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With the recent release of the movie The Martian, it seems timely to review the possibilities of sustaining human life on Mars in the long term. A recent journal article (Wieger Wamelink et al. 2014, Can Plants Grow on Mars and the Moon: A Growth Experiment on Mars and Moon Soil Simulants) suggests that it may be possible to grow crops in Martian soil. This is an important issue for those of us who dream of Martian colonies (and for science fiction authors who write about such dreams) because it will be crucial to grow food locally; the distance from Earth and high transportation costs mean that a colony would have to rapidly become self-sustaining.

So is this likely to be possible in the near future? In this essay, I'll discuss several key issues. Because each issue that I raise would require a separate essay to cover adequately, please note that I have (over)simplified many points to focus on the essence. Details will vary widely among crops, soil types, and so on. Please treat this essay only as a "general principles" overview of the subject.

A side note before we begin: why not hydroponics?



It's reasonable to ask why growing crops in soil is necessary in the first place. The answer is complex, so I'll simplify. Even though many crops such as tomatoes can be grown using hydroponics, this may not be possible for all crops because of the large space requirements (e.g., wheat, corn). The larger problem is that this won't be possible for some time at the scale of a potential Martian colony. The biggest obstacle is the prohibitive cost of shipping a sufficiently large collection of hydroponic gear to Mars; given current technology levels, it's implausible to suggest that we'll be able to manufacture the equipment on-site for the forseeable future. (3D printing may solve this problem once we're able to set up "mining" operations on Mars to provide the necessary raw materials.)

The need to produce viable seeds during many generations of hydroponics is also a concern. Nowadays, hydroponic crops are harvested and then re-established from new seeds, but those seeds are usually grown in conventional farm fields. The micronutrient composition of the solutions used to nourish hydroponic crops is a related and important issue. You've probably noted that hydroponic vegetables taste different and worse than field-grown vegetables, likely due to differences in the micronutrient supply; wine growers emphasize the importance of soil qualities as a key part of the "terroir" effect. To the best of my knowledge (and I emphasize that I have not performed a literature review to support this point), researchers have not tried to create a completely self-sustaining ("closed cycle") hydroponic crop production system and confirm it's viability over periods of years. Over long periods (several years), hydroponic crops may suffer from subtle nutrient deficiencies that eventually sabotage the crop or decrease its utility to humans. On Earth, we'd never notice this problem because our diet is primarily composed of field-grown crops.

Thus, finding ways for terrestrial crops to survive and grow in simulated Martian soil is an essential research goal, and this preliminary study by Wieger Wamelink et al. is great news for Mars fans. Unfortunately, the results are hardly definitive due to gaps in our current knowledge and some significant deficiencies in the Wieger Wamelink study. Some of these deficiencies are methodological problems that should perhaps have been fixed, and others represent defensible limitations of the study based on logistical and other constraints. (Specifically, it is never possible to study all relevant factors in a single research study; sometimes a career is too short.) Based on my training as physiological plant ecologist, here are some thoughts on the article, its limitations, and its implications for a future Martian colony:

Making light


First and foremost, the article does not explore the consequences of the light intensity and quality on the surface of Mars. Neither is a trivial issue, since Earth's plants have evolved for millennia to optimize their use of the amount and quality of the available light. It would not have been logistically possible to investigate these factors within the scope of the authors' study, so I'll frame this section in terms of needs for future research.

Light intensity on Mars would clearly differ from its values on Earth, but I can't speculate about the magnitude of the difference because this would involve a rather complex calculation: the amount would decrease greatly as a function of increased distance from the sun following the inverse-square law, and would increase due to decreased light absorption by the nearly nonexistent Martian atmosphere (i.e., there would be less light interception by molecules of air and water). Large and dense dust storms are common on Mars, and this would lead to frequent changes in the amount of light.

The spectral characteristics of the light would also change due to the different characteristics of light interception by the Martian atmosphere. Plants are keenly sensitive to subtle variations in light quality; these variations govern all plant developmental phases and plant responses to many types of environmental stress. Photoperiod (the length of the daylight period) is also an issue, since the lengths of the Martian day and of Martian seasons differ (respectively) significantly and greatly from those on Earth; plants have internal "biological (circadian) clocks" that govern every phase of their development, and those clocks are keenly sensitive to daily and seasonal changes in light intensity and quality. On this basis, Martian farmers will either need to provide large amounts of supplemental color-adjusted light, or will need to breed plants that are optimized to take advantage of the available sunlight on Mars.

A final issue is that of ionizing radiation, which will be present at much higher levels on Mars due to the lack of a dense atmosphere to absorb this radiation. This may kill plants directly, or cause ongoing mutations that will eventually kill the plants or render them useless as a food source. It may also significantly affect essential soil microbes (discussed in the next section). Providing shielding won't be trivial or maybe even possible.

As a result of these factors, plants will likely have to be grown underground, with artificial illumination. This will require significant and difficult engineering.

Soil microbes


Most (possibly all) terrestrial plants either require or benefit strongly from the presence of a diverse soil microbial community, and the characteristics of that community have resulted from millennia of coevolution between the plants and organisms in their rhizosphere. Examples include mycorrhizae, nitrogen-fixing bacteria, and others. I'm not even considering essential macroorganisms such as earthworms and collembolans, which play an important role in maintaining soil structure and promoting nutrient cycling. Although the authors quite properly did not sterilize the soils they used in their study, neither did they have the resources to monitor long-term changes in the microbial community and the consequences for plants. These changes are likely to be significant, since the composition and functional characteristics of this community are strongly determined by interactions among various characteristics of the soil and the plants being grown in the soil; in turn, these changes determine the suitability of the soil for the plants. Of particular note, they can significantly affect the risk of disease development.

Pollination


The issue of pollination is not trivial. Most important crops require some combination of insect and wind pollination. Bee-pollinated crops include some of our most important crops, and bees are just one example of insect pollinator; many other taxa contribute. To grow such crops on Mars, we'd need to confirm that the pollinators could survive under Martian conditions. Manual pollination is feasible on a research scale, but not on the scale required to grow enough crops to sustain a colony. The survival of such insects under Martian conditions is by no means guaranteed. As a specific example, I note that the geothermally warmed and powered greenhouse I recently visited in Iceland requires ongoing imports of bees from the Netherlands; I did not ask the owner, but it appears to be impossible or economically impractical to cultivate the bees in Iceland rather than importing them. Warm-climate readers may find this difficult to credit, but Iceland is actually far more hospitable an environment than Mars would be.

Soil simulation


The biggest problem I had with the Wieger Wamelink study is the use of "simulated" soils. This was required because we simply don't have access to real Martian soils, and the authors did a good job of choosing an appropriate simulation material. But in evaluating their results, it's essential to note that this is a simulation, and the underlying assumptions may turn out to be unrealistic.

The first problem with a simulation approach is that we don't know how common the simulated soils are in the Martian soil. That is, we have surveyed only the most miniscule proportion of the Martian surface, and even on Earth, soils are highly spatially variable. Thus, we don't know how good their choice of a simulation material will prove to be. The authors note this and other problems; the lack of strong evidence of abundant nitrogen in Martian soils is a particular concern, since nitrogen is crucial for plant growth, and modern crops require enormous amounts of supplemental nitrogen to produce their current high yields.

An additional problem is that what I have read of Martian soils suggests that perchlorates and other strongly oxidizing materials are abundant, which would make things rough for both plants and their associated microbes. The high aluminum levels that seem common in Martian regolith could also cause enormous damage to crops, particularly in acidic soils; this is one of the reasons why "acid rain" on Earth is so damaging; it mobilizes toxic aluminum compounds.

Another problem is that, to the best of my knowledge, the analyses of Martian soils have examined "total" amounts of elements, not the amounts of "plant-available" versions of the elements. This is an important difference, since the available level is often far less than the total level. The authors of the article suggest they analyzed total element contents, which will not provide an adequate prediction of long-term plant growth. (In their defence, it's very difficult to estimate how non-plant-available forms of elements would change into available forms as a result of chemical weathering and biological activity. That would require a long and complex series of additional studies.) Even if Martian soils would support a first crop, it's not clear from the present results whether they would support subsequent crops, since key nutrients would be removed from the soil with each harvest, and would be restored primarily by adding human feces and urine (suitably composted) to the soil before the next crop. Unfortunately, such nutrient cycling would be difficult to implement in practice; such systems are notoriously "leaky", with significant ongoing losses to the environment that would have to be replaced somehow.

The biggest problem would be losses of organic matter, whose final fate is to be converted into carbon dioxide or methane. The former would be taken up by the plants, though not without some loss; the latter would represent a net loss of carbon from the system. One consequence of this point is that you'd have to add organic matter to the soil at least as fast as it is depleted by biochemical and chemical degradation; this is necessary to increase the soil's water-retention ability, preserve the structure of the soil, and provide a nutrient supply for essential soil microbes. Human wastes would be used for this purpose, but since those wastes would initially come entirely from food supplied from Earth, it's likely to take some time to get crops growing well enough to become self-sustaining from a carbon perspective.

[A look back: I neglected something obvious, namely that you'd also need to have a significant source of CO2 to "feed" the plants. Haven't done the math, but I'm not sure that a colony of humans would generate enough CO2 from breathing to support an area of crops large enough to feed them. So you'd likely need supplemental CO2 from somewhere.]

Water


Water availability is a particularly serious issue. Water evaporates rapidly in a low-pressure atmosphere, even on Earth; on Mars, with less than 1% of Earth's atmospheric pressure, evaporation would be even faster. It's hard to imagine creating a dome large enough to grow plants on the Martian surface that would be completely airtight; even creating something underground would be difficult. Thus, you'd need a robust system for recapturing or replacing lost oxygen and water for the plants to survive in the long term. Oxygen is relatively easy, since there are large quantities of oxidized materials on the Martian surface, and all you'd need to liberate the oxygen is a large supply of (solar cell?) electricity and appropriate engineering.

The apparent presence of liquid but very salty water on the surface of Mars gives hope that water could be supplied locally, but any liquid sufficiently salty to retain its water in the near-vacuum of the Martian atmosphere, rather than having the water lost to evaporation and sublimation (as occurs in Earth's deserts), will be extremely difficult to desalinate. Water frozen into ice at the Martian poles seems abundant, but transporting it to the likely location of a colony would not be a trivial task.

Crop maturity


It's not clear why the researchers didn't grow all the plants to maturity (they stopped after 50 days) to confirm that they could successfully produce the desired final crop (seeds, fruits, etc.). Most of the agricultural researchers I work with do this even for studies based on terrestrial soils, since a great many factors can prevent successful seed production even if flowers develop and seeds appear to be produced; for example, unsuitable temperatures before, during, or after pollination can result in the production of nonviable seed. In addition, the researchers did not analyze the nutrient quality of any of the seeds that were produced, which is a significant challenge for future research. (To be fair, such analyses are clearly beyond the scope of the authors' study; I mention this point solely in the context of a need for future research.)

It's all very well to produce seeds, but if they won't germinate or prove to be severely nutrient-deficient, particularly in terms of micronutrients, this won't end well for Martian colonists who must consume them.

In conclusion...


All this being said, the Wieger Wamelink et al. study is important because its results don't rule out the possibility of growing crops in Martian soil. That's a very good thing should we want to establish a colony. But as this essay shows, there's still much research to be done before we can believe that Martian crop production will be possible on a scale large enough to support a colony.
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