The extraordinary back of the Moon

Bad Astronomy
The entire universe in blog form
March 12 2011 7:00 AM

The extraordinary back of the Moon

Remember last month when I posted that incredible super-hi-res image of the Moon's near side? A lot of folks asked if an image of the far side were coming soon.

Ask, and ye shall receive.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Holy Selene! Click to enlunanate.

This amazing mosaic is from the Lunar Reconnaissance Orbiter, circling the Moon from a height of a mere 50 km (30 miles) and mapping that entire world. This image is comprised of a whopping 15,000 images from the Wide Angle Camera -- yegads! -- and shows the side of the Moon we can never see from home.

It looks really different than the near side, doesn't it? It's almost entirely craters, while the side we see is dominated by huge lava filled basins called maria (Latin for "seas"). Why is that? Well, it's known that the crust on the near side is thinner, so it's easier for big impacts to have punched holes in the crust and allowed magma to bubble up (this was billions of years ago when the inner Moon was still molten).

But why does one side have a thick crust and the other thin? No one knows. Interestingly, it's probably tides from the Earth that forced the Moon to have this asymmetry line up with us. The Moon causes tides in the Earth, but the Earth does the same for the Moon, and the tides we create on it are much more powerful, since the Earth is far more massive than the Moon. Tides like to stretch things, and they also like to have any imbalance settle along the Earth-Moon line.

[UPDATE: Some folks were questioning my next claim, that the far side of the Moon is heavier (technically, has more mass) than the near side. The thing is, I think they're correct! The crust is thicker on the far side, but it's made of less dense material than the stuff below it. That means the half of the Moon farthest from the Earth is slightly less massive than the half facing the Earth! Or, in other words, the center of mass of the Moon is a bit closer to the Earth than the physical center of the moon (see here, and scroll down to the bit about the Moon's crust). In other other words, I was wrong. Thanks to everyone who pointed this out to me. Ironically, just tonight I was helping my daughter with her science homework. The topic? Buoyancy and density! That helped me understand my mistake here. Anyway, I've struck it through; I'll keep it up so people see the mistake I made. What I said about common sense is still right, though, and is doubly true here for me!]

You might think the heavier side would settle toward the Earth, since that's "down", but in fact tides don't care if the heavier side is pointed directly toward the Earth or directly away, as long as it's on the Earth-Moon line. In our case, the heavier side of the Moon points away from us.

Weird. But while the Universe has to follow the laws of physics, it doesn't always have to follow our common sense.

You can explore the Moon yourself; the folks at LRO also released this very cool picture of the Moon as seen from all sides, in 60° increments. Scanning over this is fascinating. I'm used to looking at close-ups, so seeing the overall picture is very, very cool.

And useful. Maps like these will be used to gauge where to send more probes, and, eventually, people. I can't help but wonder: in a few decades, will school children have to memorize the locations of Mare Orientale, Copernicus, and Rupes Recta... before taking a field trip there?

Image credit: NASA/GSFC/Arizona State University

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