Mars Once Covered by A Vast Ocean

A second study also detected around 40,000 river valleys - four times more than have previously been spotted.

The valleys were the source of sediment that was carried downstream and dumped into the deltas.

Dr Hynek said: 'The abundance of these river valleys required a significant amount of precipitation.'

The idea of a large ancient ocean on Mars has been debated for two decades. However, the new study provides compelling evidence that the sea existed - and raises tantalising questions about the planet's history.

'One of the main questions we would like to answer is where all of the water on Mars went,’ said Dr Di Achille.

Future Mars missions - including NASA's Mars Atmosphere and Volatile Evolution project, due to blast off in 2013, should shed light n the mystery.