[meteorite-list] New Capture Scenario Explains Origin of Neptune's Oddball Moon Triton

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 10 16:57:25 2006
Message-ID: <200605102055.NAA19178_at_zagami.jpl.nasa.gov>

http://www.ucsc.edu/news_events/press_releases/text.asp?pid=867

University of California Santa Cruz Press Release

Contact: Tim Stephens (831) 459-2495; stephens_at_ucsc.edu
May 10, 2006
 
New capture scenario explains origin of Neptune's oddball moon Triton

Neptune's large moon Triton may have abandoned an earlier partner to
arrive in its unusual orbit around Neptune. Triton is unique among all
the large moons in the solar system because it orbits Neptune in a
direction opposite to the planet's rotation (a "retrograde" orbit). It
is unlikely to have formed in this configuration and was probably
captured from elsewhere.

In the May 11 issue of the journal Nature, planetary scientists Craig
Agnor of the University of California, Santa Cruz, and Douglas Hamilton
of the University of Maryland describe a new model for the capture of
planetary satellites involving a three-body gravitational encounter
between a binary and a planet. According to this scenario, Triton was
originally a member of a binary pair of objects orbiting the Sun.
Gravitational interactions during a close approach to Neptune then
pulled Triton away from its binary companion to become a satellite of
Neptune.

"We've found a likely solution to the long-standing problem of how
Triton arrived in its peculiar orbit. In addition, this mechanism
introduces a new pathway for the capture of satellites by planets that
may be relevant to other objects in the solar system," said Agnor, a
researcher in UCSC's Center for the Origin, Dynamics, and Evolution of
Planets.

With properties similar to the planet Pluto and about 40 percent more
massive, Triton has an inclined, circular orbit that lies between a
group of small inner moons with prograde orbits and an outer group of
small satellites with both prograde and retrograde orbits. There are
other retrograde moons in the solar system, including the small outer
moons of Jupiter and Saturn, but all are tiny compared to Triton (less
than a few thousandths of its mass) and have much larger and more
eccentric orbits about their parent planets.

Triton may have come from a binary very similar to Pluto and its moon
Charon, Agnor said. Charon is relatively massive, about one-eighth the
mass of Pluto, he explained.

"It's not so much that Charon orbits Pluto, but rather both move around
their mutual center of mass, which lies between the two objects," Agnor
said.

In a close encounter with a giant planet like Neptune, such a system can
be pulled apart by the planet's gravitational forces. The orbital motion
of the binary usually causes one member to move more slowly than the
other. Disruption of the binary leaves each object with residual motions
that can result in a permanent change of orbital companions. This
mechanism, known as an exchange reaction, could have delivered Triton to
any of a variety of different orbits around Neptune, Agnor said.

An earlier scenario proposed for Triton is that it may have collided
with another satellite near Neptune. But this mechanism requires the
object involved in the collision to be large enough to slow Triton down,
but small enough not to destroy it. The probability of such a collision
is extremely small, Agnor said.

Another suggestion was that aerodynamic drag from a disk of gas around
Neptune slowed Triton down enough for it to be captured. But this
scenario puts constraints on the timing of the capture event, which
would have to occur early in Neptune's history when the planet was
surrounded by a gas disk, but late enough that the gas would disperse
before it slowed Triton's orbit enough to send the moon crashing into
the planet.

In the past decade, many binaries have been discovered in the Kuiper
belt and elsewhere in the solar system. Recent surveys indicate that
about 11 percent of Kuiper belt objects are binaries, as are about 16
percent of near-Earth asteroids.

"These discoveries pointed the way to our new explanation of Triton's
capture," Hamilton said. "Binaries appear to be a ubiquitous feature of
small-body populations."

The Pluto/Charon pair and binaries in the Kuiper belt are especially
relevant for Triton, as their orbits abut Neptune's, he said.

"Similar objects have probably been around for billions of years, and
their prevalence indicates that the binary-planet encounter that we
propose for Triton's capture is not particularly restrictive," Hamilton
said.

The exchange reaction described by Agnor and Hamilton may have broad
applications in understanding the evolution of the solar system, which
contains many irregular satellites. The researchers plan to explore the
implications of their findings for other satellite systems.

This research was supported by grants from NASA's Planetary Geology and
Geophysics, Outer Planet Research, and Origins of Solar Systems programs.

______

Note to reporters: You may contact Agnor at (831) 459-2426 or
cagnor_at_pmc.ucsc.edu.

#####

 
Received on Wed 10 May 2006 04:55:06 PM PDT