PHYS The Jovian Planets - Jupiter Diameter = 11.2 x Earth’s Density = 1.33 x water Rotates in 9h 50min. at equator, 9h 55min. at poles, giving violent weather. Atmospheric composition 82% hydrogen, 18% helium, traces of other elements; similar to Sun’s. Atmosphere ~1000 km thick, clouds of ammonia crystals visible, liquid ammonia and water ice lower down. Pressure and temperature increase inwards so hydrogen becomes liquid then metallic - becomes a good conductor of electricity so Jupiter has a strong magnetic field. Core may be molten rock at 40,000 K. Still cooling, Jupiter gives out twice as much heat as it receives from the Sun. Has four major “Galilean” satellites, Io, Europa, Ganymeade and Callisto, many minor moons and a ring system.

PHYS Jupiter’s Moons Io About the size of our Moon. Very thin sulphur dioxide atmosphere. Active volcanoes - hot interior due to tidal heating - spew out liquid sulphur at 400 K. Hence, young surface, no craters visible. Europa Surface made up of water ice, density implies a rocky interior. Surface is cracked and almost crater free. Colouring of cracks suggests that dirty water may have erupted from a hypothetical underlying water ocean. Varying magnetic field consistent with liquid layer beneath surface, tidally heated.

PHYS Jupiter’s Moons Ganymede Largest moon in the Solar System - bigger than Mercury. Cratered region and grooved region. Craters up to 150 km wide - flat, suggesting surface flows over time. Surface is dark from accretion of rocks and carbon-rich materials. Grooved region is lighter, most probably water ice mixed with rock; cracking up under stress. Density suggests ice crust lies over a water/ice mantle surrounding a rocky core. Callisto Heavily cratered implying an old surface - estimate 4000 million years old. Craters shallow due to flow of ice/rock crust. Density indicates ice/rock crust over water/ice mantle and rocky core.

PHYS Jupiter’s Moons - Life Signs? Best bet - Europa. Observations are consistent with a liquid water ocean below the surface. Comparison with Io suggests that there may be volcanic vents on the ocean bed. Organic cyanides (nitriles) containing CN group found on Ganymede and Callisto. Life exists on Earth under similar conditions near the Mid-Atlantic Ridge in the form of thermophylic bacteria.

PHYS The Jovian Planets - Saturn Diameter = 9.45 x Earth’s Density = 0.7 x water i.e. it would float! Atmospheric composition 96% hydrogen, 3% helium, traces of other elements. Cloud belts, differential rotation and weather similar to Jupiter (not as violent), but haze above clouds makes colours less dramatic. Interior likely to be liquid hydrogen and metallic hydrogen. Like Jupiter, it has a strong magnetic field. It radiates more heat than it receives, maybe due to falling helium rain?? Most prominent feature is the ring system. Voyager spacecraft revealed complex structure with small “shepherd” satellites stabilising rings. Rings made up of water ice and rocky particles ranging from dust grain-sized to tens of metres.

PHYS Saturn’s Moons Titan Diameter = 5120 km Surface temp. = 94 K Density = 1.9 x water Atmospheric pressure = 1.5 x Earth’s Density suggests surface crust and mantle are ice surrounding a rocky core. Atmosphere mostly nitrogen, 1% methane. Also hydrocarbons such as ethane, propane, acetylene, trace of water discovered. Covered in orange smog - cannot see surface. Ultraviolet light from sun converts methane to heavier hydrocarbons, so methane should run out in a few million years, but Titan is 4500 million years old. So, must be a source of methane - volcanism? Heavier hydrocarbons e.g. ethane are liquid at 94 K, so fall as rain  oceans?

PHYS Saturn’s Moons Titan continued... Evidence? Bounce radio waves off surface - amount reflected is too high for all liquid ocean; could be due to ice continents. As part of the Cassini Mission to Saturn, launched in 1997, the Huygens probe was dropped onto Titan at 9am on 14th January 2005  “squelch-down” Huygens landed on a surface with a texture like crème brulée - water ice grains of a sandy consistency soaked with liquid methane? Surface at landing site shows signs of weathering and erosion. Turbulent atmosphere encountered during Huygen’s descent - methane clouds. Cassini Orbiter radar images show what look like drainage channels, river beds and shore lines.

PHYS The Jovian Planets - Uranus Twice as far from the Sun as Saturn (see lecture 2). Getting colder  58 K. Density = 1.27 x water Atmosphere of hydrogen, helium, methane, ammonia and water above a mantle of ices and rock, with a rocky core at the centre. Rotation axis in plane of orbit; at present its south pole points towards the Sun. Figs. Z11.30 & K9-36 Its moons are rocky and icy and show cratering and grooving. Miranda has the most complex surface with signs of tidal heating. Has a tenuous ring system discovered in Rings are dark - probably particles contain carbon and are bare of ice.

PHYS The Jovian Planets -Neptune Warmer (60 K) than expected due to solar heating (44 K) - must still be cooling down. Density = 1.64 x water Atmosphere is hydrogen, helium and a little methane giving it a bluish colour. Neptune has four rings similar to Uranus’. Only moons known before Voyager’s fly-by in 1989 were Triton and Nereid. Triton shows volcanic activity - a cracked crust with relatively fresh ice. It is in a retrograde orbit (east-to-west) inclined at 23 o to Neptune’s equator. Nereid has a highly eccentric orbit. Perhaps Triton and Nereid were not associated with Neptune when they were formed and have since been “captured”.

PHYS Pluto and Charon Pluto’s distance from the Sun varies from 30 AU to 49 AU - it was closer to the Sun than Neptune was from Its maximum temperature is about 60 K. Density = 2.1 x water - consistent with composition being 75% rocky, 25% ices. Surface coated with solid methane, solid nitrogen and solid carbon monoxide. The same molecules form a very tenuous gaseous atmosphere. Composition shows similarities to Neptune’s moons. May be an escaped moon of Neptune - could be related to cause of Triton’s retrograde orbit if there was a close encounter? Charon is Pluto’s (largest) moon and is about half its size, in fact more like a companion dwarf planet. It may consist of ices.