Jupiter's Rings
Q: How many rings does Jupiter really have? NASA describes four but I don't know if all
of them are considered rings. I am also confused because the University of California at
Santa Barbara says three. Also, how were they created?
(Alex, Brazil)
 A: Jupiter has four rings: two
faint gossamer rings (a, in the
figure), a main, brightest ring
(b), and a doughnut-shaped
halo ring (c).
Rings
of Jupiter: a. Gossamer, b.
Main, c. Halo Figure courtesy of
JPL/NASA. The gossamer ring (a) is
probably the source of
confusion. It is actually two
rings, one enclosing the
other, spread from the outer
edge of the main ring out to about three times Jupiter's radius. The rings--incredibly tenuous--cover only one millionth of their apparent surface area.
In 1996 and 1997 NASA's Galileo spacecraft recorded events showing how Jupiter's rings are
still being formed and solves the mystery of their birth billions of years ago. It happens like this:
Comet and meteor debris smash into Jupiter's inner four moons (blue dots in the figure). Jupiter's
gigantic gravitational field drives this rubble deep into the moons' surfaces at enormous speeds.
The heat of impact vaporizes the meteoroids and they explode, flinging dark-reddish soot from
the moon like puffs of chalk-dust from bopped erasers.
The Galileo spacecraft took pictures of the dust coming off Amalthea (1, in the figure) and Thebe
(4), the two moons orbiting in the gossamer ring. That's how we know what's happening around
Jupiter today.
To continue: The sooty moon dust, that was exploded into space, travels so fast it escapes the
minute gravitational fields of the tiny moons and goes into orbit. What's more--it enters the
gossamer rings and adds to the collection that has been accumulating there over billions of years.
This dust forms the gossamer rings. The orbits of its source moons, Amalthea and Thebe, enclose
the orbit of the two gossamer rings.
The dust constituting the main ring probably exploded off the other two moons, Adrastea (2) and
Metis (3), in a fashion similar to the gossamer-ring formation. The orbit of Metis is embedded in
the main ring and that of Adrastea skims the main-ring outer orbit. The halo appears to be made
of particles escaped from the main ring.
The four moons have bizarre surfaces that appear dark, red, and heavily cratered from meteoroid
impacts. They vary from a mere 12 miles to 154 miles across--insignificant specks compared with
our 1000-mile diameter Moon.
Further Surfing:
Cornell U: How Jupiter's rings formed
NASA: Jupiter's ring system
NASA: Rings--Jupiter, Saturn, Uranus, and Neptune |
