Article Posted on : - May 04, 2009

Rembrandt, the second-largest impact basin on Mercury. NASA photo

WASHIGTON (PTI): Astronomers have discovered mysterious spokes in crater on Mercury, a feature they claim is unlike any to be found in basins on the planet or elsewhere in the solar system.

In fact, NASA's Messenger spacecraft has spotted the bizarre spoke-like pattern of troughs and ridges in Rembrandt impact basin, the second-largest impact scar on Mercury, the 'New Scientist' reported.

The basin was discovered by Messenger on 6 October 2008. And, by examining the craters that formed on top of it, the astronomers estimate that Rembrandt formed in an impact some 3.9 billion years ago, near the end of a barrage of impacts in the inner solar system, the Late Heavy Bombardment.

The impact that created Rembrandt also fractured the crust beneath it, allowing magma to flow to the surface and partly fill the 700-kilometre-wide basin.

But the astronomers cannot explain some features etched in that volcanic material -- a spoke-like pattern of troughs and ridges emanating from the centre of the basin.

"The pattern is even stranger than a mysterious spider-shaped pattern of troughs found in Mercury's Caloris basin, during Messenger's first Mercury flyby in January 2008," team member Thomas Watters of Smithsonian Institution in Washington said.

Troughs and ridges, which are thought to form through very different processes, are not expected to be found lying side by side. So-called "wrinkle ridges" are caused when the crust compresses, while troughs are formed when it's stretched causing the surface to separate.

"What's so bizarre is these features are sitting beside each other. We've never seen anything like that - not in Caloris, not anywhere," Watters said.