Mercury, Up-Close Again

Abstract

![Figure][1] CREDITS: NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/ARIZONA STATE UNIVERSITY/CARNEGIE INSTITUTION OF WASHINGTON Planetary science is much about comparisons. Studying other bodies in our solar system with different sizes and compositions provides essential context for understanding Earth's formation and evolution. Three planets—Mercury, Venus, and Mars—and the Moon, are most like Earth in their initial composition and relative size, but their differences are enlightening. The Moon, Venus, and especially Mars have all been visited and probed recently by spacecraft. Now it is Mercury's turn. The MESSENGER[*][2] spacecraft flew by the planet and observed it in January. The papers in this Special Issue feature these observations. After two more passes, it will settle into orbit around Mercury in 2011. Mercury was visited by one earlier spacecraft, Mariner 10, in the mid-1970s. Its observations, and some difficult ground-based studies, provided most of our information on the planet, but raised many enigmatic questions. Despite being the smallest planet, Mercury has an actively generated magnetic field like that on Earth, but unlike that on the Moon, Mars, and Venus, and a huge iron-rich inner and molten outer core. Images of half of the surface revealed abundant scarps thought to indicate contractional faults, implying that Mercury was originally a bit larger early in its history. The extent to which its surface was shaped by impacts or volcanism was uncertain and debated. Its proximity to the Sun means that it has intense interactions with the solar wind, which, despite the magnetic field, impacts the surface of the planet, altering it and excavating some ions. Ground-based radar data hinted that ice may be present in shadowed regions of the poles. The first MESSENGER observations provide some important early answers and a wealth of data for further study. Observations of the surface by several instruments reveal that Mercury does have a volcanic history. More contractional faults are seen in areas observed by Mariner and in an additional ∼20% of the planet seen up close for the first time. A variety of impact craters help reveal relative ages of surface units and enrich the role of impact processes in shaping the planet. Impacts and solar bombardment have greatly weathered the surface; most of the iron is not in silicate minerals but apparently in nanoscale metal or oxide grains. In addition to sodium, MESSENGER detected ablation of calcium from the planet and in its magnetotail and measured in greater detail the magnetic field and its effect on its space environment. This first pass did not cover the polar regions, so confirmation of the presence of ice will have to await future observations, which will also image other parts of the planet and come as the Sun's activity increases. But already MESSENGER has helped fill in Mercury's history and environment, allowing a better understanding of all the terrestrial planets, including Earth. [1]: pending:yes [2]: #fn-1