This radar image acquired by the Cassini radar instrument on July 22, 2006 shows shapes thought to be lakes containing liquid methane. ((NASA))

The Cassini spacecraft has discovered another similarity between Earth and Saturn's moon Titan, revealing images of what appear to be lakes on its surface.

The radar images show dark, smooth patches that appear to be lakes based on their shape and low radar reflectivity.

The images, published in the current issue of the journal Nature, are the best evidence yet of lakes of liquid on the moon's surface since they were first predicted more than 20 years ago.

Researchers speculate the liquid is methane, which is one of the few molecules to exist as a liquid in the extreme cold of the moon, where temperatures rarely rise above –179 C.

The dense haze of Titan's mostly nitrogen atmosphere had prevented earlier attempts to view the surface of the moon. Cassini's probe made its northern-most radar pass to date last July to retrieve the images.

Radar is the only way to pierce the haze surrounding Titan, which has an atmosphere 10 times denser than Earth's.

The spacecraft took an image of a narrow strip about 250 kilometres wide and more than 1,000 kilometres long that contained more than 75 lakes.

Titan's dense atmosphere and presence of carbon-based material have fascinated scientists who see it as a time vault of what Earth may have looked like billions of years ago, before life formed and introduced oxygen into the atmosphere.

Scientists had speculated Titan might also contain vast oceans, but a 2004 Cassini flyby found no evidence of large bodies of liquid.

The Cassini-Huygens mission is a co-operative project of NASA, the European Space Agency and the Italian Space Agency.