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.

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.