One example: By studying rocks on Earth, geologists know how different kinds of rocks formed and changed over time.

CNN: Curiosity opening Martian frontier?

There are curiously shaped rocks, formed over millions of years as land was gradually eroded.

UNESCO: Culture

Rocks initially formed as layers of sediment laid down in an ancient sea can be deciphered by knowing that older layers are found beneath the younger layers.

BBC: Mars south pole

This was when the iridium layer was formed (many extra-terrestrial rocks are far richer in iridium than those found on Earth, so a large impact that scattered the iridium seemed a reasonable conclusion to draw), and when the dinosaurs disappeared.

ECONOMIST: Some new ideas about why life on Earth has periodic crises

And the microfossils in the sediment over the crater are either misinterpretations of material that has recrystallised over time, or were washed in from nearby rocks just after the crater was formed.

ECONOMIST: Some new ideas about why life on Earth has periodic crises

Regolith is a fine powder formed by a constant rain of small meteorites that breaks up the rocks at the surface.

ECONOMIST: Spaceflight

Now fast-forward to 2013, where in the past few weeks the Curiosity rover has been drilling into some different rocks half a planet away, inside a deep basin formed by the Gale impact crater.

CNN: Have a drink on Mars

As researchers studied images of the McLaughlin Crater, a crater 57-miles across and 1.4 miles deep that formed as the result of an impact with a large meteor, they noticed sedimentary rocks.

FORBES: NASA Finds Remains Of Ancient Martian Lake

Besides the larger number of emigrant rocks from its surface, Mars cooled faster than the earth after it formed, and so would have reached conditions favourable to life (a dense atmosphere, allowing for surface water) earlier.

ECONOMIST: Panspermia: Interplanetary migration | The