Gas Giant Planets and Their Satellites: Jovian Planets – a Brief Inventory

Gas Giant Planets and Their Satellites: Jovian Planets – A Brief Inventory: The largest planets in the solar system are the ones we are least likely to visit in person. • The primary distinguishing feature of Jovian (or gas giant) planets is their lack of a surface. • Their composition is similar to the Sun (mostly H and He, with small amounts of everything else). This is a remnant of how they formed. • Their densities are also similar to the Sun (1 gm cm-3) • They are also fluid bodies like the Sun, which means that they rotate differentially. • Differential rotation is like water stirred in a pot. The material in the center rotates at a different speed than at the edge. • For a Gas Giant we can think of its interior as a series of cylinders each moving with its own angular speed. • The size and relative speed of the cylinders depends on the interior. • The Giant Planets also rotate very rapidly, which distorts them from a spherical sphere Oblate spheroid shape. • A cloud on the equator of Jupiter is moving 45,000 miles an hour! How are we Able to Know This? Clouds!!!! • Clouds on the Jovian planets are structurally unimportant, but they can be followed to show the larger structure of the outer layers. • In the outer solar system clouds can be made of more than just water. • They also can be made of Methane, Ammonia, Ammonium Hydrosulfide, Phosphine, Ethane, and other Aerosol compounds. • Each type of cloud has a different color. • Where the clouds form is a function of temperature and pressure in the planet’s atmosphere. • Different colors probe different levels in the atmosphere! Clouds and Mapping the Jovian Planets. The depth of the cloud and aerosol layers is determined by the temperature and pressure profiles of the atmosphere. • Jupiter is the warmest of the Jovian planets, and it’s clouds are at the highest altitudes. This enhances their contrast. • Watching the movement of the clouds tells us about the underlying convection patterns of the atmosphere. • The banded structure shows zones and belts of convective motion in the atmosphere • On Saturn the clouds are deeper and the contrast lower. • Uranus and Neptune have clouds very deep and very few visible features in their atmospehres (especially Uranus). Interiors of the Gas Giants: From the combination of shape (oblateness), Rotation Rate, and composition we are able to determine the interior properties of the gas giants. • The gas giants differ from the Sun in the way that they are organized. • While both are composed mostly of H and He, the Sun is held up by Fusion, which heats the interior to millions of degrees, while the Jovian planets are supported by pressure. • Each has differentiated to form a fluid mantle around a (possible) rocky core several times more massive than the Earth, which is in turn covered by a layer of compressed gas. • In the case of Jupiter and Saturn the mantle consists of an exotic substance called ‘metallic hydrogen’. • Uranus and Neptune have less Hydrogen, are cooler, and are smaller. Their mantle is likely to be composed of pressurized water ice. Cooling Down: The interiors of the Jovian planets are very hot (5,000-20,000 K). • Unlike the terrestrial planets, which are shielded by their crust, the heat escapes from gas giants via convection, just like on the Sun. • If this happened on Earth, it would be a cold rock today. • All the Jovian planets produce more energy than they receive from the Sun except Uranus. So where does the energy come from? • Partly it comes from the initial formation of the planets. For the much cooler Jovian worlds (50-150 K), it takes longer for this heat to escape. • Some energy is generated by on-going differentiation, where heavy material falls to the center of the planet. • And yet more energy comes from simple contraction. A cooler interior compresses more. What Happened to Uranus? Uranus is one of the strangest objects in the solar system. • It’s rotational axis is tilted almost 90° from the norm into the plane of its orbit. This makes its seasons the most extreme of any planet. • Uranus’ magnetic field is offset from its center and tilted by 59°. • Uranus produces less heat than the other jovian planets. • Its larger satellites are denser than the other objects their size in the outer solar system. • What Happened???? Giant Planets-Giant Dynamos: Convective motions of ice and liquid Hydrogen do more than just transfer heat. They produce magnetic fields. • All of the Jovian planets have strong magnetic fields. • The depth of the dynamo region determines how much like the Earth it is (dipolar and aligned to rotation). • Saturn is most like the Earth. Uranus and Neptune are VERY different. Giant Planet Magnetospheres: While all of the giant planets have strong fields, their solar wind interactions are very different. • The increasing distance from the Sun plays a role. • The magnetic field and rotation axis orientations are important for both Uranus and Neptune. • For Jupiter and Saturn, it is their satellites that define the interaction. Jupiter: The Jovian Magnetosphere is extreme in many ways. • Jupiter has the strongest magnetic field of any planet. • Its magnetosphere is also the largest continuous structure in the solar system, stretching all the way to Saturn. • The connection the Jupiter’s ionosphere produces bright aurora with an energy output equal to the entire USA. Jupiter’s Little Helper: Strangely enough, Jupiter is not responsible for the size of its magnetosphere. • The primary driver is an unlikely source. Jupiter’s moon Io! It took Voyager to discover this. • Io is the most volcanically active object in the solar system! • Io’s volcanoes don’t produce lava, they produce liquid sulfur, mixed with sodium and sulfur-dioxide! • They are recovering Io at a rate of 10 cm/yr. Stretching Io: • Its volcanoes spray material far above the surface. • What’s driving this? TIDES!!!! • Every time Io is aligned with Jupiter and the Moon Europa it is stretched. This heats the moon. • Escaping atmosphere forms a torus of gas and plasma around Jupiter Torus and Jupiter: • The Io plasma torus rotates with Jupiter’s magnetic field, which is 75 km/sec faster than Io orbits. • The plasma in the Io torus helps to inflate Jupiter’s magnetosphere and modulates the solar wind interaction. • We used to think that Io and its torus controlled the entire system, but that turns out to be only partly true. Other Jovian Moons (Ganymede): • Ganymede is the largest Moon in the solar system. • Its interior appears to be fully differentiated (like Earth) with an icy-slush mantle around a rocky core. • Its surface is relatively young, suggesting an active, warm interior similar to Earth. • Ganymede is large and cool enough to retain an atmosphere. Why doesn’t it? Ganymede’s Secret: • The big surprise from Galileo was the fact that it has a magnetic field! • This makes it the only moon in the solar system with one. • What is doubly interesting is that Ganymede is inside Jupiter’s magentosphere! • HST has observed aurora from the interaction between these two fields and the thin exosphere of Ganymede. Europa (Jupiter): Europa has generated tremendous excitement due to the possibility that it may harbor conditions favorable to LIFE. • Europa is the smallest of the Galilean (Large Jupiter) Moons, but its density is twice that of other icy moons. • It is also heated by the same process of tides as Io. • At first glance Europa isn’t terribly interesting. • But appearances are deceiving. Europa (Jupiter): • When we look closer we see chaotic terrain. • Where might we have seen this before? Europa: • It looks as though Europa’s water mantle is liquid! • Tidal heating appears to be keeping liquid perhaps via deep volcanic activity. • If this is true, then Europa has more liquid water than the rest of the solar system combined!!!! • If the volcanic theory is correct…. • Where there’s smoke…there could be LIFE!!!.

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