The surface of our Moon is comprised of many types of minerals.
Although the Apollo and Luna missions have successfully
returned lunar soil samples
back to Earth, the vast majority of the lunar surface has not been
physically explored. Thus, planetary geologists rely on reflectance
spectroscopy, in combination with analysis of research samples, to
determine the mineral content of the Moon. Understanding the mineral
content of the Moon could help us understand the Moon's, and our
Solar System's origins.
Nearside
Farside
The lunar crust is divided into two distinct units, the mare (plural maria)
and highlands. Most of the mare units are found on the nearside while the
farside crust is composed predominately of highlands.
Maria are the remains of the Moon's volcanic past - vast, flat
plains of igneous rock that were once fields of lava.
The rocks of lunar maria are made up mostly of basalt.
Basalt is more than fifty percent high-Ca pyroxene.
The pyroxene absorbs light around .98 to 1.00 microns, causing an absorption
band in the reflectance spectrum. Some types of basalt have a high olivine
content (up to 10 to 20 percent). The effect of high olivine levels is
to broaden the pyroxene band and to move it slightly towards higher
wavelengths.
One can begin to distinguish between the rocks of the lunar highlands
based on differences in pyroxene content. All rocks containing
some form of pyroxene have bands between 0.9 and 1.0 m. Norite, which
is comprised of plagioclase feldspar and low-Ca pyroxene, has an
absorbance band between 0.9 and 0.93 microns. As the low-Ca pyroxene
content increases, the strength of the absorption band (how much the
band dips from the rest of the spectrum) also increases. If an area
of lunar rock has an absorption band between 0.93 and 0.95 microns,
it is probably also norite, but its pyroxene may have a higher Ca
content, or the norite may nave some high-Ca pyroxene mixed in. Gabbro
rocks are very similar to basalts-their mineral contents are similar,
but gabbro rocks were formed intrusively (under the surface), while basalts
were formed extrusively (on the surface.) Lunar gabbros tend to have a
lower pyroxene content than lunar basalts. Lunar gabbros can be almost
pure high-Ca pyroxene, with an absorption band range of 0.97 to 1.00 m,
or they can be both high-Ca and low-Ca pyroxene, with an absorption
band range of 0.95 to 0.97 microns.
Two olivine-based rocks have absorption bands centered around 1.1 microns
dunite and troctolite. Dunite is almost pure olivine, while troctolite
is made up of both plagioclase fledspar and olivine. In both cases, the
strength of the absorption band at 1.1 microns depends on the level of
olivine as compared the the level of plagioclase. Finally, there is
anorthosite, which is made up almost entirely of plagioclase feldspar,
and as a rule contains less than 5 percent pyroxene. Anorthosite has
no readily apparent absorption bands in the visible and near-infrared
parts of the spectrum.
By comparing reflectance spectroscopic measurements taken in labs with
readings of the Moon's surface taken from orbiters, space-based
instruments, and earth telescopes, planetary scientists can determine
which of these and other rocks comprise the Moon's surface.
Lunar Surface
Regolith Closeup
Alas, the real world is not always as simple as the laboratory. On airless
bodies, such as the Moon, rocks are slowly ground to a fine powder. The
grinding occurs as micrometeorites impact the surface at hypervelocities
(10-20 km per second and faster). The Apollo astronauts brought back
may samples of this ground up rock or soil -- often called regolith.
Examination of the regolith samples revealed that not only is the surface
pulverized to a talcum powder-like texture, it also contains small
pieces of glass and iron metal. The glass and metal are formed
during the micro-meteorite impacts. The presence of these two materials
tends to make the soil redder and lower in albedo (darker). Of course
over time these materials build up in the soil. Thus young craters
that expose fresh rock are bluer and have higher albedo than older
craters.
Copernicus Crater with Rays
Maria, Highlands, Rays
In fact most crater rays are visible because they are
young. If you are very patient you could visit the Moon a couple
of 100 million years from now and see that rays visible today
have faded as the glass and iron builds up over time. As you can
see scientists have to be careful to account for aging (or maturity)
effects in their spectral analysis.
Click here to view a table of lunar rocks and their reflectance properties.
References
Englert, Peter A. and Pieters, Carle M. (1993)
Remote Geochemical Analysis: Elemental and Mineralogical Composition
New York: Cambridge University Press, 312-317.
Rencz, Andrew N.(ed.) (1999)
Remote Sensing for the Earth Sciences.
New York: John Wiley and Sons, Incorporated.
