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Title Frame Activity 2: The Other Jovians Module 15: The Jovian Planets 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. (a) Comparative Statistics The value of comparative studies • 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 . . . 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 . . . Orbit Comparisons (i) A perspective view of the almost-circular orbits. Note how our inner Solar System shrinks in comparison. Uranus Saturn Jupiter Sun to Mars Neptune 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. Neptune Uranus Saturn Jupiter Sun to Mars space is certainly a lonely place ... 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!). Uranus Saturn Jupiter Sun to Mars Neptune Orbit Comparisons (ii) 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. Neptune 165 years Uranus 84 years Saturn Jupiter 30 years 12 years Sun to Mars 5.2 AU All orbits are anti-clockwise when north is ‘up/out of the screen’ 9.5 AU 19.2 AU 30.1 AU Orbit Comparisons (iii) Comparing circularity of orbits, viewed from directly above and shown to scale J 0.048 Compare with the grey circles S 0.056 U 0.046 N 0.009 1.3o 2.5o 0.8o 1.8o Eccentricities all close to zero The same direction of orbits, the approximate circularity and common orbital planes, all point to a common origin mechanism Comparing inclination of orbits, viewed edge on 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 Sizes to scale Earth Diameter (km) 142,985 120,537 51, 119 49,528 (Earth=1) 11.2 9.4 4 3.9 Mass (Earth=1) 318 95 14.5 17 17h14m 16h07m .024 .026 98° 30° Rotation* period 9h55m (body of planet) 10h39m Oblateness .065 .108 Tilt of axis to orbital plane 3° 27° *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 Irregularities Before 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 anticlockwise 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 Retrograde* Rings# Planet Satellites Over 1000km 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 Most regular satellite orbits and rings are near-circular and inclined at a similar angle to the planet’s axial tilt; even Uranus’! * 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. # Ring system numbers relate to appearance from a distance. Close detail show subtle influence of associated satellites. ** Note that many new satellites have been announced since 1997 and are still being discovered. We’ll learn about these in the Activity Minor Jovian Satellites & Rings Photo Gallery White 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 Increasing Saturn detail showing ovals, swirls and storms Neptune’s dark spots and ‘scooter’ clouds NASA/JPL Voyager 2 images Magnetic Fields Planets are shown (not to scale) with their rotational axes tilted to their orbital planes Comparing the magnetic fields of the four Jovian planets with that of Earth . . . • 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 • 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. • 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 . . . Internal Structure Jupiter Saturn Uranus Neptune Rocky core Compressed water Liquid Hydrogen Liquid metallic Hydrogen Liquid Hydrogen & Helium 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 Rocky core Compressed water Liquid Hydrogen Liquid metallic Hydrogen Liquid Hydrogen & Helium 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 Albedo Jupiter Saturn Uranus Neptune -110oC -180oC -216oC -220oC .52 .47 .51 .41 Energy radiated nearly twice compared with that received from Sun %H:%He:%other elements in atmosphere by mass 85:14:1 ~three times 96:3:1 less than 82:15:3 greater than 79:18:3 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 Albedo Jupiter Saturn Uranus Neptune -110oC -180oC -216oC -220oC .52 .47 .51 .41 Energy radiated nearly twice compared with that received from Sun %H:%He:%other elements in atmosphere by mass 85:14:1 ~three times 96:3:1 less than greater than 82:15:3 79:18:3 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. General Descriptions 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 Uranus: 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.] 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. Cassini’s trajectory included 4 gravity assists: two at Venus, and one each at Earth and Jupiter 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. For information about the mission, visit http://saturn.jpl.nasa.gov/ 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 (d) Clues about the history of the Solar System 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.] 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. Image Credits NASA: http://www.nasa.gov Indexed status of all NASA spacecraft http://www.hq.nasa.gov/office/oss/missions/index.htm Hubble Space Telescope images indexed by subject http://oposite.stsci.edu/pubinfo/subject.html Now return to the Module 15 home page, and read more about the Jovian planets in the Textbook Readings. Hit the Esc key (escape) to return to the Module 15 Home Page