On October 26th, 2020 NASA announced that its Stratospheric Observatory for Infrared Astronomy (SOFIA) discovered water for the first time on the sunlit surface of the Moon. This is not the first time water has been detected on the Moon. A Soviet probe took samples from the Moon in 1976 that contained 0.1% water by mass[i].
Nevertheless, the announcement stirred up a great deal of excitement in the scientific community, particularly among astrobiologists. On Earth, microorganisms are found virtually everywhere there is water, even in environments considered deeply inhospitable to life[ii]. This has led many astrobiologists to speculate that water is the most important ingredient in life. Although those of us eagerly anticipating the discovery of extraterrestrial life will be disappointed to learn that life on the Moon is still an extremely remote possibility, the discovery of water on the Moon could yield important information about how water is created and how it can persist in a variety of environments.
Life is an extremely hard concept to define. There is no single characteristic of an organism that defines it as being “alive.” Instead, there is a set of characteristics that are considered properties of life. These properties include:
- The ability to take in energy
- The ability to grow
- The ability to reproduce
- The ability to adapt to the environment.
The exact definition of life is more complicated and fiercely debated, because inorganic objects, such as crystals, possess some of the same properties[iii].
Life on Earth consists of membrane-bound, water-filled cells. Within these cells, energy obtained from the environment drives chemical reactions that allow cells to grow and reproduce. Water is a vital part of life because it is the chemical solvent in which these reactions occur.
A solvent is necessary because chemical reactions can only occur if the reactants are able to make contact with each other. Chemicals dissolved in a solvent are more likely to make contact with each other than chemicals freely floating in the air. Water is considered an ideal solvent for life because it exists as a liquid over a wide range of temperatures. It is also able to dissolve salts that contain ions which are important to life on Earth, including potassium, sodium, and chlorine[iv].
Theoretically, life on other planets could use another liquid as a solvent. Ammonia and hydrofluoric acid are considered distinct possibilities. But since no lifeform has been found that uses ammonia or hydrofluoric acid as a solvent, this remains pure speculation[v].
Despite the presence of water, the Moon is an unlikely host for life because it lacks the “chemical building blocks” of life. On Earth, the elements required for the building blocks of life are
- Carbon
- Hydrogen
- Nitrogen
- Oxygen
- Phosphorous
- Sulfur
Although the Moon contains all of these elements (oxygen is particularly plentiful), there are only trace amounts of carbon and nitrogen. It is possible that life on the Moon could use different elements as building blocks. Silicon, for example, has many of the same properties as carbon. However, silicon forms extremely strong bonds with oxygen, making it chemically inert. Life requires stable bonds that are weak enough to be chemically active. It is difficult to imagine silicon-based life evolving in an environment rich with oxygen. In fact, roughly half of the oxygen found on the Moon is trapped in silica (SiO2)[vi].
So, there is likely no native life on the Moon. But could we use the water found there to grow plants and hydrate a small colony?
Even this seems out of reach. The concentration of water found by NASA was only 100 to 412 parts per million. By comparison, the Sahara Desert has a 100x greater concentration of water. It seems unlikely that this tiny amount of water could sustain plant growth, even if we had a perfect waste-water filtration system.
Additionally, there are serious questions about the ability of plant life to thrive in low gravity environments. The gravitational pull of the Earth is an important factor in plant growth. The leaves of plants grow in the direction of sunlight, but the roots of plants grow in the direction of gravitational pull. Although experiments in growing plants on the International Space Station seem to indicate that plants, with a little bit of guidance, grow just fine in low gravity environments, there are warning signs at the genetic level that plants might struggle to survive and reproduce in low gravity.
Analyses of the mRNA sequences produced by Arabidopsis thaliana plants (the “lab rats” of botany) grown on the International Space Station show that plants grown in space produce proteins typically found in response to the build-up of reactive oxygen species, a chemical that causes damage to DNA. They also produce proteins associated with nearly every stress response system the plant has, from proteins produced by plants undergoing drought to proteins produced by plants undergoing flooding. At the genetic level, it appears that something about spaceflight is driving the plant haywire, but this doesn’t seem to impact its ability to grow normally[vii]. Perhaps low gravity doesn’t affect plant growth, but it’s also possible that there is something deeply wrong with the plant that may not be immediately noticeable. There just isn’t enough research to know for sure.
All life on Earth has evolved under roughly the same gravitational conditions for at least 3.5 billion years[viii]. It shouldn’t surprise us if Earth-based life struggles to adapt to even modest differences in gravitational force.
I know this article seems pessimistic. I’ve discounted the possibility of life on the Moon. I’ve questioned the feasibility of a colony on the Moon. I’ve even presented some daunting challenges facing long-term colonization of ANY place outside planet Earth. But I believe that we should not give up in the face of these challenges. Instead, we should use these challenges to push our imagination forward, because overcoming these challenges will require innovations that would take humanity beyond what it has ever dreamed possible.
-Erik Schwerdtfeger
[i] Akhmanova, M; Dement’ev, B; Markov, M (1978). “Possible Water in Luna 24 Regolith from the Sea of Crises.” Geochemistry International. 15 (166).
[ii] Cockell, CS (2015). Astrobiology: Understanding Life in the Universe. Wiley Blackwell. (124-138).
[iii] Schulze-Makuch, D, and Irwin, LN (2008). Life in the Universe: Expectations and Constraints. Springer. (7-12).
[iv] Cockell, (43-44).
[v] Ibid. (47-48).
[vi] Taylor, SR. (1975). Lunar Science: A Post-Apollo View. Pergamon Press. (64).
[vii] Barker, R, et al. (2020) “Test of Arabidopsis Space Transcriptome: A Discovery Environment to Explore Multiple Plant Biology Spaceflight Experiments.” Frontiers in Plant Science. 11
[viii] Cowen, R. (2013). History of Life. Wiley-Blackwell. (22).
The Scientific Ecstatic 