Transcript The Gas Giants The Gas Giants Together they account for 99.5% of our planetary system Jupiter Saturn Uranus Neptune Mass (MEarth) 317.8 95.2 14.4 17.2 Distance from Sun (AU) 5.20 9.54 19.2 30.1 Equatorial Radius (REarth) 11.2 9.46 3.98 3.81 Average.

The Gas Giants
The Gas Giants
Together they account for 99.5% of our planetary system
Jupiter
Saturn
Uranus
Neptune
Mass (MEarth)
317.8
95.2
14.4
17.2
Distance from Sun (AU)
5.20
9.54
19.2
30.1
Equatorial Radius (REarth)
11.2
9.46
3.98
3.81
Average Density (kg/m3)
1330
710
1240
1670
Size-Mass Relationship
• As with terrestrials, composition can be guessed from mean
density. But the high compressibility of volatiles must be
accounted for
 Initially, as they accrete mass they grow in radius
 But at a mass of ~300 earth masses, further accretion causes the
radius to decrease.
Composition
• Jupiter and Saturn are
well matched by H+He
models
 Require some ice/rock
as well
• Uranus and Neptune are
much better represented
by an icy composition.
1
T=0 K models
T>0K
Recall: Moment of Inertia
• The moment of inertia is a measure of degree of concentration
 Related to the “inertia” (resistance) of a spinning body to external
torques
 Shows giant planets are centrally concentrated: cores?
Body
I/MR2
Sun
0.06
Mercury
0.33
Venus
0.33
Earth
0.33
Moon
0.393
Mars
0.366
Jupiter
0.254
Saturn
0.210
Uranus
0.23
Neptune
0.23
0.4 for a homogeneous sphere
I

  0.67 for a hollowshell
2
MR
 0 for a point mass

Shapes
• Rotation induces significant
flattening of compressible
material
Body
a/(a-b)
Rotation
Period (d)
Sun
10000
25.4
Earth
298
1.0
Jupiter
16
0.41
Saturn
10
0.426
Heat Balance
• The average temperature of Jupiter is 160 K. Is it in thermal
equilibrium? (Assume a visible albedo of 0.43, but a perfect
blackbody in the infrared).
• An infrared picture of Jupiter
Atmospheres
• Similar thermal structure
to terrestrial planets
• Temperature of Jupiter
and Saturn is never low
enough to form a methane
cloud deck – which
dominates Neptune and
Uranus
H at high temperatures and pressures
• Atmosphere of Jupiter and
Saturn is mostly “liquid” H2.
• At very high pressures, H atoms
dissociate from each other and
their electrons
 Forms a metallic liquid
 Good conductor
Maximum of Earth-based
laboratory experiments
STP
Interiors
• Jupiter and Saturn are dominated by an
atmosphere of fluid, metallic hydrogen
• Neptune and Uranus are dominated by an
icy mantle, probably as a fluid,
conducting ocean, surrounded by a H and
He atmosphere
Cloud patterns
• Surface features
are due to forms
and colourations of
the highest cloud
layers
Magnetospheres
• The giant planets have strong magnetic fields
 Likely due to the convective, metallic hydrogen interior
 Interact with solar wind (and atmosphere of Io) to produce
spectacular aurorae
Summary: The Gas Giants
Jupiter
Saturn
Uranus
Neptune
Mass (MEarth)
317.8
95.2
14.4
17.2
Core (MEarth)
~15?
~15?
~13?
~13?
%Mass of Core
≤5%
≤15%
≤90%
≤76%
I/MR2
0.254
0.210
0.23
0.23
Heat Out/In
2.5
2.3
~1.1
2.7
Wind Max (km/s)
~100
≥400
≤200
≤200
Prot (hrs)
9.8
10.6
17.2
16.1
Magnetic Field (vs Earth)
20,000
6000
50
25
Convection?
Y
Y
Y?
Y
Break
Jupiter
• This shows Voyager
1's approach during a
period of over 60
Jupiter days.
• Notice the difference
in speed and direction
of the various zones
of the atmosphere.
• The interaction of the
atmospheric clouds
and storms shows how
dynamic the Jovian
atmosphere is.
Cloud motions in Jupiter’s atmosphere
The Coriolis force diverts N-S motion into E-W motion, in distinct
zonal bands (5 in each hemisphere).
Infared and Optical
• Regions of white in visible light are dark in infrared
internal heat is blocked by the clouds
• Darkest visible bands are brightest in the infrared
seeing deeper into the atmosphere where it is hotter
• Red spot also dark in IR: cool, high altitude storm
Jupiter’s atmosphere
• Rising air from the deeper layers cools and forms clouds as it
rises; we see deeper where the high ammonia clouds have been
depleted by precipitation, much as on Earth rain will often mean
clearer skies.
Great Red Spot
•
•
Red colour probably from red phosphorous
A large eddy caused by rising hot gas and the Coriolis force
Oval BA
• A White oval storm, similar to the Red Spot but smaller
 Formed from colliding storms in 1998
• Recently turned Red
 May be bringing material to the upper atmosphere, where reactions with
UV solar rays change the colour.
Oval BA
• Near collision in July 2006
 Is now the size of Earth with winds reaching 645 km/s, similar to
the Great Red Spot
July 2006
Feb 2006
Saturn
Saturn… views never seen before
• From the Cassini spacecraft http://saturn.nasa.jpl.gov
Saturn… views never seen before
• From the Cassini spacecraft http://saturn.nasa.jpl.gov
• night side of Saturn is partly lit by light reflected from its own
ring system.
Earth
Faint E-ring,
created by
fountains
from
Enceladus
Earth from Saturn
Crossing the ring plane
Internal heating
•
Saturn radiates more energy than it receives, by the same
amount as Jupiter does
 But Saturn is smaller, and this cannot all be gravitational energy
 It is thought that He forms droplets and sinks downward, releasing
gravitational energy
Thermal infrared
picture of Saturn
Storms on Saturn
• Like Jupiter, Saturn
shows large storms
• These are usually harder
to see, however
Storm at the pole
Uranus and Neptune
Uranus
• Bluish green colour, with far
fewer atmospheric features
than Jupiter and Saturn
Neptune
Colours of Uranus and Neptune
Uranus and Neptune: Interiors
• Likely have a rocky core, but are
dominated by a fluid, icy and ionic
ocean
• Surrounded by an atmosphere rich in
H and He
• Uranus is the only gas giant that does
not emit much more heat than it
receives from the Sun
 Internal convection disrupted?
 Consistent with lack of storms
Uranus’ tilt
• Uranus has an axis of rotation pointing almost directly toward the
Sun
 Interestingly, prevailing winds are still E-W, so the Coriolis force
dominates the weather patterns.