Two papers in Science report the first observations of martian magnetic anomalies, made with magnetometers on board the Mars Global Surveyor spacecraft. This discovery was completely unexpected. Some of the anomalies are large (about 1,000 nanotesla) and well resolved, and they are mainly in the older, heavily cratered southern part of the planet. Although the satellite tracks are rather widely spaced, some of the anomalies are clearly lineated in an east-west direction (Fig. 1). In consequence, Connerney et al. argue that they may have been produced by plate motions early in martian history. The suggestion that plate tectonics has operated on a planet other than the Earth is of great interest, because we still can only speculate on why the process has been so important on this planet, but not on Venus, which is otherwise so similar.
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