Transcript charts_set_6

The Jovian Planets
Saturn (from Cassini
probe)
Jupiter
Uranus
Earth
(roughly to
scale)
Neptune
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.
Basic Properties
Mass
(MEarth)
Radius Orbit semi-major Orbital Period
(REarth)
axis
(years)
(AU)
Jupiter
318
11
5.2
11.9
Saturn
95
9.5
9.5
29.4
Uranus
15
4
19.2
84
Neptune
17
3.9
30.1
164
(0.001 MSun)
Jupiter's Atmosphere and Bands
Whiteish "zones" and brownish
"belts".
Optical – colors dictated by how
molecules reflect sunlight
Infrared - traces heat in
atmosphere, therefore depth
So white colors from cooler, higher clouds, brown from
warmer, lower clouds. Great Red Spot – highest.
Other Jovian planets: banded structure and
colors
More uniform haze layer
makes bands less
visible. Reason: weaker
gravity allows clouds to
rise 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!
-
- Jupiter's rapid rotation stretches them horizontally around the entire planet.
- Winds flow in opposite directions in zones
vs. belts. Differences are hundreds of km/hr.
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
Jupiter’s Composition: mostly H, some He, traces of other elements (true for
all Jovians). Gravity strong enough to retain even light elements. Mostly
molecular.
We only see the upper regions of the atmosphere. Spectroscopy of reflected
sunlight reveals which molecules present. We find Hydrogen, Helium,
Methane, Ammonia, some water, a few others.
All of these molecules should produce white clouds. The molecules
responsible for the colors we see in the bands and spots are not known.
Internal Structure
Can't observe directly. No seismic information. Must rely on physical
reasoning and connection to observable phenomena.
Core thought to be molten or partially molten rock,
maybe 25 g/cm3, and of mass about 10-15 MEarth .
Rapid rotation causes Jupiter and Saturn to
bulge:
Gravity
Gravity
without
with
rotation
rotation
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:
Near poles
At equator
Jupiter
9h 56m
9h 50m
Saturn
10h 40m
10h 14m
Uranus
16h 30m
14h 12m
How do we know? Tracking storms at various
latitudes, or using Spectroscopy and Doppler shift.
Moons of Jovian Planets
The Galilean Moons of Jupiter
(sizes to scale)
Io
Closest to Jupiter
Europa
Ganymede
Callisto
Furthest from Jupiter
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'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.
Activity causes surface to slowly change
over the years:
Voyager 2 (1979)
Galileo (1996)
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":
Jupiter
Day 0
Europa
Europa “pulls Io
outward” here.
Io
Jupiter
Day
1.77
Europa
Io
Jupiter
Day
3.55
Europa
The periodic pull on Io by
Europa makes Io's orbit
elliptical.
Io
orbital
speed
slower
Io
orbital
speed
faster
(exaggerated ellipse)
- Io “tidally locked” like our Moon. Tidal bulge always points to Jupiter. So angle of bulge
changes faster when Io is closer to Jupiter.
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
Fissures suggest tidal stresses.
Hardly any impact craters.
860 km
42 km
Icebergs 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).
Io pulls Europa
inward here.
Ganymede pulls
Europa outward here.
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
Saturn's Titan: A Moon with a Thick Atmosphere
Taken during
Huygens’ descent
From CassiniHuygens mission
Surface from
Huygens probe
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?).
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.
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:
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:
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 as old as Saturn or much younger (about 50 million
years).
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.
The Jovian Planets
Saturn (from Cassini
probe)
Jupiter
Uranus
Neptune
(roughly to
scale)
Pluto
Predicted 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.
Two more moons found in 2005
with the Hubble.
Discovery image of Pluto's
moon Charon (1978)
Basic Properties of Pluto
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)
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.
The Kuiper Belt Objects
Over 1000 found since 1992. Probably 10,000's bigger than 100 km exist.
Icy/rocky.
Orbits tend to be more tilted, like Pluto's.
Leftover planetesimals from Solar System formation?
Oort Cloud
Oort Cloud is a postulated huge, roughly spherical reservoir of
comets surrounding the Solar System. ~108 objects? Ejected
planetesimals.
A passing star may dislodge Oort cloud objects, plunging them into Solar
System, where they become long-period comets.
If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be
changed and it may plunge towards the inner Solar System and become a
short-period comet.
The New “Dwarf Planet” Eris
Radius 1200 ± 50 km or at least as big as
Pluto. Icy/rocky composition, like Pluto.
It too has a moon,
Dysnomia
(Keck telescope)
Asteroids
Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000 known.
Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The Trojan asteroids
orbit 60 o ahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits
were probably disrupted by Jupiter's gravity.
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.
Gaspra
Ida and Dactyl
The Sun
The Sun is a star:
a shining ball of gas powered by nuclear fusion.
Mass of Sun = 2 x 1033 g = 330,000 MEarth
= 1 MSun
Radius of Sun = 7 x 105 km = 109 REarth
= 1 RSun
Luminosity of Sun = 4 x 1033 erg/s = 1 LSun
(amount of energy put out each second in form of
radiation, = 1025 40W light bulbs)
The Sun in X-rays over several years
Temperature at surface = 5800 K => yellow
(Wien’s Law)
Temperature at center = 15,000,000 K
Average density = 1.4 g/cm3
Density at center = 160 g/cm3
Composition: 74% of mass is
H
25% He
1% the rest
Rotation period = 27 days at equator
31 days at poles
The Interior Structure of the Sun
(not to scale)
Let's focus on the core, where the Sun's energy is
Review of Atoms and Nuclei
Hydrogen
atom:
electron
Helium atom:
_
_
+
proton
+
+
_
The proton is the nucleus
The nucleus is 2 protons + 2
neutrons
What binds the nuclear particles?
The “strong” nuclear force.
Number of protons uniquely identifies element. Isotopes differ in number of
neutrons. Helium example: 4He: 2p + 2n. 3He: 2p + 1n
Review of Ionization
Radiative ionization of H
_
+
Energetic UV
Photon
"Collisional Ionization" of H
_
_
+
+
Core of Sun is hot: gas is completely ionized by energetic collisions
What Powers the Sun
Nuclear Fusion: An event where nuclei of two atoms join together.
Need high temperatures.
Energy is produced. Elements can be made.
nuc. 1 + nuc. 2 →
nuc. 3 + energy (radiation)
Mass of nuc. 3 is slightly less than mass of (nuc. 1 + nuc. 2). The
lost mass is converted to energy. Why? Einstein's conservation of
mass and energy, E = mc2. Sum of mass and energy always conserved in
reactions. Fusion reactions power stars.
Chain of nuclear reactions called "proton-proton chain" or p-p chain
occurs in Sun's core, and powers the Sun.
In the Sun's Core...
neutrino (weird particle)
proton
deuteron (proton + neutron
bound together)
positron (identical to electron
but positively charged)
proton
photon
proton + proton →
proton+neutron
{
1)
+
neutrino + positron
(deuteron)
+
energy (photon)
2) deuteron +
proton
→
3He
+
energy
He nucleus, only 1 neutron
3) 3He
+
3He
→
4He
+
proton + proton + energy
Net result:
4 protons
→
4He
+ neutrinos + energy
Mass of end products is less than mass of 4 protons by 0.7%.
Mass converted to energy.
600 million tons per second fused. Takes billions of years to convert p's to
4He in Sun's core. Process sets lifetime of stars.
Hydrostatic Equilibrium: pressure from fusion reactions balances gravity,
allows Sun to be stable.
How does energy get from core to surface?
photon path
core
"radiative zone":
photons scatter off
nuclei and electrons,
slowly drift outwards:
"diffusion".
"convection zone"
"surface" or photosphere:
gas density low enough
so photons can escape
into space.
some electrons bound to nuclei =>
radiation can't get through => heats
gas, hot gas rises, cool gas falls
Can see rising and falling convection cells in photosphere
=> granulation. Bright granules hotter and rising, dark ones
cooler and falling. (Remember convection in Earth's
atmosphere, interior and Jupiter).
Granules about
1000 km
across
Why are cooler granules dark? Stefan's Law: brightness 
T4
The (Visible) Solar Spectrum
Spectrum of the Sun shows:
1) The Black-body radiation
2) Absorption lines (atoms/ions absorbing photons at specific wavelengths).
10,000's of lines from 67 elements, in various excited or ionized states.
Again, this radiation comes from photosphere, the visible surface of the Sun. Elements
weren’t made in Sun, but in previous stellar generations.
Sunspots
They are darker because they are cooler (4500 K vs. 5800 K).
Related to loops of the Sun's magnetic field.
• Roughly Earth-sized
• Individual spots last ~2 months
• Usually occur in pairs
• Follow solar rotation
radiation from hot gas flowing along
magnetic field loop at limb of Sun.
Objects Seen in Transit
Transit of Venus 6/8/04
Photo: J. Lodriguss
Venus transit with bird, 4-frame composite

Rafael Navarro and Ismael Cid
Tres Cantos, Madrid, Spain
44
http://www.vt-2004.org/photos/vt-photostop01.html#iss
45
Transit of ISS and Shuttle Atlantis, 50
min after undocking, September 17th
2006 at 13h 38min 50s UT. Taken
from the ground at Mamers
(Normandy) France. Takahashi TOA150 refractor (diameter 150mm, final
focal 2300mm), Baader helioscope
and Canon 5D. Exposure of 1/8000s
at 50 ISO, extracted from a series of
14 images (3 images/s) started 2s
before the predicted time. Image
copyright Thierry Legault.
Transit of ISS and Shuttle Atlantis, 50 min after undocking,
September 17th 2006 at 13h 38min 50s UT.
46
• Sunspot numbers vary on a 11 year cycle.
• Sun's magnetic field changes direction (flips) every 11 years.
• Maximum sunspot activity occurs about halfway between
reversals.
• We just passed through a sunspot minimum (c. 2009). Sunspot
activity now on the rise again.
• High levels of sunspot activity correlate with other active Sun
behavior -- flares, coronal mass ejections (CMEs), prominences.
• Solar flares can disrupt radio communications on Earth, are
hazardous to astronauts in space (high levels of radiation), and
can even permanently damage spacecraft in orbit.
Above the photosphere, there is the
chromosphere and...
The Corona
Best viewed during
eclipses.
T=
6
10
K
Density = 10-15
3
g/cm only!
We expect X-rays from gas at this
temperature.
Yohkoh X-ray satellite
X-ray brightness varies over 11-year Solar Cycle: coronal activity and sunspot
activity go together.
The Solar Wind
At top of corona, typical gas speeds are close to
escape speed => Sun losing gas in a solar wind.
Wind escapes from "coronal holes", seen in X-ray
images.
Wind speed 500 km/sec (takes a few days to reach Earth).
106 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind.
Active Regions
Prominences: Loops of gas ejected from surface. Anchored in sunspot pairs.
Last for hours to weeks.
Flares: A more energetic eruption. Lasts for minutes. Less well understood.
Prominences and flares occur most often at maximum of Solar Cycle.