- Status Report
- Feb 2, 2023
Congressional Testimony of Timothy D. Swindle: Lunar Science & Resources: Future Options
Chairman Rohrabacher, members of the committee, ladies and gentlemen: Thank you for the invitation to talk about issues regarding lunar science and lunar resources. Today, I wish to address one topic in each. First, we are beginning to understand that lunar science is important because the Moon contains clues to the earliest history of Earth, perhaps even of the start of life on Earth. Second, there are aspects of fundamental lunar science that we need to understand better to be able to assess whether it would be worthwhile to try to exploit the Moon for certain resources that might be used on Earth. In specific example of helium-3, while it is clear that it is a prodigious undertaking, we need more information to know the real magnitude of the task.
The first evidence of life on Earth comes in rocks that are approximately 3.8 billion years old. The evidence is ambiguous, and it is unlikely that we will be able to use terrestrial rocks to learn much more about what was happening then, or earlier, because there are so few rocks this old or older. Could we use the Moon to understand what was going on on Earth?
The large circular dark features on the Moon that we see from Earth are dark lava flows that fill basins, which were formed by impacts larger than any that have occurred on Earth in the last three billion years. Perhaps surprisingly, analysis of the rocks returned by the Apollo program showed that the vast majority of impact-derived rocks are roughly the same age, between 3.8 and 4.0 billion years old. Few are younger than 3.8 billion years old; none are older than 4.0 billion. Although a number of rocks older than 4.0 billion years were brought back, extending back to the age of the Moon itself nearly 4.5 billion years ago, none of the older rocks were the types formed in large impacts.
Two possibilities have been suggested to explain the tight clustering in ages of the impact rocks from the Apollo collection. One is that the Moon actually had not had many, if any, large impacts in the previous 0.5 billion years, and then had a cataclysmic bombardment. The biggest problem with the idea of a late cataclysm is that we do not understand where the objects causing the bombardment could have been stored for that length of time before suddenly being released on the inner Solar System. The other possibility suggested is that there were many large impacts all along, but the rocks formed in the earlier impacts were all destroyed in the final few impacts, the terminal portion of this heavy bombardment. The biggest problem with the idea of a terminal heavy bombardment is the difficulty with destroying the old rocks produced in impacts while leaving intact many other rocks formed in that same period.
The question of the early impact history of the Moon has important implications for the early history of Earth. If the Moon suffered several large impacts in any given period of time, Earth probably suffered many more, since Earth is nearby (in solar system terms), but larger. This is probably why we find no intact rocks on Earth older than the time of the final basin-forming events on the Moon, whether it was a cataclysm or a terminal heavy bombardment.
Assuming that the lunar basins were only the last in a continuous string of large impacts, the end of a heavy bombardment, some scientists have suggested that there was an “impact frustration” of life. Life may have arisen long before the first evidence we find, and perhaps may have even begun more than once, but was wiped out, or “frustrated,” as one impact after another hit the Earth with enough energy to boil the oceans. In this scenario, life began on Earth virtually as soon as it became possible.
On the other hand, if there was a cataclysmic bombardment, then there may have been a long, relatively peaceful period on Earth in which life could have started. In this scenario, life might have been more abundant when the cataclysm began, but it could only have survived in some niches away from the oceans and away from the Earth’s surface. There are some primitive organisms even today that might be suitable candidates for such survival. In this scenario, it is also possible that it took hundreds of millions of years of clement conditions for life to start, reducing the chances of finding life on a planet like Mars, whose climate was only pleasant for a brief period of time.
These two alternatives provide fundamentally different expectations about the formation of life on Earth, and its likelihood elsewhere.
There is also the question of where and when Earth got its water. Water is crucial for life as we know it, but in many models of the formation of the Earth, the planet formed from material that would have lacked water because that material came from too close to the Sun. Hence, the water may have been added at some later time by the impacts of comets or water-rich asteroids. There are few clues available on Earth to address these questions, but the Moon does contain clues. The rocks formed in lunar impact basins often contain rare elements that can serve as tracers for the bodies that impacted. As we learn more about the compositions of more of the bodies that impacted the Moon, we learn more about the bodies that impacted Earth, and hence we learn more about how much material of what kind was added to Earth, and when it was added.
Although we continue to try to address these problem in a variety of ways, we are hampered by the fact that the only lunar rocks we have from known locations are the Apollo samples, and the Apollo samples all come from a rather restricted region on the near side of the Moon where the more recent basin-sized impacts occurred. One of the arguments for going back to the Moon is to try to decipher the Moon’s early impact history by studying rocks from a variety of known locations that are not near the latest basins. We can do this robotically, and it would be a great improvement over the suite of samples currently available, but having humans present is far superior. During the Apollo missions, the astronauts looked at far more rocks than they returned, and judiciously chose what were almost certainly the best samples available. Robotic technology is still far from being able to separate out the subtleties that a human easily can.
Exactly how and when life started on Earth is one of the great scientific questions of the 21st Century. Understanding the history of the bombardment of the Moon will not tell us how life started on Earth, but it will tell us far more about what the conditions were on Earth when life did start. Furthermore, the record stored within the rocks on the Moon is of times and events whose record no longer exists on Earth.
Going from the distant past to the near future, understanding basic lunar science is also crucial to understanding whether certain ideas for using lunar resources are even feasible, much less how to implement them. As an example, Gerald Kulcinski, Harrison Schmitt, and co-workers have proposed the use of the rare helium-3 from the Moon as a fuel for clean-burning fusion reactors.
Helium, and particularly its lighter stable isotope helium-3, is rare on Earth. Helium is light enough that any atom currently in the atmosphere is likely to escape from Earth’s gravity at some time over the next million years or so. Earth’s atmosphere is gaining some helium from Earth’s interior or by trapping it from space, but it is in tiny amounts, only about 10 kg per year: helium-3 is less than 1 part per trillion of the atmosphere by mass, and extraction is clearly extremely difficult.
There is, however, a body in the Solar System with huge reserves of helium-3 – the Sun itself. The Sun contains a total of about 1.4 x 1026 kg of helium-3, a larger mass than the entire mass of the Earth. Going to the Sun to get that potential fuel is far beyond our presently imagined capabilities, but the Sun expels some of its outer layer in a rather steady flow of particles, the solar wind. The solar wind flux at the distance of the Earth from the Sun is about 0.005 g of helium-3 per km2 per year. The Earth’s magnetic field deflects virtually all of this solar wind, but the Moon has no magnetic field or atmosphere of its own, so the solar wind is implanted into the surface of the Moon, except during the portion of the month when the Moon is shielded by the Earth’s magnetic field.
Kulcinski and Schmitt have suggested the possibility of mining the Moon for helium-3, and have suggested one ton per year as a target amount. In 1990, I was part of a group at the University of Arizona that evaluated the potential of this idea. We considered how much helium-3 the Moon actually contains, how well we know that amount, and how we might mine it. Although there have been some advances in lunar science since then that have caused us to revise our estimates slightly, we still have not been able to learn anything more definitive about the two most critical parameters, the distribution of helium-3 with depth in the lunar regolith and the distribution of helium-3 with location on the Moon. Our estimates of the total helium-3 contained in the lunar regolith (the lunar “soil”) ranged from 450,000 to 4.6 million tons, based just on plausible variations in these two factors.
The amount varies with depth within the regolith because solar wind is implanted only 0.1 to 0.2 microns deep (a micron is one millionth of a meter). Small impacts stir the regolith, so that helium-bearing grains on the surface can get mixed to greater depths. However, models of regolith formation predict that the amount of solar wind should generally decrease with depth, because deeper layers should have fewer grains that have spent time at the surface. At present, we have no samples from any deeper than three meters (the depth of the longest drill core taken by the Apollo astronauts), and the predicted trend is not really seen. If the abundance changes little with depth to the bottom of the regolith (typically 10 to 15 meters), then approaches to mining helium-3 would clearly need to be different than if the helium-3 is concentrated in the upper two to three meters.
The amount of helium-3 varies with location as a result of two factors, one of which we understand, and one of which we don’t. One factor is the chemical composition: helium atoms are so small, and so light, that they can escape from many minerals, even if they are implanted. Some minerals will retain less than 1% of the helium-3 implanted. By far the best, in terms of retention, is the mineral ilmenite, which also happens to be the mineral that contains most of the element titanium on the Moon. Hence, by mapping the abundance of titanium (something that has been accomplished by orbiting spacecraft), we can predict where helium might be retained well.
The other factor is the amount of helium received. Since Earth’s magnetic field shields the Moon from the solar wind during part of the month, the portion of the Moon facing the Sun at that time each month (the Near Side of the Moon) is exposed to less solar wind than the portion of the Moon that faces the Sun when the Moon is not within Earth’s magnetic field. But even though some portions are exposed to more solar wind, we do not know whether they actually receive more – it is possible (and has been suggested, based on some experiments) that individual grain surfaces become saturated, reaching a state where no more can be implanted, even if the surface is exposed to more solar wind. By far the best way to test this is to analyze samples from a variety of locations on the Moon, which we cannot do at present.
We also need to know more about the general properties of the lunar regolith if we are going to attempt to mine the Moon for helium-3. For example, a mining engineer would clearly need to know how common intact rocks are, and what sizes they are likely to be, as one moves deeper into the regolith, and if and how the properties of the deeper regolith differ from those of the surface layers studied by the Apollo missions.
It is worth noting that in any scenario, mining helium-3 from the Moon will be a massive, difficult operation. Even with our most optimistic estimates of the abundances and distribution, we found that at the most promising sites, helium-3 makes up only one part in 100 million of the regolith (by mass), so extracting one ton of helium-3 would require mining 100 million tons of regolith, even if the extraction were perfect, which it will not be. This is comparable to the annual work of some of the largest terrestrial mines. Adding in the fact that only the top few meters of the regolith contain any helium-3 at all, to mine one ton per year would require digging up seven square kilometers of the Moon’s surface each year, at the most promising site under the most optimistic set of assumptions, so it would not take many years for the mine to become large enough to be visible from Earth with even a small pair of binoculars. Although massive mining operations are increasingly mechanized, humans are still the best way to make decisions, diagnose equipment problems, and make repairs.
There are several other potential resources, most notably oxygen, whose extraction would require far less ambitious mining projects. However, they typically share the common property that we would need to know more about the basic science of the lunar regolith to be able to properly implement them, or even to properly evaluate their potential.
Once again, while it is certainly possible to imagine ways to attack these basic science questions robotically, it would require a level of sophistication far in advance of any mechanisms yet launched into space, though well within the reach of human-conducted exploration. Furthermore, in the case of helium-3, it is worth stressing that at present, a full assessment of the feasibility is far cheaper and simpler than an attempt at implementation.
In summary, the Moon holds valuable clues to the early history of Earth, at the time when life was forming, and may hold valuable resources for the future of Earth. But to evaluate either properly, there is lunar science that would need to be done, and the way to do that lunar science best is with human beings.