Something like this seems to have occurred in Tharsis, where there is evidence of ample volcanic activity, and the northern lowlands, which bear signs of widespread, thick lava flooding. In these two places, says Connerney, "the magnetism has been essentially baked out of the crust."
Another discovery from the extended survey is a distinctive pattern running east-west across Terra Meridiani. This shows a repeating pattern, plus offsets at intervals, that echoes the magnetic patterns seen along Earth's mid-ocean ridges, where the sea floor is spreading. The magnetic pattern imprints a mirror image on both sides of the spreading center.
The pattern breaks where transform faults interrupt the ridge. The faults arise from crustal stresses that build as spreading continues. "On Earth," Connerney explains, "great transform faults that span the entire Pacific plate lie about 1,200 to 1,300 kilometers [750 to 800 miles] apart." Those found on Mars that extend correspondingly far have roughly the same spacing.
Says Connerney, "I think this is about as clean an image of the magnetic field above a spreading center as we are likely to see from an orbiter spacecraft." But, he continues, "If we got a Mars airplane mission, for example, we could do a nice magnetic survey, much closer to the surface and potentially do even better."Robert Burnham works for the Mars Space Flight Facility at Arizona State University, where he develops features and other content for its Mars web site.