By Duncan Lunan

Several of the larger craters on the Moon can be seen with the naked eye, so a small telescope will show a great many of them.  They show best along the terminator, the line dividing night and darkness, so most can be seen when the Moon is half-full  (confusing called First Quarter). 

Full Moon, however, is the best time to see the bright rays of impact debris which surround the more recent craters such as Tycho and Kepler.  Material exposed on the surface of the Moon is progressively darkened by the Solar Wind, so we can tell the relative ages of craters by their brightness and by the rays of light-coloured ejecta thrown out from them.   Copernicus, Tycho and Giordano Bruno  (on the lunar Farside)  are major examples. 

The names of the lunar features follow the conventions established by Riccioli in his Moon map of 1651.  

How The Moon Was Formed

For some time it’s been believed that our Moon was formed when a protoplanet roughly the sized of Mars collided with the proto-Earth;  the collider’s core merged with ours and its superheated crust went into orbit and condensed to form the Moon, whose crust  (at least)  formed at a much higher temperature than the Earth’s.  I claim a small point in that when this hypothesis was first published, but largely ignored, I endorsed it in my second book “New Worlds for Old”  (1979). 

According to the BBC’s David Whitehouse in his book “The Moon, a Biography”  (Headline, 2001), the impact in which that happened would have been observable to a distance of 400 light-years.  Bing Zhang and Stein Sigurdsson of Pennsylvania State University have proposed a systematic search for such events in new star systems  (‘News Notes:  When Worlds Collide – and How to Spot Them’, Astronomy Now, October 2003), of which there could be as many as 50 per year.  By then the Keck telescope had found evidence that such an event has occurred in the dust disc surrounding the star BD + 20 307, which is warmer than it should be unless it’s been heated by a major internal collision  (Universe Today website, 21^st July 2005).   In 2007 another example was found around HD 23514 in the Pleiades  (Fraser Cain, ‘Planets Found Forming in the Pleiades Star Cluster’, Universe Today website, November 14^th, 2007.)   But more recent work suggests that instead of one big impact here, there may have been up to 20 of them, and they gave rise to satellites of the Moon as well as the Moon itself.

Satellites of satellites are dynamically unstable and crash into their primaries.  (This is why Venus and Mercury have no permanent moons, they’re too close to the Sun.)  It’s been suggested that our Moon’s big impact basins were formed by the fall of sub-satellites in the first billion years of its history.  That would have been after the ‘Final Bombardment’ which our Solar System had about 400 million years after its initial formation.  (F.L. Whipple suggested that bombardment coincided with the late formation of the distant planet Neptune.)   The mountainous highlands of the Moon are saturated with craters upon craters, but on the Nearside of the Moon facing the Earth the major impact basins, which we call maria, were filled with lava about 500 million years after the Final Bombardment.  The Moon’s dark plains are rich in titanium, the lighter highland rock has a high aluminium content, and there are deposits of uranium richer than any known on Earth.  What were lacking from the known moonrocks were the volatiles – hydrogen, carbon and nitrogen compounds, which are essential to life – but see below.      

It’s not clear why most of the maria formed on the side facing the Earth, but it tells us that by 3,500 million years ago the Moon’s rotation had been slowed by tidal forces until it was captured into 1:1 lock with its revolution around the Earth.  In ‘Compass Points in the Sky’, I pointed out the confusion that used to exist on the Moon, the mapping convention which meant that the Sun rose in the west even though the Moon rotates on its axis in the same direction that the Earth does – and the huge impact basin of Mare Orientale, ‘the Eastern Sea’, of which Patrick Moore was a co-discoverer, is actually on the western limb of the Moon as truly defined with reference to Sinus Medii, the Central Bay, which marks the Moon’s zero longitude.  

The Nearside Of The Moon

During the 1960s, the US Air Force mapped the Nearside of the Moon in hitherto unprecedented detail, by radar, in advance of the Moon landings.   Those maps were drawn for use by the astronauts in situations where confusion might be fatal, so they were drawn with north at the top and east and west where they should be.  Because they were the best maps available, lunar observers immediately began to use them, and to their intense chagrin, the International Astronomical Union found that what should have been their decision had been bypassed.  On the Moon, the Sun now rises in the east.  

Since the Moon keeps the same face to the Earth, Earth is always overhead at Sinus Medii in the centre of the Nearside, drifting only slightly due to the effect known as ‘libration’, a rocking motion which allows us to see a little further round to the Farside.  Sunrise and sunset on the Moon are not sudden:  because the Moon rotates on its axis only once in a lunar month, to keep the same face to the Earth, sunrise and sunset take a whole hour each.  

The Farside Of The Moon

Nevertheless “the dark side of the Moon” does not exist.  The Farside of the Moon gets every bit as much sunlight and starlight as the side which is turned towards the Earth;  the only difference is that it doesn’t get Earthlight when the Sun is down.  Leonardo da Vinci was first to explain the nature of Earthlight on the Moon, as the explanation of  ‘the old Moon in the new Moon’s arms’.  Admittedly Earthlight on the Nearside would be bright enough to read by, but when the Farside has sunshine 10% higher than at Earth’s surface for two weeks out of four, it can hardly be described as ‘dark’.

The pattern of maria on the Nearside of the Moon form the face of ‘The Man in the Moon’ as seen from the Earth’s northern hemisphere.  In modern times one of the most famous view of the Moon is one taken by Apollo 8, the first manned mission to orbit the Moon at Christmas 1968.   For example, it’s on the cover of “The Times Atlas of the Moon”  (1969).  A similar photo was taken by the crew of Apollo 11.   In this view, Mare Crisium, on the top right of the Moon’s disc as we see it from the northern hemisphere of the Earth, appears above the centre of the disc.  But for some reason, Hollywood producers have fallen in love with this image, which can never be seen from Earth, and repeatedly use it instead of ‘the Man in the Moon’ which you would think everybody knew.  The artist David A. Hardy has compiled a special collection of instances of this extraordinary howler.

The Lord of the Rings films avoided the error, using the real Moon, but because they were filmed in New Zealand it was ‘upside down’, with the southern hemisphere view in which the Man in the Moon becomes a crab.  Middle Earth is definitely in the northern hemisphere in J.R.R. Tolkien’s books, with glaciers in the north, deserts in the south and countless allusions to European mythology.  Someone must have pointed this out to Peter Jackson because in the second film, the Moon has been turned around by computer magic.  But in the first major dialogue scene of The Two Towers, Aragorn, Gimli and Legolas meet the Riders of Rohan, who give them horses.  Their leader then leaps into the saddle with the cry, “We ride north!” and they do – straight towards the Sun, because it’s midday and they’re in the southern hemisphere.

When I was in the Junior Chamber of Commerce in Irvine, in the late 1970s, I was asked to referee a management exercise, supposedly formulated by NASA but actually by a management consultant called Belbin, which the members undertook at one of their meetings.   Each participating team was supposedly stranded on the Moon by a forced landing and had to walk 100 miles to the nearest Moonbase.  Out of a list of items in the ship, only twelve could be taken.   Most of the items in the survival pack had been intended for emergency landing on Earth:  “I’ll tell you one thing, that rubber dinghy’s no use without the oars!”  But actually you were supposed to give it priority because the other items could be dragged or carried in it, which made the limit of twelve a bit arbitrary.  

Another item was a compass, but the Chamber members didn’t know whether the Moon had a magnetic field and as a referee, I wasn’t allowed to tell them.  It might give warning of an approaching solar flare stream, but it would be highly unreliable for direction:  although the Moon has no overall magnetic field, individual craters have fields generated by shock forces in the rock.  But since the phase of the Moon is always the complement of the phase of the Earth, if you were on the Moon, the phase of the Earth and the height of the Sun above the horizon give you the longitude, and the height of the Earth above the horizon gives you the latitude, so it would be hard to get yourself lost.  Even at Hadley Rille, the Apollo 15 landing site  in Mare Imbrium, the Earth is high in the sky.  The 1969 US stamp commemorating the Apollo 11 landing had the Earth on the horizon instead of overhead, but that was artistic license.  In fact there’s a famous shot of the Earth high in the sky, over the head of Apollo 17 geologist Harrison Schmitt and the US flag, at the end of David R. Scott’s article, ‘What Is It Like to Walk on the Moon?’, National Geographic Magazine, September 1973.      

Seen from the Moon or near it, the Earth would be in the opposite phase to the Moon’s;  put the two discs together and they’d make a complete circle.  All of the Apollo landings were made at sunrise, so that the obstacles on the ground were picked out by their shadows, so where the astronauts landed on the Moon determined what the Earth’s phase would be at the time and it would take days to alter.  Apollo 11 flew the same trajectory over Mare Tranquillitatis as Apollo 8 seven months earlier, so in their photos the Earth is gibbous, but Apollo 15 landed in Mare Imbrium and the Earth was a crescent.

It’s often said that the Great Wall of China is visible from the Moon, but this is due to a false claim by Nixon on his visit to China in 1970.  Because of the Great Wall’s length, it ought to be true, but in reality it isn’t visible even from orbit because it’s made of mud bricks which are the same colour as the local earth.  The only man-made thing visible from the Moon is the product of overgrazing, the Sahara Desert itself. 

Gravity

The Moon has roughly one-quarter of the Earth’s diameter, but has no nickel-iron core, so at the surface of our Moon, about the area of South America, gravity is only one-sixth of what it is here.  Despite the low gravity, with no atmosphere to hold up dust every particle follows a ballistic trajectory and falls to the ground much more rapidly than on Earth  (one of the many proofs in the Apollo footage that they really were on the Moon).  One of the big surprises of lunar exploration was that everything on the Moon was much more rounded and eroded, by thermal stresses and micrometeorites, than anyone had expected.  I remember Sandy Glover, my predecessor as President of ASTRA, looking through Bonestell’s paintings in “The Conquest of Space” (first published in Life in 1948)  and Ludek Pesek’s “The Moon and the Planets”  (1963), saying, “There’s far too much weathering.  What’s supposed to have caused all this erosion?”  In my “New Worlds for Old”  (1979), one of the pairs of paintings illustrating ‘the new look of the Solar System’ were two views by Ed Buckley of the Hadley Rille in Mare Imbrium, as space artists saw it before and after the Apollo 15 landing.  The lunar rocks are nothing like the sharp pinnacles once imagined, because everything has been worn smooth by thermal erosion.  It’s only because of the vacuum that the Victorian observers like Nasmyth and Carpenter, who built detailed models of the lunar surface, were fooled into thinking the peaks were jagged.

Tracking The Moon

At each of the Apollo landing sites the crews deployed laser retroreflectors, which are now tracked by laser beams from observatories on Earth.  Accurate tracking of the Moon’s position has practical applications in predicting the tides, but at the level of accuracy the laser tracking allows, it’s possible to measure continental drift on Earth, and the Moon’s slow retreat from the Earth due to tidal action.  Both processes take place at about the speed at which fingernails grow.   At the time of its formation the Moon was much closer to the Earth than it is now, and Earth had a much shorter day, but as the tides gradually slow down the Earth’s rotation, conservation of momentum pushes the Moon away from us.  The late Prof. Archie Roy, one of the world’s leading astrodynamicists as well as a thriller writer and President of the Scottish Society for Psychical Research, believed that the Solar System demonstrates an ongoing process which he called ‘statistical stability’, one argument for which is that the Moon’s motion should have become chaotic at several times during its slow retreat from the Earth caused by tidal forces – yet it’s demonstrably still with us, and more or less where the clockwork model says it should be.

After the Apollo 11 crew had deployed the first Apollo Scientific Experiment Package, it picked up only surface tremors and ‘instrument malfunction’ was suspected.  But when the Apollo 12 Ascent Stage was  crashed back into the Moon near its newly deployed ALSEP, the first to be powered by a nuclear battery, selenologists were amazed to discover that even this minor blow set the Moon ‘ringing like a bell’.  Reverberations from the Ascent Stage impacts lasted for 55 minutes, more than ten times longer than expected.  Perhaps the Moon’s surface was unstable, and the impact had triggered a cascade of minor avalanches, crater wall slumping and crustal fractures?  It seemed unlikely, since meteorite impacts must occur fairly often;  but it raised uneasy thoughts about crevasses opening under spacecraft, or rolling landslides, if there were to be impacts while crews were on the surface.  

Moonquakes

The majority of moonquakes detected by the Apollo seismometers were extremely mild, generated by tidal action under the bulge on the Nearside equator facing the Earth.  But while the ALSEP stations were still working, before they were switched off by order of the US Congress, they recorded several large impacts on the Farside of the Moon during the annual Beta Taurid meteor shower, big enough for the lunar interior to be mapped by the seismic waves from them.  The eventual conclusion was that the Moon’s crust was remarkably rigid, as it had to be to support the ‘mascons’ – the mass concentrations found in the big impact basins, where denser material had welled up from the interior of the Moon about a billion years after the ending of the final bombardment.  Those embedded masses increased the resonance of the rigid lunar crust.

Volcanic Activity

Plato, the ‘Great Dark Lake’ of Hevelius in 1647, is 60 miles across and flooded with dark lava.  The second edition of Patrick Moore’s “Guide to the Moon”  (Lutterworth Press, 1976)  notes that “many TLP  (Transient Lunar Phenomena)  have been reported here”.  The existence of TLP remains controversial, though many accomplished lunar observers such as Patrick himself have been convinced of their reality, and Plato, in Mare Imbrium, is one of the sites on the Moon where amateur astronomers have reported seeing mists.  (Patrick Moore, “Guide to the Moon”, Eyre & Spottiswoode, 1953;  V.A. Firsoff, “Strange World of the Moon”, Hutchinson, 1959;  H.P. Wilkins, “The True Book about the Moon”, Muller, 1960.)   

There’s still disagreement about whether any of the lunar features are volcanic, and the Apollo 17 mission was targeted to Taurus-Littrow because observations from Apollo 15 suggested there might be volcanic cones there.  On Apollo 15 the astronauts took dramatic photographs of Hadley Rille, which is 70 miles long and although it’s a thousand feet deep and looks narrow from Earth, it’s a mile wide on average.  The slopes were so gentle that the astronauts thought they could drive down to the canyon floor;  Mission Control refused to allow it but they did go part way down on foot.  (Kenneth F. Weaver, ‘To the Mountains of the Moon’, National Geographic Magazine, February 1972.)  It remained unclear whether Hadley Rille was formed by geological faulting or by the collapse of a lava tube.

The discovery of the orange soil at Shorty crater seemed at first to confirm volcanic activity, though it turned out to have been formed by impact  (but see below).  Other prime candidates for volcanism are dark-haloed craters in Alphonsus  (photographed by Ranger 9 before impact)  and the Marius Hills in Oceanus Procellarum, photographed by Lunar Orbiter and painted by Ed Buckley for my book “Man and the Planets”  (Ashgrove Press, 1983).  Ed suggested that the volcanic domes of the Marius Hills might be an early site for lunar settlement, because any domes intended for permanent occupation would need to be shielded both against solar flare storms and against long-term accumulation of primary cosmic ray damage, which would produce noticeable effects in about three years due to destruction of central nervous tissue.  The effects of impaired speech, vision and coordination, loss of memory, etc., would be very much like being punch-drunk.

In January 2009 the Japanese space agency, JAXA, released a 3-D flyround of the crater Tycho on the Moon  (scene of the black monolith in 2001), an impact crater about 60 million years old which is still bright enough to be seen with the naked eye from Earth.  Surveyor 6 landed on the ejecta blanket surrounding the ringwall and before the last two Apollo landings were cancelled, it was said that “if you had only two missions left, you’d send one to Tycho”.   The images compiled by the Selene lunar orbiter reveal that the central peaks are striated, and like the Southern Uplands of Scotland, have been rotated 90 degrees during uplift so that the strata are now vertical.  Even more remarkably they contain what’s almost certainly a large collapse caldera with a frozen lava flow pouring out of it, apparently clear evidence for vulcanism on the Moon.  V.A. Firsoff cited summit craters as evidence for vulcanism, and Patrick Moore found large numbers of them in a search from the Meudon Observatory at Paris, but they’ve had little mention since.  The JAXA discoveries have attracted remarkably little attention so far, but the upper rim of the caldera can also be seen in a striking photo of the central peak by the US Lunar Reconnaissance Orbiter.

Ice

In the 1960s, features in the dark-floored crater Tsiolkovsky on the lunar Farside were thought possibly to indicate buried glaciers, because they resembled the burial of the Sherman glacier in Alaska by the ‘Good Friday’ earthquake of 1964.  The presence of ice wasn’t confirmed in detailed studies of Tsiolkovsky by the Apollo missions  (H. Masursky, G.W. Colton, F. El-Baz, “Apollo over the Moon, a view from orbit”, NASA SP 362, GPO, 1978).  One major discovery of the Apollo missions was that the Moon’s crust is entirely lacking in ‘volatiles’, such as carbon, hydrogen and nitrogen compounds.  For this and other reasons it’s thought that the proto-Earth collided with another protoplanet whose ripped-off, superheated crust formed the Moon.  To give the Moon a lasting artificial earthlike atmosphere would require a comet-like body 80 km. in diameter, or more than 800 comets the size of the Shoemaker-Levy fragments which hit Jupiter in 2004.  Smaller impacts might create a temporary atmosphere, and in 1960 it was suggested that released volatiles could collect in ‘cold traps’ on the permanently shadowed floors of craters near the lunar poles.   The total collecting area would be very small, less than half of one per cent of the lunar surface – though even that could hold enough water to cover the Moon to a depth of one metre.  On the other hand, the small collecting area could mean that the traps would fill rapidly and any ice in them would now be very old.

The Clementine probe in 1994 found signs that there was indeed ice at the south pole, in small craters within the giant impact basin Aitken, which is 2500 km. across and at least 12 km. deep.  First results from the Lunar Prospector probe in 1998 supported that, but then suggested that at both poles there were much larger deposits of ice, very thinly mixed with dust.  Lunar dust is a very effective insulator, which could explain why the ice grains survived even in sunlit areas;  an apparent impact observed by monks at Catnerbury in 1178 suggested that a temporary atmosphere had been formed, thick enough to support the dust and carry it around the Moon.  Later results from Lunar Prospector cast doubt on the ‘frost’ interpretation, however, indicating that the ice was indeed concentrated in deep subsurface deposits, and there was still more of it.  Arguments about whether there was ice, in what form and where from continued till 2009, when the L-Cross probe impacted near the south pole and raised a plume of water vapour whose isotopic composition showed it was definitely from comets;  and at the same time India’s Chandrayaan-1 lunar orbiter discovered at least 600 million metric tons of water ice in shadowed craters at the north pole.  

Water

When Lunar Prospector discovered indications of water at the lunar south pole, my friend Bonnie Cooper calculated that even a crater 800 metres across could be worth $1.25 billion to a lunar miner  – if she could lay claim to it  (Space Resource News, March 1995).  Under the existing UN treaties governing celestial bodies, it would be difficult to claim ownership but she could still charge for the labour of extraction.  It’s not realistic to value the water at the cost of shipment from Earth, but even if it was valued at 1% of that, it might fetch $12.5 million.

When the Galileo spacecraft flew past Earth and Moon on its way to Jupiter, we now know, its instruments indicated the presence of water vapour over the sunlit face of the Moon.  It wasn’t published because nobody could believe it;  but now it turns out that during the lunar day, the impact of hydrogen particles from the Solar wind forms water molecules when they hit oxygen atoms in surface moonrocks.  Most of the molecules are split back into oxygen and hydrogen by ultraviolet radiation before the end of the lunar day, but some must find their way into the cold traps.

As we look at the Moon, the Sea of Serenity is the left ‘eye’ of the Man in the Moon, i.e. on the right as we look at him.  It has an area of 125,000 square miles, slightly more than Great Britain.  Apollo 17 was targeted to Taurus-Littrow in Mare Serenitatis, hoping to find proof of former volcanic activity on the Moon:  during the Apollo 15 mission, Command Module Pilot Al Worden had spotted what appeared to be volcanic vents there.  The hope was to find sulphur deposits or compounds, but the orange colour of the soil at Shorty crater proved to be due to glass beads, found everywhere on the Moon and formed by impacts.  In this case the colour was due to the presence of volatile elements such as zinc and chlorine, absent from normal moonrock and possibly volcanic in origin, though they might have come from a cometary or meteorite impact.  The orange soil itself is 3.7 billion years old, and was brought to the surface by a small impact 30 million years ago  (Bevan M. French, “The Moon Book”, Penguin, 1977).  More recently it’s been realised that the orange beads were formed in the presence of water, at least in water-bearing rocks;  the same applies to green glass beads found at Hadley Rille by Apollo 15, and the ‘rusty rock’ found by Apollo 14 is no longer considered to be the result of laboratory contamination  (Jason Palmer, ‘Moon’s Interior Water Casts Doubt on Formation Theory’, BBC News, Science and Environment, 26^th May 2011.)   We aren’t yet back to the sub-surface lakes postulated by Firsoff, or the habitable lunar interior envisaged by H.G. Wells, Edgar Rice Burroughs and others, but we’ve moved a long way in that direction.  

Moonrock

In June 1973, Troon had a visit by Rosalind Jackson, who worked on Apollo sample analysis including the Apollo 17 ‘orange soil’.  This was before Senator William Proxmire gave the analysis of Moonrock one of his notorious ‘golden fleece’ awards, by which he designated scientific research he considered worthless, and successfully brought in a bill to have work on the moonrock stopped.  In 1986, when I visited the Lunar Receiving Laboratory at Houston, where all the returned moonrock is stored in dry nitrogen to prevent terrestrial contamination, the huge facility was occupied only by two small university teams.  Gradually, though, more and more of it has been examined, with increasingly startling results.  

Studies of Martian meteorites at the Carnegie Institution have found uniformity in their water content which suggests that the water was taken up during the planet’s formation.  (Jason Major, ‘Mars Has Watery Insides, Just Like Earth’, Universe Today, June 22^nd, 2012).  If this proves true, it should also be true for the Earth, Venus and ‘Theia’, the body which collided with the proto-Earth to form the Moon.       

The question of how the Earth acquired its oceans  (by outgassing from volcanoes, or by cometary impacts?)  has been an issue for years.  The discovery in Australia of zircon crystals 4.2 billion years old seemed to support the impact hypothesis, because they had formed in water during the final bombardment phase;  then for a time analyses of the water content of meteorites seemed to rule out the impacts;  then meteorites were found, from further out in the Asteroid Belt, whose water content had the right isotope ratios.  Now it seems the water may have been internal after all… doubtless To Be Continued, I wrote when I first drafted this article.  Now it turns out that there may indeed be layers of hydrated rock within the Moon.  All this makes the prospect of long-term settlement there much more attractive.

See also:

Astronomy Beginner’s Guide Part 4: The Sun

Astronomy Beginner’s Guide: Part 3 Co-Ordinate Systems

Astronomy Beginner’s Guide: Part 2 Compass Points in the Sky

Amateur Involvement in Astronomy: Part 1 of Our Beginner’s Guide