It's been almost 40 years since the last (and only) time human beings have gotten a glimpse of the solar system's first sibling. Mercury, an airless, rocky world dwells in fearful proximity to the Sun. Living at just a third of the Earth-Sun distance, Mercury takes just 88 days to complete an orbit. That terrible intimacy with a star has left planet Mercury with daytime temperatures hovering around 800 F (though near the poles airless Mercury may be so cold that water ice might hide in the shadows of crater walls).
While Mercury may not look very exciting compared to a sexy planet like Mars, it holds keys to understanding how the rocky planets in our solar system formed. Living so close to the Sun, Mercury coalesced at the inner edge of a gaseous disk that birthed all the Sun's planets 5 billion or so years ago. Learning more about Mercury's history can tell us a lot about the generic process of planet assembly and is, therefore, relevant to all those 50 billion or so worlds we now believe are whirling around the galaxy's many alien Suns.