[meteorite-list] Pluto Hit By Twin to Create Moon, Study Suggests

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 27 20:00:40 2005
Message-ID: <200501280100.RAA27169_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/pluto_formation_050127.html

Pluto Hit By Twin to Create Moon, Study Suggests
By Robert Roy Britt
space.com
27 January 2005

Pluto might have been hit long ago by a virtual twin in a collision that
created the ninth planet's moon Charon, according to a new computer
simulation.

The scenario is similar to the leading theory for the creation of
Earth's Moon, another cosmic crack-up that involved a Mars-sized object
slamming into our own planet.

Charon is a whopping 10 to 15 percent as massive as Pluto. Earth's Moon
is about 1 percent as massive as our planet. No other satellites are
anywhere near as weighty in comparison to their host planets.

That similarity doesn't mean our Moon and Charon formed in the same way,
but it suggested a relationship that ought to be explored.

The new simulation, detailed in the Jan. 28 issue of the journal
Science, was generated by Robin Canup of the Southwest Research
Institute. In 2001, Canup produced a computer model that helped solidify
the impact theory for the creation of Earth's Moon,
an idea that dates back to the 1970s and is now widely accepted as the
most probable.

Now Canup has turned her attention to the Pluto-Charon system, for which
other theorists proposed a similar impact solution in the 1980s. Until
now, however, no models of the suspected Pluto collision have
successfully created Charon. The most likely alternative is that Pluto
captured Charon, just as some small moons of Saturn and Jupiter are
thought to have lured into orbit.

Glancing blow

Canup envisions a glancing blow early in the solar system's history.
Rather than a car broadsiding another at an intersection, imagine them
merging into one another at a freeway onramp, though at a rather sharp
angle of about 60 degrees.

Canup explained for SPACE.com what might have happened as the two
objects got tweaked out of shape like cosmic Silly Putty, then spin
around one another:

"After the initial oblique impact, the impactor is partially sheared
apart into an 'arm' of material," Canup said. "The impactor material
largely recoalesces into an intact moon which is torqued gravitationally
into a stable orbit through its interactions with the distorted figure
of the planet."

Pluto was temporarily contorted into the unlikely shape of an egg.

The simulation does a good job explaining the presence of a moon so
massive as Charon, and it also aptly describes the overall angular
momentum, or spin, of the two-object system. "An impact seems the
simplest way to form the pair," Canup said.

"This work suggests that despite their many differences, our Earth and
the tiny, distant Pluto may share a key element in their formation
histories," she said. "This provides further support for the emerging
view that stochastic [random] impact events may have played an important
role in shaping final planetary properties in the early solar system."

Difficult to prove

Canup cautions, however, that the scenario will be difficult to prove.

With the Earth-Moon system, a glancing collision
fits well with the objects' present-day spin and mass. It also explains
the Moon's overall iron-depleted composition, which is similar to the
outer portions of Earth of which the satellite was presumably forged.

The simulated crash at Pluto creates a satellite that has the same
overall composition and density as Pluto, or a lower density, depending
on two likely outcomes. If Charon is captured, it could be expected to
have similar traits, assuming that most objects in the Kuiper Belt, a
region beyond Neptune where Pluto resides, are made of a similar mix of
rock and ice.

"I don't see a way to easily distinguish between capture vs. impact just
based on the densities of the objects," Canup said. She goes on to point
out that other two-object systems in the Kuiper Belt can't have formed
by collision. These frozen, rocky pairs have too much overall spin or
are too far apart, so they likely were created when one captured the other.

One possible way to bolster the simulation would be to look into the
internal structure of Pluto for clues of an impact, Canup said.

The first good opportunity to explore Pluto and Charon -- and any other
hidden moons might lurk in the system -- will come in about 2015 with the
expected arrival of NASA's New Horizons spacecraft.

An animation of the suspected impact is available here
<http://www.swri.org/press/plutocharon.htm>.
Received on Thu 27 Jan 2005 08:00:28 PM PST