Astronomers have discovered 12 more moons around Saturn. And they have
evidence that these once were just 3 or 4 moons, minding their business,
orbiting the planet like all regular saturnian moons do today.

The 12 new-found moons are in irregular orbits that suggest they are the
collisional remnants of larger parent moons, once securely captured in, but
later blasted from, their saturnian orbits.

Using several medium-to-large sized telescopes, large-format CCD arrays that
photograph big areas of sky, and computers that process multiple gigabytes
of data each night, teams of astronomers collaborated last fall in a search
for so-called “irregular” moons around the gas giant.

Saturn was known to have six relatively large moons and 12 minor moons. All
except one minor moon, Phoebe, discovered in 1898, are classified as regular
satellites because they move along nearly circular orbits in the planet’s
orbital plane, revolving in the same direction as the planet spins.

The 12 new-found satellites are irregular – meaning they orbit outside the
plane of Saturn’s equator – and it appears that their orbits cluster in
three, possibly four, distinct groups, said Carl W. Hergenrother of the UA
Lunar and Planetary Laboratory (LPL).

“We think we’re seeing orbits cluster, that is, orbits of several moons fall
in the same general plane, just as asteroids cluster,” Hergenrother said.
“And with asteroids that cluster, the belief is they are pieces of what once
was a big asteroid that got hit by something. It’s possible that we’re
seeing the same thing with the satellites.”

Brett Gladman of the Observatoire de la Cote d’Azur in France, J.J.
Kavelaars of McMaster University in Canada, and Matthew Holman of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., discovered
the irregular saturnian moons in August, September and November, 2000, using
the 2.2-meter (87-inch) European Southern Observatory in Chile, the
3.6-meter (142-inch) Canada-France-Hawaii Telescope in Hawaii, and the
1.2-meter (48-inch) Mount Hopkins telescope in Arizona.

Hergenrother, Stephen M. Larson and Rob Whiteley – all of the LPL – and
Dennis Means of the UA Steward Observatory used the Steward Observatory’s
1.5-meter telescope (61-inch) in the Santa Catalina Mountains north of
Tucson and the 2.3-meter (90-inch) Bok Telescope on Kitt Peak to observe the
moons for more precise information on their orbits.

Others doing this “recovery” work to help define the satellite orbits used
the 4-meter Kitt Peak telescope, the 5-meter Palomar telescope and
2-to-3-meter class European telescopes.

The research is reported in the article, “Discovery of 12 satellites of
Saturn exhibiting orbital clustering,” in the July 12 Nature.

Astronomers in 1997 and 1999 discovered five irregular satellites around
Uranus, and in 1999 – 2000 discovered another 12 irregular satellites around
Jupiter, previously known to have eight. The UA Spacewatch on Kitt Peak
discovered one of the new-found jovian moons.

Almost all of the irregulars discovered since 1997 cluster in easily
discernible groupings, the astronomers note in their article.

“The difficult question is whether the disruptions occurred during the
capture process itself when the planets formed long ago, or whether intact
moons were captured at that time into orbits near the present grouping and
these single moons were subsequently shattered and scattered by intruding
comets or asteroids during the subsequent (more than 4-billion-year solar
system history),” they wrote.

The most probable theory is that each cluster is the remains of a
once-intact moon smashed by a collision sometime after the planets were
formed, according to their analysis.

Saturn must have captured the original parent moons during planetary
formation, as the objects passed through Saturn’s surrounding
proto-planetary gas cloud, Hergenrother said.

An alternative theory is that the moons were captured when Saturn suddenly
increased in mass – in which case the moons would all be prograde, moving
around the planet in the same direction as the planet moves around the sun.

“But we are seeing just as many retrograde as prograde irregular moons at
Saturn,” Hergenrother said. Objects captured as moons would move in either
prograde or retograde orbits depending on their direction as they passed
through and were slowed by proto-Saturn’s gas cloud.

Satellites in orbital clusters could range in size from one to 100
kilometers in diameter, he added.

“Right now, we see irregular satellites as small as 3 kilometers around
Saturn, but there may be many smaller than that. These may go on a continuum
in size all the way down to the size of dust. “

Related link

  • 12 July 2001: Discovery of 12 satellites of Saturn exhibiting orbital clustering, Nature (subscription required)