Locking in Mercury's Day

For a long time, the planet Mercury was a mystery. Studying it through telescopes was (and still is!) difficult because it is so close to the Sun. However, in 1974, the space probe Mariner 10 was sent to the tiny planet to photograph the surface. Surprisingly, Mercury was covered with craters, much like the Moon. But even before then, a surprising fact about Mercury was revealed: radar was used to determine the rotation period, and it was found to be 2/3 of its year, exactly!

It was thought before the radar experiments that Mercury's day would be exactly as long as its year. The reason for this is because of a complicated effect of gravity (called tides, it's the same effect that has locked the Earth's Moon into rotating once every time it revolves around the Earth, always showing the same face to us). No one had suspected that Mercury would rotate in any other way, but after the fact scientists realized that a 3:2 rotation to revolution ratio is not much less likely than a 1:1 ratio like our Moon has.

There is another thing about Mercury's orbit that's important too: it is fairly elliptical, which means that sometimes Mercury is a bit closer to the Sun that other times. But since Mercury's rotation is locked into its orbital period, the same spot is facing the Sun every other orbit. In other words, there is a spot on Mercury that is directly facing the Sun every second orbit, when Mercury reaches its closest point to the Sun. If you were to stand on that spot on the surface, you would suffer for it: Every second year the Sun would reach the zenith, but it would also look larger than usual because it is so close. You have the misfortune of picking the hottest spot on the planet! When Mariner 10 imaged Mercury, a tremendous impact crater was found at just that spot on the surface. It was named "Caloris Basin", or Basin of Heat, for that very reason! In the image above from Mariner 10 you can see the edge of Caloris Basin as a series of concentric arcs coming out from the dark side of the planet.