NASA Goddard

Moon’s unusual bulge may indicate early Earth had no ocean

It’s not just dads with beer bellies: Many objects in our solar system sport a bulge around their midsections, caused by fast rotations that tend to fling material outward at equatorial latitudes. But our moon’s rotational bulge—an equatorial diameter that would be, on average, about 200 meters longer than its diameter through the poles if the moon weren’t so cratered with huge basins—is about 20 times larger than expected, based on its current once-per-month rate of rotation. Now, researchers may have an explanation. A first-of-its-kind computer simulation suggests that the moon’s outer layers, and thus its rotational bulge, largely froze into their current shape to preserve the inordinately large rotational bulge about 4 billion years ago. Before that time, the moon’s spin rate was higher, the researchers report today in Geophysical Research Letters. The team’s model suggests that stems from a slower deceleration rate for Earth’s spin at the time, which affected the total amount of rotational momentum in the Earth-moon system and thus how rapidly the moon’s spin rate decelerates, among other things. And the fact that Earth’s spin rate wasn’t slowing down as quickly then as it is today hints that our planet had little or no ocean to slosh about and slow down our planet’s spin rate for its first 500 million years, the findings suggest. Or, the researchers propose, any ocean that did exist was largely frozen, possibly because of the sun’s 30% fainter output of radiation at the time.