lecithin writes: "Spaceflightnow has a story regarding a puzzle to astronomers and geologists. What makes mars magnetic? If you could pick up a rock from the surface of Mars, then the chances are it would be magnetic. And yet, Mars doesn't have a magnetic field coming from its core. These rocks are clinging to the signal of an ancient magnetic field, dating back billions of years, to the times when Mars had a magnetic field like Earth's."