Wieger scoring the first experiment Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

Can we grow food on the Moon or Mars? That was the question that started Dr Wieger Wamelink, ecologist and exobiologist at the University of Wageningen in the Netherlands, on a research quest in 2013.

The dirt on growing plants off-world

The first step on the journey was an experiment to determine whether plants could grow in Moon or Mars soil. But there’s a fairly fundamental issue at the heart of that question: we don’t have any. NASA has some samples of Moon soil (regolith, strictly speaking) that the Apollo moonwalkers brought back to Earth. Obviously, they don’t have enough to go around planting gardens in it. And we haven’t even tried to bring samples back from Mars yet, although a sample return mission is on the cards.

Moon soil simulant
Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

So scientists who would like to carry out experiments on extraterrestrial soil have to use simulants. Our off-world explorations have told us a fair amount about the mineral composition of the Moon and the Mars (at least in the places we’ve investigated), and researchers have found that there are soils on Earth that can stand in for them. Martian soil simulants tend to be made from volcanic areas on Earth (such as Hawaii); apparently, the Moon is more like Arizona. For these experiments, Wamelink got his hands on some JSC-1a (Mars-like simulant) and JSC1-1a (Moondust simulant) and used some reasonably nasty soil from beneath the Rhine river as a nutrient-poor Earth-based control soil.

“To work in this soil was very special. Nobody, not even NASA, could tell us what would happen, even just by simply adding water.” 

Wieger Wamelink

(This wasn’t the first time plants were grown in lunar soil, and I blogged about the early experiments a while back.)

Analysis of Moon and Mars soils suggests that they have all of the nutrients plants need to grow, except probably reactive nitrogen. On Earth, reactive nitrogen in the soil is mostly due to the presence of organic matter – which (as far as we know) is entirely lacking elsewhere in the solar system. Reactive nitrogen can also arise from lightning and volcanic activity, both of which exist on Mars. And it’s present in the solar wind. But probably there’s not going to be enough to feed off-world plants. On Earth, nitrogen-fixing plant species, in conjunction with their bacterial partners, can take nitrogen from the air and turn it into plant food. But there’s no atmosphere on the Moon, and the minimal one on Mars doesn’t contain much nitrogen. 

For simplicity’s sake, Wamelink designed an experiment that assumed that we would create artificial environments for growing plants off-world, in which the atmosphere, temperature and lighting were Earth-like. All of the plants grew in Earth gravity as well, of course.

What’s growing in the extraterrestrial garden?

Carrots on Mars soil simulant
Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

At the start of every growing season, each gardener has to decide what to grow. It’s no different off-world, and Wamelink chose a mixture of plant species from three different groups: crops, wild plants (essentially weeds, but I prefer wildings) and nitrogen-fixing plants. Previous experiments have also shown that bacteria, including nitrogen-fixing bacteria, can grow on Mars simulant. In this case, the team didn’t inoculate the soils with the bacteria, but neither did they sterilise them. They didn’t test for the presence of nitrogen-fixing bacteria, either; it wasn’t part of the experiment.

The four crops were: tomato (Solanum lycopersicum), Rye (Secale cereale), carrot (Daucus carota s. sativus) and Garden cress (Lepidium sativum).

Tomato seedlings on moon soil simulant
Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

The wildings were arnica (Arnica montana), Field mustard (Sinapsis arvensis), Stinging nettle (Urtica dioica), Marsh thistle (Cirsium palustre)

Reflexed stonecrop (Sedum reflexum) and Red fescue (Festuca rubra).

And the nitrogen-fixers? Common vetch (Vicia sativa sativa), Lupin (Lupinus angustifolius), Yellow sweet clover (Melilotus officinalis) and Greater bird’s foot trefoil (Lotus pedunculatus).

All of the chosen species had relatively small seeds, so that they would quickly exhaust their energy reserves and be dependent on the nutrients they could extract from the soil. 

Stinging nettle on Mars soil simulant
Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

For fifty days, the researchers recorded germination rates, first leaf production, the formation of buds, flowering and seed set. They watered, but didn’t fertilise, the plants. Then they harvested all of the plants and dried and weighed the biomass which grew above and below ground.

Their results (published in the journal Plos One), showed that seeds can germinate and grow in Martian and Moon soil simulants for 50 days with no added nutrients. The plants grew slightly better in the Martian simulant than they did in the poor Earth soil, and significantly better than plants in the Moon soil. Tomato, rye, cress and field mustard performed particularly well. The latter three flowered, and cress and field mustard even produced seeds. 

(That the crop plants germinated better is not surprising – the seeds for crop plants are controlled and carefully produced, so the quality and germination rate is higher.) 

In warmer weather, it was hard to keep the Earth and Moon soils sufficiently wet. The Martian soil had a better water-holding capacity. Analysis showed that it contained significant amounts of carbon – was that organic matter contamination? It’s not clear, but that would certainly help to retain water.

All of the soils had a high pH, meaning that they were alkaline. This isn’t ideal, particularly for crop plants. The higher pH of the Earth and Moon soils probably contributed to the poor plant growth. The Moon soil lacked nitrates; the Earth soil lacked phosphate. There was also aluminium in the Moon soil that could have affected plant growth, and even killed plants.

Big trefoil grown on Mars soil simulant after harvest
Copyright Wieger Wamelink (CC BY-NC 4.0: https://creativecommons.org/licenses/by-nc/4.0/ )

“Our expectations were very low, so we were very surprised that on the Mars soil simulant plants grew rather well and even better than on our nutrient-poor control earth soil. There were also problems, the biggest that it was very difficult to keep the soil moist and that though on Mars soil simulant there was growth it was not very good, i.e. the amount of biomass formed was low.”

Wieger Wamelink

So, in theory, it should be possible for us to learn how to grow plants in extraterrestrial soil. But we need to do a lot more research, into things like how to feed the plants, whether we can create a living soil out of regolith, and whether the resulting crops would be safe to eat. And Wamelink has done more recent research into those topics. But that’s a story for another day!

Reference:

Wamelink, GW Wieger, et al. “Can plants grow on Mars and the moon: a growth experiment on Mars and moon soil simulants.” PLoS One 9.8 (2014). DOI: 10.1371/journal.pone.0103138.

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