Habitable Exoplanets

An imagined habitable exoplanet

Scientists are hot on the trail of habitable, earth-like planets. So what are such planets? They are places where human beings could live without any genetic manipulation. Higher forms of life characteristic of earth should thrive there. 

Habitability requires a planet large enough to maintain a gravitational hold on its atmosphere, yet not so large as to generate a crushing atmospheric pressure. Planetary gravity should not be so weak that our muscles and bones atrophy, yet not so strong that easy movement is impossible. 

Average temperature should result in liquid water as the normal state, with oceans on the planetary surface. Plate tectonics should assure a stable carbon concentration in the atmosphere, preventing a runaway increase in temperature. 

Large asteroids should not be constantly plunging through the atmosphere and striking the planetary surface. Radiation should not cause life to mutate incessantly, thus the need for a strong magnetic field to deflect the solar wind. The planet should not be tidally locked with its primary star such that one hemisphere always faces its sun and one is in perpetual darkness.

Reputable sources regarding habitable exoplanets include: Habitable Exoplanets Catalog, Habitable Zone Gallery, NASA and Planetary Biology, the latter with a sortable list of exoplanets, including habitable zone candidates.

 

Habitable Zone

 

Chart showing location of  a star's habitable zone depending on stellar mass and
radius of planetary orbit relative to earth's orbit of the sun

Habitable exoplanets need to orbit within a star's habitable zone, the region of space around a star where a planet would receive roughly the same energy as our Earth. This NASA site explains the formation of habitable planets.

The graphic to the left illustrates the variation in habitable zone around different star types. The graphic shows how both the distance of the habitable zone from the star and its width are directly dependent on the star's mass. The width of the habitable zone can also be affected by assumptions, such as cloud cover. 

Planets substantially larger than earth could harbor life. In fact, according to one article, such super earths - from about 2 to 10 Earth masses - may be superior at fostering life. 

The reader is cautioned that the definition of habitable zone differs among sources. For example, at the beginning of 2020, three sources presented widely different lists of planets in their respective habitable zone. These were (1) Habitable Exoplanets Catalog [55], (2) Planetary Biology [73], and (3) Habitable Zone Gallery [131].  NASA does not list potential habitable exoplanets.

The three lists emphasize different qualities of the subject planets and their respective stars. According to Habitable Exoplanets Catalog, sixteen (16) habitable zone planets were closest to earth (within 50 light years). According to Planetary Biology, forty (40) of its listed habitable planets orbited 'G' class suns similar to earth.

Super Earths; Gravity, Radius and Density 

An apparent disadvantage of super-earths, despite the expanded real estate, is the heavy gravity. All that additional planetary mass must be crushing.

Escaping the planet will be difficult, but super-earth gravity does not necessarily squash earthlike life forms. A remedy can be found in the planetary radius. A planet's surface gravity is determined by its mass divided by the radius squared [Gravity = M/R²]. Using that formula, a planet 9 times Earth's mass with a radius 3 times that of the Earth would have a surface gravity equal to Earth. On such a hypothetical super-earth, people would weigh the same as on Earth and have nine times the planetary surface to explore [Surface Area = 4 π x R²].

But wait, another problem now arises; the planet's density [Density = Mass/Volume = M/1.33πR³]. The density of water is 1 g/cm³. The planet Earth is 5.5 g/cm³. Our hypothetical exoplanet would have a density of 1.89 grams per cubic centimeter - closer to that of water than that of our Earth.

Internal structure of Titan, largest moon of Saturn, showing its  ice layers
Courtesy NASA

This density is virtually the same as Saturn's moon Titan. Titan is composed primarily of water. A super-earth of Titan's density and composition (located in the habitable zone with water in a liquid state) would likely be covered with an ocean well over a thousand kilometers deep. People may well explore its surface, but they will never find land upon which to settle.

Mars has a density (3.93 g/cm³) a little over 71% of Earth's. It also has an iron core which at super-earth size could generate a protective magnetic belt. Using Mars as our density model rather than Titan, we find a more satisfactory exoplanet. A hypothetical super-earth, with about 2.5 times Earth's mass and a radius about 1.54 times that planet, will have a density comparable to Mars. The surface area is almost 2.5 times that of Earth. At the same time, the surface gravity will be comparable to Earth.

Other super-earth options which achieve this earth surface gravity goal are possible. I will leave the calculations to others.

Habitable Zone and Eccentrics

Eccentric exoplanet passing through and outside its star's
habitable zone, Courtesy NASA-JPL Caltech

Certain eccentric planets whose orbit extends outside the habitable zone may nevertheless be habitable. While Earth and the other planets in our solar system travel around the sun in near-circular orbits, planets in other systems can have more comet-like orbits in which the distance from the planet to star varies. Such orbits, termed eccentric, would cause the planet to move in and out of the habitable zone.

The eccentric planet would move through the habitable zone and then further out into a long, cold winter. During this phase of the orbit, any liquid water on the planet will freeze at the surface; however, the possibility remains that life could, in theory, hibernate beneath the surface. These planets in eccentric orbits could still retain lifebearing properties depending upon the percentage of the total orbit which is spent within the Habitable Zone. This paper (a PDF is downloadable) describes the development of the habitable zone concept and its potential application to planets in extreme eccentric orbits. Another source discusses the influence of eccentricity on habitability.

Terrestrial Type

Habitability for life similar to that found on Earth requires more than just being in the habitable zone. To be habitable, the planet where life might exist should be of the terrestrial type. A terrestrial planet is primarily composed of silicate rocks and is not a gas giant. By the beginning of 2020, NASA counted 161 terrestrial planets orbiting other stars.

In contrast, NASA says scientists have discovered 1302 gas giants. Hot Jupiters, are defined as gas planets orbiting extremely close (orbit of less than 10 days) to their parent star. Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets.

These 21 exoplanets listed in 2020 are more likely to have a rocky
composition and maintain surface liquid water (i.e. 0.5 < Planet Radius
≤ 1.5 Earth radii or 0.1 < Planet Minimum Mass ≤ 5 Earth masses).
Planetary Habitability Laboratory, University of Puerto Rico at Arecibo

The Habitable Exoplanets Catalog [Click here to see most recent catalog] shows potential habitable exoplanets of the terrestrial type. They are compared with Earth and Mars using the Earth Similarity Index.

This index number is a measure of Earth-likeness, where Earth is the standard of comparison with an ESI value equal to one. Exoplanets with values above 0.8 could be considered Earth-like planets but those with values down to about 0.7 might still be habitable by microbial life.

The potential habitable exoplanets bear little resemblance to Earth. Most of the habitable planets circling dwarf stars are probably tidally locked with that star. The crushing gravity of some of the super-earths compared to Earth would result in the evolution of a higher level life form quite different from what we are familiar. The likelihood of a totally earth-like environment on any of the planets discovered so far is slim.

Need for a Moon? 

One matter of controversy respecting the existence of habitable terrestrial planets is the need for a large moon (such as Earth's moon) to stabilize the planet's obliquity (or axial tilt). An unstable obliquity could produce severe climate changes, potentially affecting the development of life. Many scientists theorized that the need for such a moon, and its expected rarity, made habitable planets less likely. A recent study concluded that the need for a moon to achieve such stability is over blown. This video of a SETI Talk presentation by Jack Lissauer goes into more detail regarding the newest thinking.

Extrasolar Planets

Moon containing life orbiting a giant gas planet within the habitable zone
of an unknown star. The image involves some artistic license. Gas giants orbiting
a sun-like star at distances similar to the Earth are likely to have clouds of water
ice, with white a characteristic color. These water clouds may be obscured
by higher layers of gas, primarily methane. Methane scatters blue light
weakly, so these deeper cloud regions will have a slight bluish tinge.

The first confirmed discovery of extrasolar planets (planets around another star - also called exoplanets) occured in 1992. Radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar. In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery.

Discovery Methods

Technological advances allowed astronomers to detect exoplanets indirectly by determining their gravitational influence on the motion of their parent stars. The planets are usually discovered by measuring the change in Doppler shift of the star's light resulting from the star orbiting a common center of mass with a companion planet. The graphic to the left demonstrates this technique.

Several extrasolar planets were detected by observing the variation in a star's apparent luminosity as a planet passed or transited in front of it. The occasional transit of Venus across the sun is an example from our own Solar System. An hypothetical transit is illustrated on the right.

The European Extrasolar Planet Encyclopedia includes a number of observational programs managed out of Versoix in France near Geneva, Switzerland. According to the Encyclopaedia, 4232 exoplanets had been discovered by the end of 2020. At the same time, NASA's Exoplanet Archive counted 4141 exoplanets confirmed, with 5075 candidate planets. The Exoplanet Archive is operated by the California Institute of Technology under contract with NASA. The difference between the two databases highlights the uncertainties involved in exoplanet detection and confirmation.

Hot Jupiter

Most discovered planets are 'Gas Giants' or 'Hot Jupiters' (similar or larger in mass to Saturn or Jupiter) orbiting very close to their sun. Given the current early stage in planet searching technology, this preponderance of giant planet discoveries in close solar orbit should not be surprising. These would be the most noticeable planetary bodies given the weak degree of sensitivity of the available instruments.

As sensitivity increases in the future and new methodologies become common, the average range in planet distance from the parent star should increase and the average observed planet size should decrease. Astronomers have discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy.

Planet Searchers

Lick Observatory - Seeker of exoplanets - in a winter snow, H Graem © 2019

The Automated Planet Finder on the top of Mount Hamilton in the mountains east of San Jose, California is one of the newest and less expensive exoplanet searchers. The telescope will also be used to search for optical signals coming from laser transmissions from hypothetical extraterrestrial civilizations. Sponsored by SETI (search for extraterrestrial intelligence).

The newest searcher is TESS or Transiting Exoplanet Survey Satellite, an MIT-led NASA mission which is an all-sky survey for transiting exoplanets. TESS will monitor more than 200,000 stars for temporary drops in brightness caused by planetary transits. This first-ever spaceborne all-sky transit survey will identify planets of all sizes.The WASP or Wide Angle Search for Planets is an international consortium of several academic organisations performing an ultra-wide angle search for exoplanets using transit photometry. The array of robotic telescopes aims to survey the entire sky, simultaneously monitoring many thousands of stars.

Interstellar Travel

An obvious problem with finding exoplanets, including the holy grail of a planet habitable by human beings, is the current impossibility of interstellar travel, traveling to other stars. Some investigators are actively searching for ways to solve this problem.  Progress in revolutionary propulsion physics discusses interstellar methods currently being investigated.  Tau Zero Foundation examines the possibility of Interstellar Propulsion, giving a realistic overview of the situation and details specific efforts to find answers.

Mars

Future oceans on a terraformed Mars

Mars has been the subject of more speculation than any other planet in the Solar System. There is also a wealth of information on the web regarding all aspects of this planet. Rather than regurgitate what may have been better expressed elsewhere, the bottom of this web page points the way to the best of these resources. 

The focus of this Mars web page will be two topics of possible significance for Mars' future: caves and terraforming. Establishment of initial human bases on the planet will be greatly facilitated by the use of natural voids that may be remodeled for habitation. A permanent human presence requires some sort of terraforming of the planet. 

Tubes and Caves

On the terraformed globe image above, Arsia Mons is the last volcano to the southwest in the straight row of three. Olympus Mons, Mars' highest mountain, is the separate volcano to the northwest of the trio. The closeup to the right of the four volcanoes brings out greater detail.

The seven openings on the right were discovered on the slopes of Arsia Mons. A detailed scan of the martian surface could probably find a lot more such openings in volcanic regions of the planet. 

Evidence that the holes may be openings to cavernous spaces comes from the temperature differences detected from infrared images taken in the afternoon vs. the pre-dawn morning. "Whether these are just deep vertical shafts or openings into spacious caverns, they are entries to the subsurface of Mars," said co-author Tim Titus of the U.S. Geological Survey in Flagstaff. "Somewhere on Mars, caves might provide a protected niche for past or current life, or shelter for humans in the future."

Lava caves can be quite large. A closeup of one of the seven, 150 meters in diameter, clearly shows a vertical shaft. 

Openings to the Martian underground on the slopes of Arsia Mons

Gravity on Mars is about 38% that of Earth, allowing Martian lava tubes to be much larger in comparison. Lava caves (with surface irregularities removed) could provide sufficient space for Mars base activity after humans first land on the planet. They could provide an environment naturally sheltered from radiation and thermal extremes. Well designed entrances would keep out the Mars dust with its oxidants.

The Caves of Mars project has evaluated the feasibility of the use of caves for the initial human habitation of the planet. One approach to remodeling the caves for human habitation would be the "cured in place" technology used on earth to rehabilitate old drainage pipes. They are re-lined with a cooled, resin filled liner which hardens when heated. A similar approach could provide an air tight lining for Mars lava tubes or manmade tunnels. Unlined caves could serve as unpressurized hangars or garages. 

Once created, some sort of hardening would avoid any tube collapse, possibly a spray-on concrete-like substance made from available materials. A more porous insulating material also made in situ could then be applied. Air tight flexible lining or inflatable self-sealing fabric habitats could retain air. The habitat would be protected from solar radiation, micrometeorites, extreme temperature fluctuations (ambient temperature is believed to be stable in lava tubes), winds, and dust storms which could pose a threat to human health and technology. These natural shelters would also reduce the landed payload mass for manned missions which would be economically advantageous.^[1]

Terraforming

Hypothetical Terraformed Mars, Wikipedia

Beyond human missions to Mars in the next 25 years, what future can we expect on the red planet in 2200? Will man have altered the planet to create an environment more conducive to a successful human society, perhaps even enabling people to flourish? 

Regarding human alteration of the planet, Robert Zubrin and Christopher McKay have set forth the technological requirements for terraforming Mars. Modified Mars, an add-on to Google Mars by Frans Blok, is a detailed and vivid vision of such a future Mars.

Terraforming of Mars has been imagined or discussed on a number of websites. How Stuff Works proposes three methods to terraform Mars.  This National Academy of Sciences article proposes keeping Mars warm with new super greenhouse gases. 

One wonders if fiction, whether books or virtual worlds, can herald a future terraformed Mars reality? The Red, Green and Blue Mars trilogy is probably the most extensive literary effort to portray the eventual terraforming of the planet. The trilogy is a tale by Kim Stanley Robinson of the exploration and settlement of Mars--riven by both personal and ideological conflicts--in the early 21st century.

Image of a future Martian terraformed location beneath Olympus Mons from Modified Mars mentioned above

Other personal sites have been created to advance the concept of terraforming Mars. Martyn Fogg has created a website containing a compendium of studies regarding terraforming, primarily related to Mars. Mars Reborn is a portrait of a possible Mars one thousand years in the future. 

Martian Overview

An overview of what we know about the red planet can be found at Wikipedia. A NASA perspective on Mars has planetary facts and figures and a gallery of images of the planet. The ESA has a great portfolio of Martian images and videos. Marsnews.com provides current news articles respecting Mars.

Google Mars is the best place to start looking for maps of the Martian surface. It is comprehensive and provides an intuitive way to find various planetary features and information regarding their origin.  

Updated presentations of Martian scientific findings are provided by NASA's JPL. Other science findings (including maps and images) may be found at MOLA and Marsoweb, both NASA sites, and this ESA Mars Express scientific findings site.

Advocates of the human exploration of Mars include the Mars Society, MarsDrive, the Mars Foundation, Red Colony and the Mars Institute. A forum for Mars enthusiasts is provided by the Mars Society.

Lunar Habitats

Initial lunar base (inflatable spherical habitat) for up to twelve people, NASA

The initial lunar outpost would need to be self-sufficient. It would need to be designed for protection against extreme temperatures and high radiation levels on the Moon. The base life support system would be entirely closed. All waste gases, liquids, and solids are recycled, and there is a food production facility including a greenhouse.

NASA developed an updated concept of an initial lunar outpost in early 2009. The first structures are currently expected to be inflatable.

With certain sites near polar heights receiving sunlight more than 80% of the time, general power requirements could be satisfied by solar energy. At a more equatorial location, for the night period of 14.75 days, when the solar power would not function and for lunar manufacturing, fuel cells or the installation of a nuclear power generating station would probably be required.

First Moon Habitat

 

Conception of a lunar colony, NASA 1986

In recent years, the idea of lunar colonization gained some momentum in the form of national and international interest and actual moon missions. 

The initial moonbase habitation could be constructed by linking together metal habitat canisters and inflatable structures. For radiation protection, these units would need to be buried under a layer of lunar regolith (soil) using excavation equipment.  The basic infrastructure will include life-support structures and climate control system, a solar power plant, an oxygen generator plant, a lunar ice processing plant, a greenhouse to produce food, plus the excavation of a Lunar spaceport, and the preparation of a supply processing and storage area.

ESA Moon Village with regolith covered habitat as viewed from
the moon surface and structure cut-away below, Courtesy ESA

A major problem with inflatable structures is that in the moon's vacuum, there is little protection from micrometeorites. Catastrophic depressurization could occur if a high velocity projectile penetrates the membrane. A solution, as with radiation, would require covering the inflatable habitats with a layer of protective regolith dug out of the moon's surface.

The European Space Agency (ESA) has proposed a Moon Village approach along these lines. A cylinder with an inflatible structure would be brought to the moon and inflated. The inflated structure and cylinder would then be covered with lunar regolith. The resulting habitat interior would be protected from radiation and micrometeorites. The regolith would insulate the habitat interior from the surface environment, with its extreme day/night range in surface temperature.

A key habitation requirement will be water. The most likely source of water will be ice on the dark bottoms of craters near the poles. Shipping water to the Moon for use by humans would be extremely expensive ($2,000 to $20,000 per kg).

In addition, extensive exploration of the lunar environment will be made for nitrogen and carbon sources necessary for food production. Although present in lunar soil, the amounts are too small to be easily extracted. Carbon could be found in meteorites and nitrogen may be found locked in gas pockets under the surface. These elements must be found if the Lunar Station is to become truly self-sufficient.

Underground Approach

Illustration of a lunar robot surveying the entrance to underground lava tubes. 
(Image credit: William Whittaker, Carnegie Mellon University)

Building the lunar colony underground would give protection from radiation, extreme surface temperature changes and micrometeoroids. The area around prospective sites would first be explored for the presence of lunar lava tubes (drained conduits of underground lava rivers) for future underground habitation. Lava tubes could provide an environment naturally sheltered from radiation, thermal extremes, and micrometeoroid impact. Even more important at the early stage of human lunar settlement, there use would be less expensive than excavation of manmade tunnels.

Lava tube entrance on earth

In support of this approach, scientists at the Indian Space Research Organization in 2011 discovered a giant underground chamber on the moon, which they feel could be used as a base by astronauts on future manned missions to moon. An analysis by an instrument on Chandrayaan-1 revealed a 1.7-km long and 120-metre wide cave near the moon's equator. It is located in the Oceanus Procellarum area of the moon and has been proposed as a suitable 'base station' for future human missions. Scientists of the Space Applications Centre in Ahmedabad said that the cave provides "a safe environment from hazardous radiations, micro-meteoritic impacts, extreme temperatures and dust storms." Another possible skylight entrance to an underground cave has been discovered in the Marius Hills.

Lava tube skylight candidates in Philolaus Crater.
(NASA/Lunar Reconnaissance Orbiter/SETI Institute/
Mars Institute/Pascal Lee)

Using an orbiting radar sounder system that can examine underground structures, scientists at the Japan Aerospace Exploration Agency (Jaxa) confirmed the presence of a cave under an opening 50 meters wide and 50 meters deep. Gravity data showed that the cavern is part of a larger chain that extends for 60 kilometers (37 miles). It is at least 1 kilometer (0.6 miles) high and wide, beginning a few tens to hundreds of meters below the surface. It appears to be structurally sound and its rocks may contain ice or water deposits that could be turned into fuel, according to data sent back by the orbiter, nicknamed Kaguya.

The problem with lunar caves far from the poles is that it is not where we want it.  Human presence on the Moon requires material and energy resources sufficient to support human life and operations around the Moon.  After years of study and exploration, we now know that these locations are near the poles of the Moon.  Both poles are in the highlands and finding a lava tube in such non-volcanic terrain has been thought highly unlikely. For this reason, a discovery announced in 2018 was like music to the ears of lunar explorers.

In 2018, the SETI Institute and the Mars Institute announced the discovery of small pits on the northeastern floor of Philolaus Crater, a large, 43 mile (70 km)-diameter impact crater located about 340 miles (550 km) from the North Pole of the Moon, on the lunar near side. These pits may be entrances to an underground network of lava tubes. The pits were identified through analysis of imaging data from NASA’s Lunar Reconnaissance Orbiter (LRO).

Potential lunar tubes and rills in the Oceanus Procellarum
that could contain future habitats

If water ice is present, these potential lava tube entrances or skylights might allow future explorers easier access to subsurface ice, and therefore water, than if they had to excavate the gritty ice-rich “regolith” (surface rubble) at the actual lunar poles. The candidate pits inside Philolaus are located at such a high latitude that sunlight would never enter the underlying caves. These would be so cold that ice could be cold-trapped in them, much like it is in the permanently shadowed regions at the actual poles of the moon.

The pits appear as small rimless depressions, typically 50 to 100 feet across (15 to 30 meters), with completely shadowed interiors. The pits are located along sections of winding channels, known on the Moon as “sinuous rilles,” that crisscross the floor of Philolaus Crater. Lunar sinuous rilles are generally thought to be collapsed, or partially collapsed, lava tubes, underground tunnels that were once streams of flowing lava.

Cities

Once lunar settlement has passed beyond the initial colony stage to permanent cities, construction of labyrinthine underground corridors and greater voids in the rocky crust becomes possible. Advances in technology would need to produce fast tunnel cutters that melt through the lunar rock and form a structurally sound finished surface.

The rock overhead shields the inhabitants from radiation, changes in temperature during the day/night cycle and virtually all meteors. As an additional safety measure, airlocks could separate neighborhoods. Parks and gardens could be created in excavated voids using artificial light or beneath transparent domes roofing craters. Agriculture produce would be grown hydroponically during the lunar day in excavated channels covered with a transparent skylight that could be covered with an insulated moveable roof during the lunar night.

Von Braun Moon City

The crater walls would be excellent places to locate windows, allowing the inhabitants to look out on the actual lunar surface. The actual windows would be set back within the wall to eliminate penetration by micrometeors. The rock overhang could provide radiation and meteor protection.

Adjacent domed craters could be parks containing trees growing to great size and height in the weak gravity. With the moon's light gravity, it would be feasible for the domes to be constructed of material able to shield against small meteors and radiation.

Within most of the city, outside views would be video perspectives of lunar or earthly scenes displayed on walls. Power would most likely be generated by fission or fusion energy or the sun. The latter source would come from solar cells located on heights at the lunar poles able to catch the maximum period of daylight. Power lines laid in trenches cut in the lunar surface would connect the solar power sources to the lunar cities in non-polar regions. Two hundred years in the future, the lunar cities could be connected by underground tubes through which trains would travel.

Given the relatively light gravity compared to earth, exercise for the able bodied would be mandatory for physical health. Should long-term presence in such a light gravity be ill advised (persons would weigh less than one fifth of their earth weight), periodic stays on earth may be required.

 

Lunar Activities

Lunar base control room, Courtesy NASA, artist Rick Guidice

An obvious question related to the exploration and settlement of the moon is, "What do we do there?" America's NASA asked a similar question in 2006 and developed a set of Lunar Exploration Objectives.

Initially, lunar residents will focus on obtaining those products that would make them self-sufficient and not dependent on expensive products brought from earth. It may be impossible to completely eliminate dependence on earth products, but the goal should be to maximize the use of lunar sources that are less expensive than products transported up from earth's gravity well. Alternatively, sources elsewhere in the solar system, such as asteroids, should be investigated. 

Mining

Mining on Moon, Courtesy of NASA

For long term sustainability, a space colony should be close to self sufficient. On site mining and refining of the Moon's materials could provide an advantage over deliveries from Earth – for use both on the Moon and elsewhere in the solar system – as they can be launched into space at a much lower energy cost than from Earth. With the expected immense cost of interplanetary exploration in the 21st century, the lower cost of providing goods from the Moon could be very attractive.

 

Water and Oxygen Extraction

Water is a key resources the moon-base will require. Deposits of ice on the Moon would have many practical aspects for future manned lunar exploration. Recent discoveries confirm what planners had hoped, there is a lot of water on the moon. The water is most abundant at the poles, especially in the form of ice in crater walls and bottoms which never see the sun. 

Mining these ice deposits would save a lot of money. Shipping water to the Moon from Earth would be extremely expensive ($2,000 to $20,000 per kg). The lunar water could also serve as a source of oxygen and hydrogen. 

 

Oxygen is necessary for human life and both oxygen and hydrogen are constituents of rocket fuel. By weight, moon rocks are 40 to 45% oxygen. By heating the top meter of 1 acre of moon dust to 1300 degrees Celsius, we extract 3000 to 3500 tons of oxygen. Extracting oxygen requires 450 calories of energy per kilogram of oxygen produced.

Proposed oxygen extraction process

Recently, scientists have significantly improved the oxygen extraction process from the lunar soil (regolith). You get a lot of oxygen plus the metal alloys with which it was bound. Both of these will be really useful on any future lunar bases or colonies.

The processing was performed using a method called molten salt electrolysis. The regolith is placed in a mesh-lined basket. Calcium chloride - the electrolyte - is added, and the mix is heated to around 950 degrees Celsius, a temperature that doesn't melt the material. Then, an electrical current is applied. This extracts the oxygen, and migrates the salt to an anode, where it can be easily removed. The metal left behind is usable - the first time a lunar regolith oxygen extraction technique has produced this result.

 

Other Mining Products

By weight, in order of abundance, the chart to the right compares the percent of elements in lunar soil with the Earth. As on Earth, these percentages will differ depending on location.

The soil of the Moon, called lunar regolith by scientists, is a rich source of metals and oxygen that can be strip mined. Silicon can be used for solar panels and building construction material. Iron for construction, calcium for cement, and magnesium for vehicles.

The lunar highlands are filled with ­aluminum, a lightweight and sturdy material used in buildings, aircraft, vehicles and medical devices.

Titanium can replace steel, chromium and manganese for alloys. Strong and light titanium is found mainly in the mineral ilmenite, which also contains iron and oxy­gen.

Sodium is used to make caustic soda, important in many industries. Potassium and nitrogen for agriculture, sulfur for acid and farming, carbon and hydrogen for water and chemicals, and helium-3 for fusion energy.

The regolith can be melted in solar furnaces and cast into forms to make ceramic items. Glass, cement and concrete can be produced. If we increase the temperature to 1500 degrees Celsius, lunar soil will melt, allowing the extraction of various minerals and other elements. 

For the moon, the amount of hydrogen in the soil is listed as PPM (parts per million) as it is extremely important for human needs, but quite rare on the moon. To get a single kilogram of Hydrogen, we would have to mine 25 tons of lunar soil. Lunar soil contains very little of the other lighter elements, necessary for biology, such as sulfur, carbon, and nitrogen. They are still more abundant than hydrogen. Luckily, the discovery of large sources of water ice at the poles significantly reduces the problem.

Since about 100 million tons of regolith must be heated to about 1400 deg. F to get one ton of helium-3; 4000 tons of hydrogen; 2800 tons of helium-4. 10,000 tons of nitrogen; 20,000 tons of carbon and 54,000 tons of sulfur will also be obtained. Though helium-3 is scant in regolith, there’s still a lot more in spots like the Sea of Tranquility than on Earth.

Robot guiding mining machine on Moon, Courtesy NASA

Hydrogen and carbon do exist in amounts worth scavenging in the upper layers of Lunar soil, put there by the incessant solar wind. From Apollo samples we might expect every thousand tons of soil processed to yield ( besides over 400 tons of oxygen ) one ton of hydrogen, 230 lbs. of carbon, and even 164 lbs. of nitrogen. This is hardly abundance and not enough to support a lunar biosphere if the population on the Moon grows to a considerable size. To satisfy lunar needs for rarer elements (carbon and nitrogen) we may need to turn eventually to carbonaceous chondrite asteroids in near earth orbit.

Exporting material to Earth is problematic due to the high cost of transportation. One suggested candidate is helium-3 from the solar wind, which may have accumulated on the Moon's surface over billions of years. It may prove to be a desirable fuel in fusion reactors and is rare on Earth. The abundance of helium-3 on the lunar surface and the feasibility of its use in fusion power plants will need to be established. China has made measurement of helium-3 abundance on the lunar surface one of the goals of its exploration program.

Another export candidate might be rare earths. Fresh deposits of ­­rare-​­earth elements (17 highly conductive metals used in tech like hybrid car batteries and phones) are scarce on Earth. In spots rich in potassium and phosphorus, the moon could host REE mines on par with the best ones we have at home.

 

Power Generation

 

Solar Energy

Solar power facility, Courtesy NASA

Solar energy is a strong candidate. It could prove to be a relatively cheap source of power for a lunar base, especially since many of the raw materials needed for solar panel production can be extracted on site. However, the long lunar night (14 Earth days) is a drawback for solar power on the Moon. This might be solved by building several power plants, so that at least one of them is always in daylight. 

Another possibility could be to build such a power plant where there is constant or near-constant sunlight. The Lunar poles contain a number of such potential power generating locations. Possible locations are Shackleton Crater or Malapert mountain near the lunar south pole, or on the rim of Peary creater near the north pole. 

Near-permanent sunlit areas on the Moon shown using
Clementine spacecraft images superimposed on a Lunar
South Pole radar image. Credit: Arecibo Observatory

The solar energy converters need not be silicon solar panels. It may be more feasible to use the larger temperature difference between sun and shade to run heat engine generators. Concentrated sunlight could also be relayed via mirrors and used directly for lighting, agriculture and process heat. The focused heat can also be employed in materials processing to extract various elements from lunar surface materials.

Although water would be found at the bottom of craters permanently shadowed from the sun, near continuous sunlight for power generation would require high topography to catch the sun. Specifically, the requirement would be heights near the poles that catch sunlight more than the 75% of the time. Given the power needs associated with water extraction and lunar mining, creation of significant power sources on the moon will be a high priority.

At the lunar poles, in addition to permanently shadowed areas where water can be found, there are higher areas such as crater rims which are permanently exposed  to sunlight and could serve as a source of power. The south pole is a primary setting for such peaks of eternal light.

 

Fuel cells

PEM fuel cell

Fuel cells on the Space Shuttle have operated reliably for up to 17 days at a time. On the Moon, they would only be needed for 14.75 days - the length of the Lunar night. During the Lunar day, solar panels (either photo voltaic or Solar Thermal) could be used as well as providing the electricity necessary to convert the water ('waste' from the fuel cells) back into hydrogen and oxygen ready for the next Lunar night.

Current fuel cell technology is far more advanced than the Shuttle's cells - PEM (Proton Exchange Membrane) cells produce considerably less heat (requiring smaller, lighter radiators) and are physically lighter - more economical to launch from Earth. NASA is now actively examining its fuel cell options for future moon missions.

 

Nuclear Fission

Nuclear power facility, Courtesy NASA

A nuclear fission reactor could possibly be able to fulfill some of the need for power. The advantage it has against a fusion reactor is that it is an already existing technology. Even if the fuel had to be brought from earth, its low weight to energy generated ratio may make it competitive with solar.

 

Nuclear Fusion

Helium-3 is a non-radioactive isotope of helium available in significant quantities on the moon, but rare on Earth, which will really give value to the Moon when fusion reactors that can burn the stuff are developed. The Moon’s surface is covered with many meters of regolith that stores low but ubiquitous concentrations of helium-3, an isotope of helium that undergoes fusion reactions which may ultimately be tapped for energy. The development of lunar helium-3 could also lead to the development of fusion rocket propulsion systems, with long-term implications for interplanetary missions in terms of reduced trip times and associated reductions in astronaut exposure to weightlessness and radiation in space. However, reliable, efficient fusion reactors using helium-3 will first need to be developed.

Solar Power Satellites

Space-based Solar Power

Gerald K. O'Neill pointed out that we have on the moon's surface the materials we need to produce solar cells and other elements to build solar power stations in Earth or Lunar orbit. Lunar soils contain 20 percent silicon for solar cells, and about 20 percent metals. Much of the rest is oxygen. The moon has two other great advantages as a source of materials: a weak gravitational grip and a vacuum environment. This makes it practical to locate electric mass-drivers on its surface which would be capable of lofting a steady stream of small payloads to a precise collection point high in space where solar power satellites could function.

Manufacturing

Obviously, mining of the lunar regolith could lead to the manufacture of products containing the elements derived from the lunar soil. Some products derived from mining include metals, chemicals, solar panels, construction iron, concrete, glass, vehicles, and agriculture materials. The moon would the ideal location for manufacturing that requires a sterile, low-gravity environment in a vacuum; research and processing of potentially dangerous life forms or nanotechnology, and long-term storage of radioactive materials. Unique products may be producible in the nearly limitless extreme vacuum of the lunar surface, and these may support a high degree of lunar settlement self-sufficiency. The Moon's remoteness is the ultimate isolation for biologically hazardous experiments.

 

Agriculture

For self-sufficiency we will need to grow plants on the moon for food, recycling, and replenishing the air. Many vegetables mature within 60 days on earth. Given almost 30 earth days with 24 hours of sunlight during a lunar day, lunar greenhouses should be able to produce food for moonbase inhabitants and visitors.

Cultivation of plants for food can provide a basic diet for astronauts as well as psychological benefits for people living for years in cramped quarters. Some animals might also be taken for food.

The Moon can provide a laboratory for testing the viability of plants and animals in the space environment, particularly the possibility of multigenerational propagation of plants. Research could occur on the possibility of obtaining the nutrients needed for healthy plant growth from native moon materials. Some animal research dealing with the effects of gravity might also be undertaken. 

As shown in the graphic, agriculture produce would be grown hydroponically or in soil during the lunar day. The plants could be grown in tanks or raised beds located in excavated channels covered with transparent skylights. The channel walls could provide insulation from radiation and the great temperature changes during the day/night cycle. Radiators could remove excess heat. An insulated moveable roof could be rolled over the agriculture channel during the lunar night so as to minimize heat loss during this coldest period on the moon.

Hydroponic plant growth

Consideration should be given to growing plants in as reduced an atmospheric pressure as possible. There are two reasons. First, it'll help reduce the weight of the supplies that need to be lifted off the earth. Even air has mass. Second, Martian and lunar greenhouses must hold up in places where the outside atmospheric pressures are, at best, less than one percent of Earth-normal. Those greenhouses will be easier to construct, maintain and operate if their interior pressure is also very low -- perhaps only one-sixteenth of Earth normal.

Research will need to focus on developing plants that can thrive in this low pressure environment. Such low pressure agriculture research has already started.

The problem is, in such extreme low pressures, plants have to work hard to survive. There's no reason for them to have learned to favorably interpret the biochemical signals induced by low pressure. Low pressure makes plants act as if they're drying out. There are also benefits to a low pressure environment. In low pressure, not only water, but also plant hormones are flushed from the plant more quickly. So a hormone, for example, that causes plants to die of old age might move through the organism before it takes effect.

 

Astronomy

Since the Moon's days (about fourteen Earth days) have a dark sky, it allows for nonstop astronomical observations. Disadvantages include micrometeorite impacts, cosmic and solar radiation, lunar dust, moon quakes, and temperature shifts as large as 350° Celsius.  Regarding moon quakes, the tidal forces that the Earth exerts on the Moon are more than 20 times greater than the Moon's tidal forces on Earth, enough to cause the Moon to experience considerable moon quakes.

The moon's greatest advantage over space as a telescope site is that it offers a stable platform. Small, remote-controlled reflecting telescopes could be located at scattered locations. A number of small telescopes, separated by hundreds or thousands of kilometers yet exactly located relative to each other on the moon's stable platform, could group together to form, in effect, a telescope hundreds or thousands of kilometers across. This giant telescope could resolve earth-like planets orbiting other stars. The telescopes could also operate as individual instruments. they could even be controlled by astronomers on Earth through the Internet.

The International Lunar Observatory (ILO) is a multi-national, multi-wavelength astrophysical observatory, power station and communications center that is planned to be operational near the South Pole on the lunar surface. The southern hemisphere sky contains many objects interesting to astronomers, including the Magellanic Clouds and the Galactic Core. The mission's objective is to conduct astrophysical observations from the surface of the Moon, whose lack of atmosphere eliminates much of the need for costly adaptive optics technology.

Radio Telescopes

Observatory with a radio telescope built into the lunar surface, Courtesy NASA

GPS communications, microwave ovens, radar, cell phone and WiFi signals, and even digital cameras are among the many terrestrial sources that contaminate radio observatories. On the far side of the Moon, all of humanity's sources of interference are 100% blocked. It is the most pristine environment for radio astronomy available.

Signals of the early stages of the Big Bang, inflation, and the formation of the first stars could be discovered and recorded with a lunar radio telescope. While this may be possible either on Earth or in space, the lunar far side offers more sensitivity, due to being shielded from Earth, than any other option.

The moon would be an excellent location for SETI (search for extraterrestrial intelligence) activity. With a rarer vacuum, the view would be clearer than near-earth orbital space. Raw materials abundant on the moon, like silicon and aluminum, can be used in construction of giant telescopes. The farside is the best place in the solar system for sensitive SETI radio searches. It is insulated by 3,500 feet of rock and is always turned away from the earth and its myriad of radio frequency sources. When the sun is down for the two week night, there is no quieter radio environment.

 

Tourism

What would tourists do on the moon? Probably pretty much the same things they do on earth - visit historical and scenic spots and interesting cities. Historical places would be the Apollo landing sites of 30 or more years ago. Scenic spots include great craters, high cliffs, cracks the size of canyons, mountains and strange valleys. There is also the farside of the moon. Interesting cities are in the moon's potential future. Large, pressurized domes or caverns would permit human-powered flight in the moon's light gravity, which may result in new sports activities. The low gravity may find health uses such as enabling the physically enfeebled to enjoy life in a more rewarding environment.

Trial Run

The Moon can serve as a proving ground for a wide range of space operations and processes. This includes learning to "live off the land" (self-sufficiency) for human outposts on Mars and elsewhere in the solar system as well as on the Moon itself.

A Future Objective - Mount Olympus Mons on Mars

The outpost should prioritize not only field-testing equipment destined for Mars, but developing lunar building materials so that future outpost expansion can rely on locally produced materials. Regolith element harvesting techniques will be tested. Prototype solar collectors made wholly or almost wholly from lunar materials could be created. With ready sources of metal and a low gravity, the moon would enable the building of large space vehicles with practically no limits on cargo volume.

Technology developed for a Lunar colony would likely have application to other potential space venues, including near-Earth asteroids and Mercury, which has many similarities to the Moon. A lunar outpost program must include a program to establish the limitations of the human body for long stays on the Moon and re-adaptation when returning to Earth. These findings will be applicable to the human exploration of Mars.