The solar system’s innermost planet may look like a dead ringer for Earth’s moon, but the scientists getting the closest look ever at Mercury want you to know one thing: Mercury is not the moon. And it isn’t anything like the other rocky planets—Earth, Mars, or Venus—either. At a NASA press conference today, researchers reporting on the first three months that the MESSENGER spacecraft has been orbiting the sun’s closest neighbor emphasized many distinctive mercurian aspects. Perhaps the most fundamental is an abundance of elements that can be easily boiled out of hot rock. Team members have been gauging rock composition by measuring the x-rays and gamma-rays emitted by a surface bombarded by the sun’s x-rays and by cosmic rays. The latest analyses show ten times more sulfur than found in Earth’s or the moon’s surface rocks and as much or more potassium. That rules out some previously proposed ways of forming Mercury with its relatively huge molten iron core, such as having the nascent sun blasting away the outer rind of an Earthlike planet. Still in the running is a huge impact that blew away an outer layer of Mercury. With all systems go, MESSENGER has 9 months left in its planned mission.
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