astro 101 fall 2013 lecture 6 t. howard
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Astro 101 Fall 2013 Lecture 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. Far from the Sun Large. - PowerPoint PPT PresentationTRANSCRIPT
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.