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Early in the story of the planets, there was a beautiful water world, an oasis of hope in a sterile universe. But this was not Earth - this was the young Mars. Professor Brian Cox continues his tour of the solar system revealing that it was once home to not one, but two blue planets.

For millions of years, Mars enjoyed oceans blanketed by a thick atmosphere and a temperate climate. Whilst at the same moment Earth was a far less favourable habitat. Our planet was toxic to life today, her atmosphere choked with carbon dioxide and her oceans, acidic.

And both planets were about to face another trial. As they reached the end of their adolescence, a solar system wide cataclysm threatened them both. Called the Late Heavy Bombardment, this event saw asteroids rain down on every inch of the planets for tens of millions of years, resurfacing vast areas of both worlds. But as the dust settled, both sister worlds emerged – ripe with the conditions for life to begin.

But deep inside the molten metal in Mars’s core was rapidly cooling. And in time the planet’s protective magnetic field would fail, causing Mars’ atmosphere to be slowly stripped away by the solar wind. As temperatures plummeted, Mars’s surface froze. Her oceans gone – for a while volcanic activity occasionally melts the ice, creating the largest waterfalls the solar system has ever seen.

Today, Mars is an arid desert world that long ago had the same potential as Earth. Which begs a big question: did life get a start on both worlds? In search of the answer, plans are afoot to take humans to the red planet - so perhaps one day soon we will become the Martians.