The blue line is the Moon's present axis of rotation, but there's evidence that it used to rotate around the green line—the hot, red portion of the Moon in the right of the image could be responsible for the shift. (credit: James Tuttle Keane)
Billions of years ago, the Moon would have looked larger in the sky, as it has very gradually drifted away from the Earth over time. But in addition to its apparent size changing, the face of our companion satellite has probably tilted a smidgen.
That’s the conclusion of a new study from a group led by Matthew Siegler, Richard Miller, and James Keane, who based their analysis on some old data. In 1998, the Lunar Prospector mission was launched to map, among other things, deposits of water ice expected to exist at the Moon’s poles. Because the Moon’s axis of rotation is nearly perpendicular to the plane of its path around the Sun, it has no seasons, so the ever-dark bottoms of craters at the poles are fiercely cold. With temperatures that cold, any water ice that found its way there could be protected from turning to gas and escaping to space.
But instead of being restricted to a small circle at extreme latitudes, there’s an errant bulge of ice at both poles. If you draw a straight line through the center of the Moon—about six degrees off from its axis—you can connect the two bulges. Was that line once the Moon’s axis of rotation?
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