Mars was once covered with a planet-wide ocean, Nasa experts believe. The space agency’s two robot rovers have detected widespread deposits of chemicals called sulphates which usually form in liquid water.

A new analysis of the data from the buggies, Spirit and Opportunity, lead scientists to believe that the water was part of a sea that covered Mars millions of years ago.

The discovery was revealed as the first of the rovers to land on Mars, Spirit, celebrated its 1,000th day on the planet. Opportunity landed three weeks later in January 2004.

The latest findings came as a result of research by James Greenwood of Wesleyan University and Ruth Blake of Yale University, in Connecticut. But their work is bad news for alien hunters. The team say the ocean must have been rich in phosphorus, an elements that would normally be mopped up by any living organisms.

If life was widespread on Mars, there would not have been so much phosphorus left in the water, they say. The scientists’ results are published in the journal Geology.