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Descartes Surprise

from the Apollo Lunar Surface Journal
edited by Eric M. Jones and Ken Glover.
Copyright © 1995 by Eric M. Jones.
All rights reserved.
Last revised 14 September 2006.

Volcanics in the Lunar Highlands?

If political support for Apollo - and the funding that goes with political support - had continued at a high level into the 70's, by the time of Apollo 16 NASA might have been thinking seriously about establishing a permanent base camp on the Moon. Perhaps work would have been underway on an unmanned, cargo-only version of the LM. Such a vehicle would have been essential to the establishment of a camp and to astronaut visits longer than three or four days. NASA might not have been in a position to pick a site for a permanent base after only four landings and after only one three-day mission, and there still would have been a great deal of preliminary work to be done on Earth and on the Moon before anyone would have felt comfortable about starting construction. However, because Apollo 15 had gone so well, there would have been little hesitation about moving on to the next level of complexity. Although the second J-mission crew surely would have done some exploring and sample collection, they also might have been asked to test construction tools and procedures.

Of course, such a mission was never in the cards and, indeed, by the time of Apollo 16, NASA was already folding up its lunar tent. The Apollo assembly lines had been shut down and NASA had canceled three missions for which hardware had already been built. After Apollo 16 there would be only one more. Because Apollo was about to end, there was no new equipment to test, no new procedures to try. Nonetheless, with proven equipment and procedures at hand, the last two missions presented splendid opportunities to clear up some of the major uncertainties that remained in our understanding of the Moon as a planet.

If we had a picture of the Moon taken just before Imbrium and the other large maria began to fill, we would see huge basins excavated by enormous impacts and, everywhere else, the highlands terrain. When the mare lavas began to upwell and flow, they filled the basins and left the other seventy percent of the lunar surface uncovered. The Apollo 14 and 15 crews brought back samples of pre-mare materials, but neither crew visited a pure highlands site. As we have discussed, the 14 crew sampled a ridge of material ejected by the Imbrium impact, and the Apollo 15 crew explored some of the mountains that ring the Imbrium Basin itself. Because both sites are so closely associated with the Imbrium impact, there remained a distinct possibility that, in highlands areas far from the Imbrium rim, different geologic processes and materials might have been important. For example, some members of the geologic community thought that the central highlands looked like certain terrains on Earth built by volcanism and hoped that the Apollo 16 crew would find samples to prove the point.

(Readers interested in more details about such pre-mission hypotheses should consult Wilhelms' To a Rocky Moon, Heiken, Vaniman, and French's Lunar Sourcebook, and Jack Schmitt's discussion that concludes this Apollo 16 summary.)

A Delayed Landing

Apollo 16 was launched on April 16, 1972. The Commander, John Young, had flown twice on Gemini and had already been to lunar orbit as the Command Module Pilot on Apollo 10. Later, he would command the first Shuttle mission. The Lunar Module Pilot, Charlie Duke, had served with distinction as CapCom during the Apollo 11 landing. And, finally, the Command Module Pilot, Ken Mattingly, had been scheduled to fly on Apollo 13 but, because Duke had inadvertently exposed him to German measles, he was replaced at the last minute by his backup, Jack Swigert. Like Young, Mattingly eventually flew as a Shuttle Commander.

In lunar orbit, Young and Duke powered up the lander and separated from the Command Module. At this point, Mattingly was scheduled to perform an engine burn to put himself in a position so that he could come to the rescue in the event of an aborted landing. However, during tests of the control systems for the Service Module's steerable rocket engine, a malfunction was detected in the backup system. Mission rules dictated that, at this point, the two spacecraft rendezvous in case it was decided that the crew would have to use the LM engines to get back to Earth. However, after six hours of tests and analysis, Houston decided that the engine problem could be worked around and that the landing could proceed. (A more complete discussion of the Service Module malfunction can be found at the start of the Journal chapter on the Apollo 16 landing.)

At pitchover, Duke's first callout of an Landing Point Designator (LPD) angle - a location on a set of window scribe marks - showed Young that they were headed for a point about 600 meters north and 400 meters west of their target. Ten times, Young nudged the handcontroller to give the computer a new target. As with Apollo 15, the mobility of the Rover meant that he didn't need to land precisely on target; but the cost of this series of minor redesignations was very slight. Young's greatest concern was the fact that there were few shadows to show him where the level spots were in this rolling terrain; and it was only in the last few moments that he got some clues from the LM shadow. It was Duke who saw the shadow first as they came down through 250 feet. Seconds later, as they were coming through 200 feet, Young rotated (yawed) the LM to the right and could see the shadow out his own window and use it to estimate his altitude and the sizes of the craters ahead. Through a combination of skill and luck, he set down on a remarkably level spot. As he and Duke discovered once they got outside, had they landed "25 meters in any direction from the actual landing site, (the LM) would have been on a local slope of six to ten degrees." In particular, in the final seconds, Young had to hover so that he could fly forward and to the right, just beyond a small crater fifteen meters in diameter. Only Armstrong and Conrad landed more upright, and both had the advantage of young, lightly-cratered, mare sites.

Because of the delay in orbit, Young and Duke had already been awake for thirteen hours by the time they landed. If, as originally planned, they had tried to do a full EVA on that first day, they would have stretched the total to twenty-nine hours or so: four for EVA prep, eight for the EVA itself, and four more of post-EVA activities. But no one was interested in taking on the added risks that fatigue would have created as the day got longer and longer.

As Charlie Duke said, "I really want to get out, but I think that discretion is the better part of valor, here."

"Man, it's really tempting though," John Young added. "It really looks nice out there."

Life in the Cabin

By getting out of the suits and donning them again in the morning, they would consume a couple of extra hours worth of oxygen, electric power, and cooling water; but, even having spent an extra five unplanned hours in lunar orbit, they still had at least a half-day reserve of water and considerably more of oxygen and power beyond what they would need for a full, three-day stay. Two hours after landing, Young and Duke were out of their suits and an hour later they were in their hammocks. Both of them slept soundly; indeed, they were the first crew to do so.

On this first night, Duke was awakened twice by warning lights triggered by a leaking regulator in the primary Reaction Control System. The problem had first occurred during LM checkout in orbit and, as with the CSM engine control problem, there had been some initial concern about aborting the landing. But, eventually, a way around the problem was found; and both times during the night - after getting over the shock of being so rudely awakened - Duke assured himself that there was no new problem to worry about, reset the alarm, and then went back to sleep. Living and working on the Moon hadn't yet become an everyday experience; but the experiences of the early crews gave the later crews confidence in the LM and a sense of familiarity that allowed them to get some sleep.

The front half of the cabin, where the astronauts stood, was a space about six feet across, six high, and about three deep. The area aft of the crew stations was filled to hip height with the cover of the ascent engine. In preparation for sleep, Young and Duke had stacked their helmets and suits on the engine cover and then had strung a hammock - fore and aft at head height- for Young. Once he was bedded down, Duke then strung his own hammock close to the floor across the front of the spacecraft and climbed in. There wasn't much room to spare.

In the morning, once they'd gotten the hammocks stowed and had eaten, they then began the laborious process of getting ready to go out. They started with the Liquid Cooled Garment (LCG), a suit of underwear into which a network of thin plastic tubes had been woven. During the EVAs, cooled water would circulate through the tubes, carrying away excess body heat to sublimators in the backpacks. After donning their LCGs, they got into the pressure suits; and that was always a two-man operation. They started with Duke.

The light lunar gravity helped: "You should see me," Young said, "holding up this 50-pound pressure suit with one hand while Charlie is unzipping it with one hand. That's really neat." After getting the suit unzipped, Duke then chinned himself on an overhead guard rail and stuck both legs in while Young held the suit. Then Duke ducked to get his head into the neck ring and, finally put his arms into the sleeves. The suits were individually tailored for a tight fit and it all required a certain amount of contortion. In all, the process took about twenty minutes and, once Duke was suited, they hooked up the hoses which connected him with the LM supplies of oxygen and cooling water. For the time being, they didn't actually turn the water on, but did let oxygen flow into the suit and out the neck and wrist rings to provide some cooling. Then they repeated the process with Young.

For Apollo 16, there were a couple of unplanned steps in the EVA preparations. Because of the length of the EVA, each of the astronauts wore a drink bag inside his neck ring and had a tube he could suck on to get some refreshment. On Apollo 15, Jim Irwin's drink bag had never worked properly, causing him a severe case of dehydration. On Apollo 16, the bags leaked because of intermittent contact between the end of the tube and the microphones that each of them wore. In Charlie Duke's case, he'd been doused with four or five ounces of orange juice while he and Young were still in lunar orbit. Zero-gravity made sure that he got thoroughly soaked in the sticky stuff and, as he later told CapCom Tony England: "Tony, I wouldn't give you two cents for that orange juice as a hair tonic; it mats it down completely."

And, of course, the juice not only got into Duke's hair; it coated the inside of his helmet. Consequently, he had to wash out his helmet and dry it before he could apply a coating of anti-fogging agent; and then he had to blow some of the juice out of the microphone before it would work. Indeed, stray juice was also suspected as the cause of a sticky valve in the LM's Environmental Control System.

Young and Duke next had to put on their backpacks, the Portable Life Support Systems (PLSSs) which contained a lithium hydroxide canister to remove CO2, supplies of oxygen and cooling water, and also the associated pumps and fans and the sublimator which actually got rid of the waste heat. Then came the helmets and gloves, a communications check, and, lastly, a check of the pressure integrity of the suits. In order to do the check, the astronauts first disconnected their LM oxygen hoses and connected the corresponding PLSS hoses. They then pressurized the suits so that they could check for leaks before depressurizing the cabin. They turned on the PLSS oxygen flow until the suits were 3.8 pounds-per-square-inch (psi) higher pressure than the cabin, and then turned off the PLSS oxygen and watched the suit pressure decay. During a minute of watching, they expected a drop of about 0.1 to 0.2 psi due to their own breathing and to the slow diffusion of oxygen into various nooks and crannies in the suits. As usual, the pressure check showed that the suits were tight.

Prior to the pressure check, Young and Duke had been able to move a little, despite the small size of the cabin. In soft, unpressurized suits, they were clumsy but still could turn and bend and reach for checklists and pieces of gear with relative ease. Now, with the suits inflated and both of them wearing backpacks, the situation was dramatically different. In order to depressurize the cabin, Duke had to pull a number of circuit breakers on a panel at his right shoulder and change some valve settings on a panel behind him. The trouble was that, now, there was barely enough room for the two of them to stand, let alone turn. Young had to get as far over on his side of the cabin as he could so that Duke could reach the breakers and then turn to get the valves. Not until they were out on the surface would they have some freedom of movement again. Fortunately, they were pressurized for only a few minutes before it was time for Young to crawl, feet first, out through hatch.

Deploying the ALSEP

Because of the experience of prior crews, Young and Duke were confident that they would adapt quickly to lunar conditions and, so, got to work immediately. They took a look around, checked the condition of the spacecraft, and marveled at the five-meter-deep crater Young had overflown during the final seconds of the landing. The rear footpad was no more than three meters from the rim. Within ten minutes of Young's first step onto the Moon, they had the MESA (Modular Equipment Storage Assembly - their work table and equipment rack) adjusted to a comfortable height and were preparing to deploy the Rover.

Like the Apollo 15 crew, Young and Duke had been through training as a backup crew for a landing mission (Apollo 13) and, so, for their own mission were able to spend about forty percent of their time training for the surface operations. Consequently, they completed deployment of the Rover and, later, deployment of the ALSEP experiments with only a few problems. In particular, the re-designed drill stems on the heat flow experiment worked flawlessly and a jack-and-treadle greatly eased the job of extracting the deep core. It is noteworthy, as well, that Young used the Rover to mark a straight line of tire tracks for deployment of some geophone cables. A walking astronaut would have had some trouble keeping a straight line in the rolling terrain and, on Apollo 17, Gene Cernan would put his Rover to the same practical use.

It is perhaps unfortunate that the Apollo 16 ALSEP deployment is remembered primarily for a heat flow cable that was torn loose. On every Apollo landing mission, crews complained about cables that were forever underfoot. The bulky suits and the chest-mounted PLSS controls made it almost impossible to see one's feet and, almost inevitably, cables refused to lie flat on the surface. Time and time again, astronauts got entangled. Sometimes they would feel the tug and stop. Sometimes the other member of the crew would see them catch a cable and give warning. It was simply a matter of time before someone got a foot caught and, in the process, put enough stress on a cable to pull it loose. Such an accident finally happened on Apollo 16 and, because the cable broke at the connector to the heat flow electronics package, there was no hope of a repair. If there was a lesson to be learned, at least in the Apollo context, it was the need for less fragile cable connections. There was one more mission to be flown; and the 17 crew, too, would have plenty of encounters with foot-grabbing cables.

Flag Crater Traverse

Three and a half hours into the EVA, Young and Duke were finished with the ALSEP deployment and were ready to start on their first geology traverse. On this first trip out from the LM, they were to drive west from the spacecraft, directly away from the Sun, out to Flag Crater and then back. Inevitably, with the Sun low in the sky behind them, Young and Duke could see virtually no shadows out ahead and, consequently, they seemed to be driving into a featureless land. Young had to drive slowly in order to avoid the numerous rocks and small craters that covered the area. They were on the ejecta blanket of South Ray Crater, a huge feature about six kilometers to the south, and the debris from that impact was very much in evidence. In addition, the low Sun added to the basic problem of estimating sizes and distances and, while Young and Duke had no trouble seeing South Ray off to the south, they had some difficulty identifying craters along their route. However, they were fairly confident that they knew where they had landed and, indeed, they reached the rim of Flag Crater with the Rover navigation system giving exactly the expected range and bearing.

If, like the other crews, Young and Duke found that visual navigation required a little experience, they certainly had no wasted moments once they were off the Rover and were ready to do some geology. Both Young and Duke had picked up a great deal of practical field experience during training; and, while Young was perfectly happy to let Duke do most of the talking, occasional remarks made it clear that, behind the Commander's "country-boy" facade, there was a shrewd and skilled observer. In all, they were away from the LM for about two hours and spent an hour and a half of that collecting samples and taking pictures at two stops. Virtually all of the rocks proved to be highland breccias. There were no signs of the young volcanics that some geologists had hoped for.

Stone Mountain

For their second EVA, Young and Duke drove four kilometers south to a cluster of five craters - the Cinco Craters - on the slopes of Stone Mountain. They were to make three stops on the hillside and it was here that geologists had their fondest hopes of finding evidence of volcanism. There was none. It looked to Young and Duke as though many of the rocks they sampled on Stone Mountain had been sprayed onto the hillside by the huge South Ray impact. Nonetheless, the largest of the Cinco Craters had dug deep enough into the soil layer to bring up pieces of the underlying rock and all of these proved to be breccias as well. The absence of volcanic samples was, of course, a considerable disappointment to some of the scientists. It was beginning to look as though the hills in the area were all composed of ejecta from ancient impacts. There was always the chance of a surprise on the final EVA; but, even if there were no volcanics, the breccias gave the scientific community some detailed information about how impacts, rather than volcanoes, had shaped the Central Highlands.

The lack of scientific surprises was also matched by the relative lack of operational surprises. From a pilot's point of view, a good mission was a mission without surprises, and as the day went on, Young and Duke proved once again the value of a well-trained crew. Their confidence in themselves and in their equipment showed up in the relative ease with which they worked on the hillside. They had learned from watching Scott and Irwin and, at one point when Young decided that he hadn't parked the Rover in quite the best position, rather than take time climbing aboard, he and Duke simply picked it up and moved it. They leaned on tools as they stood facing the slope, and sometimes even knelt as they picked up samples. They were confident in their ability to get up again. As they bounced around on the inner slopes of the Cinco Craters, sliding back a few inches on every hop, their heart rates rarely went much above ninety beats per minute; and, on average, they seemed to be having an easier time working on Stone Mountain than Scott and Irwin had on the slopes of Hadley Delta. Although physical differences between the two sites undoubtedly played a role, a major factor was surely the confidence that Young and Duke had gained from having watched the 15 crew go about their work at Hadley.

There were, of course, the inevitable glitches. The pitch indicator on the Rover fell off - so they made eyeball estimates. The harnesses that held the big sample collection bags on their backpacks wouldn't stay tight. Young's bag fell off during the outbound drive on EVA-3 - fortunately when it was empty - and, generally, they had to spend far more time than they would have liked tightening the harness straps. Similarly, the chest-mounted brackets that held packs of individual sample bags failed repeatedly and, eventually, they had to hand-carry a few bags at a time. The free-swinging rod on the gnomon - the shadow device that provided a scale and a local vertical in the pictures - broke off and they had to press their long-handled scoop into service as a replacement. At one point during EVA-2, Young caught his hammer on the right-rear fender, tearing part of the dust guard completely off. From then on, they drove in a rain of dust that was not only a nuisance but also contributed to a noticeable, and somewhat worrisome, heating of the Rover batteries. And, finally, on their way north from Stone Mountain, back toward the LM, they momentarily lost power to the rear wheels. At the next stop, Young tried a number of switch settings and, in the process, disabled the navigation system. The problem wasn't noticed for awhile but, with Smoky Mountain marking the horizon to the north, they were able to steer with confidence back to the spacecraft which lay hidden by an intervening ridge.

House Rock

The third EVA had to be cut a bit short because of the late landing. On this traverse, Young and Duke drove to North Ray Crater, a kilometer-sized feature that had been dug at least a couple of hundred meters into the lower slopes of Smoky Mountain. Although there had been no very high resolution photographs available before the mission, there were hints of big boulders on the rim of North Ray and, indeed, during the landing, Duke had snuck a peek up north and had assured himself that there were, indeed, some very promising rocks at the rim of the crater. What the geologists wanted were samples from a single, huge boulder if one could be found. What they hoped for was a rock big enough to show multiple igneous (volcanic) units. Specifically, they wanted - in the specific words and punctuation of the procedures volume - a "crystalline rock, larger than 5 meters (no breccias)."

Before they had gone more than a kilometer north of the LM, Young and Duke noticed that the frequency of rocks and small craters had dropped considerably. They were now off of the South Ray ejecta blanket and the driving got a lot easier. They were climbing steadily toward North Ray and, during most of the drive, the rim was hidden by intervening ridges. Time and time again it looked as though they would find the crater just over the next rise but the navigation system said that they still had a way to go. So they pressed on and, eventually, found North Ray right where it was supposed to be. The navigation system was proving an invaluable aid.

From their perch on the slopes of Stone Mountain, Young and Duke had gotten a good look into South Ray Crater and, although most of the ejecta was a brilliant white color, here and there they could see streaks of dark material. At their various stops during the first two days, they had found white breccias with black inclusions and also black breccias with white inclusions. And, at North Ray, the pattern was even more striking. Near where they parked, there were large white boulders on the rim - again with dark inclusions - and, off to the north, a big black boulder that begged to be sampled. They examined the white rocks first and then, with only a limited amount of time left, finally had a chance to trot over to the black rock. There is a marvelous piece of film in the Apollo collection which shows people in the Backroom at Houston watching Young and Duke as they ran toward the boulder. These were the assembled scientists and astronauts - Jim Lovell, Jack Schmitt, Lee Silver, Bill Muehlberger and others - who provided advice and suggested priorities during the traverses. As Young and Duke ran, Ed Fendell - who operated the Rover TV camera remotely from Houston - steadily increased the zoom. Thanks to his efforts, the astronauts seemed to stay about the same size on the TV screen, but the rock kept getting bigger and bigger. This was a big rock - "there's your basic house(-sized) rock," Duke said - and that was the name that stuck. As Young and Duke kept running, the smiles in the Backroom got broader and broader and the laughter got louder and louder. As they finally got close, they started down a final slope, their feet now hidden from view, and Schmitt was moved to say, " as our crew slowly sinks in the west...". Because of the need to get back to the Rover and start the journey back to the LM, Young and Duke only spent a few minutes taking pictures, collecting samples, and marveling at the size of House Rock.

Because they were running short of time, Houston decided to omit a couple of other planned stops on the slopes of Smoky Mountain and have the astronauts head back toward the LM. On the way, they made a single stop, this one at another large, black boulder which had a south-facing overhang and a deep hollow underneath. The scientists wanted samples of soil protected from the solar wind and, as Duke reached into the hollow with his shovel, kneeling so that he could reach back in as far as possible, he noted that, if you reach under a rock, "in West Texas, you get a rattlesnake; here, you get permanently shadowed soil." Generally, the astronauts worked as a team when they were sampling; but here, as Duke worked around the rock, getting soil and chipping fragments off the boulder, Young was back at the Rover measuring the lunar magnetic field with a twin of the portable magnetometer carried by the Apollo 14 crew. Duke's solo sampling went fairly well, and the only problem he really had was in figuring out what to do with the individual sample bags once they were full. As Young had done at one point during the second EVA, Duke went back to the Rover, got an empty Sample Collection Bag, and set it down on the ground next to the boulder. The SCBs, as the acronym-addicted astronauts inevitably called them, were just barely suited to the job. They were 42 cm tall and, with only a 15 by 22 cm base, were a bit top heavy. Duke was working on fairly level ground and he found that, if he leaned sideways, he could place the bag upright on the ground and, later, put samples in it. But, as Jack Schmitt was to learn on Apollo 17, SCBs were all too easy to knock over; and Duke wished out loud that he'd gone down to the local supermarket in Houston to get a shopping bag with a broad base and a couple of handles.

Return to the LM

All too soon, it was time to load up the Rover and drive back to the LM. Although this final EVA had to be cut short, Young and Duke set several new records. In all, they collected 94 kilograms of samples, deployed over a half metric ton of equipment (the Rover included), and spent 20 hours and 12 minutes out on the surface. They didn't set a record for distance driven in a Rover. Scott and Irwin had managed to hold on to their pioneering record, at least for the moment, by nearly a kilometer. But there was no doubt that, although the reality of the Descartes landing site had proved a bit less dramatic than some had hoped before the mission, Young and Duke had accomplished about as much as was possible with the time and equipment available. On Apollo 17, Jack Schmitt's training as a professional geologist added an extra capability in that he could recognize subtle relationships and detail and, therefore, that he and Cernan could be a bit more focused in their sampling. However, with time at a premium, none of the crews could make many stops or spend much time looking for the subtleties. Their primary task was to identify the main features of the site, sample the dominant rock types, and look for whatever variety might be present. In this type of work, a well-trained observer was as valuable as a professional and, as observers, the crew of Apollo 16 was unsurpassed.

At the time of the Kennedy decision, the Space Age was only four years old and the era of human space flight was only six weeks old. A few dozen small satellites had been put into low orbit around the Earth. A few of those had carried dogs or monkeys as stand-ins for future human space travelers. Seven spacecraft had been launched in the general direction of the Moon: two of them had flown by at respectable distances, a third had actually impacted the Nearside, and a fourth had swung around the Moon and transmitted back humanity's first, grainy views of the Farside. Yuri Gagarin had flown one orbit of Earth and Alan Shepard had taken a 15-minute suborbital ride in a Mercury capsule. And yet, only eight years later, Armstrong and Aldrin landed on the Moon and, during the three and a half years that followed, they and five other Apollo crews proved beyond a shadow of a doubt that, even within the limits imposed by short stays, by limited equipment budgets, and by the lunar environment itself, a great deal of useful work could be done. In many ways, the weak gravity field made work easier than it would have been on Earth, and only the need to wear the stiff, bulky pressure suits proved to be a significant impediment to productivity. It was clear that, with more time, better suits and equipment, and a spacious, indoor, shirtsleeve environment for making preparations and repairs, productivity could be increased significantly. By the time of Apollo 16, the limits of the program had been approached. The prior crews had tested equipment and procedures and, if the last two missions seemed a bit routine, it was only a mark of the maturity of the program. The Moon is an enormous planet; yet, in only six missions it was possible to put together a credible description of the main themes of lunar geologic history, sketch the distribution of potential resources, and gain some valuable experience in the conduct of lunar operations.

The Truth of Descartes

The following is a brief discussion of the geology of the Apollo 16 site provided by Jack Schmitt during our review of the Apollo 16 summary.

"I think Apollo 16 was a major scientific surprise because we don't know how to interpret what we found. That's always a surprise to geologists, when they can't explain what they found. The pre-flight expectation - by some people - of finding volcanics relates back to the old issue of the Cayley formation, the light-colored plains. That still hasn't been answered today. In the lunar geologic mapping program, before we had Lunar Orbiter or anything like that, the USGS geologists had identified these smooth plains - light-colored in the highlands - that seemed relatively young. I think the first name ever applied to them was Cayley. When I was flying over the Farside of the Moon (on Apollo 17), you could see these light-colored plains, without any significant features in them, in the bottoms of large craters. They're older than the mare because, everywhere I saw them, if you had a deep-enough, young crater that got down to where the mare had reached its selenocentric level, you'd see mare in the bottom of those craters. My guess is that several different processes contributed to giving you about the same surficial features that were lumped together as the 'light-colored plains'. One was the debris flows that swept out from the really large impacts, the large basins. As it went out across the terrain, it was in the form of gasified, loose material and, when it settled, it just settled into these basins, leaving a relatively smooth surface. It was a process like an ash-flow or a landslide. Another thing that probably contributed to the formation of light-colored plains was: if you think about what would have been the logical course of eruptive events during mare formation, the major upwelling of mare basalt would have been preceded by a brief period of gas-charged debris eruption. When you begin to partially melt the differentiated, olivine/pyroxene-dominated portion of the lunar mantle, there's still going to be a little bit of volatile material left, even though most of that would have fractionated out. The trapped magma, as it crystallized around the olivine and pyroxene crystals, would have still contained some volatiles. And that's the low melting fraction. The first thing that's going to melt as that mantle got hot enough to melt at all - or evolve - would be this gaseous, early-melt material. And that would move up through a highly pulverized crust. Because, nothing had sealed the crust yet. The low-melting fraction would gather debris as it went; and it would be mostly highlands material that would erupt as debris flows and would probably settle into the same kind of areas. Now, if this had happened, you might expect to see relatively rimless eruptive centers in some of these basins and, indeed, you do - particular on the Farside. So I think several processes probably contributed to the light plains. And that's what we thought we were landing on at Descartes on Apollo 16. Others had taken it farther than I have here. They had said that the surface looked like they might have been the remains of rhyolitic ashflows and the like. I think the geologists got carried away. They were looking for something different and they over-sensitized the 16 crew to volcanics. And, indeed, there may be volcanics there. It's just they're hard to recognize because most of the material is debris taken up from the crust by whatever volcanic component was being evolved."

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