Transcript General Astronomy - Stockton University

General Astronomy
Rings ‘n’ Things
Rings are swarms of orbiting particles
• Orbits have to be very
circular
• Elliptical orbits will result
in collisions, destroying the
ring
Rings Have to be Very Flat
Inclined orbits
will result in
collisions and
destruction of
the ring
Rings have to be Equatorial
• Tilted orbits precess; the plane of the orbit
rotates due to the planet’s equatorial bulge,
other satellites, and the Sun’s gravity
A tilted ring won’t stay flat very long
• Particle orbits will precess at different rates
• Collisions will soon destroy the ring
Rings, Gaps, and Resonances
• When two objects return to the same relative
positions regularly, they are said to be in
resonance
• Some resonances are stable. Mercury’s 3:2
resonance between its rotation and its orbit is
an example.
• Pluto’s period is 3/2 that of Neptune’s. This
resonance keeps the two planets from ever
colliding even though their orbits cross.
Rings, Gaps, and Resonances
• Some resonances create gaps. There are no
asteroids with one-half Jupiter’s period
because repeated pulls by Jupiter eventually
would change the asteroid’s orbit.
• Gaps in Saturn’s rings are due to resonances
with Saturn’s satellites.
Jupiter's Ring System
Jupiter's ring was discovered by Voyager 1 in a single image that was
targeted specifically to search for a faint ring system.
The ring is now known to be composed of three major components. The
main ring is about 7000 km wide and has an abrupt outer boundary
129,130 km from the center of the planet.
The main ring encompasses the orbits of two small moons, Adrastea and
Metis, which may act as the source for the dust that makes up most of
the ring.
At its inner edge the main ring merges gradually into the halo. The halo is
a broad, faint torus of material about 20,000 km thick and extending
halfway from the main ring down to the planet's cloud tops.
Just outside the main ring is a pair of broad and exceedingly faint
gossamer rings, one bounded by the moon Amalthea and the other
bounded by the moon Thebe.
Jupiter's Ring System
Saturn, Lord of the Rings
Saturn's rings make the planet the most spectacular object in the solar system.
The Saturnian Rings
• The rings are split into a number of different parts,
which include the bright A and B rings and a fainter C
ring.
• The ring system has various gaps. The most notable
gap is the Cassini Division, which separates the A and
B rings. The Encke Division splits the A Ring.
• Space probes have shown that the main rings are
really made up of a large number of narrow ringlets.
Close up and Personal
Possible variations in
chemical composition
from one part of
Saturn's ring
system to another
are visible in this
Voyager 2 picture.
Obtained Aug. 17
from a distance of
8.9 million
kilometers (5.5
million miles).
The Voyager project is
managed for NASA by the
Jet Propulsion Laboratory,
Pasadena, Calif.
Saturn’s Rings
• The origin of the rings is obscure.
– One thought is that the rings may have been formed from
larger moons that were shattered by impacts of comets and
meteoroids.
– Another possibility is a moon/comet which was pulled apart by
tidal action
• The ring composition is not known for certain, but the rings do
show a significant amount of water.
– They may be composed of icebergs and/or snowballs from a few
centimeters to a few meters in size.
Shepherd Moons
Much of the elaborate structure of
some of the rings is due to the
gravitational effects of nearby
satellites. This phenomenon is
demonstrated by the relationship
between the F-ring and two small
moons that shepherd the ring
material.
Shepherd Moons help maintain the
sharp edges of rings
A Shepherd Moon Outside a Ring
Tends to Drag Stray Particles
Back, Causing Them to Fall Back
Into the Ring
A Shepherd Moon Inside a Ring
Tends to Speed Stray Particles
Up, Causing Them to Rise Back
Into the Ring
Outer Shepherd Moon
Inner Shepherd Moon
Spokes
Radial, spoke-like features in the broad B-ring were also found by the Voyagers. The features are
believed to be composed of fine, dust-size particles. The spokes were observed to form and
dissipate in the time-lapse images taken by the Voyagers. While electrostatic charging may
create spokes by levitating dust particles above the ring, the exact cause of the formation of
the spokes is not well understood.
Spoke Movie
Particle Size in the Rings
Purple > 5 cm
Green < 1 cm
White: too dense
Can you find Rhea?
No, That’s Mimas
Here is Rhea
The Uranian Ring System
• The Uranian ring system was discovered in 1977 during
observations of a stellar occultation by the planet.
– The star was observed to blink out briefly five times before the planet
and again five times afterward, indicating that the planet was encircled
by five narrow rings.
• Subsequent Earth-based observations indicated that there were
actually nine major rings.
• Working outward from the planet, they are named 6, 5, 4, Alpha,
Beta, Eta, Gamma, Delta, and Epsilon.
• In 1986 the Voyager spacecraft acquired additional occultations
and also our best images of the ring system.
– The Voyager cameras detected a few additional rings and also showed
that the nine major rings are surrounded by belts of fine dust.
The Uranian Ring System
IR Image of Uranus’ Rings
Another IR image (colorized)
Neptune's Ring System
•
•
Evidence for incomplete arcs around Neptune first arose in the mid1980's, when stellar occultation experiments were found to occasionally
show an extra "blink" just before or after the planet occulted the star.
Images by Voyager 2 in 1989 settled the issue, when the ring system was
found to contain several faint rings, the outermost of which, Adams,
contains three prominent arcs now named Liberty, Equality and Fraternity.
– The existence of arcs is very difficult to understand because the laws of
motion would predict that arcs spread out into a uniform ring over very short
timescales.
– The gravitational effects of Galatea, a moon just inward from the ring, are now
believed to confine the arcs.
•
Several other rings were detected by the Voyager cameras.
– Adams Ring 63,000 km from the center of Neptune
– Leverrier Ring is at 53,000 km and the broader
– Galle Ring is at 42,000 km.
– A faint outward extension to the Leverrier Ring has been named Lassell; it is
bounded at its outer edge by the Arago Ring at 57,000 km.
Neptune's Ring System
Rings Are Probably Short-Lived
• Gravity of the Sun and planet’s satellites
constantly disturbs orbits of particles
• Collisions probably happen frequently
• Particles should drift out of the ring
• Rings around outer planets have probably
come and gone repeatedly during history
of Solar System