or: How are we going to grow plants in sterile rock dust?
One of the key questions for early Lunar selenologists was whether or not the regolith of the Moon could support life. The results were pretty conclusively no, as most of the elements that we consider important for life such as carbon and nitrogen are scarce to be found. This would seem to make the Moon a pretty rotten place to try to grow plants, but there’s a strong likelihood that the Moon could turn out to be a fantastic place to grow the plants of Earth.
Early settlers are going to be looking to go deep underground on the Moon, with lots of rock above them to protect them from the vacuum and radiation. This does not mean that Moon dwellers will end up as troglodytes, as modern technology has given us many tools to work with that can help us create a subselenian paradise on the Moon.
While early plans for a Moonbase typically settled for an near-equatorial location to help keep the orbital mechanics easy and resupply cheap, newer ways of looking at how we return to the Moon, such as using an Earth-Moon L-1 platform as a staging location, are giving increasing consideration to polar locations. The difficulty of access is offset by what appears to be large supplies of hydrogen (in some form, currently unknown but hoped to be water) in the everdark craters at the North and South poles. There appears to be more at the North pole, but the more rugged terrain of the South pole, perched on the rim of the Aitken Basin, offers more interesting opportunities.
Not only are there deep, dark and cold craters where the Sun never shines, but there are also mountains and plateaus where the Sun shines nearly all the time. So instead of lugging lots of heavy batteries or a heavy nuclear reactor to the Moon to supply energy through the long Lunar night, you instead ship up Solar panels, erect tall towers to peek over the horizon, and hang your Solar arrays out to collect the abundant sunlight all the time. Other equipment can be mounted on these ‘power towers’, such as Solar ovens, which use mirrors to focus lots of sunlight onto a small area to achieve very high temperatures, and lightpipes.
Lightpipes are a technology used today here on Earth in modern buildings. An inlet for the sunlight is at the top of the building, and via a series of mirrors and ducts the sunlight is directed to the inside of the building. On the Moon these could be mounted on the power towers, directing raw sunlight deep underground that can be used in part to illuminate crops in caves deep beneath the surface. Other lighting options include light-emitting diodes, full-spectrum fluorescents, and traditional incandescent. So we don’t really lack for a light source on the Moon, and we know how to carve caves underground.
Water is problematic. It is hoped that the hydrogen abundances found at the poles are water, but even if they aren’t the hydrogen can still be harvested, and oxygen can be wrested in abundance from the Lunar rocks. Thus, the constituents for water will be available at the poles, the problem, just like here on Earth, is keeping it clean enough for humans to consume. Chemical treatments are quite advanced, and in the name of science NASA scientists have drunk water purified from their own urine. Still, maybe there are additional ways to treat the water to make it more palatable.
Which brings us back to the plants. Part of the water treatment cycle may involve watering plants with ‘grey’ or non-potable water from the treatment facilities, and then collecting the water that they transpire into the air, perhaps even controlling the humidity to encourage such transpiration.
There are two main types of seleneculture generally considered: hydroponic and soil-based. As its name implies, hydroponics relies primarily on water-delivery of nutrients to the naked roots of different plants. This style of growing plants has been extensively studied by NASA scientists as a means of providing a high-efficiency/low-mass greenhouse for a space station, and is used in industrial farming here on Earth. It may not be ideally suited for the Moon, but would certainly provide a quick way to get started.
Soil-based plant cultivation uses plants in dirt. The most frequent objection in this case is that plants can’t grow in Moon-dirt, so you’d have to ship up tons of Earth dirt for the plants to grow in. This will certainly be true in the beginning though not to the tune of tons, and we probably will be shipping up small quantities of high quality humus. Still, is it true that plants cannot grow in Moon dirt?
We’ll start the investigation with a documentary that is pretty dated in style, but definitely not in content, NASA HQ 209: “Moon, Old and New”, available on the DVD “The NASA Collection”. This documentary features a short bit (00:18:13 to 00:19:21) from Dr. Charles H. Walkinshaw, who comments that:
“Of the variety of biological systems that we tested with the Lunar material, the plants were most unique in their response. For example, the five jars of liverwort that you see illustrated on the top gave much increased growth in the presence of Lunar material. This effect was noted for ferns, a number of tissue cultures such as tobacco and corn, and certain higher plant species such as lettuce. Now the exact reasons for this beneficial response are unknown at the present. However, it is likely that some trace mineral, or perhaps even a physical property of the Lunar material is interacting with the minerals we furnish to give a more desirable medium for plant growth. This is a very exciting discovery and one that was totally unexpected in the tests conducted in the Lunar Receiving Laboratory.”
Contrast this with comments by Dr. Bevan French, who worked on both Apollo and Luna samples. In his 1977 book “The Moon Book” he states:
“Even the plants grown in [L]unar soil back on [E]arth had to have nutrient solutions added*
*Early reports that some terrestrial plants showed increased growth rates when grown in [L]unar soil have not been substantiated by later experiments. Almost everything that a plant needs for growth (water, organic compounds, potassium, etc.) has to be added to the soil to make the plants grow at all.”
I do wonder if Mr. French thinks that plants don’t often need things like water and fertilizer back here on Earth. So the question still remains, even if the video is pretty compelling evidence. So I delve deeper into the Lunar Library. Checking the papers from the Apollo 11 Lunar Science Conference I see only the search for organic elements, which IIRC didn’t yield much at all, on the order of parts per billion, mostly implanted by the Solar wind. The granddaddy of Lunar references, “Lunar Sourcebook: a user’s guide to the [M]oon”, doesn’t say anything about plants, nor really do any of the references in the Selenology section of the LL.
Time to check the Moonbase section. “Outpost on Apollo’s Moon” from 1993 has a short section on efforts at KSC during the 1980s and into the 90s and beyond that showed the viability of growing plants in an enclosed space. [I actually got to see that chamber, which is really tiny, during a field trip of the NASA Academy. It's also where I picked up a couple of packets of Perigee Wheat] The author, Eric Burgess, doesn’t really speak much to the use of regolith as a plant growth medium other than to note that it could be used in a Lunar Closed/Controlled-Environment Life Support System (CELSS) supplemented by nutrients and minerals brought from Earth.
“The Moon: Resources, Future Development, and Settlement” by Schrunk, Sharpe, Cooper (who has autographed my copy), and Thangavelu, has a short section on Lunar agriculture on p. 45, where they note that a food crop cycle has not been successfully conducted off-Earth, though plants have been grown in microgravity. They propose some active investigation across an array of variables to determine the viability of Lunar regolith for plant growth.
And here we run into one of the hurdles to experimentation. Most of the actual Moon dirt is tightly secured, and only something like 15% of the actual samples have been closely investigated, with the rest saved for the future. Until such time as we are actively getting new samples from the Moon, we probably shouldn’t be using up too much of what we do have carefully hoarded away. Furthermore, use of the regolith would be destructive to the sample since it would be changed by its interaction with the plant roots, and you would need a fair amount to test across a number of variables. So the current supply of Moon dirt isn’t necessarily going to allow any kind of real or robust investigation.
Which leads us to the second hurdle, that of the use of simulants. “The Moon: RFD&S” has an excellent appendix on the different kinds of simulants that are available, and what sorts of experimentation they’re best suited for, and I know from the last LEAG conference that knowing the kind of simulant used is important and understanding their limitations is on NASA’s radar. This is perhaps in response to some recent experimentation whereby a researcher, perhaps perusing an online article on yet another microwaved this or that, decided to plop some real regolith on a saucer and stick it in the microwave oven in the break room. Lo and behold, it melted faster than a mug of water can boil! Rock! What is going on?
Turns out that when nickel-iron metorites impact the Moon they end up in part as a mist of nano-scale (or near nano-scale) iron that ends up coating everything around the impact site. Given the number of impacts on the Moon, and if the frequency of nickel-iron asteroids is the same as what we’ve found (so far) here on Earth, then about 5% or so of those craters are from nickel-iron objects. So pretty much all of the regolith has these ultra-small bits of iron on them, which happen to react energetically with the microwaves (plasma and all that, what you see when you accidently microwave metalware) and heat up the dirt until it melts. This is not something you’d see on Earth dirt.
This will probably be very useful once we get back to the Moon, as it means we can do things like mount the travelling-wave-tube apparatus from the microwave oven on a rover and have it ‘pave’ landing sites that can be kept free of dust, or roads leading to other work sites. This also shows that there are still a lot of unknowns when it comes to our samples from the Moon, and the investigations are far from over.
“The Lunar Base Handbook” by Peter Eckart has an extensive section on food production requirements at a Lunar facility, but in the section on Plant Growth Media beginning on p. 401 the author only considers Earth-like and artificial soils, and hydroponics and aeroponics, and refers readers to…
Of a Garden on the Moon, part II
or: Let’s consult the most comprehensive text to date.