MARS MAGNETISM. The Mars Global Surveyor spacecraft has discovered patterns of magnetized surface rock—broad stripes of magnetic material pointing in one direction alternating with magnetic material pointing in the opposite direction, somewhat like the patterns seen at mid-ocean rift zones on Earth. On our planet the alternating stripes testify to the changing nature of Earth's magnetic field and to the recurring upwelling of magma resulting from the movement of tectonic plates above a seething molten planetary core. The conclusion: Mars too might have experienced tectonic activity. (Science, 30 April 1999.)