out with the data we have today.

So how do we solve the problem?

Comparing cloud colours from Hubble with
atmospheric temperatures from VLT (Fletcher et al., 2010)
Well, perhaps the only way to do this properly would be to have multiple Jupiter entry probes falling into regions of different colours and designed to sniff out the chemical and optical properties of the clouds and hazes.  We've already done this once with the Galileo probe, but we suspect that this entered a region of unusual meteorology called a hotspot that dried out the atmosphere of all its clouds.  The probe was often referred to as having entered the 'sahara desert' of Jupiter.  Failing that, we must continue the search using laboratory work (i.e., synthesising suitable jovian clouds in the lab and seeing what influences their colours) and theory (i.e., developing an accurate model of jovian chemistry).  Recent lab work has shown some promise - Carlson et al. presented results at a 2012 conference showing how tropospheric ammonia and stratospheric acetylene might be mixed together, photolysed and react to form a yellow-orange film and potentially nitriles like hydrogen cyanide (HCN) as a by-product, giving us an indirect way of studying the colouration problem.  Jupiter's episodic colour changes, such as the 2009-2012 global upheaval of the cloud structure or the recent reddening of a northern-hemisphere oval, provides a useful way of relating colour changes to alterations in environmental conditions (temperatures, humidity, opacity) that we hope will provide additional insights.

In summary, we still don't know why Jupiter's red spot is red.  And for planetary scientists, that's a little embarrassing.  But we have plenty of ideas to try, theories to test and discount.  Ultimately a combination of lab work, theory and observations will help solve this mystery once and for all.

Thursday, 12 December 2013

The Plumes of Europa

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 (HST) discovery of water vapour plumes from the south pole of this icy moon.
Illustration of icy Europan plumes
(Credit: NASA/Caltech)

Hubble's 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)
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 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)
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 sulphur being emitted by its neighbour Io to form sulphates [the sulphur 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 sulphates 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.

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)
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 HST only has one plume detection so far, in December 2012.  The plumes appear highly variable, and were not spotted when Europa was closer to pericentre 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!

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

Beneath the icy crust, these global oceans will be perpetual darkness.  Any life that exists there 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)

Monday, 2 December 2013

PhDs in Planetary Science at Oxford (2014)

With apologies for using this blog as a way of advertising, but there are plenty of opportunities here at the University of Oxford for planetary science studies.  The University of Oxford has several Planetary Physics DPhil positions (i.e., PhDs) available in the Atmospheric, Oceanic and Planetary Physics (AOPP) department from October 2014, some working on projects with me (see below).

The available projects cover a broad range of planetary atmospheric science: studies of brown dwarfs spectroscopy; Rosetta investigations of cometary atmospheres; ground-based observations and Spitzer studies of planetary processes at work on ice giants Uranus and Neptune; Cassini studies of Saturn's seasons; ground-based observations of Jupiter's weather; and experimental studies of planetary heat transport.  Please visit the following website for specifics of each project:


Specific Planetary Science Projects:
  • Modelling the spectra of Brown Dwarfs
  • Rosetta-VIRTIS cometary studies
  • Infrared remote sensing of the Ice Giants: atmospheric temperature, composition and clouds
  • Seasons of Saturn: evolution of Saturn’s temperature, clouds, chemistry and dynamics from Cassini
  • The weather of Jupiter: observation of dynamical tracers from ground-based spectroscopy
  • Testing parameterisations of large-scale planetary heat transport in the laboratory
What You Need to Know:

Deadlines:  Candidates should apply by 24 January 2014 in order to be considered for Departmental Studentships and any funding the University administers. Candidates able to secure external scholarships should apply by 14 March 2014.  Both research council and scholarship funding is available, please see our admissions website http://www2.physics.ox.ac.uk/study-here/postgraduates/atmospheric-oceanic-and-planetary-physics/funding for further details.

We would be very grateful if you could distribute this information to potentially-interested students.  If you have any queries or require further information that is not available on these pages please email:

For some great student insights into what it's like to study here in AOPP, watch the following video on Youtube:

Thursday, 28 November 2013

Hubble: Galaxies Across Space and Time

I spent the summer of 2003 working on the Great Observatories Origins Deep Survey (GOODS), supervised by Frank Summers, at the STScI in Baltimore, Maryland. I took an image of the GOODS Deep Field and converted it to a three dimensional fly-through of the galaxies. The movie was converted to an IMAX film ‘Hubble: Galaxies Across Space and Time’, which subsequently won Best Short Film at the LFCA (Large Format Cinema Association) annual film festival in Los Angeles, and can be seen at the Baltimore Science Centre.  I was happy to find that it had found it's way onto Youtube earlier this year (2013), and you can watch it below.   Below you'll also find some links to some of the news releases about the movie, but a comprehensive description can be found here: Making a Short, but Very Large, Movie

The IMAX visualisation was reported at the American Astronomical Society Meeting in 2003:

Summers, F. J., Stoke, J. M., Albert, L. J., Bacon, G. T., Barranger, C. L., Feild, A. R., Frattare, L. M., Godfrey, J. P., Levay, Z. G., Preston, B. S., L. N. Fletcher, GOODS Team. 2003. Hubble Goes IMAX: 3D Visualization of the GOODS Southern Field for a Large Format Short Film.  Bulletin of the American Astronomical Society, Vol. 35, p.1345

Hubble IMAX Film Takes Viewers on Ride Through Space and Time

Hubble Space Telescope Science Institute, June 24, 2004

Take a virtual ride to the outer reaches of the universe and explore 10 billion years of galactic history, from fully formed and majestic spiral galaxies to disheveled collections of stars just beginning to form.

This unforgettable cosmic journey is presented in the award-winning IMAX short film, "Hubble: Galaxies Across Space and Time," which transforms images and data from NASA's Hubble Space Telescope into a voyage that sweeps viewers across the cosmos. Using the 650-megapixel-mosaic image created by the Great Observatories Origins Deep Survey (GOODS), more than 11,000 galaxy images were extracted and assembled into an accurate 3-D model for the three-minute movie. The large-format film was created by a team of Hubble image and visualization experts in the Office of Public Outreach at the Space Telescope Science Institute (STScI) in Baltimore, Md. The film was directed by Frank Summers, an astrophysicist and science visualization specialist.

Galaxies are vast assemblages of stars, gas, and dust. And viewers experience these majestic cities of stars on a movie screen as tall as a five-story building. The film opens with looming images of two mature galaxies that are relatively nearby Earth, and then pans through the vibrant and diverse panorama of thousands of galaxies in the GOODS mosaic.

The ensuing 3-D journey through these galaxies provides more than just a new perspective in space, it also takes the audience back in time. Because light takes time to cross space, the galaxies farther away from Earth are seen further back in cosmic history. The virtual voyage reveals galaxies as they appeared billions of years ago, when they were still in the process of forming.

The movie has been so well received that it recently won the "Best Short Feature" award at the Large Format Cinema Association's 2004 Film Festival in Los Angeles, CA. The Hubble movie premiered in April at the Maryland Science Center in Baltimore, and is currently also playing at the Rueben H. Fleet Science Center in San Diego, Calif., and the New Detroit Science Center in Detroit, Mich. Distribution to several dozen other large-format theaters will occur over the coming months and years.

The film is based on data from the GOODS project, a collaboration between Hubble, NASA's Chandra X-ray Observatory and Spitzer Space Telescope, and several ground-based observatories. The observations with the Advanced Camera for Surveys, one of the largest Hubble projects ever, provided deep images of a small patch of sky covering about one-third of the projected area of the full moon. That patch contains nearly 30,000 galaxies, which were cross-matched against a ground-based redshift survey to get distances for the 3-D model.

Actress Barbara Feldon is the film's narrator, and space music composer Jonn Serrie wrote the surround-sound score. The STScI film team consists of John Stoke, Zoltan Levay, Lisa Frattare, Greg Bacon, John Godfrey, Bryan Preston, and summer intern Leigh Fletcher.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). DKP/70MM Productions, Inc., a subsidiary of IMAX Corporation donated their services and created a negative and first print from the Hubble digital frames.

An Award-Winning IMAX® Super Short from the home of the Hubble Space Telescope

A wonderful confluence of events has given the team that operates NASA’s Hubble Space Telescope a unique opportunity to display Hubble’s universe on the biggest of screens.

In March of 2002, during the final completed flight of the Space Shuttle Columbia, astronauts installed the Advanced Camera for Surveys aboard Hubble. This new instrument is now providing images of such resolution and clarity that large-format film screens are an ideal medium for displaying them.

With the generous support of David Keighley Productions 70MM, Inc., an IMAX company, we have created Hubble: Galaxies Across Space & Time, a journey across 9 billion years of cosmic history that takes a mere 2 minutes 51 seconds, short enough to be spliced into the “trailer space” before a main feature.

Awarded “Best Short Feature” in the Large Format Cinema Association’s 2004 Film Festival, the film is available for showing at institutional IMAX theaters in the USA and Canada at no cost. At the moment we have 5 prints available. We have the soundtrack on DTAC disc; other soundtrack formats can be procured at cost directly from the IMAX Soundtrack Mastering Facility.

The highlight of the film is a fantastic computer-generated flight through a field of over 10,000 galaxies that takes audiences on a journey back through time to an era when galaxies were newly formed. Viewers will see the universe as it appeared when it was young.

These galaxies were photographed by Hubble as part of the Great Observatory Origins Deep Survey (GOODS) project. The original source image contains over 600 million pixels. Hubble scientists and imaging specialists worked for months to extract individual galaxy images, placing them in a 3D model according to their approximate true distances as determined by ground-based photometric redshift data.