Messenger Mercury: Collision With Giant Asteroid May Have Caused Spin Change

Updated NASA Messenger probe images obtained on September 29, 2011, shows a large crater with a floor partially covered by large numbers of Coalesced Hollows.

Mercury may have a lot in common with Earth, but close-up images and data captured by NASA'S MESSENGER probe this year show it's still a bit of a planetary weirdo. REUTERS/NASA/Handout

A new study suggests that Mercury's strange spin may have been caused by collision with a giant asteroid, which may have also caused Caloris Basin, Mercury's largest impact crater.

"Mercury once had a spin rate synchronous with the sun, like the moon with the Earth," said study co-author Alexandre Correia, a planetary scientist at the University of Aveiro in Portugal.

However, a giant asteroid may have collided with planet causing it to rotate three times on its axis for every two orbits it completes around the sun.

According to the study, computer models suggest that a giant impact from an asteroid knocked the planet it into its current strange configuration.

The space rock would have been at least 43 miles wide and 550 trillion metric tons in mass, or 1/600,000 the mass of Mercury, Correia said.

Evidence of the collision could include Mercury's largest impact crater, the Caloris Basin as it matches the predicted size, age and location of the impact.

Such an impact might also explain certain hollows seen on Mercury's surface, the researchers said.

Scientists had long assumed that Mercury was tidally locked with the sun, however, radar observations revealed that the planet led a far stranger life, rotating three times on its axis for every two orbits it completes around the sun.