Scientists who staged the Deep Impact mission, which climaxed on July 4 when a projectile the size of a washing machine whacked into Comet Tempel 1, said Tuesday that the data stirred both quiet satisfaction and amazement.

They said some long-standing hypotheses about comets have been given the comfort of evidence -- but others have been swept away.

Chemical analysis from the plume of dust and gas that spewed from the impact showed largely predicted volumes of silicates, including organic (carbon-based) molecules.

Chief scientist Michael A'Hearne, presenting the research on the sidelines of a meeting of US and European astronomers, said this discovery buttressed a contested theory about the origins of life on Earth.

Under the "pan-spermia" idea, comets pounded the early Earth billions of years ago, bringing the planet organic molecules that reacted with light and heat from the sun, eventually providing the rich chemical soup from which all life began.

The pan-spermia theory, initially opposed as outlandish by some scientists, is steadily winning converts, although it remains reviled by fundamentalist Christians as an assault on creationism.

"I would argue that it (the pan-spermia theory) is (now) more likely, just because we have seen this big enhancement in organic materials coming out," A'Hearne told a teleconference.

Early analysis suggested comets were rich in carbon dioxide, acetylene, ammonium and hydrogen cyanide, and a wider range of carbon molecules was likely to be found as scrutiny of the data progressed, he said.

On the other hand, a mainstream view that comets are assemblies of rubble and ice seems destined to become a footnote of cosmic history.

Spectrometers, cameras and other instruments, wielded by a waiting spacecraft and by Earth-based astronomers as the metal missile struck Tempel 1, showed the comet has a fragile crust, underneath which lies a porous structure comprising roughly two parts microscopic dust to one part ice.

No evidence was found of any larger pieces or a cometary "core".

Until now, comets have been dubbed "dirty snowballs", but A'Hearne said it might be appropriate to call them "snowy dirtballs" given the extraordinary dominance of ultrafine dust.

"It's like a brittle sponge... which crumbles in your hand," said Uwe Keller, a scientist with the European Space Agency's comet-chasing probe Rosetta, which turned its own eyes on the collision on July 14 to add to the data flow.

Keller said the extraordinarily fragile surface of Tempel 1 could pose problems for the Rosetta team, which would have to calculate whether a proposed landing on Comet Churyumov-Gerasimenko in 2014 might end disastrously.

If the comet's crust could not withstand the impact from a fridge-sized robot lab, which was due to land and fire an anchor into the surface, the spacecraft would sink through the surface, he said.

Deep Impact's projectile gouged a crater about 100 metres (yards) across "and tens of metres" deep, said A'Hearne. Water molecules were a significant component of the plume, accounting for roughly 5,000 tonnes of it.

Millions of comets encircle the sun and as they near the star, the solar heat melts the ice, leaving a trail of dust that is illuminated as a long tail.

Comets have always been linked in superstitious minds with great events, good and bad.

They are equally beguiling for astrophysicists, who believe they are the most primitive material from the early days of the solar system.

Comets are considered to be the leftovers from the massive cloud of gas and dust that condensed and then coagulated to form the sun, the planets and other parts of the solar system about 4.6 billion years ago.

Scientists say understanding the substance and structure of comets provides clues of how Earth came into being.

The studies were published on Tuesday by the US journal Science, coinciding with the meeting in Cambridge of solar system specialists from the American Astronomical Society and Britain's Royal Astronomical Society.

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