Astro 101Fall 2013Lecture 6

T. Howard

Solar System Perspective

Sun, Planets and Moon to scale

(Jupiter’s faint rings not shown)

Two Kinds of Planets

"Terrestrial"

Mercury, Venus,Earth, Mars

"Jovian"

Jupiter, Saturn, Uranus, Neptune

Close to the SunSmall

Far from the SunLarge

Few MoonsNo Rings

Main Elements Fe, Si, C, O, N

Mostly RockyHigh Density (3.9 -5.3

g/cm3)Slow Rotation (1 - 243 days)

Mostly GaseousLow Density (0.7 -1.6 g/cm3)

Many MoonsRings

Main Elements H, He

Fast Rotation (0.41 - 0.72 days)

initial gas and dust nebula

dust grains grow by accreting gas,

colliding and sticking

continued growth of clumps of

matter, producing planetesimals

planetesimals collide and stick,

enhanced by their gravity

a few large planetsresult

Hubble observation of disk around young star

with ring structure. Unseen planet sweeping

out gap?

Terrestrial - Jovian Distinction

Jovian solid cores ~ 10-15 MEarth . Strong gravity => swept up and retained large gas envelopes of

mostly H, He.

Outer parts of disk cooler: ices form (but still much gas), also ice "mantles" on dust grains =>

much solid material for accretion => larger planetesimals => more gravity => even more

growth.

Inner parts hotter (due to forming Sun): mostly gas. Accretion of gas atoms onto dust grains

relatively inefficient.Composition of Terrestrial planets reflects that of

initial dust –not representative of Solar System, or Milky Way,

or Universe.

The Jovian Planets

Jupiter Saturn (from Cassini probe)

Uranus Neptune

(roughly to scale)

Discoveries

Jupiter and Saturn known to ancient

astronomers.Uranus discovered in

1781 by William Herschel.

Neptune discovered in 1845 by Johann Galle.

Predicted to exist by John Adams and Urbain

Leverrier because of irregularities in Uranus'

orbit.

Jupiter

Saturn

Uranus

Neptune

318

95

15

17

Mass (MEarth)

11

9.5

4

3.9

Radius (REarth)

(0.001 MSun)

Orbit semi-major axis

(AU)

Orbital Period(years)

5.2

9.5

19.2

30.1

11.9

29.4

84

164

Basic Properties

Chemical Compositions of Outer Planet AtmospheresElement / Jupiter Saturn Uranus

NeptuneMolecule

H2 86% 96% 83% 80%

He 13.6 % 3.3% 15%19%

CH4 < 1% < 1% 2.3 % 1 – 2 %

NH3 < 0.1 % < 0.1 % < 100 ppb < 1 ppm

H2O ~ 600 ppm 20 ppb ~ 10 ppb few ppb

ppm = parts per million (0.0001%) ; ppb = parts per billion (0.001 ppm)CH4 = methane; NH3 = ammonia

Jupiter has rings:photo from Voyager 2, May 1995

Galileo spacecraft photo

Optical – colors dictated by how molecules reflect sunlight

Infrared - traces heat inatmosphere, therefore depth

So white colors from cooler, higher clouds, brown from warmer, lower

clouds. Great Red Spot – highest.

Jupiter's Atmosphere and BandsWhiteish "zones" and brownish

"belts".

Other Jovian planets: banded structure and colors

More uniform haze layer makes bands

less visible. Reason: weaker gravity allows

clouds torise higher and spread

out to create more uniform layer

Blue/green of Uranus and blue of Neptune due to methane.

Colder than Jupiter and Saturn, their ammonia has frozen and

sunk lower. Methane still in gas form. It absorbs red light and

reflects blue.

- Zones and belts mark a convection cycle. Zones higher up than belts.

- - Zones were thought to be where warm gas rises, belts where cooled gas sinks. Now less clear after Cassini, which found rising gas only in the belts!

- Winds flow in opposite directions in zones vs. belts. Differences are

hundreds of km/hr.

- Jupiter's rapid rotation stretches them horizontally around the entire planet.

Storms on Jovian Planets

Jupiter's Great Red Spot: A hurricane twice the size of Earth. Has persisted for at least

340 years. Reaches highest altitudes.

New storm “Oval BA”

"white ovals" - may last decades

"brown ovals" - only seen near 20° N latitude. Not known why. May last

years or decades

Neptune's Great Dark Spot: Discovered by Voyager 2 in 1989.

But had disappeared by 1994 Hubble observations. About

Earth-sized.Why do these storms last

so long?

Jupiter's Internal StructureCan't observe directly. No seismic information. Must

rely on physical reasoning and connection to observable phenomena.

(acts like a liquid metal, conducts

electricity)Core thought to be molten or partially molten rock,maybe 25 g/cm3, and of mass about 10-15 MEarth .

Other Jovians similar. Interior temperatures, pressures and densities less extreme.

Rapid rotation causes Jupiter and Saturn to bulge:

Gravity

without rotation

with rotation

Gravity

Jupiter and Saturn rotate every ~10 hours. Radius at equator several % larger

due to bulge.

Differential Rotation

Rotation period is shorter closer to the equator:

Jupiter

Saturn

Uranus

Near poles At equator9h 56m

10h 40m

16h 30m

9h 50m

10h 14m

14h 12m

How do we know? Tracking storms at various latitudes, or using Spectroscopy and Doppler

shift.

Uranus' rotation axis is tilted by 98o

Why? Unknown. Perhaps an early, grazing collision with

another large body.

Moons of Jovian Planets

The Galilean Moons of Jupiter

Closest to Jupiter

Furthest from Jupiter

(sizes to scale)

Radii: 1570 km (Europa, slightly smaller than our Moon), to 2630 km (Ganymede - largest moon in Solar

System).

Orbital periods: 1.77 days (Io) to 16.7 days (Callisto).The closer to Jupiter, the higher the moon density: from 3.5 g/cm3 (Io) to 1.8 g/cm3 (Callisto). Higher density indicates higher rock/ice fraction.

Io Europa Ganymede Callisto

Io's Volcanism

More than 80 have been observed. Can last months or years.

Ejecta speeds up to 1000 m/s. Each volcano ejects about 10,000 tons/s

Rich in S, SO2. S can be yellow, orange, red, black depending on temperature. Frozen SO2 snowflakes are white.

Voyager 2 (1979) Galileo (1996)

Activity causes surface to slowly change over the years:

Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat?

First, Io and Europa are in a "resonance orbit":

Day 0

Europa

Io

Day 1.77

Europa

Io

Day 3.55

Europa Io

The periodic pull on Io by Europa makes Io's

orbit elliptical.

Jupiter

Jupiter

Jupiter

Europa “pulls Io outward” here.

Ioorbital speed slower

orbital speed faster

- Io “tidally locked” like our Moon. Tidal bulge always points to Jupiter. So angle of bulge changes faster when Io is closer to Jupiter.

(exaggerated ellipse)

- But Io rotates on its axis at a constant rate, so cannot keep bulge exactly- pointed at Jupiter at all times during orbit.

- So bulge moves back and forth across surface => stresses => heat => volcanoes

Europa may have Warm Water Ocean beneath Icy Surface

860 km

42 kmIcebergs or "ice rafts" suggest broken

and reassembled chunks.

Dark deposits along cracks suggest eruptions of water with dust/rock

mixed in (Europa’s density => 90%rock, 10% ice).

Fissures suggest tidal stresses.Hardly any impact craters.

What is source of heat? Same as Io: resonant orbits with Ganymede and Io make Europa's orbit elliptical

=> varying tidal stresses from Jupiter => heat.

Warm ocean => life?

Further down: rocky/metallic layers

Io pulls Europa inward here.

Ganymede pulls Europa outward

here.

Saturn's Titan: A Moon with a Thick Atmosphere

Surface pressure is 1.6 times Earth’s, T=94 K. Atmosphere 98% Nitrogen,also methane, ethane, benzene, propane, etc. Evidence for methane rain, a few lakes of methane/ethane, drainage channels, liquid-eroded rocks, an icy

volcano (active? replenishing the methane?). Mostly dry now – rain and liquid flow may be episodic (centuries?).

Surface from Huygens

probe

Origin of atmosphere: internal heat from natural radioactivity may escape surface through volcanoes. Atmosphere trapped by Titan’s cold temperature

and relatively high gravity. Interior: rocky core and water mantle.

Taken during Huygens’ descent

From Cassini-Huygens mission

Bizarre Orbits of some of Saturn's Moons

Telesto and Calypso share orbit with Tethys, and are always 60

deg. ahead and behind it! They stay there because of combined

gravity of Saturn and Tethys.

Janus and Epimethius are in close orbits. When the approach each

other, they switch orbits!

Tethys Janus and Epimethius

Saturn's Rings (all Jovians have ring systems)

- Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m!

- Composition: icy chunks, <1 mm to >10m in diameter. Most a few cm.

- A few rings and divisions distinguishable from Earth. Please read how the gaps

form.

Voyager probes found that rings divide into 10,000's of ringlets.

Structure at this level keeps changing. Waves of matter move like ripples on a pond.

Origin of Cassini Division: another resonance orbit Approximate radius of Mimas'

orbit

Mimas' orbital period is twice that of particles in Cassini division. Makes their orbits elliptical.

They collide with other particles and end up in new circular orbits at other radii. Cassini division nearly swept

clean.Other gaps have similar

origins.

Origin of Saturn's Rings:

If a large moon, held together by gravity, gets too close to Saturn,

tidal force breaks it into pieces, at a radius called the Roche Limit.

Rings inside Roche Limit => pieces can’t reassemble into

moon.

Not clear whether rings are asold as Saturn or much younger

(about 50 million years).

Unclear. Total mass of ring pieces equivalent to 250 km moon. Perhaps leftover debris from moon building? A shattering collision? A captured object? Regardless, a large moon could not survive so close to Saturn:

Rings of other Jovian Planets

The rings of Uranus.Discovered by "stellar

occultation".

Jupiter, Uranus, Neptune rings much thinner, much less material. Formed by breakup of smaller bodies? Also maybe "sandblasting" of material off

moon surfaces by impacts.

Given rings have short lifetime and all Jovian planets have them, their formation must be common.

Neptune's moon Triton is spiraling in to the planet and should produce spectacular ring system in 100 million years.

Neptune’s Rings(Voyager 2)

The Jovian Planets

Jupiter Saturn (from Cassini probe)

Uranus Neptune

(roughly to scale)

PlutoPredicted to exist by remaining irregularities in Uranus' orbit.

Discovered in 1930 by Clyde Tombaugh (1905-1997).

Irregularities later found to be incorrect!

Model created from Hubble images. This is the most detail

we have.

Discovery image of Pluto's moon Charon

(1978)

Two more moons found in 2005

with the Hubble.

Mass 0.0025 MEarth or 0.2 x mass of Moon

Radius 1150 km or 0.2 REarth

Density 2.0 g/cm3 (between Terrestrial and Jovian densities. More like a Jovian moon)

Basic Properties of Pluto

Icy/rocky composition

Moons: Charon: radius about 590 km or 0.1 REarth .

Pluto and Charon tidally locked. Nix and Hydra about 30-100 km. Origin of Pluto

Now known to be just the largest known of a class of objects in the outer reaches of the Solar System. These objects are:

Kuiper Belt Objects (KBOs)

• From Neptune’s orbit (30 AU)to about 50 AU

• Discovered 1992• Small bodies• Mostly frozen “volatiles”

water, CO2, CH4, etc.• About 200 x mass of main

asteroid belt• Known objects: ~1000• Estimated: ~ 70,000

• Several larger objects known

Green dots = KBOs Scale is in AU

Radius 1200 ± 50 km or at least as big as Pluto. Icy/rocky composition, like

Pluto.

The New “Dwarf Planet” Eris

It too has a moon,Dysnomia

(Keck telescope)

Gaspra Ida and Dactyl

Total mass of Asteroid Belt only 0.0008 MEarth or 0.07 Mmoon. So it is not debris of a planet.

Probably a planet was trying to form there, but almost all of the planetesimals were ejected from Solar System due to encounters with Jupiter.

Giant planets may be effective vacuum cleaners for Solar Systems.