The views presented so far highlight the two dominant characteristics of the lunar surface: 1) the mare/highlands dichotomy, and 2) the abundance of circular features, nearly all being impact craters and basins but some of probable volcanic (caldera) origin. This next scene emphasizes both characteristics by showing an exaggerated false-color image of the front side of the Moon, taken by the multispectral vidicon onboard the Galileo spacecraft (described later in this section). The highlands, with their higher reflectances, appear in shades of red and orange and the lower reflectance maria are in blues and greens.
After Apollo, the Moon was not specifically revisited for 22 years, until an unmanned spacecraft, Clementine (funded by the Department of Defense), orbited it to conduct mapping studies between February 19 and April 21, 1994, using UV/Visible, Near IR, and High Resolution Cameras, Lidar (a radar altimeter), and a radar-like unit that transmits in the S-band radio frequency (2.293 GHz, or 13.19 cm wavelength). Look first at a topographic map of the front and far sides of the Moon, in which stereo data provided elevation differences from high resolution photographs and radar altimetry data, acquired by the Clementine spacecraft as it orbited the lunar surface.
As explained in the next paragraph, image data at various wavelengths can be used to map compositional differences in much the same manner as with multiband data obtained by Landsat and other terrestrial spacecraft. Below is an image of the 40 km wide crater Aristarchus that is found in the southeastern part of Oceanus Procellarum. The composite image is constructed from three ratio images (input bands in units of micrometers [µm]): 0.750/0.415 = red; 0.750/1.00 = green; 0.415/0.750 = blue. The dark gray surface is mare basalt; the reddish unit is ejecta from Aristarchus; the light blue is probably anorthositic rock (common in the highlands) exposed in the crater interior:
Among specialized products were more detailed maps of lunar topography (elevations) and global maps of the distribution of several chemical elements, such as iron (Fe) and titanium (Ti), determined by analyzing reflectance variations at 0.75 m m and 0.95 m m, where these elements absorb irradiation. The Fe map, reproduced below, indicates that, while iron is widespread, its maximum concentrations are in a broad region on the nearside, roughly coincident with the vast lava outpourings into Oceanus Procellarum and several other mare basins.
Most of the iron is actually in the form of FeO (reduced iron). The Clementine results when plotted as FeO are thus:
Clementine made a controversial discovery,
which, if proved correct, has major implications for humans returning to the
Moon. Its S-band radio unit detected abnormal reflections from the rim of a
huge crater (basin) around the lunar South Pole, in areas permanently sheltered from the
Sun's rays, as seen in this Clementine image:
These reflections could be due either to water ice or to some abnormal
surface roughness condition. If indeed ice is present in significant quantity, then this precious material (which supplies water needed for life and
also oxygen, when broken down by electrolysis) might allow us to establish a
manned base on the Moon. Transport of sufficient water and oxygen for long stays
is presently beyond our technical capability. This observation, and the intriguing
results of Clementine's compositional mapping, has led to a follow-on mission.
For the first time in 25 years, NASA has returned to the Moon with a small,
but versatile orbiting satellite, called Lunar Prospector. The entire mission
including data analysis is another effort by NASA to achieve high scientific
returns at relatively low cost (for LP, $65 million). Launched on January 6,
1998, by an Agena rocket, Prospector now is operating in a 100-km high circumlunar
polar orbit, from which it can map the entire Moon over a 3-year lifetime in
more detail than Clementine provided. Here is an artist's sketch of the spacecraft:
The spacecraft, just 1.4 m (4.5 ft)
high and 1.2 m (4 ft) in diameter, weighing 300 kg (660 lbs), receives its power
from solar cells that surround its exterior. An S-band radio sensor designed
to measure lunar gravity employing a Doppler effect procedure, sits on top of
a conical communications antenna (top). At the end of the 8-ft boom or mast
extending to the front left, a Magnetometer/Electron Reflectometer will conduct
improved measurements of the Moon's magnetic and particle fields. At the end
of the left rear mast is the Gamma Ray Spectrometer, which can detect these
elements: U, Th, K, Fe, Ti, O, Si, Al, Mg, and Ca. On the right boom are the
Alpha Particle Spectrometer that will measure radon gas to assess lunar radioactivity
as a clue to volcanic and other current events, and the Neutron Spectrometer
that will determine the presence of hydrogen and can detect water ice (its confirmation
from Clementine results is a major goal). A plot of the varying thermal neutron
flux, as determined by the Neutron Spectrometer, show a wide area of low neutron
counts (resulting from high neutron capture) associated with the maria on the
frontside and near the North Pole and higher counts in the highlands. Compare the distribution of Fe as determined by Lunar Prospector with the same coverage by Clementine shown above: Information on the distribution of radioactivity on the lunar surface was one goal of Lunar Prospector. This map shows that the element thorium is highest on the front side of the Moon, mainly in the highlands south of Mare Imbrium. The correspondence with the Imbrium Basin suggests that the basaltic lavas that filled it were enriched in Th. Note that corresponding highland surfaces on the farside are lower. The first results on Lunar Prospector's detection of ice
were released during an exciting press conference, held on March 5, 1998. Around
both poles, the neutron spectrometer has indeed detected neutrons, released
from hydrogen by natural cosmic ray bombardment of water ice in craters with
sheltered shadow zones. The drop in neutrons emanating from the Moon is clearly maximal around the poles as seen in this plot.
The initial estimate of the amount, to be determined
more accurately with later observations, is 30 to 300 million metric tons (recent thinking has raised the upper limit to perhaps as high as 3 billion tons). If melted,
this larger number would fill a "lake" 10 square kilometers in area
(3.1 x 3.1 km) to a depth of 10 meters. Surprisingly, the North Pole region
contains about 50% more ice than its southern counterpart. The source of the
water ice is probably residues from cometary bodies that impacted the polar
regions, forming craters but allowing much of the comet mass to survive embedded
in the target. The implications are encouraging for future exploration of the
Moon, to the extent that we can establish and occupy a manned base facility
over extended time because of the availability of vital water (for consumption
and as a source of hydrogen, suitable as a fuel). However, landing in polar
regions is technically more difficult but doable. The dream of a permanent observation
post on our satellite is now much more feasible. More details on Lunar Prospector
are given at the National
Space Science Data Center Web site and the Mission Management Home Page
at NASA Ames Research Center. As NASA
accrues and releases data and maps, we will place them in the Web version of
this Tutorial and in later CD-ROM versions. The latest mission to the Moon is ESA's SMART-1 spacecraft. Launched September 27, 2003 as Europe's first venture in exploring beyond Earth, the spacecraft, using a novel electric propulsion system, will proceed slowly to the Moon with arrival near the end of 2004. This low cost satellite will orbit the Moon gathering information about surface composition.
Now, to summarize what we have learned about the Moon from nearly 40 years of space exploration: An excellent synopsis of the major
scientific information gained from, or supported by, the Apollo missions is
found at this
10 Top Achievements from Apollo site. 19-23: Before
you look at the site link, and based on what you have read on this page, list
or mentally note what you think were the principal findings of the Apollo program?
ANSWER One of these top achievements is/are model(s) of the lunar interior. We close with this diagram that is one of the early popular versions (Dr. Anthony Ringwood, of Australia). By now, you should have learned enough to explain the meaning of each major layer in the outer part of the Moon (note: the quartzo-feldspathic layer at the top proxies for the felsitic rocks typified by sample 12013). A similar but more recent model assumes the outer half of the Moon melted - forming the so-called "magma ocean" - early in its history and then underwent differentiation to produce the present general layering: We begin the review of the Moon's history or evolution by showing first a chart that summarizes what was known prior to Apollo: These salient points were determined both from Earth-based telescope observations and from lunar orbiters and landers. The presence of a lunar soil or regolith was confirmed by the Surveyors. The next chart encapsulates the main information on the time-marked evolution of the Moon arrived at from all sources utilizing both Apollo human observations and lunar sample analyses by Principal Investigators and other scientists: In this model, a feldspar-rich moonwide crust forms from the magma ocean. Two periods of intense bombardments by asteroids, mini-planets, and comets produce major basins which tossed materials from the highlands crust as first and then also mare surface over most of the lunar surface. These formed eventually consolidated deposits of large to small blocks and fragments making up interleaved "ejecta blankets" from 100s of meters to several kilometers thick. Off-loading and other processes mobilized subsurface rock (largely basaltic [high Fe, Mg, Ca and low Si] in composition) that melted and invaded the surface filling the maria and the interiors of larger craters. Cratering began early in lunar history, reached a maximum around 4 billion years ago, and has tapered off since. This next chart describes the changes and conditions associated with the Moon's outer reaches at the outset of the main period of basaltic lava extrusion: The ideas expressed in these charts can be presented in a different way, as shown in this timeline chart (again, courtesy of H.H. Schmitt): Jack Schmitt has a most interesting Internet site in which he uses various illustrations to show the progressive development of the Moon from its earliest history through the late stages of basaltic emplacement around 3 billion years ago. The site, accessed here, is in .pdf format, which requires Acrobat Reader. But, WARNING: you may experience some trouble with your computer - it tends to operate slower after signing off, or may hang up - especially if you try to print onscreen images or download them to a file. However, his figures on lunar evolution take into account much of the research done in the last 30 years, so it is worth a try to move through this site - nothing fatal occurred when the writer (NMS) went through each .pdf page. To entice you to work through his sequence, we put up here the last (and most complicated) of his model diagrams which have added mare basalt emplacement from a period ending 2 billion years ago. By going through his Lecture 8, you will see the evolutionary steps taken to get to this stage (after which the major changes are associated with small to large impacts). The origin (formation) of the Moon has always been a prime topic for conjecture and scientific insight among selenologists. Four main schemes for lunar origin existed before the Apollo program brought back lunar samples. One view had the Moon form from leftover debris as the Earth itself built up by aggregation. A second idea holds that debris which makes the Moon was tossed off the Earth in the latter's early days when our planet was spinning (rotating) much faster. A third proposal claims the Moon is a captured small planet once more distant from Earth.
The fourth ascribed its formation to material wrenched from the Earth's outer crust by a massive impact leaving the Pacific Ocean Basin as a scar equivalent to a huge crater (a model that would need revision and probable discounting after the ideas of plate tectonics and continental migrations took hold).None of these hypotheses adequately explains the observed balance between the combined angular momenta of the Moon and Earth which theory indicates remains constant since the two bodies became linked. Despite its greater rotation speed in the first few hundred million years of Earth's existence, this still is not enough to foster co-accretion. Nor is the speed sufficient to fission off the debris. But, that spin was too fast to allow capture of a passing body.
This leaves the impact model which became increasingly fashionable in the 1970s. (Before then, the writer around 1963 developed a model in which a massive impact produced a huge dent in the outer Earth but the Moon itself came [at least partly] from debris spalled off the Earth's opposite side by the internal shock waves reverting to rarefactions that caused material there to split off and be hurled into space, thereafter accreting as the Moon. I abandoned this idea for two reasons: 1. I could not entice a colleague who knew the appropriate math to help me with the sophisticated calculations needed to prove this hypothesis; and 2. my own calculations hinted that the energy of spallation from an impact not so large as to destroy Earth was insufficient by several orders of magnitude. I also conjectured that this direct impact could have broken the [smaller] protoearth into pieces, most of which reorganized as the Earth but some formed a second body kept in tow by gravity). By the 1970s, with the Apollo data now in hand, impact had gained favor as an integral part of lunar formation. Several impact models has since been proposed. All are constricted by the two Apollo observations that the Moon is deficient in iron (no, or a small, iron core) and by the low percentages of the volatile elements sodium and potassium. That the Moon was derived from an impact of giant magnitude on the early Earth is supported by the strong similarity in oxygen isotope compositions in the two planetary bodies. The first model was developed by scientists associated with Harvard University. But, their head-on collision model has since come up with energy and compositional problems. The most recent variation on the general impact model is illustrated by the succession of steps shown in this diagram which is the result of a computer simulation of a huge impact into the protoEarth but oriented at that moment so as to glance against or sideswipe the outer layers of an Earth whose crust had not yet fully developed. Look at this computer simulation of such an event: The model and some variants, collaboratively developed by scientists at the Southwest Research Institute (William Ward and Robin Canup; others) and the University of Arizona (A.G.W. Cameron, Jay Melosh, William Hartmann; others), considers the impact to have occurred late in the formational history of the Earth, but probably prior to the differentiation that formed an early terrestrial crust. At this time, a part, perhaps much, of the outer Earth may have been molten.A Mars-sized asteroid or small planet (about 10% of the present terrestrial mass) struck the Earth at a glancing angle. Although the Earth survived total disruption, much of the outer shell on one side was tossed into space, but held to the Earth by its larger gravity. The fragments in the ejecta plume are affected by rotational forces from Earth and within 24 hours have organized into a near circular orbit. In time these fragments (whose composition mirrors that of the primitive Earth) began to collide until the Moon was built up to its present size, large enough for it to have melted and reshaped into a sphere, developing an anorthositic crust. The Earth, still forming, healed its "wound", resumed during subsequent remelting into a near-sphere, and went on to fully differentiate into the crust, mantle, and core that has survived to the present day. The advantages of the swiping impact model are these: 1) a proper relation between Earth-Moon angular momentum comes out of the calculations; 2) the high heat of such an event boils off all water and some of the volatile elements sodium and potassium; 3) the similarity of refractory element composition between Earth and its satellite is explained; 4) only the outer mantle and any early crust are involved; 5) temperatures in a glancing event would have been higher (up to 18000° K); 6) a larger fraction of the Earth target would be ejected into orbit; 7) differences in composition could be due to incorporation of some of the impactor body, which likely varied somewhat from Earth. The resulting Moon may have been much closer to Earth, perhaps as near as 29000 km (18000 miles). This first Moon would have appeared to occupy much more of the sky than today. It is now known that the Moon is receding at a rate of about 2.4 cm/year (around an inch), to its present average distance from Earth's center of 384000 km (240000 miles). Extrapolating back in time for 4.5 billion years yields this early proximity value (which, however, may exceed the Roche Limit - the closest distance two large planetary bodies can be without one at least being disrupted). As mentioned earlier in this Section, NASA and other space agencies have started planning a return to the Moon under the mandate given by Pres. George W. Bush (which, unfortately, could be scaled back or scratched by his successors). A new, more versatile Space Transportation System will be needed, and calls for proposals are now out. The first landings will probably be more like the Apollo ones but in time it is hoped to establish a permanent (or at least long term) lunar base where astronauts can subsist and explore for extended stays. Four things are essential in making a safe, flexible base: 1) a means of replenishing oxygen; 2) water; 3) souurce(s) of power; and 4) suitable shielding from extralunar radiation. Oxygen, in principle, is extractable from the lunar silicate minerals but a reliable, practical means of obtaining this is yet to be worked out (in May 2005, NASA issued a Call for Proposals for innovative solutions). Water can, in part, be recycled from sources (such as astronaut urine) brought with the explorers. But, if substantial water is found near the polar regions, extraction should not be too difficult - thus the base would likely be located at high latitudes. Power requirements can be met with nuclear generators and/or with efficient solar arrays.
Shielding may prove difficult since the base units (presumably separate from the landing craft) need to be of light materials. Still, growing experience should aid in selecting radiation-absorbing outer components of the base. There is another strong argument for selecting polar regions for the base besides the water potential. Placing astronomical observatories at either or both poles would allow almost ideal observing conditions (better than the present Hubble Space Telescope since systems and components would be state-of-the-art). Nearly all of both celestial hemispheres would be accessible, whereas locating an observatory at lower latitudes would have some light interference from earthshine. But exploration would be curtailed somewhat by dependence on a polar station. Establishment of a Moon base will be a giant step in mankind's renewal of space exploration. Among its benefits, it could serve as the launching site for a trip to Mars. On September 18, 2005 NASA made its first public announcement of how its approach to how the Moon landings (and probably Mars later) will be made. There is a striking similarity to the Apollo approach in that landing craft will be on a large multistage rocket, with the main thrust section falling back to Earth after putting the manned vehicle on its journey. This vehicle and a companion for sending material to build a lunar base shown here, with other existing vehicles side by side for comparison: A closer look at the rocket that would carry a crew of into lunar orbit is pictured here. At its tip is the Crew Exploration Vehicle (CEV). This panel diagram shows the sequence of events or stages now envisioned in the current plan for renewed lunar exploration: As the lunar trip gets underway, the Departure Vehicle (jettisoned after burn) and Lander group have mated with the Crew Exploration Vehicle (CEV) capsule, as shown here:
This brings us to two general topics: first, a summary of lunar evolution and then a survey of the Moon's possible origins. The first and second topics are both included in an Internet site that considers an Apollo-based Evolution of the Moon. written by Harrison H. ("Jack") Schmitt. Some of the ideas and illustrations from that site are used here.
Return to the Moon: The Master Plan
The Service module remains unmanned during the days spent on the lunar surface. The lunar landing craft, housing all 4 astronauts (but, eventually, able to support 6 astronauts), as envisioned on the Moon's surface, is displayed here:
The landing units descend to the surface, much like during the Apollo program, with the larger unit consisting of a braking rocket and fuel. After the lunar stay is ended, the upper crew unit fires its rocket to put it into orbit and eventual docking with the service unit.
The schedule calls for the CEV system, without the Lander units, to be ready to fly by or shortly after 2010. It will replace the phased-out Space Shuttle program and will be NASA's means of servicing the International Space Station. This will provide extended experience in CEV use up to the first Moon landings.
Ambitious and exciting as this master plan appears to be, there are many obstacles that could delay or even cancel its execution. The earliest readiness date for a landing is set at 2018. A crew of 4 will descend on the CEV and stay for (at least) a week. Over time, the stay will be longer as the astronauts build a lunar base capable of sustaining the mission for weeks to months. This will provide the needed experience for prolonged missions that would take place on Mars at a later time. When the crew returns to Earth in the detached capsule, it will have the capability of landing either on land or at sea. If no serious damage occurs, the CEV can be used up to 10 flights.
An estimated cost for the first landings is $104 billion dollars. This is likely to be exceeded, since as a rule, such estimates are nearly always low. Additional monies must be appropriated if the undetaking is to happen. Budgetary deficits (exacerbated in 2005 by the Katrina disaster) can affect the schedule. Some monies will become available for the program after the Space Shuttle program is retired in 2010. But, NASA, and even its critics, together recognize that a Moon exploration resumption followed by Mars exploration (which would gain from the lunar experiences) may be vital to keeping the American space program healthy enough to press forward, rather than wither and diminish by loss of dedicated personnel.
To close this subsection, there are literally thousands of
informative and often exotic images of the Moon, taken by various remote sensors. Perhaps none can better convey the human emotions of having triumphantly landed astronauts on the Moon than this heart-throbbing photo taken by Michael Collins from the CSM of the about-to-dock LM containing Neil Armstrong and Edwin "Buzz" Aldrin, with Mother Earth looking so distant in the background, yet as history shows returned to successfully by these intrepid Apollo 11 explorers and ten others who set foot on the Moon's surface (watched over by five comrades in orbit) in subsequent missions:
Two very readable popular accounts of lunar exploration are The Moon Book by Bevan M. French, 1977, Penquin Books, and Lunar Science: A Post Apollo View by S. Ross Taylor, 1975, Pergamon Press. Reluctantly, we must take leave of our local satellite to begin an impressive
journey through the Solar System. We start with the two innermost planets–Mercury
and Venus. 
Primary Author: Nicholas M.
Short, Sr. email: nmshort@ptd.net