Introduction to Astronomy

• Announcements– Midterm Exam on Thursday

• Closed-book/notes/etc.

This is what happens when a massive star dies.

About 1400 ly distant, it covers an area of sky = 5 full Moons!

The Jovian Planets

Jupiter, Saturn, Uranus, Neptune

The Jovianplanets to correct scale

Also knownas the outerplanets or thegas giants

Commonalities

• All are much larger than Earth• Densities are all lower than the terrestrial

planets• All have small ice/rock cores surrounded

by liquid layer surrounded by thick gas atmosphere (which contains optical surface of planet)

• Rings• Belts and Zones (high & low pressure)

JupiterJupiterThe King of the Planets

JupiterJupiter• Name from Roman God of Gods• Largest of the planets

– 11x diameter of Earth, 318x mass of Earth

• Dense, brightly-colored atmospheric bands– Fast-moving H, He, NH3, CH4, H2O

• Fast rotation period (10 Earth hours)– Results in significant equatorial bulge

Interior

• Avg. density ~ 1.3 g/cm3 (slightly higher than water)– Much less than Earth density, so must be

made of lighter elements

• 1/6 of the way to the core, gaseous H turns to liquid H– Compression by overlying layers

• Even deeper, liquid turns to liquid metallic H

• Rocky, Iron core

Divers know this phenomenonwell…

As depth increases, pressureincreases.

Need for decompression stopsto avoid Nitrogen Narcosis(“The Bends”)…pressure on bodydrops too rapidly, allows bubblesof N2 to enter blood stream anddissolve into brain tissue,which is highly toxic.

Atmosphere

• Coriolis effect & cloud bands• Heat in interior drives convection currents

upward– Stirs up atmosphere, brings hot gases to

surface while cooler gases fall back down below surface

– Coriolis effect• Gases moving toward equator get pushed to the

west, gases moving away from equator get pushed to the east

Here on Earth, we have localizedhigh- and low-pressure systems

On Jupiter, those same high- andlow-pressure systems are stretchedinto globe-circling bands

If no rotation, hot gases in theinterior convectively rise.

In pot of boiling water, convectioncarries bubbles of WATER VAPORto surface, where it is released tocondense into STEAM

When these hot gases reach the surface, they spread out horizontally. Some travel toward the equator, some travel toward the poles

How to stretch a weather cellHow to stretch a weather cell

• Just in case you don’t believe me, here’s an example of the Coriolis deflection– Note I did not say Coriolis “force”

• Movie

Speed of cloud bands varies widely from place to place

• As opposite-moving winds blow, other uprising material may get caught between them and get “twisted” up into Jovian storms– Like ball rolling between two conveyor belts– Some transient (short-lived)– GREAT RED SPOT permanent (as far as we

know)– Brownish-red bands from Sulfur and/or

Phosphorous tinting

Smaller storms

HUGE “Jet Streams”

The Kelvin-Helmholtz Instability

• Convection & rotation in liquid Hydrogen sustains DYNAMO– BJupiter ~ 20 – 20,000 x stronger than Earth’s

magnetic field• Largest magnetosphere of all the planets• Intense auroral displays observed by Hubble and

by Voyager fly-bys• Radiation belts• Thunderstorms & lightning

Jupiter has Aurorae justlike on Earth!

Image of Jupiter & auroraein UV wavelengths (Hubble/STIS)

March 2007:

Auroral observationsby Chandra X-rayTelescope overlaidw/ HST image

Jovian Moon “footprints”

Rings

• Thin rings confirmed by Voyager I fly-by in 1979

This is a false-colorimage from Voyager 2looking back as itpassed Jupiter

Why is the planet’ssurface dark?

Cassini confirmedrings are small, irregular“chips” from nearby moons

Moons• Galilean moons

– 4 largest of Jupiter’s moons– Mini model of solar system

• In both appearance and formation• Denser, rockier moons closer in• Icier moons further out

This is why objects cannot be classified as planets based only on size

Io, Europa, and Ganymedehave a synchronous orbitalresonance: because the moonsare closely-spaced, they createa rhythmic gravitational pullon each other…

For every orbit of Ganymede,Europa orbits twice and Io orbitsfour times

Io

• Closest moon, tidally-locked– Gravitational disturbances prevent closed

orbit…makes ROSETTE pattern

• Most spectacular volcanic activity in solar system– Caused by gravitational/tidal forces that

constantly stretch and pull on interior, heating it up (like a RUBBER BAND)

– Sulfuric volcanoes: “lava” is not molten rock, but molten Sulfur

• But occasionally see evidence of silicate lava

Visible image of Io, showing a HUGEvolcanic eruption of Sulfur

This plume rose about 65 miles abovethe surface of the moon!

This puts any volcanic eruptions on Earth to shame…

Molten sulfur lava flowssown volcanic peak beforesolidifying…

IR view: simultaneous, surface-wide eruptions

Europa

• Perhaps most interesting moon in the solar system

• Icy moon– Surface layer of ice ~ 6 miles thick– LIQUID WATER underneath!

– White areas are frozen H2O, red areas from mineral-rich water that oozed through surface cracks

• Perhaps organic? Red algae?

Size comparison of Europa’sice fields

• Tidal heating by Jupiter’s gravity keeps subsurface H2O in LIQUID form– Explains lack of cratering…liquid water would

quickly smooth over any surface depressions– Also has glacier-like surface flows of ice

– LIFE?• Extremophiles: life-forms that survive in frigid

temps, high temps, high acidity, low light levels, etc…

SaturnSaturn

SaturnSaturn• Avg. density ~ 0.7 g/cm3

– This is less than density of water, so if dropped into our oceans, SATURN WOULD FLOAT!

– Mostly H & He

• Internal heat source: friction/drag on falling, condensed liquid Helium droplets

• Similar to Jupiter, but colder, so that ammonia (NH3) freezes into tiny cloud particles– Dense cloud cover masks details of surface

Saturn’s weather bands as imaged by Cassini

Rings

• Originally thought to be solid• Closer examination shows inner rings

rotate faster than outer rings– Kepler’s 3rd law again!– Therefore, rings must be made of a swarm of

small, separate bodies a few cm to a few meters in diameter

– Mapped via radar echo techniques• Determined size and composition (Voyager I)• Primarily H2O ice, but see signs of carbon

compounds

Substructure of Saturn’s rings: composition & speed differences, many gaps

• Ring gaps– Cassini’s Division: large gap caused by orbit

of moon Mimas periodically pushing particles out of that particular area

– Shepherding Satellites: small moon-like objects (larger than average) that direct ring material into narrow lanes, creating small gaps between them

These are rings aroundUranus, but principleis the same…

F-ring

Potato-shapedmoon,Prometheus

Distortion of Saturn’s F-ring by interacting moonlets(smaller than moons, but larger than ring particles)

Cassini Division

Saturnian Shadow

Filtered Sunlight

Cassini image from9 Sep 2006

Shows new ringletsnot seen in Voyager fly-bys 25 years ago

a.k.a. The Death Star

Mimas in white-lightand IR

Ring Origins• Destruction of moons & asteroids by tidal

forces• Roche Limit (RRoche = 2.44 Rplanet)

– Gravity pulls harder on near-side of orbiting object than on far-side

– If this differential gravitational force is stronger than the object’s self-gravitation (what holds the object together), it gets torn apart

– Gravity pulls pieces into orbit– Side-by-side differences in rings suggest

whole ring system formed from MANY destroyed bodies

Saturn’s B & C rings in the infrared, highlighting not onlytemperature differences, but compositional differences as well

Moons

• Mostly ice, but less dense than Jupiter’s moons

• Most interesting moon:– Titan

• D = 3000 miles > DMercury

• So cold, N2 moves slowly, doesn’t reach escape velocity

– Titan has Nitrogen atmosphere!

• Spectroscopy indicates presence of liquid methane oceans & ethane-methane “rainclouds”

Filtered sunlight

Introduction to Astronomy

• Announcements– Midterm Exam tomorrow

– HW #4 due Monday

UranusUranus

UranusUranus• Named for Ouranos• Pretty much featureless except for faint cloud

bands and faint ring system

• Rich in H (in the form of H2O and CH4)

• Methane causes blue color– Absorbs red-end of solar spectrum, scatters blue-end

off clouds of frozen methane

• Avg. density ~ 1.2 g/cm3

– Mostly light elements probably with rocky/icy core

Rings & Moons

• Rings similar to Jupiter & Saturn but darker and narrower– Probably more carbon-

based molecules– Thin rings held by

shepherding satellites• 5 large moons & 20

small ones

As a side note, all Uranus’ moonsare named for Shakespearean characters

NOT to scale…

• Miranda (large moon)– Very puzzling appearance– Large impact, destruction & reformation?– Rising & sinking motions in interior?

Tilt of Uranus• Equator nearly perpendicular to orbit (spins on

its side)• Rings & moons are similarly tilted

– Offers clues about formation– Giant impact knocked the planet over, spewed out

material that then formed rings & moons• otherwise, wouldn’t expect rings and moons to have tilted

orbits…

– One pole in perpetual day, one in perpetual night (for half of the Uranian year, 42 Earth-years)

• odd rotation + odd heating = lack of atmospheric phenomena?

Which is really the north pole?

NeptuneNeptune

NeptuneNeptune• Similar to Uranus

– Cloud bands, deep blue color– Had a “Great Dark Spot” similar to Jupiter’s Great

Red Spot

• Discovered through orbital anomaly– Uranus’ position did not quite agree with predictions– The gravity of another planet located past Uranus

explained disagreement• Leverrier & John Adams (1843)• Confirmed when Neptune first discovered (Johanne Galle,

1846)

• Discovered by Voyager 2 in August 1989

• Thought to be a “hole” in the methane atmosphere– Much like our

ozone hole

18 hours from first to last frame

Waves in overlying clouds

Atmosphere

• Cloud bands generated by Coriolis effect– 1300 mph winds!

• Methane tinting– Like Uranus

• Great Dark Spot disappeared (sometime < 1997)…why?!– not well-understood

Rings & Moons

• Very narrow, tenuous (faint) rings

• 6 moons close-in, 7 further out

• Triton– Orbits opposite to Neptune’s rotation– Highly-tilted orbit

• left-over or captured planetesimal?

– Has retained an acidic atmosphere– “wrinkly”, volcanic soot-laden surface

Outer ring is more clumpy or braided

Triton is actually spiraling-in towards Neptune…

…the Solar System will have a new ring system to rival Saturn’s!

PlutoPluto• Roman god of the underworld• Is it a planet?

– Highly-elliptical orbit out of the plane of the rest of the solar system

• Avg. density ~ 2.1 g/cm3

• Composed of H2O, Methane, Nitrogen & CO2 ices– Polar caps of frozen Methane

• Thin atmosphere of N2 & CO

• Surface temperature a constant -380 °F

Discovered by Clyde Tombaughin 1930, working at Lowell Observatoryin Flagstaff, AZ

Compared (by eye!) an enormousamount of pairs of photographicimages, looking for faint objects whose positions slowly changed…

…this is NO small task.

Even the smallestbodies in the solarsystem can havemultiple moon systems

In fact, Pluto and Charon are actuallya binary system!

The center-of-massof the two objectsis above the surfaceof Pluto

Where did this name come from?

HST, 1996

New Horizons, scheduledfor Pluto-rendezvous in 2015

NEXT TIMENEXT TIME• Meteors, Asteroids, and Comets

If Pluto were any closer to the Sun, it would be a comet!