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Module 1 - The Solar System

The Outer Planets

The outer, Jovian (named after Jupiter) planets consist of Jupiter, Saturn, Uranus, Neptune.

While these planets are large in diameter, their densities are relatively low, ranging from 687 to 1638 kg/m³ or 0.687 to 1.638 g/cm³. With a density of less than 1 g/cm³, Saturn would theoretically float on water. These planets cannot be rocky bodies and they are indeed primarily composed of gases and liquids. Their surface pressures are unknown because their surfaces are deep in their atmosphere and the locations and pressures are not known.

These planets have also managed to acquire a large number of moons, but we will see later that the really interesting moons are the inner, larger ones. This image shows the major moons of the outer planets, in relation to our moon. Also added into this size comparison are Mars' moons and Pluto's moon Charon, and of course the Earth itself.

To see how these larger moons compare to all the moons within the solar system see this image. We should point out that this image is at least 7 years old and we keep discovering more moons in the outer regions of the solar system. At present, Jupiter has at least 67 moons alone.

And even though it is no longer a planet, we'll stop at Pluto next.

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Gas Giant Interiors *Jupiter* Jupiter's composition is mainly hydrogen and helium. In contrast to planetary bodies covered with a hard surface crust (the Earth, for example), the jovian surface is gaseous-liquid, rendering the boundary between the atmosphere and the planet itself almost indistinguishable. Below the roughly 1000-kilometer-thick atmosphere, a layer of liquid hydrogen extends to a depth of 20,000 kilometers. Even deeper, it is believed that there is a layer of liquid metallic hydrogen at a pressure of 3 million bars. The planet core is believed to comprise iron-nickel alloy, rock, etc., at a temperature estimated to exceed 20,000C. *Saturn* As with Jupiter, Saturn is mainly composed of hydrogen and helium and is observed to be of extremely low density. In fact, Saturn's mean density is only about two-thirds that of water. The Saturn atmosphere comprises, in descending order of altitude, a layer of ammonia, a layer of ammonium hydrogen sulfide, and a layer of ice. Below this, the saturnian surface is a stratum of liquid hydrogen (as in the case of Jupiter) underlain with a layer of liquid metallic hydrogen. It is believed that the liquid hydrogen layer of Saturn is thicker than that of Jupiter, while the liquid metallic hydrogen layer may be thinner. The planet's core is estimated to be composed of rock and ice. *Uranus* Uranus is gaseous in composition, mainly comprising hydrogen and helium as in the case of Jupiter and Saturn. The planet atmosphere is mostly hydrogen but also includes helium and methane. The planet core is estimated to be rock and ice encompassed by an outer layer of ice comprised of water, ammonium, and methane. *Neptune * The atmosphere of Neptune consists of mainly hydrogen, methane and helium, similar to Uranus. Below it is a liquid hydrogen layer including helium and methane. The lower layer is made up of the liquid hydrogen compounds oxygen and nitrogen. It is believed that the planet core comprises rock and ice. Neptune's average density, as well as the greatest proportion of core per planet size, is the greatest among all the gaseous planets. *Image Credit:* Lunar and Planetary Institute
Moons of the Solar System All the planetary moons in our solar system are shown here at their correct relative size and true color. Their diversity of size and appearance is testament to the unique and fascinating geologic history that each of these bodies has undergone. Two of the moons are larger than the planet Mercury, and eight of them are larger than Pluto. Earth's Moon is the fifth largest of the set, with a diameter of 3476 kilometers (2160 miles). Most of the moons are thought to have formed from a disk of debris left over from formation of the planet they orbit. However Triton, Neptune's largest moon, and several of the smallest moons, including the moons of Mars, are thought to be captured planetesimals that formed elsewhere in the solar system. Earth's Moon is thought to have formed from the debris ejected from a roughly Mars-sized object colliding with the early Earth, perhaps a unique event in the history of the solar system. The moons are organized on the diagram by the planet they orbit (top to bottom with increasing distance from the Sun) and their position relative to the planet (left to right with increasing distance from the planet). Below is a listing of the names of all the moons and the planets they orbit. Most moons are named for mythological characters associated with the character the planet is named for. While most of the planets are named for Roman characters (with the exceptions of Pluto and Uranus), most of the moon have names from Greek mythology. For example, Phobos and Deimos are the sons of Ares, the Greek version of Mars. Jupiter?s moons are all named for lovers and other close associates of Zeus (Jupiter). Saturn?s moons are named for Titans, the race that included Cronos (Saturn), Zeus? father. Neptune?s moons are named for mythological characters associated with water, and Charon was the ferryman of the dead who brought people to Pluto?s realm. By tradition, the discoverer of a moon gets to name it (now subject to approval by the International Astronomical Union). The son of the discoverer of the first two moons of Uranus (Sir William Herschel) decided to name Uranus? moons not for mythological characters, but instead for the king and queen of fairies in Shakespear?s A Midsummer Night?s Dream . This began a tradition whereby all uranian satellites are named for fairy characters in English drama. To read more about the names of the planets and their satellites, go to the U.S. Geological Survey?s nomenclature guide at http://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/append7.html . *Earth* Moon *Mars * Phobos, Deimos *Jupiter* Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope *Saturn * Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Calypso, Telesto, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, Phoebe *Uranus * Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, "Ariel, Umbriel, Titania, Oberon *Neptune* Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid *Pluto * Charon *Image Credit*: Image processing by Tim Parker (Jet Propulsion Laboratory) and Paul Schenk and Robert Herrick (Lunar and Planetary Institute), based on NASA images.
Moons of the Solar System