A fly-by by a Nasa unmanned space probe has revealed evidence of "widespread" volcanism on the planet Mercury. The US Mercury Messenger spacecraft made a close pass of the first planet from the Sun on 14 January.

In addition, only 45% of Mercury's surface had previously been mapped; Messenger has already covered a further 30% of the planet.

The spacecraft also discovered a unique feature that scientists dubbed The Spider. This formation has never been seen on Mercury before, and nothing like it has been observed on the Moon.

It lies in the middle of a large impact crater called the Caloris basin and consists of more than 100 narrow, flat-floored troughs radiating from a 40km-wide central crater.

Robert Strom, professor emeritus at the University of Arizona in Tucson, said The Spider might eventually be tied to the history of volcanism on the planet.
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