ASTR 121 (O'Connell) Study Guide 20
Outer Plan Title

Jupiter from Voyager with Io and Europa in foreground


"And now for something completely different," as they used to say on Monty Python.

The Jovian planets are entirely unlike the terrestrial planets. They may have rocky cores, like larger versions of the Earth, at their centers, but these are enveloped in giant gaseous atmospheres. Only the outermost skins of these atmospheres can be studied directly. This is meteorology, instead of the geology/topography we discussed for the terrestrials. However, it can be just as extreme with respect to Earth-bound meteorology as are the canyons and mountains of Mars compared to those of Earth.

Another major distinction of the Jovians is the large number of satellites they possess. The satellites, observed at close range by spacecraft, exhibit an astonishing diversity of surface types and features. In many ways, they are more interesting than their parent planets. The ring systems of the Jovians are probably the remnants of distintegrated satellites.


A. HISTORY

    Prehistory: Jupiter, Saturn known

    1600's: Telescopic studies: satellites of J,S; red spot of J; rings of S

    1781: Herschel discovers Uranus (accidental)---first new planet in recorded history

    1790+: Deviations in U orbit, assumed caused by gravity of unknown planet, lead to prediction of its location based on Newton's law of gravity.

    1845: Neptune discovered as predicted = a triumph of Newtonian mechanics

    1930: Tombaugh discovers Pluto. Much more difficult than Neptune.

    1950+: Deeper searches: no planet larger than Neptune to 60 AU's; but many asteroidal/cometary objects

    1979-89: Voyager 1 & 2 spacecraft fly by J, S, U, N. Highly successful missions, but require elaborate planning.

    1994: Comet Shoemaker-Levy 9 collides with Jupiter; acts as an atmospheric probe.

    1995: Galileo orbiter/probe mission arrives at Jupiter; sends probe ~ 50 miles into atmosphere; orbiter continues to study J. and satellites

    2000: On its way to Saturn, the Cassini-Huygens mission passed close to Jupiter in order to pick up a velocity boost from Jupiter's gravity. Obtained images, magnetic field, and other measures of Jupiter.

    2004: Cassini-Huygens approaching Saturn.



B. JOVIAN PLANETS (J,S,U,N): PROPERTIES

    These four share gross properties. Pluto is entirely different (see below).

    Distant from Sun: 5-30 AU. Pluto at 39 AU. Outer solar system is vast (over 10,000 times the volume of the inner solar system out to Mars) and sparsely populated.

    Large: 4-11 x Earth radius. Masses 15(U)-318(J) x Earth. J. contains twice as much mass as all other planets combined. An animated image of Jupiter's rotation is shown at the right.

      Jupiter is midway (on a power of ten scale) between planets and stars. Objects only 13 times more massive are considered to be small stars.

    Structures

    • Internal structures are entirely different from terrestrial planets. A product of their formation out of the cool regions of the solar nebula, dominated by icy (H-rich) solids.

    • Low mean density (~ 1 gr/cc) ===> mainly composed of H, He; only small rocky cores

    • High internal pressures in J,S convert hydrogen to liquid "metallic" form in interiors

    Visible Surfaces

    • No solid surfaces: "gas giants"

    • Visible surfaces = cloud layers, about 150 miles deep.

    • "Spots", e.g. Jupiter's Red Spot (large oval in image above right: 22,000 mi long ~ 3x Earth). Long-lived cyclonic storms. Similar features on other planets (e.g. Neptune).

    • Banding (see Saturn image in pseudocolor, below right) caused by lateral winds and rising/falling convection currents. Winds reach 300 mph.

    • Videos of Jupiter atmosphere:

    Special Probes of Jupiter

    Magnetic Fields: strong; generated by motions in cores of liquid metallic hydrogen



Pseudo-color infrared image of Saturn

C. RING SYSTEMS

    Saturn system is brightest, but rings are present around all 4 Jovians

    Saturn's Rings
    • Not solid: the inner rings rotate faster than the outer ones, as expected for objects in Keplerian gravitational orbits

    • Composed of billions of ice-coated particles (typically about 10 cm in size). Different particle sizes, coatings produce some of the structure visible in the rings.

    • Origin: debris from tidally/collisionally fragmented satellites

        Rings lie inside the planet's Roche Limit. Inside the RL, gravity tides would pull apart a large body, such as a satellite.

    • Structure: complex! (at right). The biggest gaps are "resonance" effects produced by the cyclical gravitational tug of the satellites outside the ring. The ringlets may be produced by the self-gravity of the material in the rings.

    • Video of Saturn's rings


D. SATELLITES

    Numerous: 8-28

      Click here for Java animation of orbits of satellites

    Diverse(!) characteristics; often violent histories

    Larger moons are mixtures of rocky/icy materials

    • Some larger than Mercury

      Io
    • Form with planet

    Smaller sats rocky/irregular

    • Many are captured asteroids

  • Io (J) (at right): continual volcanic eruptions caused by heating from tidal flexing in Jupiter's gravitational field. Much more active today than Venus or Mars. Click for enlargement.

  • Europa (J): ice-coated; new evidence from Galileo orbiter of underlying oceans (kept warm by tidal flexing?). There is much speculation about a possible biosphere on Europa (see Study Guide 23).

  • Titan (S): has an atmosphere! mostly nitrogen with small amount of methane; photo-chemistry has produced many hydrocarbons that form an obscuring haze; hydrocarbon oceans? For atmospheric profile, click here.

      Titan is the target of the Cassini-Huygens Mission, now on its way to Saturn. The primary spacecraft will go into orbit around Saturn in June 2004 and release a probe to enter the atmosphere of Titan and land softly on its surface.

  • Miranda (U): shattering collision & reassembly

  • Triton (N): geysers

Artist's Concept of Huygens Probe at Titan


E. PLUTO

Pluto is entirely unlike the other four large outer planets. It is smaller by a factor of 2 than any other planet. It is a rocky/icy object rather than a gas giant. Its characteristics have more in common with the the "minor planets," "asteroids," or "comet nuclei," small rocky/icy bodies which are members of the "Kuiper Belt" and are now being found in large numbers in the outer solar system.

There was a recent debate over whether to demote Pluto from its planetary status. For now it remains the "ninth planet," according to the International Astronomical Union, which expresses its regrets for the "widespread public concern" over the status of Pluto.


Reading for this lecture:

    This Guide covers about 1.5 lectures.

    Seeds, Chapters 23 and 24

    Study Guide 20

Reading for next lecture:
    Seeds, Chapter 25

    Study Guide 21

Web Links:

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Last modified April 2004 by rwo

Text copyright © 1998-2004 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.