Download The Jovian Planets

The Jovian Planets
Dr. Ken Rice
Discovering Astronomy S
The Jovian Planets
•
The Jovian planets are the 4 outer solar system planets
– Jupiter, Saturn, Uranus, Neptune
Jupiter
Saturn
Uranus
Neptune
Distance = 19.2 AU
Distance = 30.1 AU
Mass = 14 MEarth
Mass = 17 MEarth
Distance = 5.2 AU
Distance = 9.54 AU
Density = 1.24
Mass = 318 MEarth
Mass = 95 MEarth
Composition : H
Composition : H
compounds, rock, H, He
compounds, rock, H, He
Density = 1.33 g/cm3
Density = 0.71
Composition : Mostly H, He
Composition : Mostly H, He
g/cm3
Discovering Astronomy S
g/cm3
Density = 1.67 g/cm3
Jupiter and Saturn
•
•
Central rocky core ~ 10 Earth
masses
Atmosphere
– Hydrogen and Helium
•
Both have a very similar size
– Saturn much less dense than
Jupiter.
– If Jupiter were more massive it
would get smaller.
Discovering Astronomy S
Uranus and Neptune
•
•
•
Core of hydrogen compounds, rock and metals.
Cores are bigger than Jupiter and Saturns because less compressed by the
atmosphere.
Gaseous hydrogen atmosphere
– Pressure not high enough for liquid or metallic hydrogen
Discovering Astronomy S
Internal heat
•
•
•
In terrestrial planets internal heat is important for geological activity
In Jovian planets, it is important for weather (they have no solid surface).
Jupiter and Saturn both radiate twice as much as they receive from the Sun
– Jupiter – contraction releasing gravitational potential energy
– Saturn – Helium condense in the upper atmosphere and sinks, releasing energy
(differentiation)
Discovering Astronomy S
Atmospheric structure
•
Galileo probe plunged into Jupiter’s
atmosphere in 1995
– Structure similar to the Earth’s atmosphere
– Troposphere, stratosphere, thermosphere
•
Jupiter and Saturn
– 3 clouds layers
– Water (H2O), ammonium hydrosulfide
(NH4SH), ammonia (NH4).
•
Uranus and Neptune
– 1 cloud layer (methane – CH4)
– May be additional layers lower down???
Discovering Astronomy S
Coriolis force
•
The coriolis force is a fictitious force that
occurs because the actual ground speed of
a rotating sphere varies with latitude
– Something on the equator moves faster than
something nearer the poles
– Something moving from one latitude to
another follows an apparently curved path
when viewed from the ground.
Discovering Astronomy S
Storms on Jupiter
•
•
Dynamic weather – strong winds and
powerful storms
Circulation cells
– Hot Equatorial gases expand and
move towards poles
– Polar air flows towards equator
•
Jupiter rotates every 10 hours
– Strong coriolis force
– Produces alternating bands
•
Alternating colours are different
molecules
– Reddish ammonium hydrosulfide
covers the entire planet at low altitude
– Rising air forms white ammonia
clouds at high altitudes.
•
No Seasons
Discovering Astronomy S
The Great Red Spot
•
•
Giant storm – twice as wide as the Earth
Somewhat like a hurricane
– High pressure rather than low.
•
Long lived
– Has been observed for more than 300 years!
•
Smaller storms – relative to the Great Red
Spot - are constantly brewing in Jupiter’s
atmosphere.
Discovering Astronomy S
Saturn, Uranus and
Neptune
•
Saturn
– Like Jupiter, rapid rotation creates fast eastwest winds
– Axis tilt produces some seasonal effects
•
Neptune
– Also banded and has high-pressure (but short
lived) storms like the Great Red Spot.
– Also has seasons
•
Uranus
–
No bands and no storms (very low internal
heat)
– May have some seasonal storms
Discovering Astronomy S
Jupiter’s magnetosphere
•
•
The liquid hydrogen in Jupiter’s envelope and its rapid rotation produce a
strong magnetic field.
Strong magnetic field produces an enormous magnetosphere
– Deflects the solar wind about 3 million km before ahead of Jupiter
•
Gas escaping from Jupiter’s moon Io creates a ring of charged particles
– Creates very bright aurorae
Discovering Astronomy S
Saturn, Uranus and Neptune
Discovering Astronomy S
Planetary rings
•
All the Jovian planets have ring systems
Discovering Astronomy S
Saturn’s rings
•
Made up of particles
– Ranging in size from icy dust grains to boulders
•
Frequent collisions (every few hours in the densest regions)
– Thinnest known astronomical structure
– A few tens of metres thick
– The orbits are almost perfect circles
Discovering Astronomy S
Rings and gaps
•
Moons inside the rings can open gaps
– Gravity
•
Moons outside the rings can also open gaps
– Resonances (Kepler’s laws)
•
Shepherd moons
– Two moons can produce a very narrow ring
between them
Discovering Astronomy S
Tidal forces and the Roche limit
•
If we consider a satellite (moon) orbiting a planet with mass Mpl, there is a
difference between the gravitational acceleration of a point at the centre of the
satellite, and a point on the surface.
A=
GM pl
d
2
−
GM pl
(d + r )
2
≈
2GM pl r
d3
•
The satellite is held together by its own
gravity.
•
If the satellites gravity is not strong
enough, it will be torn apart by the tidal
force from the planet.
•
The closest distance at which this will
happen is
1
 3M pl  3

d > r 
m


Discovering Astronomy S
Examples of tidal limits – moons of Saturn
distance
(km)
radius (km)
mass (kg)
tidal radius
(km)
Pan
134000
15
4.90E+15
106585.6789
Atlas
138000
14
6.60E+15
90078.19616
Prometheus
139000
46
2.70E+17
85896.64486
Pandora
142000
46
1.35E+17
108222.991
Epimetheus
151000
57
5.60E+17
83461.29844
Janus
151000
89
2.01E+18
85113.33564
Mimas
186000
196
3.80E+19
70361.40387
Enceladus
238000
260
8.40E+19
71650.60737
Tethys
295000
530
7.55E+20
70247.87251
Telesto
295000
15
2.70E+17
28009.7755
Calypso
295000
13
2.70E+17
24275.13877
Dione
377000
560
1.05E+21
66496.36176
Helene
377000
16
2.70E+17
29877.09386
Rhea
527000
765
2.49E+21
68118.94969
Titan
1222000
2575
1.35E+23
60581.34821
Hyperion
1481000
143
1.77E+19
66224.38069
Iapetus
3561000
730
1.88E+21
71385.47724
Phoebe
12952000
110
4.00E+18
83633.25085
Discovering Astronomy S
Why do Jovian planets have rings
•
The rings fall close to the Roche tidal zone
– Zone within which bodies held together by gravity (i.e. moons) are ripped apart by the
tidal forces from the Jovian planet
– Tidal forces result from the different gravitational force on either side of a large body
(this is why we have tides).
•
Ring particles are continually colliding, and being ground down
– Pressure from sunlight causes them to eventually spiral into the planet
– Must be replenished
•
Rings particles must come from small moons in the planets’s equatorial plane!
– These small moons are small enough to overcome the tidal forces.
– Small impacts release small particles from the surface of these moons
– Occasional large impacts can destroy these moons completely!
•
The above also means that all large moons lie outside the Roche tidal zone
– There are no large moons within the rings
Discovering Astronomy S