Exploring the Solar System
Lecture 9:
Jovian Planets: Jupiter and Saturn
Professor Paul Sellin
Department of Physics
University of Surrey
Guildford UK
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Paul Sellin
Overview
 Physical data for Jupiter and Saturn
 Surface features:
 Belts and zones
 Images from Pioneer, Voyager and Cassini
 Storms and the Great Red Spot
 Features of the Jovian atmosphere:
 Atmospheric composition
 Helium rain
 Atmospheric motion
 Internal structure of the planets
 Saturn’s rings
 The Jovian satellites
 Resonance phenomena
 Jupiter: Io, Europa, Ganymede, Callisto
 Saturn: Titan
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Jupiter – physical data
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Saturn – physical data
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Key physical facts
 Jupiter and Saturn are the two largest objects in the solar system, apart from the Sun
 Jupiter's diameter ~11x that of Earth, Saturn is ~9x
 In terms of mass: MJ = 318 ME, MS = 95 ME. Jupiter contains 2.5x more mass than that of
all the other objects in the solar system
 Jupiter has 4 large moons: Io, Europa, Ganymede, Callisto
 Saturn has one major moon: Titan
 Jupiter’s sidereal orbital period is 11.26 years (synodic period 399 days). As observed
from Earth, Jupiter appears to move across each of the sky’s 12 constellations at the rate of
approximately 1 constellation per year
 Saturn’s sidereal orbital period is even slower, at 29.37 years (synodic period 378 days))
Jupiter and Saturn as viewed
from Earth: although Saturn is
the most dramatic, due to its
well-defined ring structure,
Jupiter contains a more
complex and turbulent
atmosphere
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Viewing from Earth - oppositions
Like all superior planets, Jupiter and Saturn are best
viewed when in opposition:
 Jupiter becomes 3x brighter than Sirius, the
brightest star
 Only the Moon and Venus are brighter than Jupiter
when in opposition
 Saturn is 1/7 the brightness of Jupiter, but still
outshines all stars except Sirius and Copernicus
 Jupiter has an angular diameter of ~50 arc
seconds, twice that of Mars in comparable conditions
 Jupiter’s angular diameter is ~2.5x larger than
Saturn’s disk when at opposition
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Definitions of belts and zones
The ‘surface’ features of both planets the we observe
from Earth are the outer regions of their gaseous
atmospheres:
 colourful bands in the equatorial regions. The rapid
rotation of the planets twists the clouds into dark belts
and light zones that run parallel to the equator
 Jupiter contains alternating light/dark bands in shades
or orange, brown and red
 the dark orange/brown bands are called belts, the
lighter white bands are zones
 HST showed similar features on Saturn, but much
less pronounced
Jupiter also contains a large feature, the Great Red
Spot. First observed in 1664, this is a giant storm in the
upper regions of Jupiter’s atmosphere
Various oval features are also observed in Jupiter, such
as white ovals and brown ovals
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Rotation rates
Observation of surface features such as the Great Red Spot and smaller storms allow
astronomers to measure the rotation rate of Jupiter:
 Jupiter’s rotational period at the equator is 9 hrs 50’ 28”
 The fastest rotating planet (and the most massive) in the solar system
The equatorial regions rotate slightly faster than the polar regions  this is called
differential rotation:
 Jupiter’s rotational period at the poles is 9 hrs 55’ 41”
For both Jupiter and Saturn, the polar rotation rate is nearly the same as the internal rotation
rate
Differential rotation requires a substantially fluid centre for these planets. Extra information
is provided by their density:
 Average density of Jupiter: 1326 kg/m3
 Postulated in the 1930s that Jupiter contains mostly hydrogen and helium
 Other indirect evidence: terrestrial observation of absorption lines of methane (CH4) and
ammonia (NH3) in Jupiter’s atmosphere
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Saturn shows a similar differential rotation, with a period of 10 hrs 13’ 59” at the
equator, and 10 hrs 39’ 24” at the poles.
Average density of Saturn is similar to that of Jupiter: 687 kg/m3
Direct observation of H and He was very difficult, since neither element produce
spectral lines in the reflected sunlight from the planets’ outer atmospheres.
Emission spectra from H and He are emitted in the UV which do not reach Earthbased telescopes due to absorption of UV in the Earth’s atmosphere. Direct
measurements were first made in the 1970s by near-flying spacecraft.
Pioneer and Voyager observations of Jupiter
Various spacecraft have observed the atmosphere of Saturn and Jupiter from close range:
 stable, large-scale weather patterns
 dynamic changes on smaller scales
The general pattern of Jupiter’s weather appeared stable over the 4 years of observation by
Pioneer and Voyager (1973-1979). However there were some specific changes:
 During the Pioneer observations the Great Red Spot was embedded in a broad white
zone which dominated the southern hemisphere
 By the time of Voyager a dark belt had broadened and approached the Great Red Spot
from the north.
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Pioneer and Voyager flybys:
Pioneer 10: Jupiter 1973
Pioneer 11: Jupiter 1974, Saturn 1979
Voyagers 1,2: Jupiter 1979, then Saturn
Robert Hooke first observed the Great Red Spot in 1664, although it may be
much older. It has remained remarkably stable over 350 years.
During 1979 thee were strong interactions between the broad upper belt and the
spot. By the time Hubble observed the spot in 1995 it was again centered within a
white zone.
Over the last 3 centuries the spot has varied in size greatly. At its largest it was
40,000 km wide, so large that 3 Earths could fit in it side by side. At its smallest
it almost faded from view. During the Voyager flybys of 1979 the spot was
comparable in size to the Earth.
Voyager images of the Great Red Spot
Both Jupiter and Saturn emit more energy
than they receive from the Sun, consequently
both planets are still cooling
The coloured ovals visible in the Jovian
atmosphere represent gigantic storms
The Great Red Spot is the largest of these,
and has persisted for many years
This Voyager 2 image (1979) shows
circulation and atmospheric turbulence around
the Great Red Spot.
Winds within spot make it spin counterclockwise, completing a full revolution in
about 6 hours
The northern bands blow to the west, and the
southern bands blow east.
Unlike on Earth, there is no solid terrain to
dissipate energy in the atmosphere
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White ovals are mainly observed in Jupiter’s southern hemisphere, and are
typically long-lived (eg. Several decades). They have relatively low temperatures
and are areas with high cloud cover that block our view of the lower atmosphere.
Brown ovals are predominantly in the northern hemisphere.They appear dark in a
visible-light image, but bright in IR. They are hot regions, corresponding to holes
in the upper atmosphere, giving us a glimpse of the hotter inner regions of the
planet.
In the 1960s Earth based observation showed that Jupiter emits twice as much
energy by IR radiation than it absorbs from sunlight. In contrast the Earth only
emits 0.005% of its absorbed energy. The excess heat emitted from Jupiter is
energy created at the time of formation 4.56 billion years ago – mainly due to
conversion of gravitational energy from the solar nebula gases ‘falling’ into the
protoplanet.
Cassini image of the Great Red Spot
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Ref: http://zebu.uoregon.edu/~imamura/121/lecture-13/jupiter_atmosphere.html
Paul Sellin
Storms on Saturn
Saturn also has belts and zones like Jupiter, but no long-lived storm systems like Jupiter’s
Great Red Spot.
About every 30 years (~orbital period) ‘white spot’ storms are seen in Saturn’s clouds which
last for months.
These storms are caused by when
warm gases rise up from the
lower regions of the atmosphere
and cool
This causes gaseous ammonia to
crystallise and form characteristic
white clouds
This similar to the formation of hail
stones in large storm clouds on
Earth
About 20 temporary white spot
storms have been seen on Saturn
in the last 2 centuries
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White and brown ovals
Brown ovals tend to appear on Jupiter at around latitude = 20° N
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Red cloud on Saturn
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Composition of the upper atmospheres
The Jovian atmospheres
show a significant
temperature gradient,
fuelled by the high rate of
heat emission from the
planets’ centres
The black lines show
temperature vs altitude,
plus the probable
arrangement of the cloud
layers
Below the cloud layers the
atmosphere is almost
completely H and He
Zero altitude is defined as
as a pressure of 100 mbar
(10% of Earth’s atmospheric
pressure)
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Chemical composition
The chemical composition of Jupiter’s
atmosphere is (% mass in brackets):
In contrast Saturn’s atmosphere is helium
deficient:
H2 86.2% (75%)
H2 96.3% (92%)
He 13.6% (24%)
He 3.3% (6%)
CH4, NH3, H20 0.2% (1%)
CH4, NH3, H20 0.4% (2%)
This composition is quite similar to that of the
Sun
Both planets were formed in a similar way from
the solar nebula, so where did Saturn’s helium
go?
If Jupiter’s core is also included, the proportion
of heavier elements increases, by mass:
H2 (71%)
One theory is that Saturn cooled at a faster
rate than Jupiter, due to its smaller mass:
 droplets of liquid He condensed in the
colder outer regions of the atmosphere, and
rained down to the lower regions
He (24%)
Other elements (1%)
 Saturn’s outer atmosphere is therefore H
rich
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Helium rain
Saturn is smaller than Jupiter and so should have initially contained less thermal energy than
Jupiter. However it actually emits 25% more energy per unit mass than Jupiter
This additional energy comes from He condensation in the upper atmosphere – where friction
is produced by the falling ‘He rain’ droplets in the atmosphere
Saturn cools faster than Jupiter and so, after ~2.6 billion years, the atmosphere gets cool
enough for helium to condense and rain out.
As He rains through the planet it gains energy because gravity pulls it inward. As it moves
through the liquid H2, friction slows the drops down and heats up the hydrogen. This extra
heat is then radiated by Saturn.
This model confirms the low He content of
Saturn’s atmosphere compared to the
atmosphere of Jupiter:
The atmosphere of Jupiter is more than 10 %
He while the atmosphere of Saturn is 6 % He.
In the future, Jupiter may show the same
behavior.
Ref: http://zebu.uoregon.edu/~imamura/121/lecture-13/jupiter_interior.html
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Atmospheric influences on Jupiter’s belts and zones
Cassini produced high resolution images of Jupiter during its flyby in 2000-2001, showing
details less than 60km in diameter
Numerous white clouds were seen, moving upwards and located within the dark belts:
 the atmosphere is rising in the belts and sinking in the zones
Voyager 1 image
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Earth-based IR image, taken at the same time
Paul Sellin
Atmospheric motion in the zones and belts
The heat flow drives motions in the atmosphere of Jupiter and Saturn:
 Material heated deep in the atmosphere heats it becomes less dense and so rises
(carrying heat upward).
 It cools as it rises  it becomes denser and eventually reaches a point where it will start to
sink. It then returns to the deeper layers where it is heated and again starts to rise.
As in the interior of the Earth, this
leads to large scale motions in the
atmospheres of Jupiter and Saturn.
 Because different latitudes on Jupiter
and Saturn are heated by different
amounts, latitudinal motions are driven.
The equator is warmer than the poles
which tends to drive the circulation from
equator poleward.
Ref: http://zebu.uoregon.edu/~imamura/121/lecture13/jupiter_atmosphere.html
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Rocky cores of the Jovian planets
The first few 100km of Jupiter’s atmosphere has been
sampled by the Galileo probe (1995)
There is only indirect data to provide evidence about the
Jovian planets’ internal structure:
 both planets are significantly oblate, a function of
rotation rate and the distribution of mass through the
planet’s volume
 planetary models suggest that 2.6% of Jupiter’s
mass is concentrated in a dense rocky core:
 the core is 8x more massive than Earth
 enormous pressure from the outer regions of the
planet compresses the core to ~11,000 km
 the central core pressure is 70 million
atmospheres, and temperature is 22,000 K
 in contrast the outermost cloud temperature on
Jupiter is 165K
 the core is probably the original ‘seed’ around which
the planet formed, plus additional meteoric materials and
icy planetismals
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The diagrams of the two planet’s interiors are drawn to the same scale. Each
planet has a rocky core surrounded by an outer core of liquid ‘ices’, a layer of He
and liquid hydrogen, and a layer of He and ordinary molecular hydrogen (H2).
The ‘ices’ are mainly the liquids H2O, CH4, NH3, forming a layer 3000km thick
around the rocky core. They ‘float’ on top of the rocky core due to their much
lower density. Despite the very high temperatures, the pressure is so high that
these substances are probably in a liquid state,
Saturn’s rocky core contains a larger proportion of the planet’s total mass than
Jupiter, although it has a smaller volume of metallic hydrogen. Saturn’s rotation
rate is the same as Jupiter, but it’s gravity is less. Therefore, for the same internal
structure, it would be more oblate than Jupiter. Detailed models suggest that
Saturn’s core contains 10% of the total planet’s mass, compared to 2.6% for
Jupiter.
Metallic hydrogen
A large proportion of Jupiter is composed of liquid metallic hydrogen:
 H only becomes a liquid metal at a pressure of 1.4 M atmospheres, ie, at 7000km below
Jupiter’s could tops
 Metallic hydrogen consists of:
 a crystal lattice of protons, with a spacing that is significantly smaller than a Bohr radius,
comparable with an electron wavelength
 the electrons are unbound and behave like the conduction electrons in a metal
 Metallic hydrogen was first discovered on Earth in 1996, in an experiment at the Lawrence
Livermore National Laboratory. It was produced for about a microsecond and at temperatures
of thousands of kelvins and pressures of over a million atmospheres (>100 GPa)
 These measurements indicate a lower metallisation pressure than original thought
 Therefore the new data indicates that much more metallic hydrogen exists inside Jupiter
than previously modeled, and that it comes closer to the surface. Consequently Jupiter's
magnetic field is produced closer to the planet’s surface.
Ref: http://en.wikipedia.org/wiki/Metallic_hydrogen
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Jupiter’s Magnetic Field and Magnetosphere
Jupiter’s magnetic field is 14x stronger than the field at the Earth’s equator
Produced by the motion of the electrically conducting metallic hydrogen, caused by Jupiter’s
rapid rotation
 strong magnetic field produces a massive magnetosphere, encompassing Jupiter's moons
 Pioneer & Voyager reported the edge of the magnetosphere at ~7 M km from Jupiter, the
downstream edge extends ~1 billion km, beyond the orbit of Saturn
 Vast amounts of charged particles are trapped in ‘belts’, forming a current sheet which lies
on the plane of the magnetic equator
 the magnetic axis is
inclined by 11° from the
planet’s axis of rotation
 the field direction is the
opposite to the Earth – a
compass would point to the
south pole!
 Radio wave fluctuations
follow the internal rotation
period – 9 hrs 55’ 30”
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Radio astronomers first studied radio emissions from Jupiter in the 1950s, which
are observed in 2 distinct wavelength ranges:
- decametric (10m) radiation is emitted in short bursts, probably caused by
electrical discharges associated with powerful electrical currents in Jupiter’s
ionosphere. These include discharges between Jupiter and it’s moon Io
- decimetric radiation at shorter wavelengths (tenths of a metre) is emitted as a
nearly intensity, produced a synchrotron radiation by electrons spiralling around
the magnetic field lines
Saturn also has a substantial magnetic field, stronger than Earth, but only 3% that
of Jupiter. This is due to the substantially smaller volume of liquid metallic
hydrogen in Saturn compared to Jupiter (their rotation rates are comparable),
which is a result of Saturn’s smaller mass, gravitational force, and internal
pressure.
Saturn’s rings
Saturn’s rings were first observed in 1610 by Galileo, seen only as ‘lumps’ on the planet’s sides
In 1655 Huygens first suggested that Saturn is surrounded by thin rings – difficult to see when
edge-on to the Earth
In 1675 Cassini discovered a dark band separating two groups of rings – the Cassini division
In the mid 1800s a weak inner ring was also discovered
The rings lie in the
plane of Saturn’s
equator, which is tilted by
27° from it’s orbital plane
Saturn’s equatorial
plane maintains the
same orientation in space
as it orbits the Sun
So from Earth the view
of Saturn changes
significantly (29yr orbit)
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Saturn’s Rings
This image from Voyager 1 shows the ring structure in detail:
 Band A starts at 244,000 km, Band B starts at 184,000 km
 The Cassini division is from 235,000 – 244,000 km
 The very weak Band C starts at 149,000 km (hardly visible in this image)
The rings are extremely thin
(typically tens of metres)
The last edge-on view of Saturn
was 1995-1996, with the next in
2008.
At the current time Earth sees
the ‘underside’ of the rings
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Structure composition of the rings
 Saturn’s rings are not solid, but composed of many small icy particles:
 from pebble-szied fragments (1cm diameter) to large objects (5m across)
 the most common size is ~10 cm
 The icy particles reflect sunlight strongly: ~80% reflectivity compared to ~46% for Saturn
 Total amount of material is rather small – if compressed together it would be 100km across
 Close-up images showed the more
complex structure of the rings: the 270km
wide Enke gap in A ring, many fine
ringlets, the narrow inter-twined F ring
In all cases the rings are within the Roche
limit (the distance from a planet within
which a satellite would be torn apart by tidal
forces).
 Some rings seem to be maintained by the
presence of small satellites just inside and
outside of boundaries - the so-called
shepherd moons.
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Each of Saturn’s major rings is composed of
a great many narrow ringlets
Paul Sellin
The Roche limit represents the limit of dynamical stability for a satellite. All
large planetary bodies are located outside their planet’s Roche limit. Any satellite
orbiting within this limit will be torn apart by tidal forces from the parent planet –
this may be the case in the future for Triton, Neptune’s satellite.
Thus, it could be that the rings were formed by the break-up of a satellite that
strayed within this limit. Or, it may be that the rings are comprised of material
that could never form into a satellite because of tidal forces.
F ring
The faint F ring, which is 4000km
outside the A ring, is about
100km wide
The ring consists of several
intertwined strands, as narrow as
10km across
This complex structure is
probably due to the gravitational
pull of unseen ‘shepherd’
satellites
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Colour variations in the rings
Computer processing has exaggerated the subtle colour variation of the rings, which is due to
the underlying difference in their chemical composition
The different colours show that there is little mixing between rings
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The colour difference suggest that new material has been added to some of the
rings at different time, for example due to small satellites that shattered after
being hit by a comet or asteroid. This would mean that the rings were maybe
formed at different times, over an extended period.
Reflected light from the rings
False-colour view of the side of the rings away from the Sun, where the sun light has passed through the
rings. The Cassini division appears white, not black like the empty gap between A and F rings
This shows that the Cassini division is not empty, buy contains very small particles which scatter sunlight
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Satellites of Jupiter
 Jupiter has 4 planet-sized Galilean satellites: Io, Europa, Ganymede, Callisto, all
comparable in size to Mercury with orbital semi major axes 5.9-26x radius of Jupiter
 4 smaller inner planets: Metis, Adrastea, Amalthea, Thebe, with orbital SMA 1.8-3.1x radius
of Jupiter:
 All irregular in shape, like asteroids
 Amalthea is the largest, 270x150 km, reddish in colour due to sulphur plumes from Io
 55 smaller outer satellites (as at 2004), with SMA 100-360x Jupiter’s radius, all well beyond
Callisto
 The 8 inner satellites have a
prograde rotation, with orbital
planes aligned with Jupiter’s
equator  formed from the
same primordial cloud as Jupiter
 55 outer satellites have highly
inclined orbits, with 48 in
retrograde rotation  probably
asteroids captured by Jupiter
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Orbital resonances between Jupiter’s Moons*
Resonances in the Solar System are observed in two main types:
 spin-orbit coupling: fixed relationships between rotational and orbital periods of a single
body (eg. the Moon, with a synchronous spin-orbit coupling to Earth)
 orbit-orbit coupling: relationships between orbital periods of 2 or more bodies
3 of Jupiter’s Moons show striking orbit-orbit coupling:
Io has a 2:1 orbital resonance with Europa
Europa has a 2:1 orbital resonance with Ganymede
The orbital frequency n is defined as:
n
360
T
Hence, for the 3 moons, nI = 203.5 days-1
Expressed as ratios:
nE = 101.4 days-1
nI
 2.007
nE
nE
 2.014
nG
nG = 50.3 days-1
The 2:1 Io-Europa resonance is directly responsible for the active vulcanism on Io
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The ratios of the angular frequencies of Jupiter's moons are described by the
Laplace relation, where:
nI  3nE  2nG  0
This prevents the occurrence of triple conjunctions of the 3 moons, and when a
conjunction occurs between any 2 moons, the 3rd one is more than 60° away.
Other examples of resonance phenomena include:
Saturn system: Saturn has many examples of resonance phenomena, eg. the
satellites Mimas and Tethys are in 4:2 orbit-orbit resonance, and Enceladus and
Dione are in 2:1 orbit-orbit resonance.
Uranus system: the rings system of Uranus contains many examples of
resonance, although none involve the 5 major satellites. Amongst the smaller
satellites, Rosalind and Cordelia have a 5:3 resonance and are in complex
resonance with the edges of the narrow ‘’ Uranus ring.
Pluto and Charon: these objects are in a synchronous spin state, that Pluto and its
moon always keep the same face towards each other. Each object appears
‘geostationary’ when viewed from the surface of the other body. The system is
referred to as ‘totally tidally despun’.
Asteroid Belt (see lecture 2): Distinct regions of low asteroid population are
known as the Kirkwood Gaps and occur at SMAs where an asteroid’s orbital
period would have a simple relationship (1:2, 1:3, 1:4, 2:5, 3:7) with the orbital
period of Jupiter  tidal effects from Jupiter have swept these areas clean.
Conversely, where the ratio of periods is 2/3, asteroids congregate into groups or
families, eg. Jupiter has the two groups of asteroids known as the Trojans.
Io
Io shows evidence of severe and ongoing volcanism – first imaged by Voyager 1 in 1979 the
multi-coloured surface contains many volcanoes and lava flows
 the volcanoes emit massive lava plumes – more like geysers than Earth volcanoes –
reaching heights of 300km
 plumes contain sulphur and sulphur dioxide – images with IR spectrometers on Voyager 1
 liquid SO2 seeping into the hot interior produces a high pressure gas eruption
Io contains a highly molten interior and
retains a large amount of internal heat
The heat comes from tidal heating – the
tidal forces acting on Io due to Jupiter’s
massive gravitational field
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Io (2)
Galileo passed within 200 km of Io’s surface, and observed many volcanic plumes:
Plumes rising ~100km from Io’s
surface, and ~250km wide
Galileo images showing rapid changes in volcanic activity – separated
by a few months the images show sulphur ejected from Pele followed
by dark material over 400km diameter from Pillan Patera
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Europa
Europa’s surface is almost pure ice: very smooth, with no features higher than ~100m
 No crater activity, indicating a young surface affected by geologic activity
Some brown surface areas due to rocky material, probably from meteoritic impacts
 As for Io, tidal heating is responsible for Europa’s internal heat
 Minerals dissolved in this ocean may explain Europa’s induced magnetic field
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Ganymede
Ganymede is highly differentiated, and probably has a metallic core
It has a surprisingly strong magnetic field and a magnetosphere of its own
While there is at present little tidal heating of Ganymede, it may have been heated in this
fashion in the past
An induced magnetic field suggests that it, too, has a layer of liquid water beneath the surface
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Two types of terrain are found on the icy surface of Ganymede:
• areas of dark, ancient, heavily cratered surface
• regions of heavily grooved, lighter-colored, younger terrain
Paul Sellin
Callisto
Callisto has a heavily cratered crust of water ice
The surface shows little sign of geologic activity, because there was never any significant tidal
heating of Callisto
However, some unknown processes have erased the smallest craters and blanketed the
surface with a dark, dusty substance
Magnetic field data seem to suggest that Callisto has a shallow subsurface ocean
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Interiors of the Galilean satellites
Cross-sectional diagrams showing the probable internal structures of the four Galilean
satellites of Jupiter, based on information from the Galileo mission
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