Apollo Expeditions to the Moon: Chapter 14

Apollo Expeditions to the Moon

CHAPTER 14.4

THE MISSIONS OF UNDERSTANDING

The last three Apollo journeys were great missions of understanding during which our interpretation of the evolution of the Moon evolved. In July 1971 the first of these missions, Apollo 15, visited Hadley Rille at the foot of the Apennine Mountains. Apollo 15 gave lunar exploration a new scale in duration and complexity. Col. David R. Scott, Col. James B. Irwin, and Lt. Col. Alfred M. Worden looked at the whole planet for 13 days through the eyes of precision cameras and electronics as well as the eyes of men. Scott and Irwin spent nearly 67 hours on the Moon's surface, and were the first to use a wheeled surface vehicle, the Rover, to inspect a wide variety of geological features. Finally, before returning to Earth, they placed a small satellite in lunar orbit that greatly expanded our knowledge of the distribution and geological correlation of gravitational and magnetic variations within the Moon's crust.

Sampling scoop in hand, I go questing at Station 5 by Camelot Crater. At this point I had already collected a load of samples, and will shortly curve back to the Rover, off-camera at left, to unload. This was our second EVA, covering some 11 miles.

The varied samples and observations from the vicinity of Hadley Rille and the mountain ring of Imbrium called the Apennine's pushed knowledge of lunar processes back past the four-billion- year barrier we had seemed to see on previous missions. We also discovered that lunar history behind this barrier was partially masked by multiple cycles of impact melting and fragmentation. Nevertheless, the rock fragments we sampled gave vague glimpses into the first half-billion years of lunar evolution and into some details of the nature of the melted shell. Part of this view into the past was provided by the well-known "Genesis Rock" of anorthosite (a plagioclase-rich rock). In addition, we expanded our understanding of the complex volcanic processes that created the present surfaces of the maria. These processes were now seen to have included not only the internal separation of minerals within lava flows but possible processes of volcanic erosion and fracturing that could have created the rilles.

Our three traverses on Apollo 17 came very close to those we had preplanned, differing only because of unexpected findings. The fist run was a mile and a quarter to Steno Crater and back, in the 4 o'clock position above. The second was the longest, at 5 o'clock. At station 4 near Shorty Crater we found the orange soil (see here). The third run, at 12 to 1 o'clock, was more than six miles. The great fractured boulder shown here is on the slope near Station 6.

The Apollo 15 astronauts placed instruments on the Moon which, in conjunction with earlier missions, finally established a geophysical net of stations. Of particular importance was a net of seismometers by which we began to decipher the inner structure of the Moon. Correlations of information from these stations with other facts enabled us to interpret several major portions of the interior. The Moon's crustal rocks, rich in the calcium and aluminum silicate plagioclase, are broken extensively near the surface but more coherent at depths from 15 to 40 miles. The crust rests on an upper mantle 125 to 200 miles thick that contains the magnesium and iron silicates, pyroxene and olivine. From about 200 or 250 to about 400 miles deep, the lower mantle is possibly similar to some types of stony meteorites called chondrites. From about 400 miles to about 700 miles deep, the chondrite material appears to be locally melted and seismically active. There are also many reasons now to believe that the Moon has an iron-rich core from about 700 miles deep to its center at 1080 miles that produced a global magnetic field until only recent times.

Sampling by scoop was the main way we obtained the large numbers of small samples that provide good statistical information about the composition of the surface. That instrument to the left of Apollo 15's Jim Irwin is a gnomon. It provides a vertical-seeking rod of known length, a color chart, and a shadow - all useful for calibrating pictures.

Sampling tongs and coring tubes gave us other means of collecting special samples. The spring-loaded tongs below let us pick up small rocks and fragments without getting down on our knees or otherwise reaching way down with clumsy gloves. The core tubes were hammered down into the soil and then drawn back out and capped. They gave us a way to collect sections of soil that preserved the relative relationships undisturbed.

The geophysical station at Hadley-Apennines also told us that the flow of heat from the Moon was possibly two times that expected for a body having approximately the same radioisotopic composition as the Earth's mantle. If true, this tended to confirm earlier suggestions that much of the radioisotopic material in the Moon was concentrated in its crust. Otherwise, the interior of the Moon would be more fluid and show greater activity than we sense with the seismometers.

Finding orange soil near Station 4 on Apollo 17 at the time when oxygen was running low kept us on the jump. We dug a trench 8 inches deep and 35 inches long, took samples of the orange soil and nearby gray soil, drove a core tube into the deposit, sampled surrounding rocks, described and photographed the crater site in detail, and packed the samples - all in 35 minutes. The effort gave scientists a most unusual sample: very small beads of orange volcanic glass, formed in a great eruption of fire fountains over 3.5 billion years ago.

Rocks too big to bring back were studied where they were, described and photographed, and sampled by chipping pieces from their corners. If we could roll the rock over, as above, we could take soil samples underneath that had been shielded from the effects of solar and cosmic radiation.

We began with Apollo 15 to be able to correlate our landing areas around the whole Moon by virtue of very-high-quality photographs and geochemical x-ray and gamma-ray mapping from orbit. The x-ray remote sensing investigations disclosed the provincial nature of lunar chemistry, particularly by highlighting differences in aluminum-to-silicon and magnesium-to- silicon ratios within the maria and the highlands. By outlining variations in the distribution of uranium, thorium, and potassium, the gamma-ray information suggested that large basin- forming events were capable of creating geochemical provinces by the ejection of material from depths of six or more miles.

Possibly of equal importance with all these findings by Apollo 15 was the discovery - shared through television by millions of people - that there existed beauty and majesty in views of nature that had previously been outside human experience.

The mission of Apollo 16 to Descartes in April 1972 revealed that we were not yet ready to understand the earliest chapters of lunar history exposed in the southern highlands. In the samples that Capt. John W. Young, Comdr. Thomas K. Mattingly. and Col. Charles M. Duke, Jr., obtained in the Descartes area, the major central events of that history seemed to be compressed in time far more than we had guessed. There are indications that the formation of the youngest major lunar basins, the eruption of light-colored plains materials, and the earliest extrusions of mare basalts required only about 100 million years of time around 3.9 billion years ago.

The basaltic lavas of the lunar maria, like this sample photographed after return to a laboratory on Earth, tell much about the partial melting of the Moon between three and four billion years ago. This sample, 70017, is identified in its documentation photograph by the number of the counter and by the B orientation code.

Ancient beads of orange volcanic glass in the photomicrograph above have revealed secrets of the Moon's deep interior. Produced most likely by the partial melting of the lunar mantle, and discovered by Apollo 17, these beads are unusualy rich in such volatile elements as lead, zinc, tellurium, and sulfur. This indicates not only volcanic origin but also derivation from rocks possibly as deep as 200 miles in the Moon. Similar glass beads, green in color, were discovered by Apollo 15.

The extreme complexity of the problem of interpreting the lunar highland rocks and processes became evident even as the Apollo 16 mission progressed. Rather than discovering materials of clearly volcanic origin as many expected, the men found samples that suggested an interlocking, sequence of igneous and impact processes. A new chemical rock group known as "very high aluminum basalts" could be defined, although its ancestry relative to other lunar materials was obscured by later events that gave the cratered highlands their present form. The results of Apollo 16 have within them an integrated look at almost all previously and subsequently identified highland rock types. With this complexity comes a unique, as yet unexploited, opportunity to understand the formation and modification of the Moon's early crust and potentially that of the Earth.

The materials found in the Descartes region were similar to those sampled slightly earlier by Luna 20 in the Apollonius region. But there were significant differences in the aluminum content of debris representative of the two regions. Also there were differences in the abundance of fragments of distinctive crystalline rocks known as the anorthosite-norite-troctolite suite. After Apollo 15, this suite of rocks had been recognized as possibly being a much reworked leftover of at least portions of the ancient lunar crust. Luna 20 and Apollo 16 confirmed its great importance to the understanding of the ancient melted shell.