The cosmic rubber duck holds elixir of life

Imagine travelling through space for 11 years, hibernating for months in the depths of the Solar System until you finally awaken earlier this year as you home in on your goal. And then - when you arrive - you discover it's totally rancid.

It smells of rotten eggs and horse manure, coupled with the acid-sharp tang of sulphur dioxide.

The putrid stench is alleviated only by the tang of bitter almonds: but that's a sign of deadly cyanide, which is mixed with scentless but poisonous carbon monoxide.

Disappointed? Want to go home? Not so the scientists behind Rosetta, Europe's unmanned mission to a comet. They are elated. For this mix of chemicals - sniffed out for the first time at Comet Churyumov-Gerasimenko - is the very elixir of life. It's probably the original material from which all living things have been made. Rosetta arrived at Comet Churyumov-Gerasimenko in August.

As well as sniffing the gas that's erupting as the frozen comet feels the increasing heat of the Sun, the spacecraft has been sending back stunning postcards of its target. The comet looks like a rubber duck - in shape, though not in colour. It's as black as coal.

Even the experts aren't sure how the comet came to be this shape. The "head" and the "body" were perhaps separate icy objects that stuck together long ago, when the Solar System was born.

Alternatively, the comet may have been a single object all along, with a layer of volatile material through its middle that's evaporating until only a thin "neck" is left. Rosetta's cameras are already showing steam boiling away from the neck region, so it's only a matter of time till the duck's head parts company from its body.

Measure

On November 12, Rosetta drops a small lander on to the "head". Here it will measure what the comet is made of - as well as taking snapshots of the eruptions all around it, as Churyumov-Gerasimenko heads for its closest encounter with the Sun next year.

Between them, Rosetta and the *Philae* lander should answer two of the most important questions in science: where did Earth's water come from; and - ditto - the carbon atoms that make up living things? Most astronomers think Earth was baked dry billions of years ago, when a wayward planet impacted it, splashing out the rocks that made up the Moon - and leaving Earth as a ball of molten lava.

After our planet cooled, perhaps the water we see in the oceans today deluged down from space, delivered by countless comets hitting Earth - along with the smelly molecules that are now getting up *Rosetta's *scientific nose.

- The Independent