Download Giant Planets (also called jovian planets)

Jovian Planets: Jupiter and Saturn
Shortened Version Feb 15, 2011
Jovian Planets (Gas Giants)
The outer planets are
much bigger than the
terrestrial ones, and are
gigantic low-density gas
balls unlike the high
density rocky terrestrial
planets.
49,528 km
(3.8 DE)
51,118 km
(4.0 DE)
120,536 km
(9.4 DE)
142,984 km
(11 DE)
12,757 km
(1 DE)
2
3
Distance Comparison
Sun
1 AU
5.2 AU
9.6 AU
19.2 AU
30.1 AU
3.7%
as much
sunlight
as Earth
1.1%
as much
sunlight
as Earth
0.3%
as much
sunlight
as Earth
0.1%
as much
sunlight
as Earth
4
Jupiter: Atmosphere 3 - Composition
• 1930s (Rupert Wildt) based on low density
(1.3 g/cm3), suggests primarily made of
Hydrogen (H) and Helium (He)
• Confirmed by spacecraft in 1970s
– Why couldn’t we see spectral lines of H & He from the earth?
•
•
•
•
atmosphere (by mass): 75% H, 24% He, 1% other
atmosphere + interior: 71% H, 24% He, 5% other
Note Sun:
73% H, 25% He, 2% other
Hence its similar to original solar nebula!
Jupiter: Storms
5
• Great Red Spot
– about size of Earth
– At largest, has been
seen at 3x Earth size
– Rotates ccw
– more than 300 yrs old
• White ovals
– smaller, less long-lived
– also rotate ccw
Storms in Saturn’s atmosphere
seem to be shorter-lived
6
7
Jupiter’s
atmosphere is
divided into
light-colored
zones and
dark-colored
belts.
High-speed
winds blow
eats or west at
the boundaries
between them.
8
Saturn: Atmosphere 3
• Composition of clouds
same as Jupiter
• Atmosphere depleted
in He (96.3% H, 3.3%
He, 0.4% other)
=> where is missing He?
Wind speed in meters/second
• Similar easterly and
westerly winds, but at
equator, Saturn’s
winds are much faster
~1100 miles/hr!!!
9
Jupiter: Belts and Zones
• Alternating light/dark
stripes in atmosphere
• Convection – warm air
rises from below, cools
off, then sinks (same
as Earth’s troposphere)
• Rapid rotation
stretches these
convection cells across
the planet
Saturn: Atmosphere 2
10
Internal heat drives convection
•
•
•
•
Jupiter & Saturn radiate more heat than they receive from the sun.
Energy from formation (accretion).
Saturn being smaller than Jupiter should have cooled off more by now.
Helium Rain
– Saturn is cold enough that He can condense into droplets
– These droplets “rain” down through the H
• release energy (convert gravitational energy to heat)
=> Also explains why atmosphere is depleted in He
11
The oblateness of Jupiter and 12
Saturn reveals their rocky cores
• Jupiter probably has a rocky core
several times more massive than
the Earth
• The core is surrounded by a layer
of liquid “ices” (water, ammonia,
methane, and associated
compounds)
• On top of this is a layer of helium
and liquid metallic hydrogen and
an outermost layer composed
primarily of ordinary hydrogen and
helium
• Saturn’s internal structure is
similar to that of Jupiter, but its
core makes up a larger fraction of
its volume and its liquid metallic
hydrogen mantle is shallower than
that of Jupiter
13
Jupiter: Magnetic Field
• Enormous – 30 million km
across
– if we could see it from Earth, it
would appear 16x larger than the
full moon in the sky
• Also very strong, but more
variable in strength than
Earth’s
• First detected by “synchrotron radiation”
– radar signals emitted by charged particles trapped in
magnetic field lines
• These particles also cascade into Jupiter’s
atmosphere creating aurora, like on Earth
14
Saturn: Magnetic Field
• Weaker than Jupiter’s
– probably because liquid
metallic H region is
smaller
• Aligned perfectly with
rotation axis
• Contains fewer charged
particles
– no Io-like active moon to
supply
– icy ring material takes
particles out of magnetic
field lines
=> weaker aurora
Saturn’s Rings
• 1610: Galileo looks at Saturn,
notices it is not circular.
• Bulges on sides disappear in
1612, reappear in 1613
• 1655: With a better
telescope,
Huygens discovers ring, tilted
by 27°
15
Saturn’s Rings are Thin
•Last crossing 1995-6
•Next crossing 2008-9
•Rings estimated to be
only 30 meters thick!
16
17
• Huygens explains why the rings disappear. They are very thin,
and when seen edge twice a saturn year, they vanish!
Cassini Division
1675 Cassini observes the
“division” in the rings,
separating the brighter
“B” ring from the fainter
outer “A” ring
18
Earth-based observations reveal three broad rings
encircling Saturn
19
Encke Division (1838)
A more subtle division is
found in the “A” ring
20
Rings are not solid!
21
• 1857 Maxwell argues cannot be solid (centrifugal forces would tear it apart)
• 1895 Keeler measures Doppler velocities, each “ringlet” has different speed
• Rings consist of many “moonlets”, each orbiting the central planet.
• 80% reflectivity implies they are icy particles
• Probably from an icy moon pulled apart, only 100 million years ago.
Roche Limit (2.4 Saturn Radii)
• The moon’s own gravity tends to hold it together.
• Tidal forces from planet tend to pull it apart
• The “Roche Limit” is the critical distance where the two
balance
• Saturn’s moons are (mostly) outside of Roche Limit
• Saturn’s rings are (mostly) inside the Roche limit.
22
1970 Voyager Probes
• From scattering of radio
signals through the rings,
estimate average particles are
10 cm in size
• Some are as small as 1 cm,
some as big as 5 meters.
• In comparison the ring particles
of the other planets are:
– Jupiter’s are small dust
particles smaller than 1
mm
– Uranus particles are
perhaps 1 meter in size
– Neptune size unknown
(similar to Uranus?)
23
24
Cassini Saturn Orbit Insertion (SOI)
June 30, 2004, 7:30 - 9:15 PDT
25
Spectra of Saturn’s main rings
showing only H2O ice
26
Rings of Jupiter
View of Jupiter from behind, looking toward the Sun
27
•
•
•
•
Jupiter has three very thin rings
As with Neptune and Uranus, Jupiter’s rings are made of tiny black particles
The material in Jupiter’s rings is probably coming from the small moons Metis,
Jupiter’s
rings
Adrastea,
and others.
Material is being blasted off those small moons by the impact of
micrometeoroids attracted to Jupiter.
28
Saturn’s Rings:
Natural Color
Gaps in Saturn’s Rings
29
Gaps in the rings are caused by resonances with the satellites
Example: Mimas causes the Cassini Division
Mimas makes one
revolution in 23
hours. A ring
particle in The
Cassini Division
makes one
revolution in 11 ½
hours, or two
revolutions in one
Mimas period.
This is resonance.
What is “resonant
motion”?
30
When you push someone in
a swing, each time they
come back, you give a new
push to keep them going.
This is resonant motion.
Each time a ring particle
comes between Saturn and
Mimas, it gets a pull from
Mimas, causing its orbit to
become eccentric. This
increases the likelihood that
it will collide with another
particle and be destroyed.
31
Encke Division
Probably created not by
resonance, but by the
small moon “Pan”
which shares the orbit.
32
F Ring from Cassini probe close approach, July 3, 2004
33
Saturn’s F-ring from Cassini
34
F Ring Shepherd Moons
These two moons keep the narrow F ring in place.
35
Things to do
•Add more Pioneer and Voyager
history (follow “the planets” video),
along with Galileo and Cassini
probe results.
•Should we separate Jupiter from
Saturn?