The known surface of Mercury is unique among the photographed portions of the terrestrial planets in that there are no obvious tensional features, but instead some compressional wrinkling. If, as has been variously proposed, the Earth has undergone expansion due to a secular decrease in the gravitational constant G, then presumably so will all astronomical bodies. Using two alternative geophysical models for the interior of Mercury, the estimated expansion due to a decrease in G is probably in conflict with present observation. This constraint on the size of the change in G may be significant for planetary evolution.

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