Mercury and Our Moon
Inward from Venus lies Mercury, a tiny planet about 1.5 times the diameter of our moon. Because of its close proximity to the sun, Mercury is difficult to approach, and only one spacecraft, the U.S. probe Mariner 10, has flown past it. Images returned from Mariner 10 in 1974 revealed a heavily cratered surface, similar to that of our moon. Most surprising was the discovery of a magnetic field, which scientists had believed could be generated only by larger, rapidly spinning planets like Earth and Jupiter. Mercury spins so slowly that it takes about 59 Earth days for a single rotation. The existence of a magnetic field, along with information on the planet's mass, indicated that Mercury is composed largely of iron.
The body nearest to Earth—our own moon—was most closely explored by the Apollo astronauts, who returned hundreds of pounds of lunar samples. But their expeditions were limited in area, and the orbiting command modules never reached the lunar poles. A U.S. probe, Clementine, was launched on a mission to examine the poles of the moon in 1993. In 1996, radio signals from Clementine that were bounced off the poles and received on Earth indicated the presence of frozen lakes. Some scientists believe that comets colliding with the moon could have deposited water ice at the lunar poles, but it remains for future probes to confirm the presence of water on the moon.