Few objects in the terrestrial sky are as beautiful as Earth's Moon, Luna - a pale, glowing silver orb that has inspired the fancy and haunted the dreams of mankind back into the dim reaches of time. But it is also a unique place unto itself, and the only celestial body other than Earth that human beings have yet visited. Although the personalized name Luna is uncommon in English usage, I often use it here because "the Moon" does a disservice to the body by defining it entirely through its association with Earth. Apollo 11 astronaut Buzz Aldrin, staring out at the ghostly lunar surface through the faceplate of his spacesuit, remarked that it was "magnificent desolation." In part 10 of our journey through the solar system, we now visit this world of stark wonders in our planetary back yard.
The progress of our adventure so far: (current in bold)
1. The SunLuna, in all her glory:
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
12. Phobos & Deimos
23. Tethys, Dione, and Rhea
26. Rings & Minor Moons of Saturn
28. Moons of Uranus
31. The Kuiper Belt & Scattered Disk
33. The Stellar Neighborhood
As the only natural satellite of Earth, the Moon shares the solar orbit of its primary (the term for the more massive companion in an orbital relationship), so all of the general information about Earth's orbital characteristics given in Volume 1 of the Earth sub-series also applies here. Luna is quite big as far as moons go, although there are several larger than it in our solar system - specifically, Jupiter's moons Io, Callisto, and Ganymede, and Saturn's moon Titan are larger. Also, two dwarf planets of the solar system are known to be larger - Eris and Pluto. Despite its size, it is considerably more massive than these dwarf planets, as they largely consist of ices, but less massive than the gas giant moons.
However, due to the large mass imbalance between Earth and Luna (by a factor of over 80), the center of mass of their combined system is actually beneath our planet's surface, meaning that the vast majority of motion occurs on the part of the Moon. As a result, its distance from the Sun varies by a maximum of about 770,000 km over and above the eccentricity of Earth's solar orbit, and runs through one such cycle approximately once every 28 terrestrial days (the ancient month). The following diagrams are reposted from Volume 2 of the Earth sub-series:
Luna serves as something of a meteor shield for Earth, soaking up a significant number of impacts that might otherwise have reached us. This occurs because the mass of the combined system attracts objects passing nearby, and they may pass on a trajectory more strongly affected by the Moon's gravity than Earth's, bending its path into an intersection with the lunar surface. In a very small number of instances, however, this interaction may also cause Earth impacts.
As a result of the former phenomenon, the lunar surface is estimated to accrete 1.8 million kilograms of new mass per terrestrial year via impact material - a number that sounds like a lot, but is only about one-forty-quadrillionth of the total lunar mass. In other words, it would take forty quadrillion years to double in mass at this rate, and that is well beyond the expected lifetime of both the Sun and the entire Milky Way galaxy. Below are diagrams from the linked source above illustrating lunar micrometeoroid impact flux (a rate per unit area per unit time) as a function of mass on the left and the amount of accretion mass caused by given impactor masses on the right:
The meaning of the diagram on the left is that Luna receives a similar profile of impactor to those arriving on Earth, but fewer overall - a fact that makes sense in light of its much lower mass and considerably smaller size. Meanwhile, the diagram on the right tells us that the most common mass of impactor - represented by the peak of the graph - is on order of 10-5 gram, or a hundredth of a milligram, which could be a grain of sand. So, basically, the lunar sky rains down sand with the kinetic energy of bullets, and adds almost 2 million kilograms of mass to the Moon every year. This is why the surface is coated in a finely-pulverized, powdery material called regolith rather than being dominated by solid rock. Since Luna has no atmosphere, heavy-duty projectile shielding would be a necessity for designing lunar colonies.
Although it is at times slightly closer to or farther away from the Sun than Earth, Luna mainly receives about the same solar flux as its primary because the maximum diameter of its orbit is only a fraction of the difference between Earth's perhihelion and aphelion - the planet's least and greatest distances from the Sun, respectively. But because it has no temperature-regulating gases or liquids, and because it spends about two weeks at a time in sunlight, it bakes in the Sun and freezes in darkness. Its surface is also totally exposed to the full brunt of the solar spectrum, including UV wavelengths and higher, as well as having direct contact with the solar wind. In other words, the Sun is very harsh on Luna, even though it is about the same apparent size as seen from Earth.
The following images from the Apollo program (presented in order of mission) give a good sense of what the solar glare looks like on Luna, although it shows up differently depending on what type of camera was used - the famous high-quality Hasselblad shows a relatively well-constrained disk, while frames from the video camera show a lot of glare and rays. The latter is more natural, and more like what you would see if you were personally on the Moon and looking in the general direction of the Sun:
Part of the reason for the glare is that Earth's surface receives only about 2/3 of the visible-spectrum radiation that reaches the top of its atmosphere, while the lunar surface is completely unprotected and thus the light totally unfiltered. This is especially significant since it also doesn't block higher-energy radiation like UV and, to a much lower extent, X-rays and gamma rays. This is why space suits for both orbital spacewalks and lunar surface EVAs are equipped with sunshields on the faceplates - otherwise astronauts would easily become snow blind by reflected sunlight, which also affects high-altitude mountain climbers who have less atmosphere to protect them.
You may be tempted to think the Earth would appear larger in the lunar sky than the Moon in Earth's sky due to their respective sizes, but actually the distance between is so much larger than the difference in size, their relative size from each other's surfaces is quite similar:
Sometimes stars and planets are visible from the lunar surface and its orbital environs, although they don't twinkle as through an atmosphere (the light streaks are due to the camera). Usually they may be difficult to see because the sunlight is so bright, and also due to Earthshine when that's in the sky. You may have to look closely to see the stars in some of these images:
Earth may appear larger in the sky from lunar orbit by comparison with the view from the surface due to perspective, as seen in various Earthrise images from orbit:
1. Apollo 8:
2. Apollo 10:
3. Apollo 11:
Earthrise is not always equally picturesque - sometimes the phase of the Earth is such that all you see is a white crescent, as Apollo 15 saw:
4. Apollo 15:
As described in Volume 1 of the Earth sub-series, according to the currently-accepted giant impactor hypothesis, much of the mass of Luna was originally part of a single body called proto-Earth shortly after planetary formation. A Mars-sized impactor called Theia is thought to have collided with proto-Earth at an angle, adding its iron core to that of proto-Earth while blasting off substantial chunks of both bodies' mantles into orbit, part of which subsequently coalesced into the Moon while the remainder rained down on Earth. A simulation of the impact:
The impact is thought to have obliterated nearly all of the water on proto-Earth, and most likely also whatever was on Theia, leaving Earth and its newly-formed Moon desiccated. Earth, however, has the gravity to retain water subsequently gained from comet impacts and outgassed volcanically while the Moon, having far lower gravity and geologic activity that died long ago, has only been able to retain a tiny fraction of it in permanently-shadowed polar craters.
Luna formed much closer to Earth than it is today, and would have been huge in the sky with titanic tidal forces acting on the surfaces of both bodies. However much water remained on Earth or accumulated in the early era after the impact would have surged unimaginably high in the tides, like mountains, and the surfaces of both bodies would have been flowing magma long after the heat of the original impact had dissipated. In fact, the Moon was so fluid in its magma state, and the orbit so close to Earth, that tides stretched it into an ovoid - although, since the lobes of the ovoid would have been pointed toward and away from Earth, it wouldn't have looked ovoid from Earth's surface. But it would have glowed a dull, angry red with darker patches of transient rock crystallization.
For a longer period, once it had settled down a bit, it would have had a substantial crust riven in places by glowing red volcanic regions, some of which would have been newly punched through the crust by impacts. Now it's basically dead, although there are still moonquakes - sometimes quite substantial ones, potentially felt by humans if they were there - due to external forces such as tides, the heating of sunrise, and vibrations from impacts.
However, because it revolves around the Earth much
faster slower than Earth itself rotates, there is a "drag" on the Earth-Moon system that has slowly caused the Moon to migrate further away over time while also increasing the length of the terrestrial day. This is called tidal acceleration, because in essence the Earth's spin is accelerating the Moon's orbit and causing it to spiral outward. We will never lose it entirely, though - the Sun will destroy both Earth and Moon long before there would be enough time.
Much of our understanding of the history of the solar system comes from attempts to date lunar craters, and one of the most significant results has been the Late Heavy Bombardment (LHB) - a theorized period in solar system history 4.1 to 3.8 billion years ago when the amount of inner-solar system impacts dramatically spikes. The LHB is a conclusion drawn from statistical analysis of lunar craters and their correlation with craters on other bodies and is largely accepted, although there are some researchers who disagree that a concentrated period of activity is necessarily indicated.
The leading explanation for why the LHB occurred is that Jupiter and Saturn had migrated outward into 2:1 resonant orbits (i.e., Jupiter orbiting twice for every Saturn orbit) in comet- and asteroid-heavy regions of the solar system, and thus began flinging objects willy-nilly, many inward toward the inner planets. Once they had cleared out their new orbital regions and migrated out of resonance, the activity would have died down.
Lunar history is divided into
five six periods: Pre-Nectarian (formation to 3.92 billion years ago), Nectarian (3.92 to 3.85 billion years ago), Early Imbrian and Late Imbrian (3.85 to 3.2 billion years ago), Eratosthenian (3.2 to 1.1 billion years ago), and Copernican (1.1 billion years ago to the present). A diagram correlating lunar and terrestrial time periods, with units in millions of years (see Volume 1 of the Earth sub-series for descriptions of the terrestrial intervals):
The Pre-Nectarian was the period during which Luna was largely a ball of magma and cooled into a body with regular surface features, with different types of rock minerals crystallizing at different times. The Nectarian corresponds to the LHB, and is characterized by the formation of many large impact basins including the eponymous Mare Nectaris (Sea of Nectar). Mare Imbrium, Crisium, Tranquilitatis (where Neil Armstrong and Buzz Aldrin landed), Serenitatis, and Fecunditatis, among others formed at the tail-end of the LHB corresponding to the beginning of the Early Imbrian period. Lunar maria are features easily visible to the naked eye from Earth, so it's understandable that their formation would be used to define geologic eras on the Moon.
However, the interiors of the maria were not originally smooth lava plains the way they are today, so they would have just been very large craters. It is thought that they filled with lava basalt over an extended period of time due to the thinness of the crust at those locations making them more vulnerable to occasional volcanism, and this process is used to define the Late Imbrian period. The Eratosthenian period is defined as falling between the formation of craters Eratosthenes and Copernicus (which lends its name to the subsequent, current period). I don't know if there's any deeper global significance to the Eratosthenian and Copernican periods or if they're just used as convenient time-markers. A high-level map of the Near Side:
Relatively little has happened in the intervening millions of years since the last huge impact, with the most recent significant event being that a few delicate blobs of water, protein, and nucleic acid made some footprints and left behind some machinery on the lunar surface a few decades ago. In the near-total absence of erosion beyond micrometeorite impacts, the Moon is basically a museum of pristine geological artifacts going back eons. If you feel awe walking through the ancient ruins of Rome, Athens, or the Middle East, just imagine walking through a landscape that hasn't changed in hundreds of millions or even billions of years. The Apollo astronauts had the opportunity to touch rocks that were unimaginably ancient when the organic goo of the boiling terrestrial sea started self-replicating.
1. Orbital and Rotational Features
Luna is "tidally locked" to its primary, meaning that it makes one full rotation in the same time it completes one orbit around Earth - about 28 days. As a result, it always presents the same face to Earth, so the Moon is divided into two regions - the Near Side, which we all know and love, and the relatively obscure Far Side whose face humanity only first saw in the 1960s. There is no "Dark Side," contrary to the Pink Floyd album - both sides get the same amount of sunlight. When the Near Side is in New Moon phase and not illuminated, the Far Side is at the height of its day and would appear full from a point on the opposite side of the Moon from Earth. It was not always tidally locked, but the tidal acceleration mentioned above slowed it down until it stopped rotating independently of its orbit.
The Moon also appears to "wobble" during its cycle in Earth's sky (a phenomenon called libration) due to several arcane orbital factors: For one, its orbital plane is tilted from Earth's equatorial plane, so sometimes we're looking at it from a more southerly perspective than at other times; for another, its tidal locking is not perfect - sometimes it rotates a little slower than its orbit, and sometimes a little faster, but maintains the same face toward Earth only because the two deviances balance out over the complete cycle. It also occurs because a terrestrial observer can look at the Moon from locations as far apart as the diameter of the Earth over a complete terrestrial day. So even though Luna is tidally locked, we actually get to see 59% of its surface rather than just 50%, although the remainder is always on the fringes and isn't very significant to the naked eye.
There is one fact about the Earth-Moon relationship that may not be obvious, particularly given the famousness of "Earthrise" photographs from lunar orbit: Earth does not rise or set in the lunar sky - it's always in more or less the same place (within the limits of libration) depending on where you are on the Moon. It is always in the same part of the horizon if you are in a polar region or the boundary between the Near and Far Side, always near the zenith from a low latitude close to the center of the Near Side, and will never appear in the Far Side sky outside the 9% boundary area mentioned above that periodically comes into view due to libration. "Earthrise" is simply due to the spacecraft moving from the Far to the Near Side, not the orbital motions of Earth and Luna. Earth does have phases though, so sometimes it's not visible from the Near Side or may only be dully visible.
The Japanese SELENE/Kaguya lunar probe captured an Earthrise video sequence in 2007, although some of the processing versions left the image looking flat and unappealing - one of the potential pitfalls of HD imagery without properly taking dimensionality into account. However, the zoomed-in version seems to reasonably capture the sequence, although I still find that it falls short of the spooky power of the Apollo stills:
Two types of eclipse occur in the Earth-Moon relationship - solar and lunar - although their definitions depend on a terrestrial frame of reference, since a "lunar" eclipse would be a solar eclipse if you were actually on Luna. We define a solar eclipse to be when the Moon occults the Sun in part or in whole from the terrestrial surface, and a lunar eclipse to be when the Earth casts its shadow on the Moon. These events occur much less often than the lunar cycle because the Earth-Moon orbital plane is tilted from the plane of the ecliptic (the plane of Earth's orbit around the Sun), so for an eclipse to occur, the interposing body has to cross the ecliptic at a time when there is a more or less straight line between it, the other body, and the Sun: Lack of alignment will not produce eclipse, but will simply have the sunward body pass "above" or "below" the Sun, and will usually be invisible. Unfortunately, we have no photographs from the Moon of either event given their infrequency. Earth-based videos of the two forms of eclipse:
Both forms of eclipse occurred far more frequently in the past when the Moon was closer to Earth, and in early times occurred every single cycle - the Moon was large enough in the sky that it never passed "above" or "below" the Sun, but was able to completely block it with every transit. In the future, there will be no total solar eclipses because the Moon will get smaller in the sky as it recedes, and will not be able to fully obscure the Sun even when it transits in full alignment. A note about the lunar eclipse video above: The Moon looks red because of sunlight filtering through the limb of Earth's atmosphere, which is pretty much the same process that makes terrestrial sunsets appear red. So what you're seeing there is an Earth sunset shining on the Moon.
On a more abstruse note, it's worth discussing the Earth-Moon Lagrange points (EMLs) - one of the most significant and auspicious features of the E-M orbital system. A Lagrange point is a feature of all two-body orbital relationships, and is a set of five geometrically-determined points in empty space surrounding the two bodies where their respective gravitational forces achieve a dynamic balance. This sounds abstract, but it is highly useful: Traveling from one Lagrange point to another takes very little energy if you can afford to have a longer transit time - e.g., with unmanned space probes - so the amount of mass dedicated to fuel can be lower, and thus more can be dedicated to instruments.
The other important feature of Lagrange points - and one which will likely become significant to human exploration and settlement of space - is that you can "orbit" some Lagrange points even though there's nothing there. This means you can put things there (e.g., space stations, fuel depots, communications nodes, etc.) and not have to use fuel to keep them there the way you do for installations in Earth or lunar orbit. Several solar observatories are already occupying Earth-Sun Lagrange points, and the EMLs will likely come into greater use as lunar exploration ramps up. The way a Lagrange point works is that it's a region where the two bodies trade gravitational dominance as they move, so an object under their combined influence in the region moves back and forth as if being acted upon by some invisible object at the point, but it's an illusion. A diagram of Lagrange points:
The configuration above is standard for all two-body systems, and results from the simple geometric interplay of their respective gravitational forces. L1, L2, and L3 of a system are called the linear Lagrange points, since they lie on a line between the two bodies, while L4 and L5 are called triangular Lagrange points because they each occur 60 degrees along the orbital path, forming the third point of an equilateral triangle with the two bodies. To cite another reason why this is significant, Trojan asteroids are asteroid swarms occupying the L4 and L5 points of gas giant planets, and will some day be sources of raw material and potentially colonization.
However, not all Lagrange points are created equal - the linear variety tend to be much less stable than the triangular, but the triangular are farther away from bodies of interest. Since L4 and L5 points form vertices of equilateral triangles, they are the same distance from both bodies that the bodies are from each other, so EML-4 and EML-5 are as far away from both the Earth and Moon as the Earth and Moon are from each other. Still, this is purely a problem of distance rather than effort - because forces are balanced at L4 and L5, it doesn't take nearly as much energy to get from one of them to one of the bodies in the system than it would take to move directly between the bodies. This is why EML-4 and EML-5 have long been considered ideal locations for space stations - far more suitable than Earth orbit, which is difficult to access, difficult to stay in, and difficult to leave for destinations farther out.
This does not mean, however, that linear Lagrange points are not potentially useful. In particular, EML-2's proximity to the Moon makes it very attractive, and the fact that it's on the Far Side means it could be particularly useful for observatories seeking to avoid the radio noise of Earth. The instability of linear points is that they are only stable in some planes of motion, while in others they are perturbed, but the presence of some degree of stability is still useful. The EMLs are most likely to be humanity's training ground for understanding and using Lagrange points in general, which will likely be very helpful in other locations in the solar system.
2. Size and Mass Characteristics
Luna has slightly more than a quarter of the Earth's radius, but only a bit more than 1.2% of its mass - a fact due to its origins as relatively low-density mantle material. However, gravity doesn't change linearly with mass, so lunar surface gravity is about 1/6 of that on Earth even though it only has an eightieth the mass, so if you weigh 150 lbs here you would (inside a habitat, without a bulky spacesuit) weigh around 25 lbs on the lunar surface. It wouldn't be as bad as weightlessness, but you would still want to move around more slowly than on Earth to avoid making yourself dizzy. Some fun size comparisons of Luna with other major solid bodies in the solar system - mouse over to see the name if you don't recognize them:
Without the insulating effects of gases and liquids, temperature swings radically between light and shadow, and even more extremely between day and night when lighting or absence thereof is persistent. The effect is intensified by the fact that the Moon only rotates roughly once every 28 days, meaning that any given point on its surface spends about two weeks basking in the Sun and then two weeks radiating into frigid vacuum at night. The day/night cycle is most radical at the lunar equator, where high noon temperatures can be 370 K (97 °C / 207 ºF), and midnight can be 100K (-173 °C / -279 ºF). So astronauts walking on the lunar surface at low latitudes don't only need their suits to protect them from vacuum, but also from boiling heat and frigid cold.
Higher latitudes are colder both day and night than equatorial regions, but experience a much lower range - a fact that makes them attractive for human exploration and settlement, since it's easier to design systems for a consistently challenging environment than one that swings radically between extremes. In polar regions there are even permanently-shadowed craters where orbiter probes have detected water ice deposits, and of most interest is Shackleton crater at the South pole - a 19 km-wide crater whose interior is permanently shadowed while its rim is permanently sunlit (a good location for solar panels to power systems operating in the permanent shade):
Unfortunately, I haven't found any temperature maps, although I'm sure the raw data for any given latitude is out there somewhere.
4. Internal Structure
The interior of Luna is dominated by its crust and mantle, since these were overwhelmingly the materials it formed from. However, it does possess a small iron core less than half the size expected of a body that formed independently. The exact layering and proportions of the interior have not been strongly proven yet, but researchers are in the process of doing so via gravity-mapping - a process whereby two spacecraft orbit a body and use comparisons of their orbital motions to determine internal structure. This is the purpose of the Gravity Recovery and Interior Laboratory (GRAIL) mission, whose twin spacecraft entered lunar orbit last month. Interesting note: The GRAIL spacecraft made their way to the Moon slowly using a low-energy Lagrange point trajectory, although it was the Earth-Sun L1 point that was used rather than any of the EMLs.
We do have a pretty good bead on the elemental composition of the crust, although the molecular composition is more difficult to determine - e.g., hydrogen was detected in the polar regions many years before it could be proven that it indicated water. Elemental composition:
As you can see, generating a breathable air supply is not going to be a problem on the Moon for long-term habitation. But as you can also see from the absence of significant hydrogen (at least outside of polar craters), water will definitely be a critical constraint on both the location and growth patterns of settlements - at least in the early history of lunar colonization. It is, however, likely that technology will eventually (I use the term loosely) enable affordable bulk shipping of water from Earth or elsewhere, or at least hydrogen (in the form of some safe, convenient, H-rich compounds) to combine with locally mined oxygen to obtain both water and energy. But this is speculation.
5. Surface Features
The complete map of the lunar surface is obtainable in two noteworthy places: The US Geological Survey (a cornucopia of geological lunar maps here) and Google Moon, which has a cylindrical map and notes all of the Apollo landing sites. I'll start out this section with some images of the Far Side, a global topographic map, and then a map of Apollo landing sites with some choice imagery.
For the first image, you'll note there appears to be a "seam" running from North to South - this is just an artifact of the mosaic process used to compose the image. Secondly, you'll notice that the dark, smooth maria that play such a crucial role in defining the face of the Near Side are relatively absent here, and consequently there are a lot more craters. For whatever reason, impacts on the Far Side were less able to break through the crust and unleash lava to resurface their interiors. Although there's no strict boundary between the Far and Near sides, the second image clearly shows the transition taking place as the maria give way to multiply-cratered terrain where several ages and sizes of craters overlay each other. About 1% of the Far Side consists of maria, as compared to 31% of the Near Side.
It is not clear what caused the radical difference in terrain between the two sides, but one theory holds that the Theia impact that created the Moon also created a smaller Trojan companion at a Lagrange point that was subsequently perturbed and collided with the Near Side, drastically weakening its crust and allowing basalt magma to upwell and form the maria. This is just one theory, and has not achieved consensus. However, the Far Side does offer opportunities for radio astronomy, since it is shielded by the entire bulk of the Moon from Earth's radio chatter. A full orbital view and topographic map of the Moon:
There are 9,099 named features on the Moon, according to the official naming body for celestial objects and features, the International Astronomical Union (IAU - link leads to database of lunar features). Craters are named after great explorers, scholars, and scientists of history, in addition to deceased astronauts and cosmonauts in a few areas whose overall names are associated with either Russia or the United States. The maria and some linear features are named after weather patterns or abstract concepts. Montes (mountains) are named after mountains on Earth, or else in accordance with the names of nearby craters. Rupes (scarps) and rilles are features caused by the contraction of the crust due to internal cooling, and are named after nearby mountain ranges. Valles (valleys) and other features are named for nearby areas in general. Given the long history of lunar observation, it is difficult to make generalizations about nomenclature, but the IAU has sought to impose standards over time.
Now for some awesome images from the Lunar Reconnaissance Orbiter (LRO) - if I don't specifically note what they are, just mouse over for their names. Tycho crater should be popular with lunar mountain climbers some day:
Boulders on a hill in Anaxagoras crater:
Boulder-strewn crater near Mare Australe:
Cracked Western floor of Giordano Bruno crater:
A pit in the Marius Hills:
The eroded crater wall of Far Side crater Moore F:
Pit in Mare Tranquilitatis:
Two images of pit in Mare Ingenii:
Linné crater full-view, followed by a zoom-in:
An oblique view of Cabeus Crater:
Some orbital views from Apollo 8 and 10 (Apollo 9 was strictly an Earth-orbit mission to demonstrate hardware):
Now we can descend all the way to the surface, beginning with a map of the Apollo landing sites - naturally all on the Near Side:
Apollo 11, from Mare Tranquilitatis:
We learned a few things on Apollo 11 worth mentioning here: One, the maria are boring - exactly as the mission planners had hoped they would be. They're not totally featureless, as the various craters and hillocks in the images above show, but they are relatively flat and uneventful. Two, shadows on the Moon are inky, near-absolute black, creating a spooky but majestic scene. Three, the lunar regolith is surprisingly cohesive, as can be seen in the smoothness of the footprints. And, of course, the most important discovery of all: IT CAN BE DONE. Such a finding can never be un-learned, however far we fall from the optimism and dedication of the Space Race.
On a side note about regolith, it wouldn't be known until later but Moondust has a very sharp, brittle micro-structure similar to asbestos that would probably make long-term exposure dangerous. It's a particularly thorny issue, because it also has a tendency to stick to everything, so it will be difficult for future explorers to clean off their suits and equipment when entering habitats from outside. Part of its cohesiveness is also a tendency to build up electrical charges, which might in itself prove problematic to the reliability of equipment. I'm sure, however, that people will figure out some effective cleaning procedure.
It should also be noted at this point that the Moon is, in fact, as black as coal - its silvery, shining appearance is an optical illusion due to the fact that even coal is brighter in sunlight than the black of space. If by some magic we were to put the Moon in front of something that is actually silver, it would look like a huge lump of coal.
Apollo 12, from Oceanus Procellarum:
Apollo 14, from the Fra Mauro Highlands:
Apollo 15, from Hadley Rille - a sinuous system of rolling hills and ridges (mission planners were no longer insistent on landing in maria):
With the introduction of the more mountainous environment and the lunar rover, people began to get a better appreciation of what the Moon is all about. It also introduced humanity to the perspective-defying properties of vacuum, because here were these landforms that could have been a few hundred meters away or multiple kilometers; could have been not much taller than the crew, or tower over them once they reached the base; and you could never quite tell because the characteristic dimming with distance that comes from having an atmosphere is absent. You simply cannot know if it's a big thing far away or a small thing closer just by looking at it.
Apollo 16, from the Descartes Highlands:
Apollo 17 from the Taurus-Littrow valley:
Luna has no atmosphere, but that doesn't mean there aren't isolated gas molecules floating around it. In particular, rocky bodies in vacuum exhibit a property called sputtering whereby micrometeorite impacts and the constant effects of high-energy particles from the solar wind bombard the surface and vaporize small amounts of regolith. As a result, there are very low levels of sodium, potassium, oxygen, etc. gases wisping around that either return to the surface or are energized by solar energy to the point of escaping lunar gravity. However, this is not significant to human affairs. The International Space Station in low-Earth orbit actually passes through thicker gases than one could find on the lunar surface.
7. Magnetic Field
The Moon probably once had a magnetic field when its core convected, but due both to its small size and relatively rapid cooling, it no longer has a geodynamo to product a magnetic field. However, there are weak local electromagnetic fields on the surface that were induced by the original field and remain today. The tendency of lunar regolith to build up electrical charge may have unforeseen consequences when it interacts with these local fields. We should not be surprised if weird phenomena in this department are discovered by future explorers.
IV. Past Relevance to Humanity
The cycles of the Moon are deeply bound into terrestrial biology because of its primeval influence on tides, although there is no clear scientific understanding of exactly how this influence operates - it may occur in a number of different ways simultaneously, including the simple fact that full Moons provide more light. However, one thing we do know is that the phases of the Moon were essential to early timekeeping, since it was one of the few reliable celestial markers humans had to mark timescales longer than a single day. With the advent of agriculture, it became crucial to the ability of farmers to know when to plant and harvest their crops, and with that came profound religious associations, mythologies, and rituals.
In the early centuries of the modern era, it became instead a fanciful frontier populated with any number of exotic creatures. The technology didn't exist at the time to definitively rule out such notions, but industrialization and growing confidence in the ideals of progress awakened people's imaginations to the possibility of other worlds being actual places rather than just lights in the sky. Since the Moon is the closest and by far most obviously a world unto itself, it became the natural setting for many early works of fantasy, adventure, and speculation. Even today it is still attractive as a setting for imaginative works, although now the focus is overwhelmingly on what people do with it rather than what already exists there.
V. Modern Relevance to Humanity
Luna continues to be an object of beauty and romantic associations, a perennially popular setting for science fiction, and a place of great national prestige for those countries involved in exploring it or planning to do so. It is, however, more significant today as an inspirational object rather than as a direct goal, since the current reasons for going are highly specialized - e.g., astronomy, tourism, theoretical applications of Helium-3, etc. - or else articulated in terms of being a "stepping stone" to elsewhere rather than a destination unto itself. However, I do think this will change.
The Moon is also the focus of the Google Lunar X-Prize - a $30 million prize competition to the first private team to successfully deploy an unmanned rover to the surface of the Moon and complete various tasks. There are over two dozen registered teams in the competition, although only a handful are considered "serious."
VI. Future Relevance to Humanity
Economically speaking, it will probably be cheaper in the very-long-run to build things on the Moon and then send it to Earth than vice-versa, and the same logic may apply to other destinations as well. What you have on the Moon is a significant gravity field that is nonetheless much easier to escape than Earth's, in addition to have no interfering atmosphere, so once an industrial base is located there you could build things and send them anywhere at a fraction of the cost and risk of doing it on Earth.
For instance, you could manufacture rockets intended for use in vacuum already in that state, and flight-test them right there without having to contend with the enormous complexities of weightlessness during the manufacturing process. In fact, every system and component designed to operate in vacuum would be easier to make and test where vacuum is ubiquitous rather than having to recreate it in expensive, power-hungry test chambers on Earth. It would probably be more economical, in the final analysis, to build things in vacuum designed to survive atmosphere than vice-versa, so I think it's reasonable that the terrestrial manufacturing base would migrate to Luna in coming centuries.
Now, it's important to note that these are unit costs I'm talking about - obviously the fixed costs involved in implementing such an industrial base in the first place would be gigantic. That's why I'm placing this in the context of the medium-term future rather than citing it as a justification for lunar exploration, because simply going there, surviving, and doing science would not enable this capability. The relative scarcity of water would hold back such a development for some time, but other than agriculture and the direct needs of colonists, it seems likely that innovators would find more locally efficient ways of cooling machinery or suspending chemicals than using water. And ultimately, I don't think exporting water from Earth to support a mutually beneficial industrial base on the Moon would be more expensive than trying to export high-mass finished products.
This is perhaps the more rigorously defensible version of Luna as a "stepping stone" - not merely a training ground for exploring further destinations, but an actual economic support base for manufacturing the vehicles, habitats, components, and maybe even bulk food that is sent beyond the environs of the Earth-Moon system.
I will say this categorically: The fates of Earth and Luna are entwined. They are simply too close, and too mutually accessible for lunar civilizations to ever be fully independent - they will always be subject to terrestrial political and economic forces of one kind or another, and in prosperous times exposed to the temptations of conquerors looking down on the wealth of much older civilizations than theirs. However, within this fact is a very wide diversity of possibilities - the Moon need not ever evolve into a distinctive, long-lasting culture: It could just as easily become a soulless company town or resort stocked with temporary workers, mercenary security soldiers, and the rootless flotsam of idle high society, all ultimately with their loyalties and pocketbooks aimed back at Earth.
It could also become the Wild West, or a refuge for those fleeing terrestrial nations in their hours of trial, or the private palace and Death Star of some tyrant and his sycophants, brimming with weapons all pointed at the blue jewel in his crown. You never quite know with history: Would the Moon become 18th-century Britain to Earth's Continental Europe, or 20th-century Central America to Earth's United States? A powerful, thriving, innovative society with a small population and influential sociopolitics, or a world raped and battered by callous external forces? Frankly, we don't have the precedents to know: Colonizing other worlds would be more like life crawling on to land for the first time than people moving from one continent to another.
Longer-term, terraformation is a possibility for the Moon, and for precisely the reasons that are normally cited as making it sub-optimal for colonization. Basically, the Moon has no atmosphere not because it can't hold one at all - it just can't hold one for millions of years without some sort of process replenishing it, and human beings hardly need to concern themselves with timescales that large. Meanwhile, there is plenty of gas locked up in lunar rocks that could be released and accumulate, provided settlers were willing to completely annihilate the eerily beautiful vacuum-desiccated environment they had inherited. That is ultimately the moral bargain of terraformation: You can create a new world, but only at the price of obliterating an existing one. Here is Daein Ballard's artistic interpretation of a terraformed Luna:
It's a beautiful vision, and perhaps a possible one, but I doubt the outcome would be so cut-and-dried as to result in a blue world that looks like Earth. The Moon has properties that differentiate it from Earth, one of which is the fact that it has no magnetosphere to shield a hypothetical atmosphere from the solar wind. What that means is that, most likely, the upper layers of a lunar atmosphere would be continually breaking down and reforming into smog-like chemicals that diffuse light - in other words, there would be a thick global cloud layer. That doesn't mean the surface still couldn't be quite pleasant, but it might not look like much from Earth or space.
Then there's the matter of getting from here to there - a process that would likely involve extraordinarily violent environmental events. Going from a sharp, microcrystalline dust in vacuum to an environment with an atmosphere and liquids would cause tremendous upheavals during which much of the Moon would be unpredictable and dangerous. Landslides, sink-holes, flash floods, hurricanes, massive lightning discharges, etc. And that's really the danger of terraforming any world that doesn't already have an atmosphere and some level of hydrosphere - the willingness to wait it out while the system reaches a new equilibrium. On balance, lunar colonists probably would not be willing to waste so much time, effort, and resources on a potentially chaotic outcome whose benefits, if any, would only come to their distant descendents. More likely the Moon will become a manicured indoor suburb of Earth, like some affluent planned community where "nature" consists of lawns and pruned hedges.
I do think, however, that the Moon will serve as a stepping-stone to more distant destinations, even if nobody plans it that way. It's always peninsulas that give birth to the intrepid maritime civilizations, and not the bulky continental empires, and that's what I think the Moon is - a peninsula of Earth. An extension of the terrestrial sphere of influence into an environment more like what the rest of the solar system experiences. Once human beings are comfortable, safe, and proficient living in sub-g artificial habitats and making use of locally-available resources, it's not a big leap to Mars, the Asteroid Belt, the Jovian moons, etc. They have their unique challenges, of course, but the Moon is halfway to anywhere.
That is not to say that I endorse a "Moon first" ideology - I see no reason to fixate on destinations. Rather, we should just go everywhere we can simultaneously, and the natural result will be that humans flourish more quickly in the places that are most convenient - and the Moon is probably at the top of the list because of its strategic placement on the Road to Everywhere.
In the longest-term, it probably doesn't have a very good fate: It seems destined to be an object of strategic and economic interest in the middle period of solar system expansion when Earth is waning but still powerful, which spells War Zone if not utter ruin. But that's many centuries into the future, and I am after all just speculating. We may instead find that it becomes a totally irrelevant backwater, like some town that had depended on a railroad only to see it diverted to another path. In the furthest future, however, it probably will be gobbled up for raw material.
VII. Future of Luna
Absent human intervention, the Moon's rate of outward migration will slow as greater distance reduces the intensity of tidal acceleration, so it will still be visible in the sky as a small (but still visibly round) object by the time the Sun engulfs it and the Earth billions of years from now.
VIII. Catalog of Exploration
Exploration of the Moon has been quite extensive compared to other solar system bodies, so I won't try to comprehensively cover it here. The Wikipedia page for exploration of the Moon covers it quite nicely, including past, current, and future missions. All told, there have been 102 separate missions to the Moon to date, 67 of which have been at least partial successes, and 3 of which are currently operating successfully in ongoing missions. There are also six missions planned for launch within the next two years. However, I continue to be irked that none of them involve human crew.
10:17 PM PT: I'm informed that uncertainty about the sizes of Pluto and Eris calls into question the earlier figures I'd seen indicating they're bigger than the Moon. The issue should be reasonably cleared up by the time I get around to the Kuiper Belt and Scattered Disk diary where the two will be be discussed.