Page 295 - NS-2 Textbook

290 NAUTICAL SCIENCES

Many smaller craters are ahnost certainly the result
of early volcanic activity, as gases and dusts escaped
from the Moon's interior. Some of these can be compared
to volcanic craters on Earth, formed when the surface of
the Earth collapses into an underlying cavity from which
lava has flowed. (Often a central peak remains in the cen-
ter of such craters-for example, the island in Crater
Lake, Oregon.)
The most conspicuous crater is Tycho, in the Moon's
southern hemisphere. It is easily seen when the Moon is
full. Tycho has a great system of rays, which radiate as
far as 1,500 miles out from the edges of the crater. The
crater Copernicus has a similar system. Rays are thought
to be fine surface material that was splattered out of the
most recent impact craters when they were formed.
Some rays are chains of small craters, created by the ex-
plosive ejection of material during the formation of the
main crater.
1n late 1996 a spectacular discovery of possible water Rilles are cracks in the lunar surface similar to shallow, meandering
river beds on the western deserts of the United States.
ice on the Moon was announced by U.S. scientists. Radar
signals originated by a Defense Department satellite
called Clementine indicated the presence of the ice in a tains are very rugged, since they are not eroded by wind,
large shady crater near the Moon's south pole, where the water, or ice.
temperature is about -387 degrees F (-197 degrees C). A large telescope will also show that the Moon's sur-
The ice is thought to have been deposited there by a comet face is covered with many cracks, called rilles. They are
impact in the distant past. If it does in fact exist, the ice similar to shallow, flat-bottomed river beds on Earth. There
could be used by fuhrre human explorers as a source of seems to be no connection between rilles and other surface
both potable water and fuel. The Clementine satellite was features because they sometimes extend hundreds of
the first U.s. Moon exploration effort since the last Apollo miles, uninterrupted by mountains} valleys, or craters.
mission in 1972.
As a follow-up to Clementine, the LlInar Prospector
satellite was placed in orbit around the Moon in January
1998. It was equipped with a neutron spectrometer that MOON QUAKES
could detect the presence of hydrogen plus nine other el-
The Apollo 11 astronauts set up a moonquake detector at
ements including iron, titanium, and ahuninum. It did in
the Sea of Tranquility. This detector was an instrument
fact detect large amounts of hydrogen at the Moon's poles,
called a passive seismol1lete)~ a device that transmits re-
thus supporting the possibility of water there. After com-
ports of tremors on the Moon's surface. Scientists had
pleting its mission in July 1999 it was intentionally
expected the Moon to experience quakes similar to our
crashed into a crater at the lunar south pole, to try to kick
earthquakes. But they found out that a moonquake
up enough material to prove the presence of water. How-
causes the Moon to vibrate in an entirely different way.
evel~ scientists observing the crash site from more than
twenty observatories around Earth and the HlIbble Space Earth tremors are severe only for seconds: beyond the
rather small area of the quake, only the finest instru-
Telescope could not detect any signs of the impact. Scien-
ments can record them. Moonquakes, however, cause
tists are presently drawing up a detailed mineral map of
the whole Moon to vibrate for extended periods.
the Moon's surface based on the data gathered by LlInar
In only three weeks after its placement, the Tranquil-
Prospector.
ity Base seismometer registered twenty-five different
tremors on the Moon's surface. Fourteen of them were
from avalanches of lunar rocks falling down the slopes of
MOON MOUNTAINS AND RILLES
crater walls. When the Apollo 12 lunar module was pur-
The Moon's mountain ranges lie in great arcs bordering posely crashed back on the surface of the Moon in 1969,
the circular maria. Some of their peaks are as tall as the the shock set the whole Moon vibrating for nearly an
highest Earth mountains. They are concentrated in the hour. It will take many years and many seismograph sta-
Moon's southern hemisphere. With peaks sometimes ris- tions to explain this and to find out how the interior of
ing more than 20,000 feet above the plains, lunar moun- the Moon is struchued.