THE HISTORY OF MERCURY EXPLORATION

The History of Mercury Exploration Robert G. Strom1 and Sean C. Solomon2 The first known recorded observation of Mercury was Nov. 15, 265 BC, in cuneiform writings from Mesopotamia, although it was undoubtedly seen much earlier. Telescopic observations began with the use of the first telescopes by Thomas Harriot and Galileo in 1609. From Earth-based telescopic observations, dark patches thought to be similar to the lunar maria were discerned, and some information on the exosphere and surface composition was derived, but because of the difficult observing conditions the observations were of relatively poor quality and left many questions unanswered. After the Mariner 10 mission, high-resolution radar images revealed radar-bright polar deposits, possibly water ice, in permanently shadowed craters, and showed that previously known bright spots were rayed craters and that large parts of the side not seen by Mariner 10 were heavily cratered. Until the Mariner 10 mission, Mercury was one of the least understood planets in the Solar System, and thus a high priority for spacecraft exploration. Mariner 10 was launched on Nov. 3, 1973. The science instruments were selected to characterize the surface and near-planet environment of Mercury, but they were also used to explore the atmosphere of Venus. Mariner 10 viewed only 45% of the planet but discovered such similarities to the Moon as heavily cratered terrain and smooth plains associated with impact basins. However, it also discovered major differences, including a dipolar magnetic field, large areas of intercrater plains (the major terrain type), and a tectonic framework and interior basin structures unlike those seen elsewhere. Although it provided our first close-up look at the planet and made numerous discoveries, as well as more information on the exosphere and field and particle environment, Mariner 10 left open such questions as the role of volcanism and the origin of the tectonic deformation. There were no other missions to Mercury for almost 30 years until the launch in 2004 of MESSENGER , which will orbit the planet in 2011 after three flybys. This mission will fill in the blanks left over from Mariner 10. 1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721; 2 Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015