NASA’s Hubble Space Telescope Finds Rings and Moons of Uranus

NASA's Hubble Space Telescope photographed a new pair of rings around
Uranus and two new, small moons orbiting the planet.

The largest ring is twice the diameter of the planet's previously
known rings. The rings are so far from the planet, they are being
called Uranus' "second ring system." One of the new moons shares its
orbit with one of the rings. Analysis of the Hubble data also reveals
the orbits of Uranus' family of inner moons have changed
significantly over the past decade.

"The detection of these new interacting rings and moons will help us
better understand how planetary systems are formed and sustained,
which is of key importance to NASA's scientific exploration goals,"
said Dr. Jennifer Wiseman, program scientist for Hubble at NASA
Headquarters.

Since dust orbiting Uranus is expected to be depleted by spiraling
away, the planet's rings must be continually replenished with fresh
material. "The new discoveries demonstrate that Uranus has a youthful
and dynamic system of rings and moons," said Mark Showalter of the
SETI Institute, Mountainview, California.

Showalter and Jack Lissauer of NASA's Ames Research Center, Moffet
Field, Calif., propose that the outermost ring is replenished by a
12-mile-wide newly discovered moon, named Mab, which they first
observed using Hubble in 2003.

Meteoroid impacts continually blast dust off the surface of Mab. The
dust then spreads out into a ring around Uranus. Mab's ring receives
a fresh infusion of dust from each impact. Nature keeps the ring
supplied with new dust while older dust spirals away or bangs back
into the moon.

Showalter and Lissauer have measured numerous changes to the orbits of
Uranus' inner moons since 1994. The moon's motions were derived from
earlier Hubble and Voyager observations. "This appears to be a random
or chaotic process, where there is a continual exchange of energy and
angular momentum between the moons," Lissauer said. His calculations
predict moons would begin to collide as often as every few million
years, which is extraordinarily short compared to the 4.5 billion
year age of the Uranian system.

Showalter and Lissauer believe the discovery of the second ring, which
orbits closer to the planet than the outer ring, provides further
evidence that collisions affect the evolution of the system. This
second ring has no visible body to re-supply it with dust. The ring
may be a telltale sign of an unseen belt of bodies a few feet to a
few miles in size. Showalter proposes that a previous impact to one
of Uranus' moons could have produced the observed debris ring.

Hubble uncovered the rings in August 2004 during a series of 80,
four-minute exposures of Uranus. The team later recognized the faint
new rings in 24 similar images taken a year earlier. Images from
September 2005 reveal the rings even more clearly.

Showalter also found the rings in archival images taken during Voyager
2's flyby of Uranus in 1986. Uranus's first nine rings were
discovered in 1977 during observations of the planet's atmosphere.
During the Voyager encounters, two other inner rings and 10 moons
were discovered. However, no one noticed the outer rings, because
they are extremely faint and much farther from the planet than
expected. Showalter was able to find them by a careful analysis of
nearly 100 Voyager images.

Because the new rings are nearly transparent, they will be easier to
see when they tilt edge-on. The new rings will increase in brightness
every year as Uranus approaches its equinox, when the sun shines
directly over the planet's equator. When it happens in 2007, all of
the rings will be tilted edge-on toward Earth and easier to study.
These research data will appear in an upcoming issue of the journal
Science.

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