Isostasy, gravity, and the Moon: an explainer of the first results of the GRAIL mission

Posted By Emily Lakdawalla

2012/12/11 01:04 CST

Last week the GRAIL lunar gravity mission published their first scientific results, and what they have found will send many geophysicists back to the drawing board to explain how the Moon formed and why it looks the way it does now. To explain how, I’m going to have to back way up, and explain the basic science behind gravity data. It’s a really important field because it’s often the only way that we can learn about the deep interiors of other worlds. Gravity data gives us a kind of x-ray vision into what’s happening inside a planet, and so helps us understand the planet’s past.

A paragraph just for the pedants out there (you know who you are): when I talk geophysics, I need a word that means “anything big enough to have internal geology.” I’m going to just say “planet” when I mean that; it’s much easier than “planets and dwarf planets and round moons.” It’s the geophysicist’s definition of the word “planet,” which is quite a bit different from an astronomer’s use of the same word. And yes, when I say “geophysics,” I’m not just talking about Earth, I’m talking about all planets.

Okay. So, first of all, let’s talk about how planets are built. Most planets are differentiated: they have layers of different materials separated by their densities. Broadly speaking, planets have cores (densest), mantles (less dense), and crusts (least dense). The terrestrial planets — Mercury, Venus, Earth, the Moon, Mars, and Io — have metallic cores, mantles made of iron and magnesium silicate minerals like olivine and pyroxene, and lighter-weight silicate crusts. All of what I will be talking about in this post concerns the upper two layers, the mantle and crust.

Read more: Isostasy, gravity, and the Moon: an explainer of the first results of the GRAIL mission | The Planetary Society.

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