Forget Ziggy Stardust’s Spiders from Mars. Space scientists have discovered a giant alien arachnid on the planet Mercury. It is lurking at the centre of one of the biggest impact craters in the solar system.

The spider is a rock formation quite unlike anything ever observed before, and there is nothing like it on our own heavily-cratered Moon.

It is made up of more than 100 raised, narrow troughs that radiate from a smaller crater inside the 800-mile wide Caloris Basin.

The peculiar formation is among features snapped by a passing unmanned Nasa spaceprobe, called Messenger. Other images show huge cliffs snaking for hundreds of miles across Mercury’s face.

The probe took more than 1,200 photos of nearly a third of the 3,031 mile-wide planet during its close encounter on January 14. They are now being closely studied by the space agency.

The latest results, from the first visit to Mercury in nearly 33 years, are forcing scientists into a radical rethink of how the closest planet to the sun evolved.

Unlike the Moon, which is believed to be geologically dead and sterile, Mercury appears to have volcanoes and there are clear signs of lava oozing from the planet’s crust. Interestingly, radar observations from Earth had suggested that Mercury has a liquid core.

Messenger also detected a weaker magnetic field than that discovered by the last visitor, Mariner 10 in 1975, believed due to the craft flying different trajectories.

Messenger’s chief scientist Sean Solomon, of the Carnegie Institution, Washington, said: “This flyby allowed us to see a part of the planet never before viewed by spacecraft, and our little craft has returned a gold mine of exciting data.”

Messenger flew 124 miles above the closest planet to the Sun, taking close-up pictures of its Moon-like mountains and craters. The probe, which was launched in August 2004, will eventually go into orbit around Mercury in 2011 after a 4.9 billion mile journey looping through the solar system.

A joint European-Japanese mission to Mercury, called BepiColombo, is due to launch in 2013.

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