Lecture 7

Astro 1001

6/13/07

Jovian Planets Overview• Much larger than terrestrial

planets– Jupiter is over 300x more

massive than Earth

– About 1/5 of the density of Earth

• We know a lot about the planets now– Pioneer and Voyager visits

in the 70s

– Recently, Galileo and Cassini visits

Composition

• Jupiter and Saturn are almost entirely hydrogen and helium– Jupiter is often called a “failed

star”

• Uranus and Neptune are smaller, contain much less hydrogen and helium– Made up of hydrogen

compounds

• Why are they different sizes?

Planets and Pillows

• Difference between densities in Neptune and Uranus vs Saturn and Jupiter should now be obvious

• Size is not necessarily an indication of mass– More mass compresses things more, increasing

density but not planetary radius

Rotation Rates

• Jovian planets rotate very quickly– Difficult to measure

• Fast rotation makes the planets bulge– Saturn’s equator is about

10% wider than its poles

– Extra equatorial material keeps moons and rings aligned with bulge

The Interior• We can say things

about the interior of jovian planets because of experiments and computer simulations

• Interior is very dense and hot– Galileo dropped

probe that only survived for 200 km

The Other Jovian Planets

• Saturn is very similar to Jupiter

• Uranus and Neptune don’t have metallic or liquid layers, although their cores might be liquid

Internal Heat

• Jupiter emits about twice as much energy as it gets from the Sun

• Cannot be accounted for by accretion, differentiation

• Heat is probably coming from gravitational contraction

• Neptune is a bit mysterious

Weather and Clouds• Jovian planets are typically

colorful due to their clouds– We see lots of methane,

ammonia, water bands

• Jupiter has similar layers to what the Earth has

• Neptune and Uranus have different behaviors and can form methane snow

Color and Winds• Trace amounts of chemicals

produce the colors and Saturn and Jupiter– Saturn has more subdued colors

because its clouds are lower

• Methane is responsible for the bluishness of Uranus and Neptune

• Stripes on Jupiter due to Coriolis effect

• Great Red Spot is a giant hurricane

Magnetic Fields• The jovian planets all have

magnetic fields– Jupiter’s magnetic field is

20,000 stronger than Earth’s– Jupiter has very spectacular

aurora– Causes atmospheres on the

moons

• Neptune and Uranus have odd magnetic fields in that they aren’t aligned with the poles

Moons

• All of the jovian planets have moons (and rings)– Jupiter has over 60 moons– Ganymede and Titan are larger than Mercury– Lots of ice– Larger moons probably formed near the planets– Smaller moons are probably captured asteroids

Io

• Most volcanically active world in the solar system

• Shoots some of its volcanic materials into space

• Tidal heating is why Io is so active

Europa

• Covered by water ice

• Interior might be water or convecting ice

• Magnetic field data indicates its probably liquid water

• Might be heat sources beneath the surface

Group Work

• Europa is a fascination moon that has a fairly strong chance for supporting life. However, NASA no longer has any plans to visit it with a probe. Why do you think there are no plans to visit Europa?

Ganymede and Callisto

• Ganymede– Largest moon in the solar system

– Has both young and old features

– Features probably erased due to liquid water welling up

• Callisto– Heavily cratered

– No significant internal heat

Titan

• Second largest moon in the solar system

• Atmosphere is very thick– Mostly nitrogen

– No oxygen, but lots of hydrogen compounds

– Created by the surface sublimation or evaporating

• Recently explored with the Huygens probe

Triton

• Coldest world in the solar system

• Almost certainly captured

• Probably had past geological activity

Rings• Made up of countless

small particles• Particles are icy in nature• Might be thinnest known

astronomical structure• Rings have gaps

– Might be because of shepherd moons

– Might be because of orbital resonances

Other Ring Systems

• Other ring systems are much darker, smaller

• Always go about the planet’s equator

• Uranus’s rings are slightly tilted, elliptical

How Did the Rings Form?

• Large planets have enough gravity to rip things apart that get too close– Difficult to explain why this would frequently

happen

• Rings were formed from leftover material, but are resupplied from the moons of the planets

Asteroids

• Small rocky bodies– Discovered only about 200 years ago– Took 50 years to discover the first 10– 150,000 known asteroids now

• Ceres is the largest, under 1000km in diameter

• Total mass is probably less than that of the moon

Asteroids Continued

• Shape depends on mass

• Thousands of asteroids have been analyzed through spectroscopy– Made up of metal and

rock, with perhaps a big of ice

• Asteroids vary greatly in density

The Asteroid Belt

• Vast majority of asteroid are in the asteroid belt– Individual asteroids

are separated by millions of kilometers

• Jupiter’s gravity causes two groups of asteroids called Trojan Asteroids

Meteorites

• Meteors are just particles coming through the atmosphere– Usually around pea sized

objects

• Meteorites actually hit the ground– Can tell use a great deal

about how the solar system formed

Types of Meteorites

• Primitive– Very old (4.6 billion years old)– Are made up of stone, or a combination of

stone and carbon compounds

• Processed– Were once a part of another object– Can be made of metals, or rock

Comets• Comets formed outside of the

frost line– Thus, are made up of lots of ice

• Comets do not race across the sky

• Vast majority of comets do not have tails and never get anywhere close to Earth

• Some comets are knocked into the inner solar system by various sources of gravity

Composition

• Made up of chunks of ice mixed with rocky dust and complex chemicals– “Dirty snowballs”

• Starting to get more and more details about comets– Deep Impact

– Stardust

Structure of a Comet• Nucleus is the actual icy core

– Typically about 20km across

• Might have a dusty atmosphere called a Coma

• Two tails– Plasma tail consists of the

gasses escaping the comet

– Dust tail is made up of dust sized particles

Where Do Comets Come From?

• Left over material was flung far out into the solar system by the gravity from the jovian planets

• This lead to the Oort Cloud

• Slightly beyond the solar system, remnants stayed put and formed the Kuiper Belt

Big Icy Things• Lots of Pluto type objects

in the Kuiper belt– Perhaps 1000s of km in

diameter– “Xena” (Eris) is a good

example– Not really comets

• These objects are very cold, but might have atmospheres

• Will be visited by a probe in a decade or so

Collisions• In 1994, a comet whacked

into Jupiter– Each fragment had the

energy of a million H-bombs

• A massive collision is probably (at least partly) responsible for the extinction of the dinosaurs– A layer of irridium was the

initial evidence– Later found the Chicxulub

crater

Do We Have to Worry?

• We witnessed a probably collision in 1908– Had the force of

several atomic bombs– No more than 40

meters across

• The asteroid Apophis will pass very close to Earth soon