The challenges for growing plants on Mars are many and unpredictable. To troubleshoot, we look towards controlled studies under similar conditions. Fortunately, there is plenty of research available detailing the effects of radiation on plant growth. These studies can be extrapolated to make reasonable assumptions about the effects of the radiation environment on space crops. From this information, we can also make some suggestions for how we might deal with a radioactive environment on Earth as well, in the event of nuclear fall-out.
Before digging into the research, we need to lay the foundation for understanding a little bit about radiation. Radiation comes in a wide range of intensities, and low levels are relatively harmless to living things. However, at higher levels, radiation has the ability to disrupt the structure of certain atoms and eject electrons. This is called ionization and results in an unstable atom called a free radical. Free radicals are very reactive and when they interact with DNA, they cause genetic changes that can lead to tumors if the immune system is not able to keep up with destroying mutated cells. On Mars, we would expect radiation to come from three sources: the sun (especially during a solar flare event), cosmic rays, and the Martian surface (aka regolith). Because the Martian atmosphere has very little protection from cosmic and solar radiation, the Martian surface is exposed to high levels of radiation from above. As a result, electrons in the regolith become excited and begin to emit ionizing radiation of their own.
From a space farming perspective, radiation from above is not as much of a concern as radiation from the regolith below. This is because the aboveground parts of plants are already adapted to resist damage from solar and cosmic radiation due to the fact that they have been exposed to extreme levels of this type of radiation throughout their evolutionary history on Earth. Although we have a wonderfully protective magnetic field on Earth right now, during past geomagnetic reversal events the protection from solar and cosmic radiation was reduced to a fraction of what we experience today. Plants have survived these events several times over the course of their evolutionary history on Earth and are therefore fairly equipt to deal with rather high levels of radiation from above.
Experiments on Earth have shown that crops exhibit a wide range of responses to radiation depending on the type of crop and the levels of radiation to which they are exposed. For example, in response to experimental radiation, some crops increase starches, some decrease starches, some increase proteins, some decrease proteins, etc. Very high levels of radiation can kill plants, but the daily levels we expect to see on Mars are more likely to be an addressable problem rather than an unapologetically lethal scenario. This is because the aboveground parts of plants have already evolved several built in system for dealing with radiation. In the face of free radical damage from ionizing radiation, plants begin to produce Reactive Oxygen Species (ROS) which are molecules that are able to pair with and essentially consume free radicals, rendering them harmless. ROS are actually of medical benefit to human as well, because when we consume plants that contain unpaired ROS, these compounds are able to pair with and neutralize free radicals in our own bodies as well. This protects us from free radical damage that would occur as a result of exposure to ionizing radiation, both on Earth and on Mars.
This is not to say that solar and cosmic radiation is not an issue at all, because high levels of radiation can certainly damage plants and impact the way that they grow. However, radiation from above is manageable from a space farming perspective, and there are couple of tricks Martian colonists can use to help their crops resist radiation damage.
For example, in the event of a radical increase in radiation from a solar flare, farmers can help their plants recover by reducing the ambient temperature in their controlled environment. Normally, controlled environment systems favor the warmer end of the crop’s growth range because this maximizes plant productivity. However, in a high radiation environment the lower end of their temperature range for growth would be preferred. This is because cold temperatures facilitate higher efficiencies in the plant’s natural DNA repair mechanisms. All cell have the ability to identify and eliminate mutations to their DNA. However, this DNA repair “machinery” functions best at lower temperatures. In a high radiation event, it may be best to temporarily trade off plant productivity for plant protection.
In addition, experiments have shown that an increase in photosynthetically active radiation (PAR) (This is what LED grow lights are designed produce) will reduce the negative effects of ionizing radiation. In essence, PAR gives plants the energy that it needs to deal with ionizing radiation through a myriad of processes, including DNA repair mechanisms and production of ROS. In the event of a solar flare, Martian farmers will be wise to divert energy resources away from heating and toward increased PAR to help their crops recover from damaging levels of solar radiation.
Although the aboveground parts of plants show many adaptations to radiation exposure, plant roots, in contrast, are much more susceptible to ionizing damage from radioactive soils. Crops on Mars will need to be produced using minerals from regolith that does not contain radioactive ionizing particles. In order to farm on Mars, the top layers of ionized regolith will need to be removed and discarded, exposing unionized regolith from further down in the ground. Consider though, removing the top layers of Martian soil is an energy intensive endeavor and this activity will need to be minimized to save power. Instead of digging new regolith for every crop, it will be more energetically conservative to reuse regolith derived minerals from human and plant waste, and then use this recycled nutrient broth to fertilize new crops.
What does this mean for farming on Earth in the event of a nuclear fallout? There are many similarities between radioactive regolith on Mars and the effects of nuclear fall-out on Earth’s soils. One could conceivably remove the top layers of ionized particles from fallout soil and still use the unaffected layers below, but there are complications on Earth to consider. On Mars, there is no precipitation to be concerned with, meaning that ionizing particles formed on the surface remain on the surface. However, elements in Earth soils are very mobile due to infiltration caused by rain. We would could possibly removed the fallout from the top layers of soil, but the likelihood of doing this on a large scale prior to infiltration is very small.
Furthermore, removing the top layers of soil means that you would be working with undeveloped soil below. When soil lacks development, air, water, and roots have difficulty penetrating and plant productivity suffers as a consequence. In the early stages of soil development, we would need to rely on a closed-looped controlled environment agricultural system to grow crops. This is a trend we have seen emerge in Japan after Fukushima. By growing their crops in a clean lab type of setting, they can ensure that no radioactive fallout particles are uptaken into their crops and then transferred into humans during consumption.
In the agriculture industry, a large scale controlled environment agricultural system is called a Plant Factory with Artificial Lighting (PFAL), and they are very high tech. In the event of a fallout, it would behoove us to find DIY hacks for this type of technology. If individuals are able to grow food on the small scale for themselves and their communities, without fallout contamination, the likelihood of large scale radiation exposure will be reduced.
The best way to obtain radioactive free closed looped systems is to build them now, build them in places that are protected from gamma radiation, and build them to exclude dust from the atmosphere. Because we can’t assume that we will be able to find uncontaminated nutrients and water after fallout, the systems we build now need to be capable of recycling water and nutrients indefinitely. I think that we will all be much better off in the event of a fallout if we already have an extensive network of DIY closed loop systems growing clean food in every home.
A Particle of Hope
As a farmer myself, I have grown crops in CEA systems for fun and curiosity, but I will always prefer to spend my agricultural energy outdoors. The idea of growing food in a cramped Martian colony or underground in a nuclear fallout bunker both sound like total hellscapes. But I have that prepper ethic in my DNA. It gives me peace of mind to know that there are survival options for humanity, should we choose to be the brave ones to voyage to the stars, or outlive the apocalypse here on Earth.