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Pre-Apollo Exploration of the Moon

Without question, the greatest part of early remote sensing investigations of the planets, as expected from its proximity, centered on the Moon. (For a quick synopsis of the Moon's general characteristics, read through this review by Bill Arnett: nineplanets/nineplanets/luna.html). * The first telephoto image of its surface occurred in the 1860s. Prior to the space program, almost all studies used the telescope. At first, emphasis was on optical observations (and photographs) within the visible spectrum. Among the first measurements was the Moon's albedo, the ratio (stated as a percentage) of the light reflected to the light incident on a surface. (The Earth's albedo is 39% on average [with slight fluctuations due to varying cloud cover] and Venus falls between 70% and 90% [from strong reflections of sunlight by a thick planetwide cloud cover].) By comparing albedos measured on possible terrestrial counterparts, the lunar albedo offers insights into the composition, texture, and structure of its surface materials. At full Moon, dark areas known as maria have albedos of 5% to 8%, while brighter areas, generally coincident with elevated terrain called highlands or (in Latin) terrae, range between 9% and 12%. These albedo differences are evident in this full Moon photograph, taken from the Lick Observatory in California (M. stands for Mare in the labels; A refers to Apollo Landing Sites [in red]; and major craters show in yellow):


B/W photograph of the full Moon taken from the Lick Observatory in California.

19-2: How many of the Apollo landings were in the maria; in the highlands? ANSWER

Once established, the trick is to interpret these albedo values. We can measure the albedos of likely surface materials in the laboratory, where we can vary the textures and illumination conditions. In this way, lunar scientists postulated that the maria were some type of volcanic material, most likely basalt. The highlands were more enigmatic. Several other volcanic types could produce albedos in the proper range, but uncertainties about particle size and other variables inhibited definitive identification. The first unmanned landers, the five Surveyors, resolved the problem. The answer was that the highlands included a different group of rock types, of which anorthosite was prevalent, containing larger amounts of light-colored feldspars. Investigators judged that the albedo values for the maria and highlands were consistent with scattering by granular or fragmental textures associated with unconsolidated surface materials (some thought the craft would sink into loose, almost powdery rock debris, which, fortunately, didn't happen).

19-3: What would have happened to the Apollo program if the lunar surface indeed had a thick cover of powdery dust? ANSWER

The total intensity or brightness of reflecting surfaces at specific points or areas varies with the phases of the Moon (progressive waxing and waning due to illumination changes, depending on relative positions of the Sun, Earth, and Moon). The lunar photometric function, a plot of intensity versus phase angle, represents the intensity mathematically. The degree of polarization of light from the Moon also shifts with the phase angle and can show considerable variations in different regions of the Moon, again related to composition and textural changes.


19-4: What are the vernacular names applied to the phases of the Moon? ANSWER

By selecting certain major features on the Moon's front face as reference points, one notes that the same face of the lunar sphere is seen during the entire period of its revolution around the Earth. This curiosity is the result of the Moon's sidereal revolution period of 27.3 days being equal to its axial rotation period. In effect, a center point on the lunar frontside is seemingly locked on the Earth, i.e., an imaginary line perpendicular to the lunar surface at that point would extend towards the Earth's center at all times during the Moon's orbital transit. This remarkable condition describes a state of captured or synchronous rotation. The synodic lunar month is 29.5 days. This increase in time by approximately 2.2 days results from the combined relative motions of both Earth and Moon forward in orbit around the Sun during which the illumination angle with respect to the Sun will be shifted; the Moon thus requires extra time for the first sliver of reflected light at New Moon to be seen from a reference point on Earth to compensate for the angular advance of the Earth-Moon system since the previous lunar month began.

19-5: What is the difference between rotation and revolution of a planetary body? ANSWER

Although the Moon is nearly a sphere, one side always facing Earth, inspection of photographs taken on different dates discloses an apparent shift of major landmarks relative to reference point at the lunar limbs. Mapping demonstrates that almost 60% of the total surface (front and back) of the lunar sphere has been seen from Earth through telescopes. Because this seems to be a "wobbling" or "rocking back and forth", the term libration (from a Latin word for "balance") is applied to the phenomenon. Librations result in part from apparent (optical) displacements that bring more of both equatorial and polar regions into view. The motions involve variations in angular velocity of the Moon's revolution in elliptical orbit, tilt of its rotational axis relative to its orbital plane (inclined at 5°09' to the ecliptic), and parallax effects for different observation points on Earth. There is also a real libration owing to varying gravitational pull by Earth as the Earth-Moon distance varies and by the Moon's departure from perfect sphericity (it has an equatorial bulge of +2 km resulting from gravitational attraction to Earth).

19-6: Most people live in the Northern Hemisphere of Earth. It is a little known fact to most such individuals that the Moon is "upside-down" in the Southern Hemisphere, with Tycho near the top and Mare Imbrium towards the bottom. Can you explain why? ANSWER

Even before the pre-Apollo and Apollo programs, scientists examined the Moon in the multispectral mode (we show a recent example on page 19-6). The simplest approach was to use photofilters that pass limited spectral ranges. Thus, they examined the Moon in the UV, blue, green, red, IR, and other spectral intervals. This technique brought out variations in color shades that appeared to distinguish different surface units. Some maria tended to have stronger blue components, while others were more reddish. Differences between maria and highlands were accentuated. These subtle variations related to the influence of elements, such ascalcium, iron, and titanium, on the behavior of reflected light at different wavelengths. Some investigators made more exacting measurements by passing the light through a spectrometer, which provided semiquantitative estimates of variations in percentages of several of the common elements. Higher values for iron, magnesium, and calcium supported the surmise that lunar rocks were most probably of basic igneous composition (silica-low, but Fe/Mg-rich).

The astronauts orbiting the Moon described its color as dominantly medium to dark grays with tan overtones. We have obtained many pictures of the lunar surface, starting with unmanned pre-Apollo missions, then through numerous Apollo shots, and, afterwards, in a series of images sent back by the Galileo spacecraft as it passed Earth enroute for its rendezvous with Jupiter, and then by the Clementine orbiter (page 19-6). A typical natural color image of the full Moon, taken from the Apollo 17 Command Module, shows much of the farside (some have called this the "backside" but its connotation rules this out) and part of the eastern limb visible from Earth. Visible maria include Smythii, Marginis, Crisium (near circular), Fecunditatis, and part of Tranquillitatis (site of first Apollo [11] landing).


Natural color image of the full Moon taken from the Apollo 17 Command Module.

19-7: Considering the upper diagonal half of the above full Moon image to be typical of the farside, how does it differ from the full Moon view seen from Earth? ANSWER

Prior to the landings, as early as 1946, radar had provided more images of the full Moon and selected regions. Sent from Earth transmitters, signals returned as polarized backscatter, in which we measure time delays and Doppler frequency shifts. Here are three mosaics in Mercator (top), Lambert Conformal, and Polar Stereographic projections made from reflections of 3.8 cm pulses sent by the Haystack Observatory at the Massachusetts Institute of Technology:


Illustration of three mosaics of the Moon - Mercator, Lambert Conformal, and Polar Stereographic.

Infrared scanners, operating through telescopes, can acquire Earth-based thermal emission images. The ideal time to sense the full Moon is during a total eclipse:

Illustration of Earth-based thermal emission images of the full Moon.

Under this condition, bright areas called "hotspots" appear, many of which, correlate with large lunar craters. These craters often contain dark lavas that absorb solar radiation (for more on this black body effect, see Section 9) and re-emit it at higher radiant temperatures. During such times, observers also look for "lunar transients" (localized short-lived bright, often reddish glows visible to the eye) that some believe are evidence of volcanic activity and other thermal phenomena. Some people claim to see these volcanic events, even when the Moon is normally illuminated, particularly in the shadowed areas (dark phases).

19-8: Think of another possible explanation for these glowing transient phenomena. ANSWER

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Primary Author: Nicholas M. Short, Sr. email:


Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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