▪ terracing - A feature seen in the inner walls of some impact craters. It is a step-like feature caused by the collapse of material after an impact. ▪ transit - the event where one astronomical body passes in front of another. Most interesting are the somewhat rare transits of Mercury across the Sun and even rarer transits of Venus across the Sun. ▪ troposphere - the layer of a planet’s atmosphere where vertical air movement occurs. It’s pretty much unique to Earth. We get our tremendous thunderstorms because of this vertical mixing.