Title Frame Module 16: Jovian Satellites & Rings

Activity 1: Major Jovian Satellites

Summary:

In this Activity, we will investigate

(a) vital statistics of satellites greater than 1000km diameter

(b) properties - detailed information from spacecraft

(c) modelling the formation and evolution of the major satellites

Io (above the Great Red Spot) and Europa transiting the face of Jupiter

Note:

There are many clues to be found for later work on the formation and evolution of Jupiter’s system of satellites.

Unless otherwise noted, orbits are shown to scale and planet and satellite sizes are to scale.

Work through the next frame slowly, a step at a time, as it starts off simply but ends up looking rather complex.

However it interactively reveals a great deal and each mouse click will add new information.

Remember the and keys will step you back and forward to review details.

(a) Vital Statistics

Note that this Activity does not include the new satellites of Jupiter discovered after 1999 (the

current number of satellites is 63!). They will be discussed in the next Activity Minor

Jovian Satellites & Rings

The (old) Jupiter System of 4x4 satellites

Jupiter as seen in a small telescope, with its 4 brightest ‘Galilean’ satellites in a line in their edge-on orbits as viewed from Earth

The (old) Jupiter System of 4x4 satellites

Europa

Ganymede

Callisto

4 small inner satellites

The Earth and Moon enlarged 10 times. Note their much greater relative separation compared with Jupiter and its Galilean satellites.

5000km

The (old) Jupiter System of 4x4 satellites

IoJupiter

Our Moon

Viewed from above, their orbits are near-circular

1 million km

Viewed from the side, the 4+4 inner satellites all share the same plane. The outer 4+4 satellites are less than 200km diameter and have highly inclined elliptical orbits

4 small satellites with orbits shown half size to fit on screen (and they are not circular). There are 4 more small satellites at about double this distance

Formation and Evolution (1)

Factors for consideration in formation of satellites:

• Orbit distance from parent planet, eccentricity and inclination

• Satellite size, shape, appearance (and composition) and rotation rate

As with the Solar System overall, a system of satellites with near-circular orbits and orbiting in the same direction as the planet and in its equatorial plane is strong evidence for a common formation from a condensing nebula of gas and dust.

Small satellites cannot (gravitationally) retain their lighter elements and do not differentiate to form spherical shapes - such as Jupiter’s inner 4 satellites.

The 4 large Galilean satellites retain lighter elements and tidal and other (heating) causes determine surface activity and differentiation of interior structure into layers.

The outer 8 small satellites, all with inclined, elliptical orbits, (the outer 4 orbiting against the general rotation), strongly suggest they are captured asteroids.

These factors will be reviewed at the end of this Activity.

Jupiter’s 8 inner satellites generally meet these properties. If all formed from the same material, then their mass, distance from the parent planet and internal heat will be the main factors determining their subsequent evolution.

The Galilean SatellitesThe Galilean Satellites

They are bright enough to be seen with the naked eye (if much brighter Jupiter would not steal the show!) and can be seen in steadily held binoculars.

Diagrams, such as at left, show, for a given date, the position of the four satellites about Jupiter (the orange central strip).

Since Galileo first observed them on January 10, 1610, the four satellites of Jupiter - in order of their distance from Jupiter - Io, Europa, Ganymede and Callisto, have been observed regularly winding their way around the planet.

April 2000West East

April 2000West East The Galilean SatellitesThe Galilean Satellites

Here for example, at about 12:00hrs (GMT) on April 11, 2000, only Callisto is on the east side of Jupiter.

April 2000West East The Galilean SatellitesThe Galilean Satellites

Ganymede, Jupiter with Io about to transit in front of it, Europa, and Callisto at ~12:00hrs on April 26, 2000.

Here, left to right are

The Galilean SatellitesThe Galilean Satellites

The transit times are so predicable that, depending on whether Earth was moving toward or away from Jupiter, the different light travel times enabled the first measurement of the speed of light.

April 2000West East

A different set of images, displayed in order of increasing orbit radius out from Jupiter ...

Low resolution ImagesFirst Voyager and later the Galileo spacecraft images show the remarkable differences between the four Galilean satellites of Jupiter. Firstly displayed in scale order of size including our Moon . . .

Ganymede Callisto Io Moon Europa

Io Europa Ganymede Callisto

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Galilean Satellite General Descriptions

Io is subject to the strongest tidal stresses from Jupiter. These stresses generate internal heating which is released at the surface and makes Io the most volcanically active body in our Solar System. This is the side of Io which always faces away from Jupiter.

Europa appears to be strongly differentiated with a rock/iron core, an ice layer at its surface, and there is evidence of local or perhaps even global zones of salty water below the icy surface.

Galilean Satellite General Descriptions

Tectonic resurfacing brightens terrain on the less active and partially differentiated moon Ganymede. This side shown always faces Jupiter.

Callisto, furthest from Jupiter, appears heavily cratered at low resolutions and shows no evidence of internal activity.

Observations from the NASA Galileo spacecraft have indicated that both Ganymede and Callisto may also harbour salty liquid water under their icy surfaces. See http://www.jpl.nasa.gov/galileo/ for further details.

A numbers gameThe Galilean satellites orbit Jupiter in (near-circular) ellipses which give another wonderful example of Kepler’s Laws - or, as Newton showed, that they orbit under the common influence of Jupiter’s gravity.

Check for yourself that, for each satellite in the table below,orbit radius cubed divided by its orbit time (period) squared gives the same result - as you would expect from Kepler’s 3rd Law.

Satellite Radius of orbit Period million km in days

Io 0.422 1.77

Europa 0.671 3.55

Ganymede 1.070 7.15

Callisto 1.883 16.69 0.024

Tidal locked rotations

Like our own Moon, the four Galilean satellites orbit with their same faces oriented toward Jupiter - in other words their rotation periods are the same as their orbit periods shown in the previous frame.

We know, from the Earth-Moon tidal interaction, that this matching of periods is due to tidal drag slowly varying a satellite’s rotation period until it is ‘not rotating’ from the planet’s ‘point of view’. It of course rotates once in the course of one orbit of the planet.

Tidal locked rotations

The tidal distortion and gravitational locking is vividly displayed by the inner satellite Amalthea which orbits 110,000 km above the cloud tops of mighty Jupiter in only 12 hours. It has been distorted into a shape 270 km by 200 km by 155 km with its long axis always pointing to Jupiter.

Amalthea’s size Greatly Exaggerated

1997 HST image of Io and Jupiter

Tidal locked rotations

In the case of Earth’s liquid water, tidal effects are obvious. For satellites, their composition was less solid in past ages and even now they may have non-solid interiors or ‘malleable’ crusts. This is visibly evident on Io as we will see.

Surprising Io

Prior to Voyager 1, Io, like other satellites and planets, was expected to have a dead, crater covered surface.

Voyager 1 images of Io revealed an apparent release of material from a volcano, given the name Pele.

Around 11 volcanoes have been subsequently detected and confirmed by Voyager 2 and Galileo spacecraft.

(b) Properties

Galileo (1997) shows changes in Io over just a few months around volcanoes Pele (red plume) and Pillan Patera (dark material).

Sulphur and sulphur dioxide have been detected in Io’s volcanic plumes.

Sulphur assumes colours from orange to red to black when heated and cooled - accounting for Io’s colours.

Io’s source of energy

A theory that Io should have tidally generated heating was confirmed by Voyager’s discovery of Io’s volcanism . . .

Io is so close to massive Jupiter (relatively, compared with Earth and Moon) that, though its orbit is generally circular, minor influences, as Io passes satellites Europa and Ganymede, result in tidal squeezing and flexing, generating heat which rises to the surface and releases material volcanically.

Io’s source of energy

The graph (like we saw earlier) showing the Galilean satellite positions around Jupiter in September 1999, clearly shows the regular relations in the positions of Io, Europa and Ganymede.

Io passes Europa every 2nd orbit and Ganymede every 4th orbit

1 2 3 4 5 6 7 8 9

Interior Structures

Europa and Ganymede, and possibly even Callisto, are thought to contain salty liquid water oceans below their icy surfaces

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Gravity and magnetic field measurements by the Galileo spacecraft lead to possible interior structures . . .

Metallic (iron, nickel) cores (except Callisto)

Rocky mantles (except Callisto)

Europa and Ganymede with layers of ice or liquid water

Callisto with a more uniform mixture of ice and rock, with possible slight differentiation into layers near the centre

The surface layers of Ganymede and Callisto may differ from the underlying ice/rock layers in, for example, the amount of rock

Europa and Ganymede

The next few frames show just a representative few of the hundreds of detailed images returned by the Galileo spacecraft. They have been chosen for interesting features. . .

Ganymede

With the hindsight

of what Jupiter did to Comet

Shoemaker Levy 9 this chain

of 13 craters about 200 km in length suggests a similar cause.

Fresh impact craters (32-38 km diameter) in bright grooved terrain.

EuropaMitten shaped formation possibly caused by upwelling of ice. Typical icy surface - brown ridges perhaps

from refreezing upwelling material. Underlying blue surface is water ice.

Callisto

Progressively higher resolution is shown in these Galileo images of Callisto, showing an inactive, cratered surface.

The Valhalla multi-ring structure, consisting of a bright inner region about 600 km across. Valhalla's 4,000 km diameter makes it one of the largest impact features in the solar system.

Callisto

The 105km diameter double ring crater Har. In an otherwise lunar-like terrain, Har shows an unusual rounded central mound perhaps caused by an uplift of ice-rich materials from below. Imaged from a distance of 14,000 km in June 1997 by Galileo on its 9th orbit of Jupiter.

Detail down to 9 metres is visible in this image of the Pillan area, showing varied terrain with pits and domes.

Galileo Spacecraft Close Flyby’s

On January 3, 2000, the spacecraft passed just 351 km above Europa. Check Galileo’s website for more information and images http://www.jpl.nasa.gov/galileo/

On its 25th orbit of Jupiter, Galileo passed Io on October 11, 1999 at a height of just 611 km.

8 small inner satellites in near circular orbits in the same direction and plane as Saturn’s rings and equator, the last and largest being Mimas* (392km) and Enceladus* (500km)

Tethys* 1050km diameter with same properties as above

2 small satellites also with the same orbital properties

Dione* 1120km diameter also ‘well behaved’ as above

1 small well behaved satellite

Rhea* (1530km) and Titan** (5150km) again as above

Hyperion (405x260x220km) as above but with more elliptical orbit (e=0.104)

Iapetus* (1440km) orbit inclined at 14.7°

1 small satellite with the highest (e=0.163) elliptical orbit, inclined at 175°

in the reverse (retrograde) direction to all the others; nearly 4 times further from Saturn than Iapetus

Satellites of SaturnLet’s apply our investigative concepts learnt at the Jupiter system to the satellites of the further planets . . .

*tidally locked with one side facing Saturn

Viewed from the side, orbits of 5 bold named satellites, to scale with Saturn

At Saturn, we find a total of 18 (+13 new) satellites. Again, we will discuss the new satellites to the next Activity Minor Jovian Satellites & Rings. Working out from Saturn:

** may be

Larger Satellites of Saturn

Titan 5150km

Tethys 1050km

As at Jupiter, Saturn’s larger satellites exhibit a great variation in features - in cratering, colour, ice flows, rifts and cracks

Rhea 1530km

Shown in relative size order but not to scale

Iapetus 1440km

Iapetus has a partial covering of dark material

Dione 1120km

Dione has unusual wispy streaks

Titan

5150km diameter Titan shows no surface detail due to its smoggy 200km deep atmosphere of nitrogen, methane and hydrocarbons.

The extent of its atmosphere is evident in this image by Voyager 2 from Titan’s night side.

From Earth orbit, the Hubble Space Telescope was able to reveal variations in Titan’s surface and atmospheric radiation in this infrared image.

Some of the hydrocarbons in Titan’s atmosphere are components of organic molecules required for life - inviting further study of such early-solar system material.

The Cassini spacecraft, which arrived at Saturn in July 2004, will reveal new information about Titan through its Huygens probe descent to the surface in December 2004 (see previous Activity).

Satellites of Uranus

Working out from Uranus, its satellites comprise:

10 small inner satellites out to Miranda (472km), all in circular orbits in the same direction and plane as Uranus’ equator and faint rings

Uranus, its (faint) ring system, and the 5 large named satellite orbits to scale

Plane of Uranus’ solar orbit

[Two further satellites were identified in 1997. In mid 1999 another satellite (small in inner orbit) was found on Voyager 2’s 1986 images, and in July 1999 another three satellites were detected with the 3.5m Canada-France-Hawaii telescope. Six more satellites have been discovered since (the last in 2003), bringing the number to a total of 27 satellites. We will look at these new satellites in more detail in the next Activity Minor Jovian Satellites & Rings.]

Ariel (1158km)

Umbriel (1172km)

Titania (1580km)

Oberon (1524km)

Larger satellites of Uranus

Titania1580km

Oberon1524km

Umbriel1172km

Ariel1158km

Cratered and rifted satellites show less variation than those of Saturn and Jupiter

Satellites of Neptune

Working out from Neptune, its satellites comprise:

6 small inner satellites all in circular orbits in the same direction and plane as Neptune’s equator and faint rings, the last being Proteus (415km)

Triton (2700km) in a circular orbit but showing the greatest break with normal orbital directions for major satellites in that it has a retrograde orbit inclined at 157° (or -23° off Neptune’s equatorial plane)

A further small satellite (Nereid) has the highest eccentricity (e=0.75) for planetary satellites and its orbit is inclined at 27.6° Its distance averages 15 times further from Neptune than Triton.

Neptune & orbits of the 2 satellites identified above

Plane of Neptune’s solar orbit

[Four new satellites were identified in 2002 and one in 2003, taking the tally up to 13 satellites for Neptune. We will look at these new satellites in more detail in the

next Activity Minor Jovian Satellites & Rings.]

Triton

Mosaic of Voyager 2 images

Triton’s surface shows few craters.

One theory to explain its retrograde orbit is that it is a captured satellite and its now-circular orbit resulted from tidal effects. With its spherical form this invites debate as to where it was captured from.

Voyager 2 detected dark plumes being ejected from the surface suggesting there is still a source of internal heat despite its -236°C surface.

Interesting surface detail on Triton suggesting flooding from interior - imaged by Voyager 2

Mosaic of Voyager 2 images

Voyager 2 detected an atmosphere as thin as Earth’s at 100km altitude; enough to support winds evident from windblown dark material.

Tidal interaction with Neptune and its retrograde orbit means that (unlike our Moon) Triton will spiral slowly toward Neptune, breaking up to form new rings in perhaps 100 million years.

We anticipated, in an earlier frame, factors which might affect conditions for the major Jovian satellites.

In general we find those factors can be invoked to explain the different features we found - such as the heating leading to Io’s volcanic action, the lack of internal layering of Callisto etc.

But we also find the unexpected - such as the thick atmosphere of Titan - eagerly anticipated by the Huygens probe.

(c) Formation and Evolution Models

Under familiar laws of Physics subtly different combinations of factors such as internal heating, solar radiation, tidal effects, gravity and rotation lead to the great variety of surface and atmospheric conditions found on the major satellites.

We should not be surprised. The same laws of Physics and those same factors for our Earth give us our infinite variety of weather conditions, cloud patterns, sunsets, beach sculptures, river systems, mountain ranges and landscapes.

The elements and molecules comprising the atmospheres and satellites of the distant Jovian planets represent those present in the early solar system and further exploration, by Cassini for example, will shed further light on the origins of life’s essential compounds.

This ends our consideration of the major satellites of the Jovian planets. Some of the material (e.g. on origins and evolution) overlaps the material in the following Activity on the Minor Satellites and Ring Systems of these planets.

NASA: http://www.nasa.gov

Indexed status of all NASA spacecrafthttp://www.hq.nasa.gov/office/oss/missions/index.htm

Galileo Spacecrafthttp://www.jpl.nasa.gov/galileo

Io & Jupiter - HSThttp://oposite.stsci.edu/pubinfo/pr/1999/13/extra-photos.html

Image Credits

Now return to the Module 16 home page, and read more about the major Jovian satellites in the Textbook Readings.

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