At first glance, Mercury seems to be a planet with few mysteries: it’s small, cratered, and geologically moribund. But Mercury has a weak magnetic field--which is presumably generated, like Earth’s, by the circulation of molten, electrically conducting metal at the planet’s core. Yet Mercury is so small that its core should have frozen eons ago. Last June a team of scientists suggested a way to resolve the contradiction. The researchers measured infrared and microwave radiation coming from Mercury’s surface. They found essentially no radiation characteristic of basalt, the most common type of volcanic rock. This means that volcanism, the main means of venting heat from a rocky planet’s interior, must always have been rare on Mercury. The magma that might be present at depth doesn’t appear to make it to the surface, says Raymond Jeanloz, a geophysicist at the University of California at Berkeley. Mercury’s crust insulates the planet’s interior, so its core has apparently remained hot, liquid, and capable of generating a magnetic field.