Title Frame Module 15: The Jovian Planets

Activity 2: The Other Jovians

Summary:

In this Activity, we will investigate

(a) comparative statistics for the four Jovian planets

(b) properties - and the underlying factors determining the properties of these planets, extended from our Jupiter Activity,

(c) spacecraft missions, and

(d) clues about the history of the Solar System.

With a little artistic licence we look inwards to the ‘backlit’ Jovian planets.

The value of comparative studies

Rotation

rate

(day/night

cycle)

Orbit size

(solar

radiation)

Mass (ability to maintain atmosphere)

• Some of the observed properties of planets lead to testable predictions about what conditions might be experienced at their surfaces and in their atmospheres. These include . . .

(a) Comparative Statistics

Tilt of axis

(seasons)

The value of comparative studies

• Other properties (and some of those in the previous slide) lead to possible conclusions about the formation and development of the Solar System in the outer region of the gas giants. These include . . .

Composition of atmosphere

Nature of

satellites and

ring systems

Planet and satellite orbit eccentricities

Orbit Comparisons (i)

Neptune

Uranus

Saturn

Jupiter

Sun to Mars

A perspective view of the almost-circular orbits.

Note how our inner Solar System shrinks in comparison.

Neptune

Uranus

Saturn

Jupiter

Sun to Mars

Here we are looking down on the Solar System from about 30 AU away. In fact, from here we would not detect any size to the planets; the Sun would appear as a star about one-thousandth of its current intensity; and Jupiter and Saturn would only appear as average stars to the naked eye.

space is certainly a lonely place ...

Neptune

Uranus

Saturn

Jupiter

Sun to Mars

In the previous frame we emphasized the insignificance, in size and brightness, of the planets when viewed with their orbits to scale. Often their size is exaggerated - as in this frame - giving the wrong impression about their potential for mutual attraction (or worse, collision!).

Orbit Comparisons (ii)

12 years30 years

84 years

165 years

All orbits are anti-clockwise when north is ‘up/out of the screen’

Neptune

Uranus

Saturn

Jupiter

Sun to Mars5.2 AU

9.5 AU

19.2 AU

30.1 AU

However, with this diagram in front of us, we will add the orbit sizes (relative to the Earth-Sun unit of 1AU) and durations to the nearest Earth-year.

J

SU

N

0.8o1.8o2.5o

Comparing circularity of orbits, viewed from directly above and

shown to scale

0.048

0.056

0.046

0.009

Eccentricities all close to zero

1.3o Comparing inclination of orbits, viewed edge on

The same direction of orbits, the approximate circularity and common orbital planes, all point to a common origin mechanism

Compare with the grey circles

Orbit Comparisons (iii)

Comment - science at work

• It was once thought that orbits had to be circular. Benefitting from the careful observations of others, Kepler and Newton showed that the ellipse was the natural orbit and the circle was but one example (that itself needed the explanation!).

• That most of the planetary orbits are so close to circular suggests a well ordered mechanism for the original condensation of the planetary material orbiting the newly forming Sun, (without much interference from external bodies such as comets, passing stars etc.)

• Scientific theories arise from regular patterns such as seen in the last frame, but must also be able to explain irregularities such as you will see in the next frame.

Size and Rotational properties

Oblateness

Tilt of axis to orbital plane

Mass (Earth=1) 318 95 14.5 17

.065 .108 .024 .026

3° 27° 98° 30°

Rotation* period (body of planet)

9h55m 10h39m 17h14m 16h07m

142,985 120,537 51, 119 49,528

11.2 9.4 4 3.9

Diameter (km)

(Earth=1)

Sizes to scale

Earth

*As is usual throughout solar system, rotation directions are the same as orbit directions with the technical exception of Uranus with its high axial tilt

(b) Properties - Consistencies and IrregularitiesBefore we move on to more detailed examination of the gas giants, are some of the inter-relationships of the previous frame what we might expect?

• Yes, the higher rotation rate planets have higher oblateness. This, along with the relatively low, but consistent mass with planet size, suggest, as we saw with Jupiter, small rocky cores with overlying liquid and gaseous atmospheres.

• Even in the small images used so far, the variation in atmospheric detail and colour invites speculation about the causes. As well as composition, we may expect axial tilt (which causes our familiar seasons) to be a contributing factor to weather systems and, already, Uranus stands out as nearly featureless.

• If Uranus’ tilt was 90° we could not say whether its rotation was clockwise or anticlockwise compared with its anti- clockwise orbital direction.

• So, though its 98° tilt technically means its rotation and orbital directions differ, not too much can be made of this. It is believed that the whole Uranian system was perturbed by some passing body early in its formation history. However, we might look to its different seasonal heating effects to help explain its featureless appearance.

Satellites and Rings

Satellites and Ring systems are covered in detail in the next Module on Satellites & Rings of the Jovian Planets. However one cannot look at Saturn without noticing its spectacular rings, and a planet’s system of satellites also provides information about its environment now and in its formative period:

Satellites and Rings

Jupiter 63** 4 14 3 tenuous

Saturn 31** 5 6 7 prominent

Uranus 27** 4 5 11 faint and dark

Neptune 13** 1 1 4 faint and dark

# Ring system numbers relate to appearance from a distance. Close detail show subtle influence of associated satellites.

* Retrograde means orbiting in a direction opposite to that of the parent planet. In the few cases above, they are small, outer satellites with high orbital inclinations and eccentricities.

Planet Satellites Over Retrograde* Rings# 1000km

Most regular satellite orbits and rings are near-circular and inclined at a similar angle to the planet’s axial tilt; even Uranus’!

** Note that many new satellites have been announced since 1997 and are still being discovered. We’ll learnabout these in the Activity Minor Jovian Satellites & Rings

Photo GalleryWhite oval and turbulence in Jupiter’s atmosphere

Saturn’s thin rings, shadows and almost featureless belts in atmosphere

False colour and enhancement reveals clouds in Uranus’ otherwise featureless atmosphere

Neptune’s dark spots and ‘scooter’ clouds

Increasing Saturn detail showing ovals, swirls and storms

NASA/JPL Voyager 2 images

Magnetic Fields

Comparing the magnetic fields of the four Jovian planets with that of Earth . . .

Planets are shown (not to scale) with their rotational axes tilted to their orbital planes

• The magnetic axes (where a compass needle would point) are tilted with respect to the rotational axis (except for Saturn). The fields are measured by the magnetometer of passing spacecraft such as Voyager 2.

Magnetic Fields

• The field strength gives information about the size and nature of internal materials such as iron and metallic hydrogen as shown in the next frame . . .

• For Uranus and Neptune they are offset from the planet centre. For Neptune this could mean the magnetic axis is slowly reversing (as it has done for the Earth in the past). For Uranus the misalignment could be a further result of the

past perturbation that caused its dramatic axial tilt.

Internal Structure

Jupiter Saturn Uranus Neptune

Liquid metallic Hydrogen Liquid Hydrogen & HeliumLiquid Hydrogen

Rocky core

Compressed water

Relative masses and sizes of cores and higher layers is deduced from oblateness, rotation rate, overall mass and magnetic field of the planets.

Internal Structure

Jupiter Saturn Uranus Neptune

Liquid metallic Hydrogen Liquid Hydrogen & HeliumLiquid Hydrogen

Rocky core

Compressed water

Massive Jupiter is thought to be most similar in composition to the original nebula from which the Sun and planets condensed. Lower mass and other factors have been invoked to explain differences in the other three planets.

Temperature and radiation

Temperature at cloud tops -110oC -180oC -216oC -220oC

Albedo .52 .47 .51 .41

Energy radiated nearly twice ~three times less than greater than compared with that received from Sun

%H:%He:%other elements 85:14:1 96:3:1 82:15:3 79:18:3in atmosphere by mass

Jupiter Saturn Uranus Neptune

Voyager found that Saturn’s atmosphere had less helium than expected. One theory that explains both the lower helium in the atmosphere and Saturn’s excess energy radiation is that, since Saturn’s formation, Helium has been slowly dropping toward the core and its gravitational energy is converted to the observed excess heat.

Temperature and radiation

Temperature at cloud tops -110oC -180oC -216oC -220oC

Albedo .52 .47 .51 .41

Energy radiated nearly twice ~three times less than greater than compared with that received from Sun

%H:%He:%other elements 85:14:1 96:3:1 82:15:3 79:18:3in atmosphere by mass

Jupiter Saturn Uranus Neptune

Neptune’s excess radiated energy is also attributed to slow gravitational contraction.

Uranus’ lack of an internal energy source is a further contributor to its featureless weather systems.

Jupiter: The most massive planet, comprising 71% of all the planetary matter in our Solar System. With its high rotation rate, internal energy source, and impurities which colour its atmosphere at different depths, it exhibits a spectacularly detailed turbulent atmosphere with belts, storms, eddies and small ovals. It has a gravitational influence on objects such as comets and asteroids if they pass close enough.

Saturn: With a slightly smaller size and rotation rate than Jupiter; and less than a third of its mass, but a stronger internal source of energy, Saturn exhibits more subtle variations of Jupiter’s belts and storms in a similar three layered atmosphere.

General Descriptions

General DescriptionsUranus: Less than half the diameter of Saturn and twice as distant from the Sun, Uranus is much colder and lacking a strong internal energy source. Though it has high speed winds its atmosphere is generally featureless. Traces of methane give it its blue-green tint. Uranus’ unique feature is its 98° tilt to its orbit plane. That its ring system and most of its satellite orbits are circular and in its equatorial plane suggests it was perturbed early in its formation to a stable but inclined system.

Neptune: Slightly smaller but more massive than Uranus and with an internal heat source driving a high-wind atmosphere with rotating storms seen as spots and clouds. The blue colour of the atmosphere is because of its methane content which, like Earth’s air, scatters blue light more than red.

(c) Spacecraft Missions

Voyager 2, launched August 1977, reached Jupiter in July 1979. It went on to photograph Saturn (August 1981), Uranus (January 1986) and Neptune (August 1989), before heading out of the Solar System. Most of the photos in this Activity are from Voyager 2.

The Galileo spacecraft, launched October 1989, reached Jupiter in December 1995, dropping a probe into Jupiter’s atmosphere. It went on to an extensive photographic tour of Jupiter’s satellites. Galileo’s mission ended in September 2003. [See previous and following Activities.]

For information about the mission, visithttp://saturn.jpl.nasa.gov/

Cassini’s trajectory included 4 gravity assists: two at Venus, and one each at Earth and Jupiter

The Cassini-Huygens Mission was launched in October 1997 to arrived at Saturn in July 2004. It has begun its 4 year mission, which will include over 30 orbits of Saturn and its moons.

The Cassini-Huygens mission will study Saturn’s composition and atmosphere, magnetosphere, rings and satellites – specifically Titan. The Cassini spacecraft orbiter has 12 instruments, while the Huygens probe, which will descend to Titan, has another 6 instruments.

The Cassini Spacecraft

At 2150 kg, the spacecraft carries instruments for imaging, remote sensing and measuring magnetic fields and particles.

Cassini needs 600-700 watts to operate the science instruments and transmit their data. It must be able to produce power reliably for 11 or more years at up to 1.6 billion km from the Sun. Power is provided by 3 Radioisotope Thermoelectric Generators requiring 33kg of plutonium in total - a controversial aspect in the case of any launch or mission failure.

Cassini’s Huygens Probe

The Huygen’s Probe, supplied by the European Space Agency, is planned to be released from Cassini in December 2004. It will study the clouds, atmosphere, and surface of Saturn’s satellite Titan.

The probe will enter and brake in Titan’s atmosphere and parachute a fully instrumented robotic laboratory down to the surface.

It is designed for a maximum descent time of 2.5 hours and will spend at least 3 additional minutes (and possibly a half hour or more) on Titan’s surface.

Huygens Probe release over Titan

The near circular orbits of all the Sun’s family of planets; in the same direction and (apart from Pluto) generally close to the same plane; and (apart from Venus, Uranus and Pluto) the rotation of the planets in the same direction, are strong evidence that all formed along with the Sun from a condensing nebula of gas and dust.

[That we now see dust disks around hundreds of other stars and planets around tens of others is additional evidence that this is a normal occurrence for, at least, the Sun’s type of star.]

(d) Clues about the history of the Solar System

The Jovian gas giants, being the most massive planets and furthest from the heating effects of the Sun, should have retained the original material of the solar nebula better than the inner planets.

Their further study (by the Cassini spacecraft) and the study of their satellites (in the following Activities) thus adds to our knowledge of the possible formation and evolution of our Solar System.

In the next Module we will look at the satellites and rings of the Jovian gas giant planets.

NASA: http://www.nasa.gov

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

Hubble Space Telescope images indexed by subjecthttp://oposite.stsci.edu/pubinfo/subject.html

Image Credits

Now return to the Module 15 home page, and read more about the Jovian planets in the Textbook Readings.

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