TERENCE DICKINSON
THE UNIVERSE

The Mars rover Opportunity's examination of Martian rocks last week provided the first convincing evidence that our neighbour world was once "awash" in water, as one NASA scientist described it.

But where did the water come from? And why does Mars have no liquid water now, while Earth apparently has been covered with the stuff for 4 billion years?

Scientists are just beginning to piece the story together, and it goes right back to the beginning.

Mars, like Earth, was formed from dusty and rocky debris left over after the sun was born 4.57 billion years ago.

Initially, there were more planets in our solar system than the nine we recognize today, perhaps twice as many.

Earth suffered an especially brutal encounter with one of them 4.52 billion years ago, when a wayward body the size of Mars smashed into it. Our planet was almost split in two. Molten rock was splashed out into space and later condensed in orbit to form the moon.

The impact blasted the Earth's atmosphere into space, boiled off any water and turned our planet's surface into a sea of molten rock. Venus, Mercury and Mars, the other approximately Earth-sized planets, likely suffered similar collisions around the same time, though no large moons remain orbiting those worlds.

By 4.4 billion years ago, the Earth's surface had cooled enough to have a solid crust.

The formation of the planets was an inefficient process and for millions of years the Earth and the other planets were bombarded by what astronomers call planetesimals — essentially leftover chunks from the birth of the solar system, up to a few hundred kilometres in diameter.

By 3.9 billion years ago, the bombardment began to subside, but evidence for it is visible in binoculars when you look at the craters on the moon, the majority of which date from 3.9 billion to 4.4 billion years ago, when the planetesimals were raining down.

The puzzling part of this is that Earth's oceans, and now probably the water that was on Mars, both date from this period. Where did it come from?

The only reasonable answer is comets. Comets were formed farther out from the sun than Earth, but in such abundance that they also rained down in the early solar system.

They came into the inner solar system as frozen water — giant snowballs — depositing vast amounts of liquid water on Earth and apparently on Mars too.

Because of Earth's distance from the sun, our planet's surface temperature remains, on average, between the freezing and boiling points of water.

Moreover, Earth's atmosphere acts like a lid, trapping most of the moisture.

Mars, on the other hand, is too far from the sun to stay warm and too small to gravitationally trap a dense enough atmosphere to bottle up what warmth it does have.

The comet-fed oceans it likely had either escaped into space or ended up trapped in cold storage as permafrost.

To test these ideas, the Mars rovers will continue their explorations.

The comet part of the equation will be investigated this summer, when two comets float into Earth's sky in May.

They are expected to be bright enough to allow astronomers to examine them for further clues to the origin of water on Earth and Mars.