A research paper on the planet Mercury is an example of a new discipline: comparative geomagnetic core studies. You see, Mercury is the most Earthlike planet in one respect—it has a large, fluid iron core that produces a magnetic field. The paper, in April's Geophysical Research Letters, analyzes high-pressure experiments on hot iron-sulfur mixtures and suggests that iron crystallizes, leaving behind a sulfur-enriched liquid, and rains—rather, snows—down onto a solid inner core, like Earth's. Not only that, the iron snows from two places in the core; from its top and from a zone partway down. Earth's core is not so exotic; in it, iron snow forms at the top of the core taking nickel along with it, leaving an iron-enriched liquid behind. (Sulfur is not involved.) But Earth has a far more energetic magnetic field, probably related to its much greater mass and much greater angular momentum due to its rapid spin. After all, Mercury is very un-Earthlike in its rotational spin, making its day longer than its year. And let's mention the other two terrestrial planets while we're talking: Venus has a large liquid core but an extremely slow spin (it actually rotates backward) while Mars has a small solid core but a day length uncannily close to Earth's, however neither planet has a significant magnetic field.