(8) Planets Geophysics 178 Introduction to Geophysics and Planetary Physics

The New Solar System – Episode 1 For a short time in Sept our Solar System com- prised 11 planets (the usual suspects plus Ceres – the largest asteroid belt object, Charon – the largest moon of Pluto, and the recently discovered 2003 UB 313 ). A ballot at an IAU meeting changed everything: Geophysics 179

The „New Solar System“ knows only 8 Planets, 4 terrestrial planets and 4 gas planets (Source: IAU). Pluto is now only a Dwarf Planet, like Ceres, the largest body in the Asteroid Belt and 2003 UB 313 (Eris), the largest body in the Kuiper Belt (thereby “dwarfing” Pluto). The New Solar System – Episode 2 Geophysics 180

Pluto – Postscript 2006 was a very eventful year for Pluto. Two new moons were discovered based on a Hubble Space Telescope image: Nix and Hydra (left: HST), but then Pluto lost its planetary status due to a decision of the IAU (International Astronomical Union). Meanwhile he is a „Dwarf Planet“, with a new name: Pluto. The IAU decision is, however, under dispute – and resistance is active (e.g. below: Geophysics 181

Eris The cause for Plutos „degradation“ was the discovery of an even larger Kuiper belt object: 2003 UB 313, meanwhile (aptly) baptized Eris, after the Greek goddess of discord (the one with the apple). Eris is orbited by a moon: Dysnomia (after Eris’ daughter, the demon of lawlessness (Picture: HST). Geophysics 182

(8b) Gas Planets Geophysics 183 Introduction to Geophysics and Planetary Physics

Jupiter Mean radius: km = RE Mass: ·10 24 kg = ME Mean density: kg/m 3 Rotation period: 9.93 h Orbital period: years Sun distance: 5.2 AU Moons: 67 (status: 2015) upiterfact.html Pictures: NASA Geophysics 184

Saturn Mean radius: km = 9.14 RE Mass: 568.5·10 24 kg = ME Mean density: 687 kg/m 3 Rotation period: h Orbital period: years Sun distance: 9.6 AU Moons: 62 (status: 2015) saturnfact.html Picture: HST Geophysics 185

Uranus Mean radius: km = 3.98 RE Mass: 86.83·10 24 kg = ME Mean density: kg/m 3 Rotation period: h Orbital period: 84.0 years Sun distance: 19.2 AU Moons: 27 (status: 2015) uranusfact.html Picture: L. Sromovsky, Keck Telescope Uranus has an axial tilt of 97.8 ° – its rotation axis is almost parallel to the orbit plane. Geophysics 186

Neptune Mean radius: km = 3.86 RE Mass: 102.4·10 24 kg = ME Mean density: kg/m 3 Rotation period: h Orbital period: years Sun distance: 30.0 AU Moons: 14 (status: 2015) neptunefact.html Picture: NASA Neptune’s largest moon Triton shows cryo-volcanism. Geophysics 187

Saturn (1) The „Lord of the Rings“. With an equator diameter of ~ km he is the second largest planet in our solar system. Besides a small rocky core, the main constituents are Hydrogen and Helium, yielding a density of just 0.7 g/cm 3 – less than water. The short rotation period (10½ hours) leads to an oblateness of ~1/10 and to impressive wind speeds of up to km/h (Source, also for the next slides: Cassini-Spacecraft, NASA/ESA). Geophysics 188

Saturn (2) Saturn‘s most striking features are its rings, which show an impressive radial extent – but only a surprisingly small thickness of 10 – 100 m (right, eith the moon Dione immediately above). Galileo Galilei was the first to observe them (in 1610), but Christiaan Huygens discovered their nature as a disk surrounding Saturn. The rings A and B are separated by the Cassini Division (left, discovered by Giovanni Domenico Cassini in 1675). Geophysics 189

Saturn (3) The moon Dione just above the ring plane (note the ring‘s shadows on Saturn‘s cloud cover). The rings are predominantly composed of water ice (particles and boulders from ~1 cm to ~10 m diameter). Geophysics 190

Saturn (4) The material depletion in the Cassini Division is caused by the gravitation of the “Death Star Moon” Mimas (left: above the rings, right: in detail). The central peak of Herschel Crater is 7 km high. Geophysics 191

Saturn (5) The total mass of the rings amazingly small (similar to the mass of Mimas km diameter). The rings are probably remnants of a destroyed moon. Geophysics 192

Saturn (6) The seven main rings have been named in chronological order (of discovery) – with increasing distance from Saturn they are therefore termed: D, C, B, A, F, G and E (above). The small walnut-shaped moon Pan (35 km x 25 km, left) acts as a “shepherd moon” – right inside the Enke-Gap, close to the outer edge of the A-ring. Geophysics 193

Saturn (7) Enceladus, with a diameter of ~ 500 km, is characterized by a surface of pure water ice (and snow) yielding a huge albedo of 99 % – and as a consequence a surface temperature of just about –200 °C. Large areas with no craters at all are geologically young – the moon is obviously geologically active. Geophysics 194

Saturn (8) Striking features on Enceladus are the „Tiger stripes“ near the South Pole. Geophysics 195

Saturn (9) As a big surprise the temperatures at the South Pole turned out to be higher than at the Equator (upper left). The „tiger stripes“ are warmer than the surrounding (above, IR data). Here are also source regions of water vapor eruptions (cryo-volcanism) (left) – delivering material (in form of ice crystals) for the E-Ring – and snowfall on the surface. Geophysics 196

Saturn (10) The moon Iapetus (with a diameter of 1400 km) has a (real) Equatorial Bulge, which is about 13 km high and 20 km wide. Its cause is currently entirely unclear. Iapetus has – literally – two different sides The leading one is one of the darkest surfaces in our solar system – with an albedo of 3-5 %, while the other hemisphere (below) shows an albedo of 60 %. There is obviously some work left for young planetary scientists. Geophysics 197

Saturn (11) The moon Hyperion is (for a moon of its size) amazingly un-spherical (painstakingly not following lecture rules). With dimension 360 × 280 × 225 km it is the largest (clearly) non- spherical body in our solar system, Even more – Hyperion’s rotation is chaotic, its rotation axis changes – in an unpredictable manner. Hyperion seems to be the remnant of a formerly larger body, which suffered a huge impact. Is surface shows a impressive crater – about 10 km deep, with a diameter of 120 km. Geophysics 198

Saturn (12) With a diameter of 5150 km the moon Titan is larger than the planet Mercury. Titan has a surprisingly dense atmosphere with (similar to Earth) nitrogen as main constituent (94 %) and a surface pressure of about 1.5 bar. Titan is entirely covered by organic dust. The surface temperature of about –180°C allows for methane rain. Geophysics 199

Saturn (13) In January 2005 the space-probe Huygens successfully landed on Titan (as first European lander on a planet or moon). Middle: the first look through the cloud cover, right: the surrounding of the landing place. Radar-images of Cassini (left) confirmed the existence of Methane lakes (with diameters from von 3 to 70 km). Geophysics 200

Saturn (14) Geophysics 201

Einführung Geophysik The End Introduction to Geophysics and Planetary Physics