Smallest planet yields big surprises

Mercury has a complicated inside and an active geologic past

THE WOODLANDS, Texas — Mercury is even weirder than expected, NASA’s MESSENGER probe is showing.

Purple marks low elevation and white high elevation in this rendering of ancient volcanic plains in Mercury’s northern hemisphere. Images like these suggest the planet had an active geologic past. NASA, JHUAPL, CIW-DTM, GSFC, MIT, Brown University.