[meteorite-list] New Moons Suggest Brutal Beginnings

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 18 17:53:32 2004
Message-ID: <200408182153.OAA17781_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/news/news.jsp?id=ns99996298

New moons suggest brutal beginnings
Maggie McKee
New Scientist
August 18, 2004

Seven moons discovered recently around Saturn and Neptune have complex
and violent pasts, suggest two teams of astronomers.

Five confirmed moons - and one candidate moon - around Neptune appear to
have been captured by the distant planet after they were born from
collisions in the outer Solar System, according to a new study.

And the strange orbits of two tiny moons around Saturn - discovered by
the European-US Cassini spacecraft on Monday - hint at brutal beginnings.

Researchers now report detailed orbital analyses of Neptune's moons,
whose discovery was announced in 2003. Their orbits, though stable, are
stretched out, tilted, and in three cases, backwards - with the
satellites moving in the opposite direction to Neptune's rotation. That
suggests the moons could not have formed by simply condensing from disks
of material around their host planet.

Two other moons, Triton and Nereid, also have unusual orbits, so now
seven of Neptune's 13 moons are classified as "irregular".

"There's been a real debate in the literature about what happened to
Neptune's satellites," says JJ Kavelaars, a planetary scientist at the
National Research Council of Canada in Victoria and an author of the new
study.

Parent body

He and his colleagues believe one or more collisions - possibly between
a comet and Nereid itself - spawned the five moons, which are all
between 30 and 55 kilometres wide. Then the system was disturbed when
Neptune captured the 2700-km-wide Triton from the Kuiper Belt, a
collection of primordial space rocks beyond Neptune.

To test their theory, the researchers will try to study the colours of
the faint objects. "If we find they all have very similar surface
compositions, it would make it more reasonable to say they were
collisional fragments of a parent body," Kavelaars told New Scientist.
Such features have linked irregular moons around Jupiter and Saturn into
families.

Kavelaars says 40 or so smaller moons - a few kilometres wide - may
orbit Neptune. But these are undetectable by even the best ground-based
telescopes, which were pushed to their limits to find the five new moons.

But such tiny moons have just been discovered around Saturn, thanks to
the Cassini spacecraft now in orbit around the ringed planet.

The moons are the planet's smallest known satellites. They span just
three and four kilometres across, sizes approaching those of the largest
ring particles, say astronomers.

Debris disks

The moons are located about 200,000 km from Saturn's centre, between the
orbits of two of Saturn's 33 larger moons, Mimas and Enceladus.

"It is surprising to find satellites in this region," says discoverer
Sebastien Charnoz, a planetary scientist at the University of Paris,
France. Gravitational tugs from the larger satellites make their
positions precarious, so the mini-moons will likely be forced into
different orbits in the future.

This destabilising force also has implications for the moons' past.
"One thing is almost sure - they did not form where they are seen
today," Charnoz told New Scientist.

Collisions may have also created these satellites, says Kavelaars,
when larger moons crashed into each other in their orbits around
Saturn.

"The actual rings of Saturn are probably debris disks from the
breakup of satellites," he says. "So it's pretty obvious there's
a continuous process churning up the area around Saturn and
creating new small moons."

Journal reference: Nature (vol 430, p 865)
Received on Wed 18 Aug 2004 05:53:29 PM PDT