Life linked to icy barrage

By SIMON COLLINS

Comets have always been a source of wonder. Now scientists believe that they may also be a major source of the water that sustains life.

American scientist Dr Martha Hanner, in Auckland to give a public lecture tonight, says icy comets that hit the Earth in its first 500 million years may have brought frozen water from the outer reaches of space.

The ice would have melted when it hit the Earth, but much of it would have been captured as steam by the Earth's gravity, giving rise eventually to rain and forming the oceans.

Water was probably also produced from steam spewing out of volcanoes. But constant bombardment by comets and asteroids "could have made a significant part of the water on the surface of the Earth".

Dr Hanner says it was also possible that some of the complex carbon-based molecules on comets had been brought to Earth to help form the building-block molecules from which life developed.

She has worked for 25 years for California's Jet Propulsion Laboratory, a branch of the Californian Institute of Technology whose main client is the US National Aeronautics and Space Administration (Nasa).

She is an originator of a small desk-sized robotic spacecraft called Stardust, which flew close to the asteroid Annefrank this week on its way to collect dust particles from a comet called Wild-2 in 2004.

It is due to land in the Utah desert in January 2006 - bringing back the first extraterrestrial material from beyond the Moon.

"We study rocks with telescopes. That is mainly what I do. But there is only so much you can learn with a spectrometer," Dr Hanner said.

"A lot of information is in the structure of the particles on a micron-sized scale [one-thousandth of a millimetre] - what are the minerals that are present, how they fit together with each other, whether there is hydrocarbon material, and what is the form of the carbon.

"We want to see whether some of the structure is what we have inferred that interstellar gas should look like, and the crystal structure to tell us what temperature it was formed at."

The solar system took shape out of a formless cloud of tiny particles 4.5 billion years ago.

Evidence suggests that the Earth, the Moon and all the planets continued to be hit by comets and other bits of "leftover debris" for a further 500 million years. Many bits of debris may have contained water and complex carbon molecules.

However, literally billions of other comets - small "planetesimals" typically just 5km to 10km across - stayed away from the planets, and have continued to circle the sun independently at spans of up to 50 times the distance between the sun and the Earth.

"Some of us believe that the material that froze into the comets was actually material that predated the solar system," Dr Hanner said.

"So we believe the samples [from Comet Wild-2] will be interstellar dust, or stardust, that is older than the solar system - at least 5 billion years old."

If the specks from the Stardust spacecraft turn out to contain frozen water and complex carbon molecules, they will be the kind of proof that Dr Hanner needs to show that water - and eventually life - may have come here, at least in part, from outer space.

* Dr Hanner gives a free lecture on "a new view of our solar system" in the McGhie Theatre, Auckland College of Education, Gate 3, Epsom Ave, at 7.30 tonight.

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