Space.com notes a study published in the May 7th issue of Science (abstract) which concludes not only that Mercury has a magnetic history dating back billions of years, but also that the strength of that field means that it once rivaled Earth's own magnetic field, though it is now "about 100 times weaker." The source of this conclusion is data gathered prior to the crash of NASA's Messenger probe into Mercury's surface on April 30 of this year. Says the story: The researchers analyzed magnetic data collected by MESSENGER in the fall of 2014 and 2015, when the spacecraft flew incredibly close to the planet's surface, at altitudes as low as 9 miles (15 kilometers). In contrast, the lowest that MESSENGER flew in previous years was between 125 and 250 miles (200 and 400 km). ... The scientists detected magnetized rocks in a part of Mercury's crust that, due to the presence of many craters from cosmic impacts, appears to be quite ancient. The researchers suggest the rocks were once magnetized by the planet's magnetic field, and based on the age and amount of the magnetized rocks, as well as how strongly they were magnetized, the investigators deduced that Mercury's magnetic field has persisted for 3.8 billion years.