Mercury's atmosphere thickened as we watched. The likely perp? Meteoroids.

Mercury is the smallest planet in the solar system. It’s a ball of rock and metal just under 5,000 kilometers across (about 40% wider than our Moon), and the closest planet to the Sun. That means it’s hot, of course, with a surface temperature that reaches well over 400° C (750° F). Lead melts at 327°C, by the way.

It’s common to describe Mercury as airless, lacking an atmosphere, and that’s true, more or less… though more less than more. It doesn’t have an atmosphere per se, but it does have a...