MYSTERIOUS 鈥渁cne spots鈥� on the moon may be the splattered remains of a giant impactor.
Hundreds of regions of magnetised rock dot the moon鈥檚 surface, mostly on the far side. This is a real puzzle, since the lunar crust is made up of minerals that are hard to magnetise. These minerals floated to the surface when the infant moon was still molten after its birth in a violent run-in between Earth and another planet. Heavy, magnetisable metals such as iron would have sunk down to its core.
Now of Paris Diderot University in France and colleagues say the magnetic rash came from a giant, metal-rich space rock that slammed into the moon early in its history. The collision left behind the solar system鈥檚 largest crater: the South Pole-Aitken basin (black oval at centre, below).
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Their computer simulations show that most of the molten ejecta from the impact should have settled on the crater鈥檚 rim 鈥� exactly where most of the acne spots (red splotches at left) are found. These metallic remains then recorded the early moon鈥檚 magnetic field, produced as a result of conducting metals sloshing in its core ().
It鈥檚 a 鈥渘eat鈥� idea, says at the University of British Columbia in Vancouver, Canada.