Monday, August 8, 2011

When a Moon Hits Your Eye Like a Big Pizza Pie, That's a Mare

Ever wondered why the near side of the Moon is home to pretty much all of its flat areas (called maria, singular mare, from the Latin for "sea")?

I can't say I have, frankly.  Although maybe if I'd seen this picture before today, the thought might have crossed my mind.  It's a cylindrical projection of the surface of the Moon, with the middle of the near side in the center.

image credit: U.S. Geological Survey

Fortunately, we have scientists to think about these things.  And two of them, Martin Jutzi of the University of Berne, Switzerland, and Erik Asphaug of the University of California, Santa Cruz, have an interesting new theory about it.

For some time now, the theory about how Earth came to have a large satellite, when none of the other terrestrial planets do, has gone like this: 4.5 billion years ago or so, gravitational interaction between forming planets sent some of them hurtling out of the solar system, and others hurtling across it.  One of the latter, a protoplanet the size of present-day Mars, managed to strike proto-Earth.  The heat of collision liquefied the surface of proto-Earth, and a spray of debris blew off, some of it staying in orbit.  This debris coalesced, and the Moon was born.

But, Jutzi and Asphaug ask, why should the debris become one satellite?  Why not two, a larger and a smaller, in the same orbit?  As the orbit of the two moons slowly climbed away from the Earth, the balance between them would destabilize.  They would draw closer, and eventually meet.  It seems odd to describe the crash of two moons as "gentle," but being in the same orbit, they would have the same orbital velocity, and so when they collided, it would only be with the fairly low speed given them by their mutual gravitational attraction.  Rather than leaving a crater, the smaller moon splashes over the near surface of the larger, like an egg hitting a bowling ball.

Magma, forced out by the collision, covers the other surface, leaving it smoother.  Eventually, the collision side, being heavier thanks to the contributed mass of the ex-moon, swings away from the Earth, leaving the smoother, opposite surface facing the planet.

This theory, incidentally, also explains why the far side of the Moon is, as we know, tens of kilometers thicker than the near side.  But a theory isn't much good without a potential test, and Jutzi and Asphaug have one of those, too.  The smaller moon would have finished forming before the larger.  Therefore rocks on the far side of the Moon, having once been part of the smaller moon, should be older than rocks on the original surface of the near side.

Additionally, when NASA's Gravity Recovery And Interior Laboratory (GRAIL) mission, launching next year, will produce high-resolution gravity maps of the Moon, and Jutzi and Asphaug can see whether the Moon conforms to their computer simulations of the long-ago collision.  It may be that when the Soviet Luna 3 probe first photographed the far side of the Moon in 1959, we were unknowingly getting our first look at what's left of an entirely different moon.

And no, I'm not apologizing for the title pun.

"Second moon may have collided with our moon, say scientists,"
Wikipedia (for the photo)

1 comment:

Kevin Martin King said...

LOVED the title pun!

No apologies necessary, in my book.