[meteorite-list] Ice Layers Record Comet Creation (Deep Impact)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Mar 17 13:26:35 2006
Message-ID: <200603171659.k2HGxAw07431_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/2/hi/science/nature/4816712.stm

Ice layers record comet creation
By Paul Rincon
BBC News
March 17, 2006

The Deep Impact mission is casting new light on how comets formed and
how they shed their ice in space.

The US space agency probe sent a 370kg projectile crashing into Comet
Tempel 1 and then studied the plume of debris with its suite of
instruments.

Nasa's mission scientists say images from last July's encounter reveal
as many as seven different layers on the comet's surface.

Their results were presented at a major science conference in Houston, US.

Team member Mike Belton told the meeting he thought the layering was a
sign of how comets like Tempel 1 were built up from lesser objects.

Growing 'snowball'

In the outer part of the early Solar System, smaller bodies called
cometesimals collided and merged, gradually piling up to form the larger
objects we know as comets.

Similar collisions in the inner Solar System led to a loose accumulation
of fragments that largely retained their internal structure.

But primordial material in the outer regions was travelling at
relatively lower speeds and contained less solid material.

As the cometesimals hit the surface of a growing comet nucleus, they
"flowed" on to the surface, researchers believe.

Deep Impact's scientists think the interior structure of Tempel 1
resembles layers of material piled up on one another - a signature of
the process that formed the icy body.

Model conflict

Data from the mission is also helping scientists understand how comets
shed water-ice through sublimation, the phenomenon which sees a solid
become a gas without first melting.

When comets are heated by the Sun, ice sublimes and is lost to space in
a process known as outgassing. Some scientists have proposed that this
material is coming from deep below the surface crust of the comet.

But temperature data from Tempel 1's nucleus suggests the material must
be lost from only a few centimetres below the surface.

"The normal outgassing of the comet has been modelled by different
people as coming from bare ice on the surface to subsurface ice that
migrates through pores to escape, or from 40-50m below the surface,"
Deep Impact's chief scientist Mike A'Hearn told the BBC News website.

"I think it is clear from what we have here that the ice that is
subliming is within the upper metre. Whether it's 5cm or 20cm below, I
wouldn't want to say; but it's not below the top metre. That rules out a
lot of the models."

The new results from the mission were presented here at the Lunar and
Planetary Science Conference in Houston, Texas.
Received on Fri 17 Mar 2006 11:59:10 AM PST