URANUS: A featureless blue-green sphere
Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.
It was the first planet discovered in modern times, by William Herschel
in 1781. Uranus is the third largest planet (by diameter), larger in
diameter than Neptune but smaller in mass. The planet rotates once
every 16.7 hours and a Uranus year is 84 Earth years.
Uranus' axis is almost inline with the ecliptic (the plane of the planets revolution around the Sun). Uranus still has seasons: the south summer lasts for 1/4 of the orbit, about 21 years. Then gradually, for the next 21 years, the Sun would appear over the equator, passing through equinox. Next, the north pole of Uranus would be fully illuminated for about 21 years, and so on. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun.
|
|
How did the rotational axis of Uranus get tilted?
When something strange is found in the Solar System, the scientists like to
suggest the "collision theory".. And this is indeed what may have happened to
Uranus: It is known from the cratering evidence we see on a variety of planetary
surfaces, that soon after the planets were formed, there were still some mighty
large mini-planets orbiting the Sun. One of these may have collided with the
Earth, dredging up material that later solidified into our Moon.
The material from the Uranus collision didn't form a separate moon, but was
absorbed into Uranus. In addition to mass and kinetic energy, the
collision would also have contributed angular momentum. The result is
that the spin direction and speed for Uranus could have been seriously
altered from its initial state. This theory has also been suggested to
explain why Venus rotates backwards.
The composition of Uranus
The Uranus atmosphere is composed of about 83 percent hydrogen, 15 percent helium, 2
percent methane, and tiny amounts of ethane and other gases. The methane
gives Uranus is blue-green color, as this molecule absorbs red light
more efficiently than blue. Very faint bands do exist within the
clouds parallel to the equator, as well as "hurricane" spots (like Jupiter
and Saturn), but are only seen in high contrast computer images.
Uranus appears to have a core, or center, of melted rock, which changes
into a dirty ocean made of water, ammonia and other elements further
from the center. The planet layers are not separate but gradually
blend with one another.
For a recent article about Uranus's Weather, click here.