14 December 2009

Cuddling up to the Moon

Posted by Michael McFadden

Earth/Moon composite, from NASA/JPL

To keep itself warm on the cold nights of its youth, the Earth snuggled up to its honey–the Moon. It may have changed the way our planet developed, according to a late morning talk by Lindy Elkins-Tanton of the Massachusetts Institute of Technology.

Early in the evolution of the Earth and Mars, magma oceans, hot baths of molten minerals formed from core heating, radiation or meteor impacts, cooled to form planetary mantles. But magma oceans didn’t cool in the same way on Earth as on Mars.

Our mantle, a sticky layer of half-melted rock sitting over the core, has its minerals mixed relatively evenly throughout. Mars’ mantle, on the other hand, is stratified like a jaw breaker. This stratification is important–it acts like a blanket and keeps heat from flowing. Without heat flow, Mars doesn’t have the plate tectonics that give our turf earthquakes, volcanoes and the nutrient cycles important for life.

Lindy Elkins-Tanton, an assistant professor of geology at MIT, thinks that the differences between the mantles of Earth and Mars may be, in part, due to the Moon.

The Earth didn’t always snub its satellite like it does today. For a short time after a stray piece of galactic junk knocked the Moon free from the Earth, the two spun close to one another and spun quickly, Elkins-Tanton said. The gravitational fields from this speedy spinning may have slightly heated the Earth by squishing the planet’s surface when the Moon was close and relaxing it when the Moon was far.

This planet baking helped to mix the Earth’s mantle, Elkins-Tanton said. Other factors may have played a part too. As heavy metals cooled in the magma ocean, they dropped, pulling water along for the ride. But as soon as these metals hit a region of a dry mineral called perovskite, the water escaped its metal captors and mixed with the surrounding mantle. The released water may also have melted surrounding rocks, preventing stratification, she said.

The Moon in the night sky has long been a draw for cuddling companions. If only these cuddlers had known that without the Moon’s embrace, our home may not be what it is today.

–Daniel Strain, UC Santa Cruz Science Communication Graduate Student