Two men were collecting rocks at a place called Hadley Rill one sunny day in August 1971. Truly, they were working in broad sunlight and it was daytime somewhere on Earth, but Hadley Rill is on the Moon. Extensive pre-flight training devoted to geology in the field transformed these military test pilots into competent lunar explorers. Apollo 15 was dedicated to lunar geology with advanced equipment and goals not possible on previous missions that were more about flight engineering than science. Important questions remained about the origin and history of the Moon. More geologically complex places needed to be explored and that generally meant more difficult places to land. However, Congress and the public were losing interest in the space program. Three of the last planned missions were already cancelled due to budget-cuts. With two more missions remaining after Apollo 15, the clock was ticking.
During the second day of excursions from their Lunar Module, Dave Scott and Jim Irwin drove their rover, the ultimate sports utility vehicle, to a crater named Spur. After collecting some unusual green-colored rock fragments, they noticed a little white rock reflecting sunlight more brightly than most of the other rocks scattered on the surface. It was perched on a little pedestal like a trophy.
Scott exclaimed, “Oh boy. Look at that!”
Irwin replied, “Oh man. I got it. Look at that glint! I can almost see twining in there!”
Irwin chuckled over the discovery as Scott announced to mission control on Earth, “….. Guess what we just found! I think we found what we came for. Look at the plage in there. I think we might have ourselves something close to anorthosite….. What a beaut.” Geologists at mission control were as excited as the moonwalkers.1
Dave Scott was holding a fragment of anorthosite, an igneous rock composed of the mineral plagioclase (what he called “plage”). Anorthosite rocks on Earth are generally very old. For example, based upon various radiometric-dating techniques, the anorthosite bedrock of the Adirondack Mountains of New York yields ages of 1160 to 1150 million years.2 Older terrestrial anorthosite at Mt. Narryer, Western Australia is dated at 3730 million years.
Geologists hoped to find anorthosite on the Moon, expecting it to be the composition of the oldest lunar crust. They were testing various hypotheses for the origin of the Moon. One idea was that the Earth, Moon and meteorites (rocks that fall to the earth from space) were formed at about the same time and from the same materials early in the formation of the solar system. If so, the oldest Earth and Moon rocks and meteorites should be close to the same age. Geologists who trained the astronauts in the field told them to look for anorthosite! Even before the sample returned to earth with the crew, journalists started calling it the Genesis Rock.
Indeed, the Genesis Rock, officially named sample 15415, was among the oldest Moon rocks recovered by the Apollo expeditions (1969-1972), dated to about 4100 million years. Careful examination revealed that the rock experienced some shock and metamorphism (exposure to high temperature) after it formed.3 Perhaps the impact that formed Spur Crater excavated it from the Moon’s crystalline basement, prompting some lunar geologists to rename it the Exodus Rock!4 The Apollo 16 crew collected an older 4360-million-years-old anorthosite.
How do these dates from Moon rocks compare with the oldest earth rocks and meteorites? The oldest known earth rocks, located in Northern Quebec, Canada are 4280 million years old. The oldest date from an earth mineral is 4375 million years from a zircon crystal from rocks in the Jack Hills of Western Australia. Meteorites are dated between 4530 and 4580 million years (4,567 million years is generally taken as the average of these measurements). The oldest meteorite dates probably represent the time in which rocky material was condensing in the early solar system, or at least when material started accreting into asteroid-size planetesimals and protoplanets. The oldest Earth rocks are younger than meteorites by several hundred millions of years because virtually all of the original Earth crust (if not all!) was removed through plate tectonic recycling. But, the convergence of the oldest Earth and Moon rocks at about 4400 million years and chemical similarities between the Earth’s mantle and Moon are evidence that the Moon formed from material ejected from the Earth’s mantle after a collision with a smaller planetary body, after the Earth reached its maximum size some 100 million years after the solar system began forming. Coarse, plagioclase crystals such as found in the Genesis Rockcrystallized out of a magma ocean that solidified into the original crust of the nascent Moon.
The Moon is called “the lesser light” in the biblical account of origins. This declaration overturned contemporaneous pagan cosmologies giving the Moon the status of a deity. God created the lesser light in order to govern the night, to assist the other lights in the expanse of the sky by separating light from darkness and marking the days, seasons and years. Ancient astronomers tracked the heavenly bodies with sophistication. They understood the cycles of the Moon provided a precise clock for agriculture and religious observances. The Israelites learned that God provided them the gift of this celestial timekeeper. Yet, there is more. We know now that the Moon gives Earth a number of unique benefits that enhanced the ability of life to thrive here.5 The impact that formed the Moon probably influenced the composition of the Earth’s atmosphere by reducing original carbon dioxide levels. Tidal forces were greater early in Earth-Moon history, promoting erosion along the coasts that delivered nutrients to the early oceans and providing a regular mechanism for cleansing and oxygenating the oceans. In the search for life beyond our solar system, modern astronomers may find an identical Earth-like planet, but lacking an Earth-like Moon may decrease the chances that it contains life beyond “pond scum.”
Naming sample 15415 the Genesis Rock gave it a powerful connection to the wider human project of understanding creation. The general public, the media, the astronauts, and even the scientists embraced the connection. After Apollo 15, Jim Irwin dedicated his life to Christian evangelistic ministry. He carried a replica of the Genesis Rock and frequently referred to it in his presentations to audiences around the world. His message was that seeking a personal relationship with the God who created the Moon is still relevant in the Space Age.6 The story of the Genesis Rock reminds us of how scientific exploration makes creation all the more remarkable and how biblical and scientific accounts, despite their different purviews and purposes, remain inexorably linked in our understanding of origins.