Large asteroid strike may have changed Mercury's rotation

Planets orbiting a sun generally, but not always spin on their axis. Some spin east to west, others west to east.

Those that don’t spin are said to be tidal locked with their sun; always showing it the same face, such as the moon does with planet Earth.

In these instances, the planet tends to develop different properties than it would were it spinning.

One side is cold, the other hot, for example leading to unique geophysical properties.

Also because of the sun’s gravitational pull, more asteroids are likely to hit the far side of the planet, leaving more craters.

It’s partly because of evidence like this that a group of European researchers is proposing that Mercury, the planet closest to the sun in our solar system, was once tidal locked with the sun, but now is not, because, as they assert in their paper published in Nature Geoscience, it was knocked into its current spin by a large asteroid. They even have a likely crater made by the impact to back up their theory.

It’s called Caloris Basin, the largest impact crater on Mercury's surface, and it appears to be just the right size to fit the computer model the team has created.

What happened, they suggest, is that long ago, Mercury had an east to west (retrograde) spin, but then over millions of years, slowed till it spun no more.

Then, at some later time, the planet was struck by an asteroid large enough to cause it to begin spinning again but this time, in the opposite direction, though not very fast, causing Mercury’s current 3/2 (three spins on its axis for every two trips around the sun) spin it has today.

The team points out that one side of Mercury clearly has more craters than the other, but perhaps more concretely suggest that the thus far inexplicable hollows inside the Caloris Basin could be the result of ice that was once buried by matter from the asteroid when it hit, then melted as sunlight began to fall on it as a result of the planet spinning again.

And finally, they say, the side that would have been the hot side before being struck by the asteroid would have been flatter than the opposite, more cratered side, do to molten activity. Evidence from Mariner 10 and MESSENGER, suggest that this is the case as well.

It all adds up, the team concludes, to a planet that once spun one way, stopped, then was knocked into spinning the other way.