Apollo
11 was the first mission to the Moon on which samples were collected
and brought to Earth. In July of 1969, the Apollo
11 astronauts landed in Mare
Tranquilitatis, a huge impact crater formed in the lunar crust
about 4 billion years ago that had been flooded by basaltic lavas
3.6-3.9 billion years ago. Between the time the lavas cooled and the
astronauts collected samples, the area was impacted by countless meteoroids,
big and small. The impacts both shattered large rocks into small rocks
and welded (melting and shock compaction) fine-grained material into
coarser-grained material, forming impact breccias.
The medium-gray, crystalline fragments in the photo are the basalts,
usually called "mare basalts." There are 2 black, glassy,
impact-melt spheroids (one shiny, one not). The other darkest fragments
are glassy soil breccias (now usually called regolith
breccias) consisting of fine-grained soil that was "glued"
into a rock by a meteoroid impact. The light-colored fragments are
anorthosites and impact breccias composed mainly of anorthosite. |
anorthosite:
An igneous or plutonic rock consisting mainly of the mineral anorthite,
a feldspar with a high ratio of calcium (Ca) to sodium (Na)
feldspar: A common mineral ranging in composition from sodium
aluminum silicate (albite) to calcium aluminum silicate (anorthite)
basalt [pronounced ba-SALT]: One form of solidified lava
or magma from a volcano
breccia [pronounced BRETCH-ee-a]: A rock made up
from bits and pieces of older rocks
maria [pronounced MAR-ee-a; Latin for "seas"]:
The dark, circular features on the surface of the Moon that are easily
visible from
Earth
mare [pronounced MAR-ay; Latin for "sea"]:
Singular form of mare |
The Apollo 11 mission landed in a mare because mare are flatter and, consequently,
safer places to land than the surrounding highlands.
At the time of the Apollo 11 mission, little was know about the crust of
the more rugged lunar highlands, although the unmanned Surveyor
VII mission in 1968 had suggested that the highlands were rich in aluminum.
When the material of the photo was first examined in late 1969, a team of
scientists from the Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, made an important observation and leapt to a conclusion that
was to become one of the most significant to be made from the first Apollo
samples. In the abstract to the paper* describing their findings, John Wood,
John Dickey, Ursula Marvin, and Ben Powell state the following:
"We
prepared and studied thin section of 1676 rock fragments (dia. range
1-5 mm) from the Apollo 11 bulk sample. Most are basaltic rocks, glasses,
and soil breccias, but about 4 per cent of the fragments are of a
totally unanticipated material: anorthositic rocks, breccias, and
glasses. The anorthositic rocks are similar to terrestrial anorthosites
except for the fine grain-size (typically 20-100 µ) and low
content of Na (the feldspar is usually anorthite). Many have what
we feel are are true cumulate textures. Because of their light color,
low density, and chemical resemblance to the Surveyor VII analysis
of material in the lunar highlands, we believe this anorthositic suite
is derived from the highlands (portions of which lie only ~50 km from
Tranquility Base). The finer-grained soil fraction from the Apollo
11 sample must contain more than 4 per cent of the anorthositic component
(about 20 per cent) to account for the bulk composition of soils and
breccias.
The highlands would have to be underlain by ~25 km of anorthosite,
if their 3 km of mean relief above the maria is isostatically compensated.
We propose a lunar model in which such an anorthositic layer "floats"
on gabbro, which in early times welled up into major impact craters
to form maria. Since basaltic lava contracts upon on solidification,
faster cooling and earlier solidification of the material in the maria
than in the highlands would promote transfer of additional magma from
beneath the highlands into the maria to fill the volume vacated upon
solidification, and this would give rise to mascons. The model entails
extensive melting and crystals fractionation in the moon, with anorthosite
crystals floating to the surface through a denser magma." |
We now know that the "belief" expressed in the first paragraph
is correct: The typical crust of the Moon consists of anorthosite. The
"extensive melting" suggested by the researchers in the second
paragraph has become what is now called the "magma ocean" model.
In this model, much or all of the Moon was molten very early in the history
of the Solar System. The earliest minerals to crystallize from the melt
(olivine and pyroxene) were rich in iron and sank to form the lunar mantle.
When plagioclase feldspar began to crystallize, it was less dense and
floated in the melt to form the anorthositic (plagioclase-rich) crust.
* Wood J. A., Dickey J. S. Jr., Marvin U. B., and Powell B. N. (1970)
Lunar
anorthosites and a geophysical model of the moon, Proceedings
of the Apollo 11 Lunar Science Conference, pages 965–988.
See also: "How
Do We Know That It's a Rock from the Moon?" and "Lunar
Meteorites" |
