Before men had gone to the Moon there were many questions that needed answering. We didn’t know whether volcanism or meteorite impacts had determined the topography of the Moon, or whether there were any useful volatiles such as water to be found. Perhaps most importantly, we didn’t know how the Moon had been formed.
Early theories
In the early 17th century, Galileo had shown that the Moon’s surface was divided into two distinct terrains. The large dark areas that he called ‘maria’ or seas, and the lighter areas which he called ‘terrae’ or land.
These names were simply convenient labels since there was no clear evidence for topographic differentiation.
So, with an unknown and alien topography to contend with, before the Apollo missions could think about landing they had to know more about the surface of the Moon. Were the maria lowlands, as they appeared to be, and were the terrae highlands? Crucially, would the surface of the Moon support the weight of a lander or would it sink into thick piles of dust, never to be seen again?
Scientifically, the Moon was thought to be a ‘dead’ world. A world, where, unlike the Earth with its constant rumble of tectonic activity, nothing happened to recycle the surface, where dust accumulated over aeons waiting to trap an unwary spacecraft.
By the mid-1960s the unmanned Ranger and Surveyor probes had returned data showing that the surface of the Moon – regolith – was not covered with deadly oceans of dust and would support the weight of a lander. The Apollo series of manned landings was ready to proceed and would be divided into an initial phase to prove the engineering technology and a later phase with chemistry and geology as the primary focus.
As Jim Lovell, commander of the ill-fated Apollo 13, explained: ‘The first two flights to the moon were … essentially … to put a man on the moon and get him back safely with no other major purpose. But after that, the whole purpose of the lunar flights was for science – to learn more about the moon itself.’
Nobel chemistry prizewinner, geochemist Harold Urey was fascinated by the strange topography of the Moon. He believed that the Moon’s craters had been formed solely by meteorite bombardment – the ‘cold-Moon’ theory. This was in contrast to University of Arizona professor Gerard Kuiper’s competing idea that the Moon had once been intensely volcanically active and that it was this that had formed the Moon’s craters –the ‘hot-Moon’ theory.
Farouk El-Baz, geological mentor to the Apollo astronauts and closely involved in selecting the landing sites, comments: ‘Apollo 11 … answered [the hot Moon/cold Moon] debate early on. As soon as we got the samples from Apollo 11 [back to Earth] and found out that it was basalt there was no question that it was from volcanism.’ The cold Moon theory was disproved. And yet there was still the bigger question of how the Moon been formed.
Before the Apollo mission there were several competing theories. One suggested that the Moon been formed when a chunk of the Earth broke free through rotational forces, leaving the Pacific basin behind as the scar of its departure. Another purported that the Moon was a deep-space body that had been captured by the Earth’s gravitation and yet another hypothesised that it had been formed at the same time as the Earth from the accretionary disk of primordial dust and gases during the birth of the solar system. The problem with all of these ideas, however, was that none could explain why the Earth is the only one of the inner, terrestrial planets to have such a large satellite.
In the 1970s and 1980s, as a direct result of studying lunar samples, another theory was put forward by George Wetherill of the Carnegie Institution, US. Wetherill, using multivariate statistical techniques made possible by the then new generation of digital computers, showed mathematically that hundreds of bodies with roughly the same mass as the Moon could form one or two Earth-mass planets – such as Venus and Earth – and perhaps a few smaller rocky planets – Mercury and Mars – orbiting at their observed distances from the Sun.
Wetherill’s work inspired the young cosmochemist William Hartmann. Hartmann and his colleague Don Davis suggested the Moon’s formation could have involved one or two of Wetherill’s proto-planets colliding with the Earth while it was still forming from primordial accretionary disk. If such a giant impact occurred off-centre, they suggested this would coalesce into the Moon. Hartmann recalls: ‘It occurred to me that if a big enough object hit the Earth in the past it could have blown off the mantle material – assuming that the Earth’s core had already formed – and that would explain why the Moon has [a] lower mean density; it has no material from the Earth’s core.’
Chemical proof
The proof came when they looked at the chemistry. Hartmann explains: ‘The big change came when we looked at the ratio of oxygen isotopes on the Moon. They were identical to those of the Earth, whereas every other sample that we have from every other part of the solar system is very different to those of the Earth. And those data were not just from the samples returned by the Apollo missions but were also supported by the three Russian robot lander missions that had returned samples to Earth as well.’
By 1970, however, enthusiasm for the Apollo mission was waning. Apollo 13 had proved it to be highly dangerous and it was expensive – $24bn at 1970s prices – especially when the questions that were being asked were so abstruse that only a handful of academics seemed to care about the answers.
The later missions of Apollo had to operate under the shadow of waning interest and the knowledge that missions 18 through 20 had just been cancelled. The pressure was on to maximise the scientific return from these last missions.
During Apollo 15, commander Al Worden had been examining an area to the north of Sea of Tranquillity where Apollo 11 had landed. This area, on the southern edge of the Sea of Serenity, had some interesting geological features. In photographs, El-Baz had seen craters that were surrounded by darker material which led him to believe that these might be cinder cones. In the summer of 1971 Worden confirmed that there was a whole field of cinder cones at Taurus/Littrow.
The existence of the cinder cones was evidence of recent geological activity. Radiometric dating of volcanic deposits from previous Apollo missions suggested strongly that volcanism had ceased on the Moon 3.5bn years ago, but if these cones still had cinder residue it suggested that volcanism might have occurred much more recently. This, in turn, would suggest that the Moon was not the dead world that it had been thought to be prior to the Apollo programme.
So Taurus/Littrow assumed overwhelming importance for the vanishing Apollo programme. With a landing site still to be selected for Apollo 17, the only mission that would carry a scientist on board – Harrison ‘Jack’ Schmitt was a geologist – it became the obvious choice. El-Baz explains: ‘The visual observation of “cinder cones” in the Littrow region elevated the region above all others for Apollo 17. It was believed that the dark ‘mare’ material at the site represented the latest extrusion of volcanic rock from the lunar interior. Thus, the recognition of dark halo craters atop volcanic cones confirmed that.’
In fact, there were other scientific reasons for visiting the region. El-Baz says: ‘[As well as] sampling the ‘dark – presumably younger – mare deposits in the plane, there was also the opportunity to sample … one of the oldest ejecta [deposits] as represented [by] the ridge – part of Montes Apenninus – and sampling the edges of the sinuous rill, possibly a caved in lava tube, all at the same place.’
Schmitt, enthusiastically concurred that Taurus/Littrow was the place to go and lobbied extensively for no less than four extra vehicular activities (EVAs) to explore the region. This was one more than either Apollo’s 15 or 16 and significantly increased the hazards that Schmitt and his fellow astronaut Gene Cernan would be exposed to. Eventually, Chris Kraft, the mission director vetoed the fourth EVA and reminded the astronauts that, with the budget cutbacks and safety concerns following Apollo 13, they were lucky to be going at all. Schmitt and Cernan backed down.
And so just before Christmas 1973, the two astronauts were sampling Taurus/Littrow. Perhaps the biggest surprise of the mission was the discovery of a band of bright orange soil paralleling the rim of a crater that they had named ‘Shorty’. Schmitt and Cernan took a core of it. Later it would be shown that the soil was comprised of the finest particles ever returned from Moon and that the orange coloration was of volcanic origin and not the result of a meteorite impact. Explosive volcanism is often associated with the extrusion of hydrous compounds and it was a moment for much excitement. In fact, only in the past few years, the Lunar Reconnaissance Orbiter has shown that the Moon has enough water bound up in its regolith to cover the surface with a layer 1m deep. This more than anything else is fuelling hopes that the Moon may be an important stop-off point on our way to the other planets of the solar system.
By the end of Apollo 17, the big questions that had taxed scientists before the Apollo mission had been answered. Big basins like the Sea of Tranquillity and the Sea of Serenity had been blasted out by impacts, the mare had been formed by flows of lava episodically welling up from the lunar interior, the Moon had been formed by an impact with Earth and there are volatiles on the Moon.
As the crew of Apollo 17 prepared to leave the lunar surface for the last time, Cernan unveiled a plaque on the descent stage of the lunar module. It reads: ‘Here man completed his first explorations of the moon.’ The plaque and the LM Descent Stage remains in the valley of Taurus/Littrow today, perfectly preserved in the vacuum of the lunar surface.
‘It is a testament to the ingenuity of the human race. Cernan comments: ‘[Before Apollo] the technology it took to get to the moon did not exist, and in a few short years we had to develop what it took to go where man had only dreamed of going in the past.’
The power behind Apollo
Without the chemical industry, the Apollo missions would never have happened.
The Saturn V’s five first-stage F1 engines each burnt 788kg of RP-1 (kerosene) rocket fuel and 1789kg of liquid oxygen per second. So temperamental were they that it took Rocketdyne’s engineers three years to developed them to the stage where they would not shake themselves to pieces from the vibration set up by the pressure waves in the engine bells. Combined, the five engines produced 1,522,000 foot-pounds (6.77MN) of thrust.
Another crucial fuel that made the Apollo missions possible were the hypergolic propellants used in the command/service module (CSM) and lunar module (LM) engines. The CSM used a combination of Aerozine 50 – a 50/50 mix of hydrazine and unsymmetrical dimethylhydrazine (UDMH)) with nitrogen tetroxide (N2O4) as the oxidiser. The LM descent engine used UDMH and N2O4 while the LM ascent engine used Aerozine 50 and N2O4. The primary advantage of hypergolic fuels is that they spontaneously ignite on contact, all that is necessary for explosive thrust is that two valves open and the propellants are allowed to mix. This was extremely desirable for the Apollo missions where components had to be ‘no-fail’.
Richard Corfield is a science writer based in Oxford, UK.
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