Mercury's thin skin and inner warmth.
The heavily cratered surfaces of Mercury and the Moon may look alike, but recent observations reveal that the similarity is strictly superficial. David L. Mitchell and Imke de Pater (University of California, Berkeley) used radio telescopes at Hat Creek Observatory and the Very Large Array as microwave interferometers to create maps of the innermost planet at wavelengths between 0.3 and 20.5 centimeters. The astronomers determined that the composition of Mercury's outer surface differs considerably from that of the Moon, a conclusion that will force planetologists to reevaluate theories of how the planet formed. Both the Moon and Mercury are covered with a layer of fine particles. This material, called regolith, results from millions of years of meteoritic bombardment, which has turned the top few centimeters of surface into a light powder and has compacted lower material several meters down. Presenting their findings at last June's meeting of the American Astronomical Society, Mitchell and de Pater demonstrated that the microwave emission they mapped originated 2 1/2 to 400 cm beneath Mercury's surface, depending on the wavelength. These depths are at least twice those penetrated on the Moon, implying that Mercury's surface is poorer in titanium-and iron-bearing minerals, which block or absorb microwaves.
A widely held theory for Mercury's formation proposes that soon after the planet coalesced, a large impact ripped off most of its outer layers and left behind a heavy iron core. Thus the resulting new crust would be metal-rich, which is at odds with the new observations.
The microwave maps also improved our view of Mercury's "hot poles" (S&T: November 1988, page 457). The planet's spin and orbit are coupled so that three rotations exactly equal two revolutions. This linkage causes two regions near the equator to receive an especially heavy dose of sunshine. Over time, the absorption and radiation of heat equalized, producing two hot spots, one in each hemisphere of the planet.