nature.net 



New Moon 



By Robert Anderson 



The Moon reveals just one side to 

 its admirers on Earth, yet our 

 satellite seems an object with a thou- 

 sand faces. It smiles with romantic light 

 and winks at armchair space travelers. 

 For me, most of all, it is the place where 

 the Apollo 1 1 astronauts set foot in 

 1969, when I was eight. But as an adult, 

 I also see it as our planet s dynamic part- 

 ner, without which life on Earth would 

 never have flourished. Isaac Asimov's 

 "Triumph of the Moon" (at mountain 

 man.com.au/i_asimov.html), 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 "gi- 

 ant impact" hypothesis with paintings 

 by William K. Hartmann, one of the 

 astronomers who originated the idea 

 in 1975 (psi.edu/projects/moon/moon. 

 htmi). Alistair G.W Cameron, another 

 pioneer in the study of giant impacts, 

 has a site at xtec.es/recursos/astronom/ 

 moon/camerone.htm with 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 Planetol- 

 ogy (www.psrd.hawaii.edu/Dec98/Origin 

 EarthMoon.html) and H.Jay Melosh of 

 the University of Arizona in Tucson 

 (www.lpl.arizona.edu/outreach/origin). 

 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 over- 



whelming 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 tecton- 

 ics, another likely prerequisite for com- 

 plex life to evolve. 



At the Internet encyclopedia 

 Wikipedia (en.wikipedia.org/wiki/Giant_ 

 impact theory), an animation at the bot- 

 tom 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 suit- 

 ably low speed. Edward Belbruno and 

 J. Pochard Gott III calculated that this 

 mechanism increases the likelihood of 

 such planet-size impacts. While look- 

 ing 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 Califor- 

 nia, Riverside (math.ucr.edu/home/ 

 baez/lagrange.html). In his section ti- 

 tled "Mars Trojans, Neptune Trojans, 

 and Earth's strange companions," I was 

 surprised to learn that Earth has sev- 

 eral other "moons" tagging along. 

 Relative to our planet, asteroid 3753 

 Cruithne, for instance, moves in a 

 complicated spiraling orbit whose ex- 

 tremities resemble horseshoes. 



On the Internet you can find many 

 new faces of the Moon, but I still en- 

 joy the images the astronauts brought 

 back almost four decades ago. At the 

 Lunar and Planetary Institute Web 

 site (www.lpi.usra.edu/resources/apollo), 

 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 mis- 

 sions may be added to the old. Go to 

 the lunar exploration page of the 

 Goddard Space Flight Center (nssdc. 

 gsfc.nasa.gov/planetary/lunar/apollo_25 

 th.html) for a chronology of lunar ex- 

 ploration past, present, and future. 



Robert Anderson is a freelance science 

 writer living in Los Angeles. 



April 2006 NATURAL HISTORY 



67 



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