Having landed Chang’e 4 in South Pole-Aitken basin, China intends its sample-return mission, Chang’e 5, for Mons Rumker in Oceanus Procellarum, an intriguing plateau studded with apparently young volcanic domes that was at one point a potential Apollo landing site.
India’s Vikram lander and rover, part of its Chandrayaan-2 mission, is also set to land on the nearside, at a crater called Manzinus. Only about 600km from the South Pole, it will use a ground-penetrating radar to look for buried ice. Beresheet, the lander developed by an Israeli organisation, SpaceIL, is destined for a northerly nearside crater, Berzelius, which sits between Mare Serenitatis and Mare Crisium. The site is interesting because it is a place where the crust has a faint magnetic field. These little remnant magnetic fields are an enduring lunar mystery; Beresheet, which means “In the beginning”, is hoped to mark a first step towards solving it.
Beresheet’s range of possible landing sites is, unsurprisingly for a modest mission, quite constrained. Were it not, it might be heading for one of the particularly intriguing magnetic anomalies that are associated with “lunar swirls”, pale ribbon-like loops overlaid on the darker regolith.* The patterns and the fields seem sure to be related in some way—no swirls have been seen without magnetic fields—but as yet no one knows how. One possibility is that the magnetic fields provide partial shelter from the solar wind, and thus slow the weathering of the regolith. Another is that they are formed through interactions between the magnetic fields and dust electrified by ultraviolet radiation. Or it may be that the swirls are meteor showers in reverse.
Meteor showers in the skies of Earth occur when tiny particles of comet dust hit the edge of the atmosphere at high speed. The lunar swirls may be created when a passing atmosphere hits tiny particles of sedentary moondust just as fast. When a comet’s nucleus passes very close to the lunar surface, its tenuous atmosphere (called a coma, in a comet’s case) may burn up the smallest particles in the regolith.
To sort out the possibilities requires some way of getting very close to the swirls. Just landing won’t work: the structure of the magnetic fields matters, and that structure is only perceptible if you move across them—the closer to the surface, the better. Hence the rather wonderfully named BOLAS mission (Bi-sat Observations of Lunar Atmosphere above Swirls) developed at Goddard Space Flight Center for NASA. It is one of many lunar missions currently being suggested that make use of very small spacecraft called cubesats.
Two almost identical satellites, Bolas-L and Bolas-H, are launched clasped together and put into a “frozen” orbit around the Moon, an elliptical path which at times brings them to an altitude of less than 15km. Once settled in, they let each other go—but remain attached by a very light tether. They line up radially with respect to the Moon, Bolas-L below, Bolas-H above. As the tether is paid out, Bolas-L begins to sink and Bolas-H to rise.
This means that neither satellite is moving at the orbital speed appropriate to its altitude. Bolas-L is moving too slowly; it should be falling towards the Moon. Bolas-H is moving too quickly; it should be flying off into space. But thanks to the tether, Bolas-H’s centrifugal tendency is pulling Bolas-L up, and Bolas-L’s tendency to fall is weighing Bolas-H down. The tension in the tether keeps them lined up one above the other even as they move farther and farther apart. They are like the two tidal bulges in Earth’s oceans, one pulled by the Moon more strongly, one less, but lined up together.
At the end of the unspooling, the two spacecraft will be 25km apart—and Bolas-L will, at closest approach, be just two and a half kilometres above the surface. That is far lower than any stable orbit: but thanks to the tether, Bolas-L and Bolas-H are still, as far as the force of gravity is concerned, in the original “frozen” orbit; their centre of mass remains at the mid-point of the tether.
One of the things that BOLAS could study as it skims the surface is the rate at which hydrogen from the solar wind gets into the regolith and how much water-making chemistry it might get up to once there. In 2020 Moon Express intends to look at some of the same processes when it lands the first of its intended series of Lunar Scouts in a region called Rima Bode, quite close to the valley of Taurus Littrow, the landing site of Apollo 17’s Challenger. In his fieldwork at Taurus Littrow, Harrison Schmitt identified “pyroclastic” deposits in the regolith—deposits formed when an eruption sprays lava high above the surface in something like a fire fountain, giving it time to freeze into droplets of glass before it gets back to the surface. Rima Bode, too, seems to be rich in pyroclastics, but of a different chemical make-up, and much more weathered; the ones at Taurus Littrow, which had been preserved under a lava flow then uncovered by a recent impact, were a bright and distinctive orange.
The Rima Bode pyroclastics offer potentially fascinating science, because they offer samples from deep inside the Moon. They might also be of practical interest. Paul Spudis, the American scientist who did more than any other to argue the scientific case for returning to the Moon, and sadly died before seeing it, believed that well-weathered pyroclastics with a lot of titanium in their make-up, like those of Rima Bode, were likely to be particularly good at absorbing hydrogen from the solar wind, and that the fairly uniform size of the particles would make regolith rich in them relatively easy to process. There is thus a hint of practical prospecting in the choice of the landing site. But, more importantly, there is an appreciation by the people at Moon Express of Spudis’s devotion to the Moon.
Astrobotic intends to land at Lacus Mortis, which lies between Mare Frigorum and Mare Serenitatis, next to a peculiar pit that might be the opening of a lava tube. When lava below the surface keeps flowing after the surface has solidified, it can leave a long cylindrical void behind it. Most Earthly cavemaking relies on water’s powers of erosion and dissolution, but lava tubes do not. They may thus be the only caves the dry Moon has to offer.
In 2015 Japanese radar studies of the Marius Hills, another set of volcanic domes in Oceanus Procellarum, found an intriguing double return, as though some of the radar signal was bouncing off the surface and some off a second surface some way below. NASA’s Grail mission, a pair of satellites which measured the Moon’s gravity field with exquisite accuracy, showed that the crust is of lower-than-average density in the same area. And there is a pit in the surface which looks as if the roof of a subterranean void has collapsed. Put it all together and you have one of the best candidate caves yet found on the Moon. And it looks big. Though Earthly lava tubes may be kilometres long, they are typically just a few metres across. The tubes in the Marius Hills, if tubes they be, could be hundreds of metres across, and maybe 75m high—twice the height of the nave of Chartres Cathedral. Computer models suggest that fast-flowing lavas and low gravity might allow some lunar lava tubes to be larger still, perhaps a kilometre or more in height and two or three times that in breadth. The low gravity is crucial; it means that the weight of the rock above the void is much less than it would be on Earth.
This is not just an opportunity for off-Earth spelunking. The tubes might be good places to live. A lunar settlement is very unlikely to be a trailer park of spacecraft-like buildings on the surface. A structure that is heated to well above 100°C and then chilled to liquid-nitrogen temperatures every month faces an alarming amount of stress and strain. And the surface is peppered not just with micrometeoroids but also with cosmic rays—high-energy protons—that the Earth is protected from by its magnetosphere. Worse, there are barrages of protons thrown off the Sun by events known as coronal mass ejections.
Over a 100-day stay on the Moon’s surface, even one spent in a hab shielded against background radiation of cosmic rays, astronauts would be exposed to a 13% chance of a “solar proton event”, as they are known, strong enough to raise their cancer risk significantly. There would be a 5% chance of one strong enough to cause prompt radiation sickness, and a 0.5% risk of one that would be fatal. One such particularly savage event took place in early August 1972. If the Sun had lashed out in the same way four months e
arlier, Charlie Duke, Ken Mattingly and John Young, the crew of Apollo 16, would have been killed. If it had done so four months later it would have killed Apollo 17’s Gene Cernan, Ronald Evans and Harrison Schmitt. There are ways of providing advance warning of such events using satellites hanging between the Earth and the Sun at the Sun-Earth L1 point. But such warnings are only useful if there is somewhere to go for shelter.
A surface hab thus needs a thickly shielded inner sanctum into which the crew can retreat—something which adds to the mass. Putting your living quarters in a cave provides shielding from all such radiation throughout the living area. It also offers a pretty constant temperature, and shelter from micrometeorites, too. Hence some of the interest in lunar lava tubes. If the wormholes in the Moon’s green cheese could be made airtight, they might be underground analogues for O’Neill’s “Islands” in the sky, voluminous enough for towns, maybe even cities. Boa Vista, the estate of the helium-mining Cortes dynasty in Ian McDonald’s novel “New Moon”, is a fetching example: stretched out below Mare Fecunditatis, 100m across and fully pressurised, its lush vegetation watered by the spray of fountains and the streams running down its gently inclined length, studded with grand haciendas, graceful pavilions and private glades, with stairs, apartments and balconies built into the basalt walls beneath vast bas reliefs of the Orixas of the Umbanda religion and, higher still, the bright blue fusion-lit sky, it is a samba-inflected mixture of Mar-a-Lago, Rivendell, the cave dwelling of the Grand Lunar in H. G. Wells’s “First Men in the Moon” (1901) and a Bond villain’s lair. The parties are great.
Is Boa Vista any more unlikely in the 22nd or 23rd century than the sprawling high rises of São Paulo were 500 years ago? I cannot say (though I am pretty sure it would not be financed off helium-3 exports). For the time being, though, there is no prospect of sealing such a cavity off and filling it with air—not least because the air would freeze. Keeping a whole lava tube warm would be a power-hungry undertaking. Far easier, at least in the early years, to bring habs up from Earth, manoeuvre them into trenches dug for the purpose or modestly reshaped small craters, and then cover them with a few metres of loose regolith: they would stay a lot warmer than in a deep cave and be just as well shielded from radiation.
In time, baked-regolith bricks and melted-regolith glass might be added to the architect’s repertoire. An interesting set of studies of such dwellings, built over and around inflated balloons, has been carried out by Foster + Partners; its founder, Norman Foster, is an architect particularly attuned to the simple forms of flight and space. It seems likely that most of life will still take place under such mantles, if not fully underground. The transparent surface domes which delight science fiction artists have little to recommend them. Light for the crops, which any decent-sized base will need to grow, is better provided by light-emitting diodes tuned to the most photosynthetically efficient wavelengths than by windows that see no Sun for two weeks at a time.
The likelihood that bases will be built in burrows of their own, though, is not to say that lava tubes are not worth looking into, figuratively and literally. They are cold because, like the permanently shadowed craters, they are never sunlit. That means water vapour and other volatiles released by impacts will refreeze in the caves just as they do in those craters. In general the craters are a better bet for ice miners; a very tenuous vapour will not get very far into an airless cave. But you can imagine circumstances where a particular coming together of an impact, or impacts, and a specific cave system might create something interesting and valuable, such as the once-ice-filled lava bubble under Oceanus Procellarum used as a scientific base in Greg Bear’s “Heads” (1993).
As robotic exploration continues, expect more interest in the search for such oddities, for unforeseen structures created by chains of independent or rare events. Or, just possibly, intelligence. To expect to find alien artefacts on the Moon would be to go too far. But if there are or have ever been alien intelligences in this part of the universe, and if over the four billion years or so before humankind came along they ever visited this solar system, and if they wanted to leave some sign of their passing for some future intelligence to find, the unchanging Moon that stands close by the only living world in the system would seem the most obvious place to leave it. It was this idea, as developed in Arthur C. Clarke’s short story “The Sentinel” (1951), that provided the seed for Clarke and Stanley Kubrick’s “2001: A Space Odyssey”. The idea served as a narrative bridge that let the film jump from the plausible near future of solar system exploration to the interstellar weirdness of higher intelligences. But both men also knew it was a good speculative idea in itself.
Do I think it is worth searching for such artefacts in a deliberate, diligent and expensive way? No. But just as I think it is worth examining radio signals from the universe at large to see if any show signs of intelligence, so I think it is at least worth keeping an open mind about the possibilities of extraterrestrial intelligence when humans and robots look for oddities on the Moon.
And oddities there will be. The processes that shape the Moon are admittedly few compared to those that shape the Earth, the raw materials they work on far more limited, the ability of the environment to push and shove and mould things in interesting ways almost non-existent. But it still has a surface bigger than Africa’s, a surface that has had billions of years to develop quirks and oddities, for coincidence to pile on coincidence so as to provide truly unlikely accidents. Not all these oddities will be as easily distinguishable from orbit as the magnetic swirls or as intriguing as the lava tubes. Few if any may be of much practical use. But scientifically they may prove intriguing, at least to cognoscenti. And they might be more than that.
NEVERTHELESS, AS THAT PAPER BY MR WINGO SHOWS, THE poles—the Moon’s most striking oddities—remain the best bet for human bases early on in the Return, and perhaps even for its first human landings. This is for reasons of practicality, potential and politics.
The practical reason is power. Power on the Moon is going to be solar or nuclear, and as yet there are no suitable nuclear options. Lunar nuclear reactors would need to be very light, by the standards of such things, if they were to be shipped up from Earth. They would need to work with little if any water (of which most power reactors use quite a lot) and to have very low maintenance requirements. They would also have to be so safe that a government would be willing to license one being launched from its territory.
For the moment that leaves solar. And for solar power, as for crops, 14-day nights are a problem. A solar-powered lunar settlement would need enough panels to provide more than twice the power it needs during the day and enough batteries to store the unused half of that power for use in the night. That’s a lot of capital expenditure.
After working on a study of the costs of solar power on the Moon, Geoff Landis, a NASA engineer who is also a poet and science fiction writer, took to fiction to explore a quirky alternative to batteries: mobility. “A Walk in the Sun” (1992), one of the nicest lunar iterations of science fiction’s perennial resourceful-individual-against-the-hard-facts-of-the-cosmos trope, tells the story of Trish Milligan who, having survived a crash landing on the Moon, needs to hold out for a month as a rescue mission is mounted. She has a spacesuit with big solar panels and a lot of protein bars, but not much by way of batteries. So she decides there is nothing for it other than to walk all the way round the Moon, keeping pace with the Sun.
Unluckily for her, she crashed near the equator, meaning she has to walk a distance the same as that from New York to Los Angeles at an average of 16 kilometres per hour if she is to stay ahead of the night-edge. Easy enough when bounding along at a sixth of her terrestrial weight on the smooth maria; harder in mountains and the unrelenting highlands of the farside.
If she had crashed at a higher latitude, she could have set an easier pace. Up at 70°N, around the latitude of Mare Frigorum, so named because it is the Moon’s northernmost maria, you can stroll west at less than six kilo
metres an hour and keep the Sun on your shoulders forever. And right up at the poles there are places where you hardly need to move at all.
The Moon’s upright posture with respect to the plane of the ecliptic means that, at its poles, the Sun sits near permanently on the horizon. This tangential lighting is what allows the depths of polar craters to be forever cold and dark; it also allows polar uplands near perpetual day. These sunlit uplands are not a new idea. In “The Moon” (1837), Wilhelm Beer and Johan Madler pointed out that the Moon’s obliquity meant there could be places at the poles that saw little or no night; Claude Flammarion, taking up the idea some decades later, dubbed such places “peaks of eternal light”. Robert Goddard wrote about the possibility, too, as well as that of permanent cold spots in craters below the peaks. Now both types of feature have been seen and quantified.
Today’s maps of the Moon show that there are raised areas of, if not eternal, then very long-lasting, light at both poles—ribbons of raised land on crater rims which see the Sun more than 80% of the time, and where solar panels mounted vertically, like the sails of a ship, could realistically provide three times the average power of systems elsewhere that would get sunshine only half the time. For a solar-powered moonbase, you would need a very good reason not to go to one of the poles.
Beyond power, the poles also offer potential, in the form of those frozen volatiles. The presence of water in significant amounts will, other things being equal, make it easier to sustain a moonbase and to refuel rockets there. There are other sources of propellant on the Moon—you could pair solar-wind hydrogen absorbed in the regolith with oxygen torn out of various minerals. But though hydrogen is thousands of times more concentrated in the regolith than helium-3, you would still need to process quite a lot of regolith for serious amounts. And pulling apart minerals for their oxygen takes a lot of energy. Energy itself is cheap on the Moon, at least during daylight; the capital equipment needed to gather it up and use it will probably not be. Hence the attraction of the volatiles at the poles that can be liberated with the equivalent of a kettle.
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