Alan Longstaff‘s talk “What are Terrestrial Planets made of?“ began with a short lesson in basic geology; it’s as well to know a bit about the way the Earth has formed first. Alan explained the structure of the Earth from its core to its crust, and described some of the processes involved in plate tectonics and the formation of ocean floor, volcanos and mountain ranges. He pointed out the role that Earth’s abundant water has had to play, and how it led to a planet with unique features. The Moon’s formation led to it having a crust mostly made of anorthosite. Meterorite impacts allowed basaltic rocks to flow from below that crust to form the “seas”. Little is known about Mercury, but in some ways it’s the Moon’s opposite, apparently having a large metallic core, and a surface composition more like the Earth’s mantle than the Moon’s crust. Venus is also somewhat mysterious, but it does have a great many volcanos, and some impact craters. The distribution of the volcanos suggests that it, like Earth, has hot convecting plumes in its mantle, but unlike Earth it does not have plate-tectonic features. Mars was Alan’s final subject. It too shows evidence of having mantle plumes, but there remains some doubt about the role water has played in its formation. Recently the NASA rovers Spirit and Opportunity have found minerals that are most likely to have formed in the presence of liquid water. However, some of them would also have been destroyed if they’d been in the presence of liquid water for an extended period of time. It is possible that liquid water was only present for very brief periods of time, maybe only a few weeks or months. Alan left us with the impression that, while much is known, there’s a lot more yet to be discovered about the terrestrial planets.