To a Rocky Moon - Four Revolutions in the Earth Sciences - page 166

closer to than 240,000 miles? In 1961, Shoemaker and his colleagues at the USGS
proposed that in principle one could use terrestrial impact craters to estimate the
age of features on the Moon. 2 First of course, one would have to accept that met-
eorite impact has created craters both onthe Earth and the Moon.One whodid was
a remarkably versatile and clever young man named William Hartmann, a scient-
ist, artist, and science-fiction author.
Before he looked to the Moon, Hartmann turned to the Canadian Shield, that
vast area of Precambrian rock that forms the ancient core of the North American
continent and underlies most of the eastern half of Canada. If impact craters exist
on Earth, and if they fall at random times and places, then the oldest terrains on
Earth have had time to accumulate the largest number. Indeed, from studying aeri-
al photographs, Canadian scientists had already identified several circular features
as likely impact craters.
From the estimated number and age of the putative Canadian craters, Hartmann
calculated the rate of crater formation on Earth, which he then extrapolated to the
probablerateofcratering ontheMoon.UsingphotographsoftheMoon,Hartmann
counted the number of craters on the lunar seas: the maria. In effect, he then had
a space-age hourglass: he had both the number of lunar craters and an estimate of
the rate at which they had formed. Dividing number by rate gave the age. It turned
out to be 3.6 billion years—as old as any terrestrial rock known at the time and
“very consistent with current isotopic ages of meteorites of about 4.5 to 4.7 billion
years,” as Hartmann putit. Knowing full well that hehadmade many assumptions,
Hartmann estimated that his result was “probably correct within a factor of 4.” 3
Nothing could be settled until a Moon rock and the astronauts who had collected
it were safely back on Earth. But the question of where to make their field trip re-
mained.
It's Basalt, Isn't It?
In order to choose the safest landing site, NASA's engineers knew they needed
more information than could be provided by a spacecraft crashing into the Moon's
surface. One that settled gently enough to survive the landing could telemeter sci-
entific data about the surface back to Earth. Thus was born Surveyor .
Tom Gold criticized Surveyor 's design, claiming that when it landed “even the
antenna would sink out of sight.” 4 But when Surveyor I dropped onto Oceanus
Procellarum on June 2, 1966, it bounced gently a few times, came to rest, and
began transmitting its first picture: an image showing the craft's footpad resting
high and handsome in a small dimple. The next few Surveyor missions were