THE DUSTY LUNAR SURFACE
Some visitors to Kilauea Volcano, Hawaii, have been overheard to say, upon
seeing a vast landscape covered with fresh lava, "It looks just like
the Moon." Well, it doesn't. The fresh lava flows of Kilauea and other
active volcanoes are usually dark grayish and barren like the Moon, but
the resemblance ends there. The lunar surface is charcoal gray and sandy,
with a sizable supply of fine sediment. Meteorite impacts over billions
of years have ground up the formerly fresh surfaces into powder. Because
the Moon has virtually no atmosphere, even the tiniest meteorite strikes
a defenseless surface at its full cosmic velocity, at least 20 km/sec. Some
rocks lie strewn about the surface, resembling boulders sticking up through
fresh snow on the slopes of Aspen or Vail. Even these boulders won’t last
long, maybe a few hundred million years, before they are ground up into
powder by the relentless rain of high-speed projectiles. Of course, an occasional
larger impactor arrives, say the size of a car, and excavates fresh rock
from beneath the blanket of powdery sediment. The meteoritic rain then begins
to grind the fresh boulders down, slowly but inevitably.|
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The powdery blanket that covers the Moon is called the lunar regolith,
a term for mechanically produced debris layers on planetary surfaces.
Many scientists also call it the "lunar soil," but it contains
none of the organic matter that occurs in soils on Earth. Some people
use the term "sediment" for regolith. Be forewarned that the
regolith samples in the Lunar Sample Disk are labeled “soil.” Although
it is everywhere, the regolith is thin, ranging from about two meters
on the youngest maria to perhaps 20 meters in the oldest surfaces in the
highlands. [See the "Regolith Formation" activity on Pages 47-52.]
Lunar regolith is a mixed blessing. On the one hand, it has mixed local
material so that a shovelful contains most of the rock types that occur
in an area. It even contains some rock fragments tossed in by impacts
in remote regions. Thus, the regolith is a great rock collection. It also
contains the record of impacts during the past several hundred million
to a billion years, crucial information for understanding the rate of
impact on Earth during that time. On the other hand, this impact record
is not written very clearly and we have not come close to figuring it
out as yet. The blanket of regolith also greatly obscures the details
of the bedrock geology. This made field work during Apollo difficulty
and hinders our understanding of lunar history.
The regolith consists of what you’d expect from an impact-generated pile of debris. It contains rock and mineral fragments derived from the original bedrock. It also contains glassy particles formed by the impacts. In many lunar regoliths, half of the particles are composed of mineral fragments that are bound together by impact glass; scientists call these objects agglutinates. The chemical composition of the regolith reflects the composition of the bedrock underneath. Regolith in the highlands is rich in aluminum, as are highland rocks. Regolith in the maria is rich in iron and magnesium, major constituents of basalt. A little bit of mixing from beneath basalt layers or from distant highland locales occurs, but not enough to obscure the basic difference between the highlands and the maria.
One of the great potential bits of information stored in the complex pile atop the lunar surface is the history of the Sun. The nearest star puts out prodigious amounts of particles called the solar wind. Composed mostly of hydrogen, helium, neon, carbon, and nitrogen, the solar wind particles strike the lunar surface and are implanted into mineral grains. The amounts build up with time. In principle, we can determine if conditions inside the Sun have changed over time by analyzing these solar wind products, especially the isotopic composition of them.
The same solar wind gases may prove useful when people establish permanent settlements on the Moon. Life support systems require the life-giving elements: hydrogen and oxygen (for water), carbon, and nitrogen. Plenty of oxygen is bound in the silicate, minerals of lunar rocks (about 50% by volume) and the solar wind provided the rest. So, when the astronauts were digging up lunar regolith for return to Earth, they were not merely samplingÐthey were prospecting!
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