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After this time of volcanism, the Moon cooled down, and has since been nearly unchanged, except for a steady
rain of "hits" by metreorites and comets. The Moon's surface is charcoal gray and sandy, with much fine soil. This powdery
blanket is called the lunar regolith, a term for mechanically produced debris layers on planetary surfaces. The regolith is
thin, ranging from abour 2 meters on the youngest maria to perhaps 20 meters on the oldest surfaces in the highlands.
Unlike Earth, the Moon does not have moving crustal plates or active volcanoes. However, seismometers planted
by the Apollo astronauts in the 1970s have recorded small quakes at depths of several hundred kilometers. The quakes are probably
triggered by tides resulting from Earth's gravitational pull. Small eruptions of gas from some craters, but the Moon does
not have a magnetic field resembling Earth's.
A suprising discovery from the tracking of the Lunar Orbiter spacecraft in the 1960s revealed strong areas
of high gravitational acceleration located over the circukar maria. These mass concentrations may be caused by layers of denser,
basaltic lavas that fill the mare basin.
In 1998, the Lunar Prospector spacecraft team reported finding water ice at both poles. Comet impacts deposited
water on the Moon. Some of it migrated to very dark, very cold areas at the poles.
Much remains to be learned about our Moon. Researchers continue to study the samples and data returned by
Apollo and other missions, as well as lunar meteorites.
NASA'S HUBBLE LOOKS FOR MOON RESOURCES.
NASA is using the unique capabilities of the Hubble Space Telescope for a new class of scientific observations
of the Earth's Moon.
Hubble's resolution and sensitivity to ultraviolet light have allowed the telescope to search for important
oxygen bearing minerals on the moon. Since the moon does not have a breathable atmosphere, such as ilmenite (titanium and
iron oxide), may be critical for a substained human lunar presence. Ilmenite is a potential source of oxygen for breathing
or to power rockets.
The new Hubble observations are the first high-resolution, ultraviolet images ever acquired of the moon.
The images provide scientists with a new tool to study mineral variations within the lunar crust. As NASA plans futureexpeditions
to the moon, such data, in combination with other measurements, will help ensure the most valuable sites are targeted for
robotic and human missions.
"These observations of the moon have been a challenging and highly successful technological acheivement
for NASA and the Hubble team, since the telescope was not originally designed for lunar observations."
Hubble's Advanced Camera for Surveys snapped ultraviolet and visible light images of known geologically
diverse areas on the side of the moon nearest Earth. These include the Aristarchus impact crater and the adjacent Schroter's
Valley. Hubble also photographed the Apollo 15 and 17 landing sites, where astronauts collected rock and soil samples in 1971
and 1972.
Scientists are comparing the properties of the rock and soil from the Apollo sites with the new Hubble images,
and the Aristarachus regions, which neither humans nor robotic spacecraft have visited. The Hubble observations of Aristatachus
crater and Schroter's Valley will help refine researcher's understanding of the diverse,scientifically interesting materials
in the region and to unravel their full resource potential.
The potential existence of some unique varieties of oxygen-rich glassy soils in both the Aristarchus and
Apollo 17 regions. They could be well-suited for visits by robots and human explorers in efforts to learn how to live off
the land on the moon.
The Moon of course has been known since prehistoric times. It is the second brightest object in the sky
after the Sun. As the Moon orbits around the Earth once per month, the angle betweenthe Earth, and the Moon and the Sun changes;
we see this as the cycle of the Moon's phases, The time betweensuccessive new moons is 29.5 days (709 hours), slightly different
from the Moons orbital period since the Earth moves a significant distance in its orbit around the Sun in that time.
Due to its size and composition, the Moon is sometimes classified as a terestrial "planet" along with Mercury,
Venus, Earth and Mars.
The Moon was first visitied by the Soviet Spacecraft Luna 2 in 1959. It is the only exraterrestrial body to have been
visited by humans. The first landing was on July 20, 1969, the last was in December 1972. The Moon is also the only body from
which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft
Clementine and again in 1999 by Lunar Prospector.
The gravitational forces between the Earth and the Moon causes some interesting effects. The most obvious si the tides.
The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weeker on the opposite side.
Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line towards the Moon.
From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite.
The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher. And because the Earth
rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides
per day. (This is a greatly simplified model; actual tides, especially near the coasts, are much more complicated.) But the
Earth is not completely fluid, either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly
beneath the Moon. This means that the forces between the Earth and the Moon is not exactly along the line between their centers
producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotation energy from
the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/centry and raising the Moon into a higher
orbit by about 3.8 centimeters per year.
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