Page 294 - NS-2 Textbook

ASTRONOMY 289

one-third of the lunar dust. Many scientists believe that
the beads are congealed rock droplets, formed when me-
teorites blasted the Moon and sent out a spray of fine
molten particles. The hmar dust layer, called the regolith,
covers the entire surface of the Moon to a depth ranging
from a few centimeters to 100 meters or more. Various
lunar landers and the Apollo astronauts found that it is
rather loose on top and compacted underneath.
The Lunar Receiving Laboratory of NASA in Hous-
ton carefully examined all the hmar dust and rock sam-
ples brought back by the Apollo expeditions. Some rocks
appeared to be rich in magnesium; others sparkled with
colorful crystals. When scientists burned a sample of
Moon soil and studied its spectrograph, they fOlmd
about sixty elements. TI,e Moon rocks appear to be rich
in titanium, but with less than one-third of the sodium
and potassium found in comparable igneous rocks on
Earth.

THE SURFACE OF THE MOON

The surface of the Moon is pockmarked with craters. The A photo of the Moon's back side, taken from the Apollo 10 com-
larger circular craters, easily seen through binoculars, mand and service module from an altitude of 60 nautical miles.
have been visible for centuries. There are also smooth
plains and mountain ranges on the Moon's surface.
the Moon. The term maria has been retained, though now
Galileo mistook the plains for bodies of water and called
each one a lIlare (plural, lIlaria), the Latin word for "sea." they are known to be filled with lava or volcanic ash, not
water. Through a telescope the maria look much darker
The craters have been named in honor of scientists and
than the craters or TI10lmtains. 111is is because the hmar
philosophers. The mountain ranges are named after
plains have a lower reflectance. An object that reflected
mountain ranges on Earth.
The great black maria are younger than the rest of all light would have a reflectance of 100 percent, while
one that absorbed all light would have 0 percent. The
the Moon. They cover up older craters and show fewer
Moon is actually a rather poor reflector, 'with a re-
signs of meteorite bombardment. Radioactivity measure-
ments of rocks from the Sea of Tranquility show that they flectance of only about 11 percent. The Moon gives off no
light of its own, but reflects the Sun's light; moonlight,
were made 3.6 billion years ago-l billion years later
therefore, is reflected sunlight.
than the Moon or Earth.
Some maria have definite lnagnetic fields. This ·was
discovered when Mariner reconnaissance spacecraft or-
CRATERS
biting the Moon experienced a greater pull of gravity
over certain maria. Probably a massive body, such as an It is still not known hmv the Moon's craters 'were formed.
iron asteroid, lies under such maria. It is thought that the One theOlY says they were formed by the inlpact of huge
"seas" welled up when the lunar crust was punctured by meteorites. TIlis theory is supported by the fact that
a swarm of asteroids, about 2-3 billion years ago. craters on the Moon look much like craters formed by
These buried super-heavy magnetic concentrations meteorite collisions on Earth. Another theory states that
beneath the hmar surface have been named I1/aSCOIlS. craters 'were formed by volcanoes. If so, these volcanoes
Mascons are much too heavy to remain on the slu'face of had to be far bigger than any on Earth. Another theory
a molten body. But they are known to be near the Moon's suggests that the craters were formed by the bubbling ac-
surface, because of the extra gravitational pull they exert tion of the molten Moon as it cooled.
on spacecraft in hmar orbit. TI,is lends strength to the be- A seemingly infinite number of craters cover the
lief that the Moon's crust is strong and very thick. If this Moon's surface. The largest on the near side is Bailly, 183
were not so, the nlascons vvould have sunk into the deep- lniles in diameter, but several unnamed ones on the far
est core of the Moon-especially if the Moon were a soft, side are larger than 200 nllies in diameter. TI,e craters are
molten body. the most striking formations on the Moon, and they are
There are about twenty maria, and they cover about present in all sizes. The typical crater has a surrounding
half of the Moon's surface. Most are on the near side of ring, which is from 1,000 feet to 20,000 feet high.