Post-Apollo Lunar Exploration Part-3 - Remote Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Post-Apollo Lunar Exploration Part-3

Summary of Lunar Exploration Science

This subsection includes both a review of the sequential or evolutionary history of the Moon and a survey of ideas pertaining to the origin of our lunar satellite.

The list you created - and that in the answer - certainly does not tell the whole story. Here are some key items that complement the listing:

1. Probably the top achievement, by consensus, is this: that human intellect and commitment combined to make the technical feat of astronauts reaching and landing on the Moon - and RETURNING safely - six times stand out as perhaps the greatest accomplishment of mankind to date.

2. Close behind is the singular success in the face of dire adversity of bringing back the Apollo 13 astronauts to Earth after the near fatal catastrophe that almost doomed them to a dire ending in outer space.

3. The many orbiting and lander missions leading up to Apollo 11 proved the value of unmanned flights designed to gather critical data as part of an overall exploration program.

4. The question of the Moon's origin has been settled by analysis of lunar samples, which disclosed a strong kinship with Earth such that the Moon had to be derived from its terrestrial parent - most likely from a huge impact event (see below).

5. Impact and volcanism, which dominate the lunar surface, are adjudicated to be among the most fundamental of planetary processes.

Another of these top achievements is/are model(s) of the lunar interior. We cite this diagram that is one of the early popular versions (Dr. Anthony Ringwood, of Australia) of the Moon's structure. 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:

Model of the Moon in which it melted to considerable depths (left) and then cooled and solidified the differentiated layers shown on the right; courtesy H.H. Schmitt.
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.

We begin that review of the Moon's history or evolution by showing first a chart that summarizes what was known prior to Apollo:

Pre-Apollo Lunar Facts.

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:

Major findings attributed to the Apollo program.

The ages cited in the above chart are determined from relative cratering frequencies, calibrated by radiometric dating of the Apollo samples. 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:

Changes at onset of main period of mare basalt emplacement.

The ideas expressed in these charts can be presented in a different way, as shown in this timeline chart (again, courtesy of H.H. [Jack] Schmitt):

Timeline chart for major lunar events.

This diagram has a deceptively inconspicuous word, "cataclysm", which calls attention to a major defining event in lunar history. First proposed in the early 1970s by Dr. Gerald Wasserburg of Cal Tech, the Lunar Cataclysm (also called the "Late Heavy Bombardment [LHB]), occurred during a hundred million year span centered around a 3.9 billion year age. Wasserburg found that glasses in lunar breccias had prevailing radiometric ages (Argon-40/Argon-39) in that time frame. These glasses are best interpreted as resulting from impact processes. He, and later workers, attributed the impacts that caused large basins such as Imbrium, Serenitatis, and Crisium, together with many of the smaller craters, to swarms of asteroids striking the lunar surface. (Later work on lunar meteorites [found mainly in the ice surfaces of the Antarctic], which represent sampling over the entire lunar terrains, have confirmed this clustering of ages around 3.9 b.y.) The bulk of the asteroids are believed derived from the Asteroid Belt between Mars and Jupiter; these asteroids are postulated to have been perturbed out of their prevailing orbits by a "resonance" process related to Jovian gravitational influence.

The LHB has a strong corollary implication for the Earth itself. The asteroids would also have hit Earth during the bombardment period. This would have had a profound effect on the early crust of Earth, but since almost all of that crust has since been destroyed by subduction and erosion the evidence for a corresponding terrestrial LHB has been erased.

The Moon has obviously changed its appearance over time. This set of three paintings depict (left to right) 1) the surface following the major bombardment up to the present, 2) the surface after the Imbrium lavas were emplaced, and 3) the early lunar surface before lavas entered the impact basins.

Three stages of lunar evolution.

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. 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. To entice you to work through his sequence, we put up here the last (and most complicated) of his model diagrams which has 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).

Cross-section through the Moon showing its state from a beginning 4.6 billion years ago to the last stages of mare basalt emplacement 2 b.y. ago; interpretation by Dr. H.H. Schmitt.

Another summary of lunar history has been put online by Prof. Stephen Dutch of the University of Wisconsin-Green Bay. From his web site we have extracted this figure and key:

Steps in the developmental evolution of the Moon.

A. Initial Accretion of the Moon, probably from debris launched into Earth orbit by a mega-impact.

B. In the last stages of accretion, so much heat accumulates that the outermost 100 km of the lunar crust melts to form a magma ocean. Light feldspars rise to accumulate an anorthosite crust.

C. Late impacts excavate giant basins. One of the earliest is the South-Pole-Aitken Basin.

D. Mare Nectaris and other bains form.

E. Mare Imbrium forms.

F. Mare Orientale forms

G. Mare basalts erupt and flood many of the impact basins.

H. Since 3000 Ma, only a few large rayed craters like Tycho and Copernicus have formed.

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.

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. This diagram neatly summarizes this idea and the key features of subsequent lunar evolution:

The birth and development of the Moon.

All lunar genesis models are constricted by the two Apollo observations that the entire Moon is deficient in iron (Fe can be high in some mare basalts, but is very low in its interior, with 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:

Computer-simulation of the formation of the Moon by a giant impact on Earth.

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's outer shell(s)) 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 its organization 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).

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:

The Apollo 11 LM approaching the CSM, with Earth above the horizon.

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.