300 The Poetry of Physics and The Physics of Poetry
The most interesting feature of Saturn is its many concentric
equatorial rings that stretch from 7,000 km to 80,000 km above Saturn’s
surface with an estimated local thickness of only 10 to 20 meters. The
rings are composed of particles ranging in size from 0.01 to 10 meters
and are made primarily of water in the form of ice. In addition to these
particles of ice there are tiny moonlets. The mass of all the material in
the rings is only about 3 × 10^19 kg. This is a miniscule fraction of the
total mass of Saturn or 0.05 of 1% of the mass of our moon. While only
two gaps in the rings can be seen from Earth a fly by of the Voyager
spacecraft revealed that the rings are riddled with thousands of gaps
some of which are caused by tiny moonlets sweeping out any debris in
their orbit. The rings of Saturn have their own atmosphere independent
of the planet due to the ultraviolet rays of sunlight interacting with the
water to produce molecular oxygen in the form O 2.
In addition to the equatorial rings Saturn also has the Phoebe ring,
which extends from 128 to 207 times the radius of Saturn and is tilted at
27° to the equatorial plane of Saturn. The rings seems to have been
created by tiny meteorites impacting the moon Phoebe which orbits
Saturn at a distance of 215 Saturn radii in an orbit tilted at 27° just like
the ring.
In addition to the hundreds of moonlets in the rings Saturn has 61
moons. Titan, the largest moon of Saturn, is the second largest moon in
the Solar System after Jupiter’s moon Ganymede. It exceeds the planet
Mercury in size. It is also the only moon in the Solar System that has a
significant atmosphere.
Uranus
Uranus was the first planet discovered with a telescope. It orbits the
Sun once every 84 years at a distance of 19.1 AU. It has a radius of
23,800 kilometers. Slightly denser than Jupiter with a mass that is
14.5 times that of Earth. Uranus rotates on its axis once every 10 hours
and 50 minutes from east to west like Venus. The equator of most planets
is inclined to the plane of their orbits by some 20 or 30 degrees. This
accounts for the seasonal change of Earth and Mars. The equator of
Uranus, on the other hand, is extremely unusual as it is inclined 97.8
degrees to the plane of its orbit and therefore rotates perpendicular to the
line of its orbit about the Sun. The consequence of this is that the poles