In the 1970s, the first spacecraft to Mercury surprised scientists by discovering that the small world sports a global magnetic field, something Venus and Mars both lack. Now, as planetary scientists report online today in Science, the second spacecraft to Mercury has found that this magnetic field formed billions of years ago. NASA deliberately crashed the MESSENGER spacecraft into the planet last week; during its final year of life, the craft repeatedly skirted close to the surface, detecting magnetized terrain, such as over the volcanic plains shown in blue here (blue denotes low elevations and red, high elevations). This magnetized terrain dates back at least 3.7 billion to 3.9 billion years, so the researchers deduce that ancient Mercury had a dynamo in which electrically conducting fluids inside its iron-rich core generated the field. Earth’s magnetic field shields us from the solar wind, but with a daytime temperature hot enough to melt lead and a night cold enough to freeze carbon dioxide gas, Mercury probably isn’t hosting any creatures that need such protection.