SATURN
Key Concepts
- The interior structure of Saturn is generally similar to that of Jupiter.
- Saturn's giant moon Titan has a thick atmosphere of
nitrogen and methane.
- Saturn is circled by rings, consisting of chunks of ice
orbiting the planet.
(1) The interior structure of Saturn is generally similar to that of
Jupiter.
Saturn's orbit has a semimajor axis of 9.5 A.U., about twice the size of
Jupiter's orbit. The orbital period of Saturn is about 29 years. The rotation
period of Saturn is 10.5 days, only slightly longer than that of Jupiter. The
rapid rotation of Saturn has flattened it noticeably; the equatorial diameter of
Saturn is 10% larger than its polar diameter. The oblate shape of Saturn can be
seen in the image below, returned by one of the Voyager
spacecraft.
Subtract Saturn's spectacular rings, and what have you got left? A downsized
Jupiter. The internal structure of Saturn has the same four layers as that of
Jupiter:
- Ordinary molecular hydrogen (with He mixed in)
- Liquid metallic hydrogen (with He mixed in)
- Liquid water, methane, and ammonia
- Solid rocky core
The differences that exist between Saturn and Jupiter arise mainly from Saturn's
lower mass. For instance, because of its lower mass, Saturn experiences less
gravitational compression than Jupiter. Thus, the density of Saturn is very low
(less than 700 kg/m3). The low density makes Saturn more easily
flattened by rotation than Jupiter, despite the fact that its rotation period is
slightly longer. The smaller gravitational compression of Saturn means that it
contains less metallic hydrogen than Jupiter, since hydrogen becomes a metal
only when strongly compressed.
Like Jupiter, Saturn has an internal heat source. The heat of Saturn's
interior comes from gravitational potential energy, as helium in the liquid
interior of Saturn condenses into dense drops and falls inward to the core. The
atmosphere of Saturn, heated from above by the Sun and from below by the
internal heat source, has belts and zones, like the atmosphere of Jupiter. The
west to east winds on Saturn are very strong; at the equator, the westerly winds
of Saturn blow at speeds of 1800 kilometers per hour (1100 mph, the fastest
winds in the solar system). There are few circular storms on Saturn, though; the
few that exist are smaller and much shorter lived than the Great Red Spot on
Jupiter.
So far, Saturn is just a miniature Jupiter. Big deal. It is only in its
entourage of moons and rings that Saturn is more interesting than Jupiter.
(2) Saturn's giant moon Titan has a thick atmosphere of nitrogen and
methane.
At the moment, Saturn has 30 known moons. Twenty-nine of these moons are small
objects made of ice and rock, with diameters ranging from a few kilometers to
1500 kilometers. The moons larger than 300 kilometers in diameter are all
spherical; their gravitational pull is large enough to compress them into a
spherical shape. The moons smaller than 300 kilometers across are irregular in
shape; their smaller gravitational pull is unable to squeeze the stiff rock and
ice into a sphere. (The dividing line of 300 kilometers - about 200 miles - is
only a rough one; the actual value depends on the stiffness of the material of
which the moon is made.)
Life is too short (alas!) to study all the moons of Saturn in
detail, so I will only focus on Saturn's giant moon, Titan.
Titan is the only moon in the solar system that has a substantial
atmosphere. In the image of a crescent Titan seen below, the horns of the
crescent are elongated by the light which is refracted through the atmosphere of
Titan.
Titan's diameter is only 2% smaller than that of Ganymede, the largest of
Jupiter's Galilean moons. The density of Titan is 1900 kg/m3, the
same as that of Ganymede, hinting that Titan, like Ganymede, has an icy mantle
over a rocky core. Is Titan, then, a twin of Ganymede? NO! Titan differs from
Ganymede in one key respect; it is almost twice as far from the Sun, and thus
receives about a quarter as much energy per unit area. The temperature
of Titan is lower than that of Ganymede. The escape velocity of
Titan is the same as that of Ganymede. The cooler moon, Titan, has hung onto the
heavier molecules in its atmosphere, whereas Ganymede, being warmer than Titan,
has lost all its atmosphere.
Titan's atmosphere is about 90% nitrogen, with the remainder consisting
mainly of methane (CH4), with traces of more complex hydrocarbons,
such as ethane (C2 H6). The surface pressure on Titan is
50% greater than the air pressure at sea level on Earth. The surface temperature
on Titan averages about 95 Kelvin (-288 Fahrenheit), slightly warmer than it
would be in the absence of methane, which is a greenhouse gas. We cannot see the
surface of Titan, because it is hidden by a layer of ``smog'', made of tiny
droplets of hydrocarbon polymers. Infrared light can pierce the smog;
observations at infrared wavelengths hint that Titan might have regions that are
covered with lakes of liquid hydrocarbons.
Hydrocarbons are to Titan
as water is to Earth.
On Earth, the average temperature of 300 Kelvin is between the melting point and
boiling point of water. On Earth, water exists as a gas in the atmosphere, as a
liquid in the oceans, and as a solid in the icecaps.
On Titan, the average temperature of 95 Kelvin is between the melting point and
boiling point of methane. On Titan, methane and ethane exist as gases in the
atmosphere. Methane and ethane exist as liquid droplets in clouds and as liquid
lakes at the surface of Titan. Gooey complex hydrocarbons exist as tarry
deposits on the solid surface of Titan.
The surface of Titan is probably an interesting place, with a layer of
hydrocarbon ``goo'' covering the solid surface, and lakes of liquid natural gas.
All the
oxygen on Titan is locked up in the form of water ice, far beneath the gooey
surface.
(3) Saturn is circled by rings, consisting of chunks of ice orbiting the
planet.
A history of the discovery of Saturn's rings:
- 1610: Galileo looks at Saturn through his telescope. He notes that the
planet is not circular, but has an odd bulge on either side.
- 1655: The astronomer Huygens looks at Saturn through a better telescope.
He is able to resolve the bulges seen by Galileo into a ring completely
encircling the planet in its equatorial plane.
- 1675: The astronomer Cassini looks at Saturn through a still better
telescope. He notice that the ring is actually two rings (now called the A
ring and the B ring), separated by an empty gap (now called the Cassini
division).
- 1980: At last, the Voyager 1 spacecraft visits Saturn, and reveals that
the rings of Saturn are made of thousands of individual narrow ``ringlets''.
The ringlets are not solid objects, like hula hoops, encircling the
planet. Instead, the rings of Saturn are made of literally billions of
individual chunks of ice, each a tiny moon of Saturn, orbiting the planet on its
own elliptical orbit. The chunks are too small to be seen individually by the
Voyager spacecraft. However, the size of the chunks was determined by an
experiment which looked at how effectively the rings of Saturn scattered
electromagnetic radiation of different wavelengths. (An opaque object 1
centimeter across, for instance, will not affect radiation with wavelengths much
longer than 1 cm, will absorb radiation with wavelengths much shorter than 1 cm,
and will scatter radiation with wavelengths comparable to 1 cm.) Bottom line:
the lumps of ice which make up Saturn's rings vary in size. Most of the lumps
are between one centimeter and a few meters in diameter. The ice chunks are so
closely packed together, they frequently collide, giving them battered,
irregular shapes.
The Cassini spacecraft, launched on October 15, 1997, is on its way to Saturn.
After reaching Saturn, Cassini will drop a probe into the atmosphere of Titan in
December 2004. The probe is pictured on the surface of Titan in the artist's
rendition below; Saturn is seen in the background, through the smoggy haze of
Titan's atmosphere.
[Image credit: European Space Agency]
For more information, visit the Cassini
home page. (Want to know where the Cassini spacecraft is located right now? Click
here.)
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