A close-up view of the ‘Genesis Rock’ collected by
astronauts Dave Scott and Jim Irwin on the mission’s
second moonwalk. Credit: NASA
described, “we could look over and
see some of this white rock. Immediately, I saw white, I saw light green
and I saw brown. But there was one
piece of white rock that looked
different from any of the others. We
didn’t rush over to it; we went about
our job the usual way. First I took
down-Sun shots and a locator shot
about 45 degrees from the Sun-line
and Dave took a couple of cross-Sun
shots.”
Scott and Irwin slowly threaded
their way between the craters to
the strange white rock. “It was lifted
up on a pedestal,” Irwin wrote. “The
base was a dirty old rock covered
with lots of dust that sat there by
Working at Spur Crater during the second EVA.
Credit: NASA via Retro Space Images
atmosphere or haze, almost knocked
Dave Scott’s socks off.
Their first task was to find a small
“drill hole” crater that could have excavated material from the mountain,
but the flank was remarkably clean.
Scott curtailed the planned drive
and they sampled a small crater and
then an isolated boulder which was
coated in greenish material.
The green hue captivated Jim
Irwin, whose Irish descent and birthday on St. Patrick’s Day—and the
fact that he had stowed some shamrocks in the lunar module—made this
a special find.
At first, the two men wondered
if their eyes or Sun visors were playing tricks on them, but when it was
unpacked a few weeks later in the
Lunar Receiving Laboratory (LRL),
their initial impressions would be confirmed: it was green, made entirely
of minuscule spheres of glass, tiny
droplets of magma spewed from a
fissure by a “fire fountain”.
In time, it and other samples would
contribute to making Apollo 15 one
of the greatest voyages of discovery
ever undertaken in human history.
Finally, they headed for Spur Crater. “As soon as we got there,” Irwin
30
30
itself, almost like an outstretched
hand. Sitting on top of it was a white
rock almost free of dust. From four
feet away I could see unique long
crystals with parallel lines, forming
striations.” Scott used tongs to pick
it up and held it close to his visor to
inspect it. The rock was about the
same size as his fist and even as he
lifted it, some of its dusty coating
crumbled away…and he saw large,
white crystals.
“Aaaahh!” he exulted.
“Oh, man!” added Irwin.
The rock was almost entirely “plagioclase”—an important tectosilicate
feldspar mineral used by petrologists
on Earth to help determine the composition, origin and evolution of igneous rocks—and from their expeditions
into the hills of the San Gabriels, Scott
recognized it as “anorthosite”, which
is the purest form of plagioclase.
For some time, lunar geologists
had suspected that anorthosite
formed the Moon’s original, primordial crust; indeed, data from the unmanned Surveyor 7 lander had suggested its presence in the ejecta of
the crater Tycho and tiny fragments
of it had actually been found in
samples from both Apollo 11’s landing site at Tranquility Base and Apollo
12’s site in the Ocean of Storms.
“Explaining why most of the
Moon’s crust should be composed of
anorthosite,” wrote Andrew Chaikin
in A Man on the Moon, “led some
geologists to an extraordinary scenario. Within the infant satellite, they
proposed, there was so much heat
that the entire outer shell became
an ocean of molten rock. As this
“magma ocean” cooled, minerals
crystallized. The heavier species,
including the iron- and magnesiumrich crystals, sank to the bottom.
The lighter crystals, specifically, the
mineral [aluminium-rich] plagioclase
floated to the top.”
Recognizing the find as probably a
piece of the Moon’s primordial crust,
Scott could hardly contain his enthusiasm. “Guess what we just found!”
he radioed. “I think we just found
what we came for!”
“Crystalline rock, huh?” said Irwin.
“Yessir,” replied Scott.
After briefly describing the rock’s
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