MadSci Network: Astronomy |
I believe that the answer is that Mercury stays in an almost locked orbit with the sun - tidal friction over time has slowed the rotation of Mercury so that the planet only turns three times for every two Mercurian years (orbits around the sun). Although Mercury is very close to the sun, the side facing away from the sun will be receiving no source of heat and will radiate away its heat rapidly - there is no atmosphere or ocean like on the Earth which can carry heat around the planet. It can therefore cool rapidly as it points away from the sun for long periods of time.
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