FIRE AND ICE
In 1974 and 1975, the Mariner 10 spacecraft photographed about half of Mercury. It revealed an ancient Moon-like face with craters and scarps, but without the Moon's dark lava sheets. Mercury has an unusually large iron core, perhaps the result of a catastrophic collision early in its history that removed part of the planet's mantle.
Mercury also has a magnetic field, but with only 1 percent the strength of Earth's. Although the planet's gravity is too weak to retain any significant atmosphere, there is a tenuous layer of helium and hydrogen (probably captured from the solar wind), as well as sodium (possibly from the surface rocks).
With a diameter of only 3,024 miles (4,878 km), Mercury is the smallest planet apart from Pluto. It also has the second-most eccentric orbit, again after Pluto's. Mercury's temperatures range more than any other planet. In daytime, the equatorial regions can reachj about 800 degrees Fahrenheit (430° C), while at night, the surface temperature can fall to —300 degrees Fahrenheit (-185° C). Mercury may have subsurface ice in permanently shadowed craters at the poles. If so, the ice probably condensed from water vapor released in comet impacts.