Lecture 25 The JOVIAN PLANETS They formed beyond

Lecture 25

The JOVIAN PLANETS

They formed beyond the frost line to form large, icy planetesimals which were massive enough to… Capture H/He far from Sun to form gaseous planets. Each Jovian planet formed its own “miniature” solar nebula. Moons formed out of these disks.

The Jovian planets are principally made of hydrogen & helium, with composition very similar to the Sun. • Moving from the surface to the core: • temperature increases • pressure & density increases At low temperatures and pressures characteristic of the outer layers of Jovian planets: Hydrogen is a diatomic gas (H 2) At higher pressures (few thousand km below the upper cloud deck): H 2 becomes dissociated and undergoes a phase transition from the gaseous to a liquid state.

At pressures greater than 3 million atmospheres, H is squeezed so tightly that the atoms are separated into freely moving protons and electrons: liquid metallic hydrogen (LMH). LMH is highly conducting (the electrons are highly mobile) The combination of a metallic hydrogen interior and high rotation rates give the Jovian planets (especially Jupiter) strong magnetic fields.

• All Jovian cores appear to be similar. • • • Uranus & Neptune captured less gas from the Solar nebula. • • ● made of rock, metal, and Hydrogen compounds 10 x the mass of Earth accretion of planetesimals took longer not much time for gas capture before nebula was cleared out by Solar wind Only Jupiter and Saturn have high enough pressure for H & He to exist in liquid and metallic states.

• Jupiter emits almost twice as much energy as it absorbs from the Sun. • • • accretion, differentiation, radioactivity can not account for it Jupiter must still be contracting Jupiter has 3 x more mass than Saturn, but is not much larger! • the added weight of H & He compresses the core to a higher density, just like stacking pillows Uranus & Neptune have less mass than Saturn, yet they have higher densities they must be made of denser material

Jovian Planet Atmospheres

Jupiter’s Atmosphere • • In 1995, the Galileo space probe plunged into the planet Jupiter! It measured the atmospheric structure of Jupiter • thermosphere {absorbs Solar X-rays} • stratosphere {absorbs Solar UV} • troposphere {greenhouse gases trap heat from both Jupiter and the Sun} These are the same structures found in Earth’s atmosphere. • Atmospheres are governed by interactions between sunlight and gases.

Jupiter’s Cloud Layers Ø Convection in the troposphere causes Jovian weather. Ø Warm gas rises to cooler altitudes, where it condenses to form clouds. Ø Three gases condense in the Jovian atmosphere: v ammonia (NH 3) v ammonium hydrosulfide (NH 4 SH) v water (H 2 O) Ø They condense at different temperatures, so their differently colored clouds form at different altitudes.

The Jovian Atmospheres • • • The temperature profile of each planet determines the color of its appearance. Cloud layers form where a particular gas condenses. Saturn has the same cloud layers as Jupiter. they form deeper since Saturn is colder overall • they are spread farther apart since Saturn has lower gravity Uranus & Neptune • ● • cold enough to form methane clouds

• Convection in the troposphere, where the clouds form, coupled with rapid rotation of the Jovian planets leads to numerous bands of rising and falling air. • These are the colored “stripes” which we see in Jovian cloud structure.

• We also see high pressure storms • • ● analogous to hurricanes, but they rotate in the opposite direction Jupiter • the Great Red Spot • we are not sure why it is red Neptune • the Great Dark Spot

Saturn’s Atmosphere Saturn is not as colorful or turbulent as Jupiter. This may be due to the fact that Saturn is cooler because it is further away from the Sun. It does not contain storms as large as those seen on Jupiter, nor are they permanent. It also has enough gravity to hold on to all the gasses and consists of 92. 4% hydrogen, 7. 4 % helium, and traces of methane and ammonia.

Atmospheres of Uranus and Neptune Both planets have similar makeup to Jupiter and Saturn Unlike Jupiter and Saturn, ammonia is not significant in either of these planets. However, both planets have significant amounts of methane in their upper atmospheres - makes them appear blue.

Magnetospheres ● A planet’s magnetic field attracts and diverts the charged particles of the solar wind to its magnetic poles. • particles spiral along magnetic field lines and emit light (aurora); protective “bubble” is called the magnetosphere • ● The Earth is only terrestrial world with a strong magnetic field • • solar wind particles impact the exospheres of Venus & Mars solar wind particles impact the surfaces of Mercury & Moon

Jovian Planet Rings

The Rings of Saturn • From Earth, they look solid. • concentric rings & Cassini division • From spacecraft flybys, we see thousands of individual rings. • separated by narrow gaps • differ in brightness & transparency • From within the rings, we would see many individual particles • size ranges from boulders to dust • reflective H 2 O ice (snowballs) • many collisions keep ring thin

The other ring systems: fewer particles, smaller in extent, darker particles

Origin of Planetary Rings • Within 2 or 3 planetary radii of a planet, tidal forces will be greater than the gravity holding a moon together. A moon which wanders too close will be torn apart. Matter from the mini-nebula at this distance will not form moon. • Rings can not last the age of the Solar System. Particles will be ground to dust by micrometeorite collisions. Atmospheric drag will cause ring particles to fall into planet. ● There must be a source to replenish ring particles. gradual dismantling of small moons by collisions, tidal forces, etc. ● The appearance of ring systems must change dramatically over millions or billions of years.

Jovian Planets have Numerous Moons • small moons (< 300 km across) • not spherical • probably captured asteroids • Medium/large moons formed like planets out of the “mini-Solar nebulae” surrounding the Jovian planets

Are these moons are too small for active geology to occur? • No! • terrestrial planets made mostly of rock; Jovian moons mostly ice Ices melt at lower temperatures than rock. ● less heating is required to have molten cores • volcanism and tectonics can occur There is another heat source. • • tidal heating plays a more important role There is very little erosion due to lack of substantial atmospheres with the exception of Titan. • ●

The Jovian Moons • The moons of Jupiter become less dense as you get farther from Jupiter • • “mini Solar System” Gravitational tidal heating keeps the interiors of the inner moons hot.

The Large Jovian Moons • Jupiter - Galilean Moons sulfur volcanoes world of water ice (and liquid? ) active ice world dead & dirty ice world ● Io • Europa • Ganymede • Callisto Saturn thick atmosphere (N 2 & CH 4) ● Titan Neptune • • • Triton nitrogen volcanoes, retrograde orbit

Io �� • Jupiter’s tidal forces flex Io like a ball of silly putty. • friction generates heat • interior of Io is molten • Volcanoes erupt frequently. �� • sulfur in lava accounts for yellow color • surface ice vaporizes and jets away • Evidence of tectonics & impact cratering erased.

• Io loses volcanic gases into space. Ø ions of Sulfur, Oxygen, Sodium form a donut-shaped belt of charged particles, called the Io torus Ø they follow Io’s orbit & are a source of charged particles for the auroras The Io Torus

Io Torus as imaged by the Galileo Spacecraft

Europa • metallic core, rocky mantle, & a crust made of H 2 O ice • fractured surface -> tectonics. • few impact craters seen • double-ridged cracks • jumbled icebergs • • (evidence of a subsurface ocean). • Europa has a magnetic field. • implies liquid salt water beneath the icy crust • Where liquid water exists, there could be life!

Ganymede • largest moon in the Solar System • Two types of terrain: • heavily cratered, implies old • long grooves, few craters, implies young like Europa • It also has a magnetic field. • Could it have subsurface ocean? • case not as strong as Europa’s • tidal heating would be weaker • would need additional heating from radioactive decay

Callisto • It has an old surface. • heavily cratered, dirty ice • cratering reveals clean, white ice • no evidence of tectonics • Its interior did not differentiate. • rock mixed with ice • It does not experience tidal heating. • Yet it has a magnetic field. • Could it have a subsurface ocean anyway?

Titan • largest of Saturn’s moons • Huygens landed in January 2005 • has a thick atmosphere. • Nitrogen (from dissociated NH 3; 90%), Argon, methane, ethane • methane, ethane are greenhouse gases: surface is warmer than it should be • Ethane/methane may condense to form clouds and rain • Atmosphere blocks view of surface • may have oceans of ethane/methane • erosion may be important

http: //saturn. jpl. nasa. gov

Liquid Lakes on Titan: Some regions on Titan reflect very little radar. The leading explanation for this is that these regions are lakes, possibly composed of liquid methane. This image is a false-color radar map of a northern region of Titan taken by the Cassini spacecraft. On this map, which spans about 150 kilometers across, dark regions reflect relatively little of the broadcast radar signal. Titan is the only body in the Solar System, other than the Earth, known to possess liquids on its surface.

Cold, windy, surface like wet clay, ice “rocks”

Enceladus

Triton • orbits in the opposite direction of Neptune's rotation in a highly inclined orbit. • implies that it was probably captured by Neptune • Thin Nitrogen atmosphere • Volcanic activity