After reading the comments to a question recently posed at Reddit, I’m once again struck by how quickly a serious discussion about space can fly off the rails without knowledge of basic facts and their implications. The question that was raised is “Why is everyone so eager to colonize Mars, while the Moon, with its proximity and low gravity, sits empty?” As you might expect, the comments on this question vary widely in their relevance and cognizance. I thought it might be useful to collate some of the relevant facts that must be considered in determining which body is most useful for learning the life skills of an off-planet species.
Of course, the word “colonize” is loaded with different interpretations, but in this case, I take it to mean the establishment of permanent human settlements on either world. As is so often the case, the discussion at Reddit quickly turns toward comparing the two objects in terms of their resources and surface environments. While some of the comments are well informed, many misconceptions about the properties of both objects are readily evident – both confusing the casual reader and inhibiting the discussion.
Humans need raw materials, wherever they live, including light elements (e.g., oxygen, hydrogen and carbon (see page 4 of this paper), usually associated with the needs of life support, such as air, water and food) and heavier elements (needed to make things, including structures and machines). Energy is required to process this material into whatever form is required. Fortunately, all of the objects of the inner Solar System are rich in materials, although their concentrations vary from place to place. The critical controlling factor on whether a place can be inhabited is the availability of a reliable and continuous source of energy.
There is no “Second Eden” in our Solar System. Wherever people travel in the space around our Sun, they will have to create a protective environment to shield their bodies from the harsh conditions that they encounter. Because we are talking about not merely exploring, but rather living off the Earth, we need to be able to make what we need to survive from locally available materials. Naturally, some places are easier to settle than others, but when deciding which locations have more merit, it is important to fully understand all the requirements for habitation, not just the most obvious (albeit critical ones), such as the availability of water or the depth of the local gravity well. The key light element materials needed to support life are the so-called CHON elements (carbon, hydrogen, oxygen and nitrogen). Water supplies the middle two, but sources of both carbon and nitrogen must be found and available for harvest. After collection, we must be able to find or synthesize the substances needed, which involves a lot of chemical processing, time, and energy.
The Moon is depleted in light elements (although large quantities are present near the poles) but is well endowed in the heavier rock-forming elements (e.g., iron and aluminum). Over billions of years of micrometeorite bombardment, the lunar surface has been ground into a fine, grain-sized dust of jagged, angular fragments of minerals and glass. Moving parts quickly become immobile when coated with this talcum powder-like, abrasive dust. Future lunar inhabitants will need to mitigate these effects, as well as protect themselves from the transfer and inhalation of the local surface dust. The Moon has no appreciable atmosphere (its exosphere has a surface pressure of 10-15 bar, or about one-thousandth of a trillionth of the atmospheric surface pressure of Earth). The lack of a global magnetic field means that the lunar surface is a hard radiation environment. Both solar particles (including coronal mass ejections) and galactic cosmic rays bombard its surface. Over the course of a single lunar day (28 Earth days) at the equator, the Moon experiences thermal extremes ranging from 100° C to -150° C, while at the near-permanently lit areas near the lunar poles, the temperature is a constant -50° C. Compared to the planets, the Moon’s low gravity (about 1/6 that of the Earth) makes it a relatively easy object to access and leave (something we did successfully on six occasions, 45 years ago).
Mars appears to be richer in light elements than the Moon. We know very little about the nature and abundance of the heavier elements on Mars, but meteorites (that we believe come from Mars) suggest that its crust is made of rocks quite similar to those that make up both Earth and Moon. Thus, it is likely that iron is very abundant, and it is probable that aluminum and other metals can also be found in quantity. Like the Moon, Mars also has very fine dust, but it appears to be composed of clay minerals and thus, it is likely to be both softer and less abrasive than lunar dust. However, analysis of data from landed probes suggests that Mars dust may be highly reactive chemically (including the presence of toxic substances, like peroxides). Future Mars inhabitants will need to protect themselves from these substances.
Mars has an atmosphere but it is extremely thin (surface pressure is about 6 millibars, or six thousandths of an Earth atmosphere) and is composed almost completely of carbon dioxide. The martian atmosphere can be used to aerobrake (i.e., slow down a spacecraft during landings) but its atmosphere is not thick enough to eliminate the need for significant propulsive braking. This is a problem since the martian gravity is more than twice that of the Moon, or about 3/8 (0.38) the gravity of the Earth. Landing on Mars with heavy (i.e., human-sized) landers remains an important, unsolved issue (called the Entry-Descent-Landing (EDL) problem). The deep gravity well of Mars means that bigger, more energetic spacecraft will be required to get off the planet (and streamlined, as initial passage will be through an atmosphere that, while thin, is still significant). Mars is cold, but warmer periods occur in some areas (temperatures range from about -150° C near the poles, up to almost 20° C during summer at the equator). Although its atmosphere provides some protection, the surface of Mars remains a hard radiation environment, roughly equivalent to what is received by the equipment and crew on board the International Space Station.
On both planets, humans must be protected from the local environment. Pressurized habitats are needed and must include shielding from radiation. Such protection will likely be accomplished through the use of local material as shielding, either water (an excellent radiation protective) or local soil, requiring high-power machinery to excavate and move large amounts of material. Both Moon and Mars contain significant deposits of water. On the Moon, water is found in quantity within the permanently dark floors of polar craters. Hydrogen is also implanted on the grains of the lunar soil in extremely small quantities. Water appears to be more widely distributed on Mars, being found as vapor in the atmosphere, chemically bound in clay minerals everywhere, and in some localities at higher latitudes, near the surface as ground ice.
Energy is the critical pacing item for colonization. Wherever people go in space, they will need energy and lots of it. We must create a special environment to protect ourselves, something we get naturally here on Earth. The principal sources of electrical energy in space travel are solar and nuclear. The closer you are to the Sun, the more solar energy is available. Because Mars is about 1.5 times as far from the Sun as the Earth-Moon system, solar energy is less than twice as intense there (inverse-square law). This allows small robotic spacecraft to operate on Mars with solar panels, but solar electric, as the sole source of energy for larger vehicles and facilities (such as human habitats), is not practical. It is certainly inadequate for the amounts of energy needed for resource processing necessary to support a human colony. For this reason, credible plans for the colonization of Mars rely on the continuous operation of nuclear reactors.
On the Moon, a day/night cycle of two weeks duration (at the equator) means lunar inhabitants must survive a very long, cold night without solar power. In the past, ideas about lunar habitation have always collided with this reality, leading to a requirement of a nuclear reactor. Recently, however, mapping of the Moon’s surface found areas near the poles of the Moon that remain in sunlight almost continuously. This is possible because the Moon’s spin axis is nearly perpendicular to its plane of orbit around the Sun. This discovery makes lunar habitation much more likely. We can now envision an initial human presence off-planet without the need for the near-term development of a practical space nuclear reactor (an item that does not currently exist and will require several billions of dollars for development).
One last consideration is the distance from Earth. The Moon has the advantage of being relatively close – about three days away on typical trajectories. Moreover, as it is in orbit around the Earth, the Moon is constantly available for both arrival and departure, so a quick bug-out is always an option. In contrast, launch windows to Mars occur infrequently, on the order of every two years with current technology. Transit times (one-way) are several months in duration and do not offer easy abort options. The proximity of the Moon results in instantaneous RF communications (3 seconds round trip) while the distance of Mars means that communications between Earth and Mars have time-lags of tens of minutes. Thus, habitation requires much more local autonomy at Mars than the Moon. Unless the first colonists have a death wish, these issues of proximity and access must be addressed.
We know about de-conditioning of the human body in zero gravity, but we are completely ignorant of such effects in the fractional gravity the Moon and Mars. We think that problems from radiation can be minimized, but the long-term effects of living in a shielded environment are unknown. Some focus on initial access as the biggest problem, but gravity is only one factor and consideration among many. Any debate about where to “settle” in space must be cognizant of these and many other facts. Both the Moon and Mars have their respective advantages and disadvantages. The decision over where to focus our limited resources in the near term must take into account the relative abundance of materials needed, their locations on the object and our ability to access and process them into a form that we can use.
Debate is good and is to be encouraged but only informed debate is useful and essential.
Related: A comparison of asteroids vs the Moon as a space destination can be found in this 3-part series: