Question

why are we most likely to learn about the early history of the early by studying the rocs from the moon rather than those on the Earth ?

Most planetary scientists now believe that our moon was created approximately 4.5 billion years ago when a Mars-sized body collided into the Earth; this event is often labeled the giant impact hypothesis (see http://www.psi.edu/projects/moon/moon.html). Thus, the moon actually originated from materials from the early Earth. The moon and the Earth, however, have had two very different histories: the Earth’s surface has been constantly reworked by plate tectonic motions, weathering, erosion, volcanic outpouring, etc. Since the moon is not geologically active and it doesn’t have an atmosphere, its surface is much more pristine than the Earth’s (although, the lunar surface is altered by solar and cosmic radiation, impacts, etc.). For these reasons, many of the lunar rocks are very old (about 4.5 billion years old!), while Earth rocks are in general much younger due to their ages being reset by numerous active processes at the surface (e.g. volcanism, plate tectonic subduction, etc.). Lunar rocks thus provide a glimpse into processes occurring on the newly formed moon and on early Earth. But scientists can also find out about the early Earth by looking for materials that originated from the Earth after the moon had formed. Researchers believe that during a time called the Late Heavy Bombardment, large impacts on Earth would have caused some Earth rocks to be ejected onto our lunar orbiter. In this case, we wouldn’t just have analogues (i.e. lunar evolved rocks to compare to Earth evolved rocks), we’d have direct samples of early Earth rocks unaffected by destructive processes occurring at Earth’s surface. More is available on this at: http://www.space.com/scienceastronomy/moon_earth_020723.html For more on how scientists determine the age of rocks, check out: http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=91&search=dating http://pubs.usgs.gov/gip/geotime/radiometric.html David Lamb, Science Projects Specialist, NAI
November 26, 2002