Uranus'
upper atmosphere (Click on the image for a larger
view) |
Radio
emissions detected several days before closest approach
provided the first conclusive indication that Uranus actually
possesses an magnetosphere.
Not
only does a Uranian magnetic field exist; it is intense
and skewed with its axis tilted at a 60-degree angle to
rotational axis. At Earth, by comparison, the two axes are
offset by about 12 degrees.
The
intensity of the magnetic field at Uranus's surface is roughly
comparable to that of Earth's, though it varies much more
from point to point because of its large offset from the
center of Uranus. The magnetic field source is unknown;
the electrically conductive, super-pressurized ocean of
water and ammonia once thought to lie between the core and
the atmosphere now appears to be nonexistent. The magnetic
fields of Earth and other planets are believed to arise
from electrical currents produced in their molten cores.
As at
Mercury, Earth, Jupiter and Saturn, there is a magnetic
tail extending millions of miles behind Uranus. Voyager
measured the magnetotail to at least 10 million kilometers
(6.2 million miles) behind the planet. The extreme tilt
of the magnetic axis, combined with the tilt of the rotational
axis, causes the field lines in this cylindrical magnetotail
to be wound into a corkscrew shape.
Voyager
2 found radiation belts at Uranus of an intensity similar
to those at Saturn, although they differ in composition.
The radiation belts at Uranus appear to be dominated by
hydrogen ions, without any evidence of heavier ions (charged
atoms) that might have been sputtered from the surfaces
of the moons. Uranus's radiation belts are so intense that
irradiation would quickly darken (within 100,000 years)
any methane trapped in the icy surfaces of the inner moons
and ring particles. This may have contributed to the darkened
surfaces of the moons and ring particles.
Voyager
detected radio emissions from Uranus that, along with imaging
data, helped narrow the planet's rate of rotation to about
17 hours, 14 minutes.
