SPIN ORBIT COUPLING

For many years it was assumed that Mercury always showed the same face to the Sun in the same way that the Moon always shows the same face to the Earth. This would be caused by the large tidal forces exerted on Mercury by the Sun. Synchronous rotation occurs when the period of revolution equals the period of rotation (i.e. a ratio of 1:1). Accurate measurements of Mercury using radar echoes bounced of the planet revealed that its period of rotation (58.65 days) is equal to two thirds its orbital period (87.97 days), the ratio is 3:2). The general term applied to a tidally locked body such as the Moon or Mercury is spin orbit coupling.

The image to the right shows the motion of Mercury around the Sun. The green mark on Mercury represents the position of a large crater called Caloris Planitia (or the Caloris Basin). The animation shows that Mercury spins on its axis one and a half times during each orbit of the Sun. When Mercury is at perihelion (closest point to the Sun) the Caloris Basin is always pointing either directly towards of directly away from the Sun. It is theorised that the meteor that caused the Caloris Basin is embedded within the surface of Mercury and as such makes one side denser than the other. The tidal forces exerted by the Sun will act to make the denser side point towards the Sun at each perihelion, but the elongated orbit of Mercury probably prevented this and resulted in the current, stable, situation. At each perihelion the tidal forces are balanced so there is no tendency to alter the rotation.