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Last Updated: Friday, 17 March 2006, 11:50 GMT
Ice layers record comet creation
By Paul Rincon
BBC News science reporter, in Houston, Texas

Projectile crashes into Comet Tempel 1 (Nasa)
Comets hold materials unchanged since the Solar System's formation
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.

The impactor approaches Tempel 1

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.

Comet shakes conventional wisdom
07 Sep 05 |  Science/Nature
Comet impact gave 'powder plume'
11 Jul 05 |  Science/Nature
Comet's huge plume hides crater
06 Jul 05 |  Science/Nature
Nasa probe strikes Comet Tempel 1
04 Jul 05 |  Science/Nature
Q&A: Deep Impact comet mission
04 Jul 05 |  Science/Nature

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