The Solar System and the Planet Earth 301
are in total darkness for 42 Earth years or one half a Uranus year and in
light the other half of the planet’s orbit.
Uranus differs in composition from Jupiter and Saturn and is more
similar to Neptune. Its core is rocky but it also contains ices. Uranus’s
atmosphere, like that of Jupiter’s and Saturn’s, is composed primarily of
hydrogen and helium but it also contains the ices of water, ammonia
and methane along with traces of hydrocarbons. The lower clouds in
its atmosphere are made up of water and while those of the upper
atmosphere consist of methane. It has the coldest planetary atmosphere
in the Solar System, with a temperature as low as 50 K. Winds on Uranus
can reach speeds of 900 km per hour.
Uranus has 27 known moons and a complex planetary ring system
like that of Saturn. The moons are rather small. The largest Uranian
moon has a radius of only 789 km, which is less than half the radius of
our Moon. The moons are made up of approximately 50% rock and 50%
ice. The 13 known rings are very narrow and are only a few kilometers
wide. They are composed of small particles less than a meter in diameter
and are thought to be the debris form a moon that disintegrated from a
collision of some sort. The rings also contain ice particles.
Neptune
A search for the planet Neptune was made because of the irregularities of
the orbit of Uranus. It was concluded that an eighth planet beyond the
orbit of Uranus was causing this perturbation. The position of Neptune
was correctly predicted on the basis of the perturbation of the orbit of
Uranus. When the telescope was directed at the expected position of
Neptune it was discovered in its proper place. Neptune orbits the Sun
once every 165 years at a mean distance of 30 AU. It rotates on its axis
once every 15 hours and 40 minutes. The planet is very similar to Uranus
slightly smaller in size, but a wee bit heavier due to its greater density.
Neptune, like Uranus, also appears greenish in colour.
Of Neptune’s 13 known moons the largest is Triton. It accounts for
more than 99.5 percent of the mass in orbit around Neptune. It is massive
enough to be spheroidal unlike the other moons of Neptune. Triton is the
only large moon in the Solar System with a retrograde orbit indicating
that it was captured rather than formed with the planet. It is likely that it
once was a dwarf planet in the Kuiper Belt.