“I Can’t Breathe in Mercury!”

Like the earth’s moon, Mercury possesses insufficient mass to hold—by gravitation—an atmosphere for very long. In the same way that mass attracting mass built up planetesimals, so the early planets built up atmospheres by hanging on to them with their gravitational pull. If an atmosphere was ever associated with Mercury, the heating of the sun and the planet’s small mass helped it to escape long ago. Without an atmosphere to speak of, the planet is vulnerable to bombardment by meteoroids, x-rays, and ultraviolet radiation, as well as extremes of heat and cold. In sunlight, the planet heats to 700 K. In darkness, with no atmosphere to retain heat, it cools to 100 K. Despite the absence of atmosphere, regions at the poles of Mercury may remain permanently in shadow, with temperatures as low as 125 K. These regions, and similar regions on the earth’s moon, may have retained some water ice.