Transcript The Outer Solar System

The Outer Solar System
The “Gas Giants”
The outer planets have no solid surface, and are much bigger. The primary gases
are hydrogen and helium (like the Sun), although Uranus and Neptune have
substantial (15 Earth mass) rock/ice cores.
Jupiter – King of the Planets
Mass = 0.001 solar (300 earths), Radius = 11.2 Earths, density = 1.3 x water
Distance: 5.2AU; Orbital Period: 11.8 years; Rotation period: 9.9 hours.
Hydrostatic Equilibrium – Pressure balance
“Hydrostatic equilibrium” governs the structure of all gaseous bodies
(planets or stars). The inside has higher temperature and density
because of the weight of the overlying material.
Giant Interiors
Jupiter and Saturn have similar structures. Both are still collapsing
slowly, and the gravitational energy released makes them “shine”
more heat out than they get from the Sun. It is carried out by
convection. Helium is also slowly settling faster than hydrogen.
Atmospheres of Jupiter and Saturn
Colorful Clouds
The Great Red Spot is a cyclone the
size of the Earth that has lasted at
least 300 years.
Red Spot Movie
Banded Structure of Clouds
Jupiter’s Magnetosphere – Bigger than the Sun
Making Magnetic Fields
Magnetic field arises when there is a conducting,
convecting medium in a rapidly rotating body.
This makes a “dynamo”; the same mechanism in
the Sun, the Earth and other contexts.
Auroral Zones
The high energy particles come down the field lines and hit the
atmosphere near the poles, causing the gases to glow. Just like on
the Earth, this makes an “aurora” in a ring-like zone.
Saturn
Mass = 95 Earths, Radius = 9.4 Earths, density = 0.7 x water (floats)
Distance: 9.5 AU; Orbital Period: 29.4 years; Rotation period: 10.6 hours.
Although it is impossible to think of Saturn without its rings, they are
of no planetary consequence, and are temporary. All the other outer
planets also have rings systems (but not as nice).
The Cassini Mission to Saturn
Already passed Jupiter,
will reach Saturn in
July 2004. We will
learn much more about
the planet, rings, and
moons.
The Huygens probe will drop into
Titan’s atmosphere, hopefully
reaching and analysing the
surface.
Uranus
Mass = 14.5 Earths, Radius = 4.0 Earths, density = 1.3 x water
Distance: 19.2 AU; Orbital Period: 84 years; Rotation period: 17.2 hours.
The Interior of Uranus (and Neptune)
Uranus and Neptune have relatively thin hydrogen layers on top;
they may have just begun to accrete large amounts of gas when
the solar nebula dissipated, interrupting their formation.
A Very Tilted Pole
The seasons on Uranus are extreme. Half the planet shares the fate of
the pole in not seeing the Sun for half the Uranus year (40 years).
Nonetheless, the temperature is fairly uniform around the planet; the
gases redistribute the heat.
Neptune
Mass = 17 Earths, Radius = 3.9 Earths, density = 1.76 x water
Distance: 30 AU; Orbital Period: 163 years; Rotation period: 16.1 hours.
Voyager showed it is more like
Jupiter than Uranus in
appearance. Recently, we have
developed the ability to see its
storms from Earth, using
“adaptive optics” in infrared.
Clouds and Storms on Neptune
High clouds are made of methane ice crystals. The heat flow is greater
than expected, giving more storms. The Great Dark spot was an
upwelling, but has already disappeared.
Pluto
Discovered in 1930 by Clyde Tombaugh. Charon discovered at USNO in 1978.
Pluto: radius = 1145km, mass = 0.002 Earths, density = 2.1x water
Charon: radius = 600 km, mass = 0.1-0.2 Plutos
Pluto was closer than Neptune in the late 1990s, but never crosses
Neptune’s actual path (2:3 orbital resonance, tilted).
Adaptive Optics image
Pluto and Charon
Last decade Charon’s
orientation carried it
through a number of
eclipses, giving us good
sizes and even maps for
the 2 bodies.
Inclination of Pluto's equator
to its orbital plane
About 112 degrees
(between 98° and
122°)
Charon's rotation period
6.387 days
retrograde
6.387 days
Charon's orbital period
6.387 days
Charon's average distance from Pluto
19,130 km
(11,889 mi)
Pluto's rotation period
What Pluto probably looks like
NASA is considering a Pluto mission now. Neptune’s moon Triton
is likely a good model for Pluto now (Triton is twice its size);
eventually Pluto’s atmosphere will freeze out onto its surface as it
recedes from the Sun. Triton has ice volcanoes erupting methane.