nature.net
April 2006
New Moon
By Robert Anderson
The Moon reveals just one side to its admirers on Earth, yet our satellite seems an object with a thousand faces. It smiles with romantic light and winks at armchair space travelers. For me, most of all, it is the place where the Apollo 11 astronauts set foot in 1969, when I was eight. But as an adult, I also see it as our planet’s dynamic partner, without which life on Earth would never have flourished. Isaac Asimov’s “The Triple Triumph of the Moon,” written shortly after he watched the launch of Apollo 17, sets forth his reasons for thinking we would not have evolved without the Moon, and how the Moon was crucial to the development of mathematics, science, and space travel.
The Moon, as the leading theory goes, was born in the aftermath of a titanic collision between a Mars-size planet named Theia and the early Earth. A Web page at the Planetary Science Institute introduces the “giant impact” hypothesis with paintings by William K. Hartmann, one of the astronomers who originated the idea in 1975. The Web site of Alistair G.W. Cameron, another pioneer in the study of giant impacts, has a number of his early computer simulations of the collision.
Collision theories also enliven Web pages by G. Jeffrey Taylor of the Hawai'i Institute of Geophysics and Planetology and H. Jay Melosh of the University of Arizona in Tucson. Their simulations show lighter mantle rock from both bodies blasted into orbit, while Theia’s dense iron core merges with that of the proto-Earth to form our planet’s present massive core. That core was key to life’s overwhelming success: a smaller core could not have generated a magnetic field strong enough to shield us from lethal cosmic rays. Furthermore, the internal heat of our planet’s enlarged core has been the driving force of plate tectonics, another likely prerequisite for complex life to evolve.
At the Internet encyclopedia Wikipedia, an animation at the bottom of the Web page shows how Theia may have formed in the same orbit as Earth, at what is called a Lagrange point, before it drifted into us at a suitably low speed. Edward Belbruno and J. Richard Gott III calculated that this mechanism increases the likelihood of such planet-size impacts. While looking for more about Lagrange points, I came across a Web page on the topic by John C. Baez, a mathematical physicist at the University of California, Riverside. In his section titled “Mars Trojans, Neptune Trojans, and Earth’s strange companions,” I was surprised to learn that Earth has several other “moons” tagging along. Relative to our planet, asteroid 3753 Cruithne, for instance, moves in a complicated spiraling orbit whose extremities resemble horseshoes.
On the Internet you can find many new faces of the Moon, but I still enjoy the images the astronauts brought back almost four decades ago. At the Lunar and Planetary Institute Web site, click on “70 mm Hasselblad” to view a complete collection of the ultimate tourist snapshots. Who is not still amazed by the images of Earth, rising moonlike over that barren surface? In the next few years, new lunar missions may be added to the old. Go to the lunar exploration page of the Goddard Space Flight Center for a chronology of lunar exploration past, present, and future.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2006