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Exploring the Solar System

Lecture 9: Jovian Planets: Jupiter and Saturn

Professor Paul Sellin Department of Physics

University of SurreyGuildford UK

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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

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 Earth-based telescopes due to absorption of UV in the Earth’s atmosphere. Direct measurements were first made in the 1970s by near-flying spacecraft.

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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.

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.

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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 counter-clockwise, 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

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.

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Cassini image of the Great Red Spot

Ref: http://zebu.uoregon.edu/~imamura/121/lecture-13/jupiter_atmosphere.html

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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):

H2 86.2% (75%)

He 13.6% (24%)

CH4, NH3, H20 0.2% (1%)

This composition is quite similar to that of the Sun

If Jupiter’s core is also included, the proportion of heavier elements increases, by mass:

H2 (71%)

He (24%)

Other elements (1%)

In contrast Saturn’s atmosphere is helium deficient:

H2 96.3% (92%)

He 3.3% (6%)

CH4, NH3, H20 0.4% (2%)

Both planets were formed in a similar way from the solar nebula, so where did Saturn’s helium go?

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

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 extraheat 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 Earth-based IR image, taken at the same time

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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/lecture-13/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

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.

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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”

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.

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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

Each of Saturn’s major rings is composed of a great many narrow ringlets

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.

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.

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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

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.

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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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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:

Hence, for the 3 moons, nI = 203.5 days-1

nE = 101.4 days-1

nG = 50.3 days-1

The 2:1 Io-Europa resonance is directly responsible for the active vulcanism on Io

Orbital resonances between Jupiter’s Moons*

Tn

360

007.2E

I

n

n

014.2G

E

n

n

Expressed as ratios:

The ratios of the angular frequencies of Jupiter's moons are described by the Laplace relation, where:

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.

023 GEI nnn

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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

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

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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