Lunar Terrarium


NASA Apollo 11 on the Moon

Space station size comparison
Plants have been grown on Earth orbiting space stations

Project Horizon
USAF proposed Project Horizon

A lunar terrarium would be a container containing plants and everything they need to survive on the Moon. No terrariums or greenhouses or permanent bases had been placed on the Moon until 2019. Before 2019, the only example of something close to a lunar terrarium would be the Apollo Lunar Module. The Apollo Lunar Module provided a habitat for astronauts, not for plants. Similarly, the Apollo spacesuits also provided a habitat for astronauts. During Apollo missions, experiments involving seeds or mice stayed in orbit similar to the seeds and tortois in Zond missions. However, an Apollo Lunar Module could provide the atmoshphere, water, and temperature that a plant needs in its environment to survive. NASA had plans for a Lunar plant growth experiment. In addition to atmoshphere, water, and temperature, this craft would have provided the light and nutrients needed by the plants being grown. The Lunar plant growth experiment never flew to the Moon.

In 2019 the Chinese Chang'e 4 spacecraft landed on the far side of the Moon. The Chang'e 4 has a sealed container with seeds and insect eggs to test whether plants and insects can live in lunar gravity. The climate controlled lunar biosphere experiment container includes potatos, arabidopsis thaliana seeds, cotton, rapeseed, fruit fly pupa, and yeast. If the plant seeds germinate and start photosynthesis they will use carbon dioxide and release oxygen, while the fruit flies and yeast use oxygen and produce carbon dioxide. After landing on January 3 2019, the containers temperature was adjusted to 24 degrees C and the seeds were watered. The China National Space Administration indicated that potato, cotton, and rapeseed had sprouted in the biosphere experiment, but only cotton sprout images were published. However, the biosphere experiment was not able to last over the cold lunar night, so it was ended after 9 days on January 12 2019.

While not on the Moon or in Lunar orbit, plant experiments have been conducted in Earth orbit. By being in Earth orbit, plants in these environments would have experienced weightlessness and not Lunar gravity and they would have been protected from radiation by the Van Allen radiation belt which wouldn't protect them on the Moon. Skylab was NASA's first space station, and it used an atmosphere of 74% oxygen and 26% nitrogen at 5 psi. Carbon dioxide would have been present from astronauts exhaling. A student proposed Skylab experiment involved the study of rice seeds that were planted and grown on the space station. It was the first time plants were studied in space in an experiment that lasted longer than three days. The rice took longer than usual to sprout, and then the stems sometimes grew away from the light. Salyut was the USSR's first series of space station, and they used an air like atmosphere at approximately sea-level pressures of 93.1 kPa (13.5psi) to 129 kPa (18.8 psi) with an oxygen content between 21% to 40%. Salyut-6 and Salyut-7 each included an Oazis 'orbital garden' experiment for plants to be grown on the space stations. Salyut-7 set a record by growing Arabidopsis plants which were the first to flower and produce seeds in space. Plant growth experiments have also been conducted on Mir Space Station, Space Shuttles, and on the International Space Station.

space terrarium examples
Vessel mission type country of development pressure (atm) atmosphere notes
Zond (aka Soyuz 7K-L1) circumlunar trajectory USSR unknown, but probably 1 Atm unknown, but probably O2 + N2 Lunar radiation environment, but not lunar gravity
Apollo Lunar landing USA 1/3 Atm 100% O2 Lunar radiation environment, Lunar gravity, but no plants, just humans
Skylab low Earth orbit USA 1/3 Atm 74% O2 + 26% N2
Salyut low Earth orbit USSR 1 Atm 21% O2 + 78% N2
Mir low Earth orbit USSR / Russia 1 Atm 21% O2 + 78% N2
Space Shuttle low Earth orbit USA 1 Atm 21% O2 + 78% N2
ISS low Earth orbit international 1 Atm 21% O2 + 78% N2
Chang'e 4 Lunar landing China unknown unknown Lunar radiation environment, Lunar gravity, plants sprouted, but not lunar regolith/soil

The Apollo Lunar Module, proposed Lunar plant growth experiment, and Chang'e experiment, are all examples of above Lunar surface terrariums. Alternatively a lunar terrarium could be placed on the surface of the Moon or under the surface of the Moon. By placing a lunar terrarium on or below the surface of the Moon, lunar regolith might be able to be used for soil nutrients. Also, if there is frozen water or frozen carbon dioxide under the lunar surface, these approaches might be able to leverage it. Past lunar base proposals such as Project Horizon, Zvezda, and Lunex have focused on below surface structures.

Placing a lunar terrarium on or below the surface of the Moon would require an environment manipulator to dig into the lunar regolith. An environment manipulator is a tool or instrument designed to manipulate the environment outside of a spacecraft lander or rover on a celestial body. For the US Apollo program to the Moon the astronauts used a number or environment manipulators that they brought to the Moon in the Apollo Lunar Module. The Apollo Lunar Surface Drill was used to obtain a continuous regolith column up to 3 meters in length and to provide holes for the placement of heat flow probes. During the Apollo missions, the astronauts were surprised by the difficulty of extracting subsurface samples. While the top layer of the Moons surface was powdery and soft, attempts to drill into the surface and extract subsurface material resulted in seizing of drill tubes which could not be removed and had to be abandoned. They were ultimately successful and that's how data on temperature at depth of the lunar surface was obtained. However, this indicates that constructing a burried terrarium might require a large spacecraft, lander, and rover in order to dig a hole more than a meter below the surface.

Lunar Plant Growth Experiment
NASA proposed Lunar Plant Growth Experiment
Lunar Biological Payload
This container carried by Chang'e 4 was mankind's first plant biological payload on the Moon. / Chongqing University Photo

For a terrarium to provide the carbon dioxide needed by plants, it could be transported to the Moon from Earth as an atmosphere mix or as pure dry ice. The mass of Earth atmosphere needed to fill a 10cm x 10cm x 10cm cube would only be about 1.22 grams. Alternately, it is hypothesised that carbon could be extracted by heating the Lunar soil, causing carbon dioxide (CO2) and other carbon based molecules to form. There is a threshold of how much carbon dioxide in the atmosphere plants can or will use. Ambient level of carbon dioxide hover around 400 ppm in Earth atmosphere. If the level of carbon dioxide is increased to around 1,000-1,500 ppm, plants will be healthier. However, if the level rises to 2,000 ppm or higher, plants will experience negative effects such as carbon dioxide burn. Thus, some other substance such as nitrogen or oxygen must be present in the terrariums atmosphere. Also, during photosynthesis, carbon dioxide is consumed and replace by oxygen, so the terrarium would need to replace the carbon dioxide as the plant uses it. For this reason, a local source of carbon dioxide would be preferable. For a terrarium to provide the water needed by plants, it could be transported to the Moon from Earth as ice. As another option, the small amounts of water believed to exist closer to the Lunar poles could be leveraged. Again, during photosynthesis, water is consumed, so the terrarium would need to replace the water as the plant uses it. For this reason, a local source of water would be preferable.

Not only do plants require a certain temperature range to grow, most machinery and electronics need to stay within a temperature range or they will break down. For example, the Apollo Lunar Rover's signal processing unit could only survive between -50 C (-65 F) and 85 C (185 F). With days and nights that last about 14 Earth days each and no atmosphere to distribute heat, the Moon goes from very hot to very cold. This is differnt than for Earth orbiting space stations which go from about 45 minutes of sunlight to 45 minutes of shadow every orbit. Temperatures on the Moon vary from approximately -170 C (-275 F) at night to around 140 C (280 F) in the day. As a gauge of that range, while the lunar high temperatures can be simulated with a kitchen stove, the lunar low temperatures are significantly lower than you can get with a kitchen freezer. Past lunar base proposals such as Project Horizon, Zvezda, and Lunex that focused on below surface structures planned to take advantage of the characteristic that beyond a meter below the Lunar surface the temperature doesn't rise or fall as extremely. Also, anything blocking sunlight from hitting a surface will allow that item to radiate heat rather than absorb heat. The Apollo Lunar Module was only used during Lunar daytime and used insulation to prevent it from getting to hot. The Chang'e 4 biological payload was able to regulate its temperature during the day, but it was terminated at the first lunar nightfall. It's unclear if the Chang'e 4 biological payload had a means designed into it to keep the environment warm over Lunar night. Just like how an electric battery stores electricity, a thermal battery is used to store and release thermal energy. It might be possible to use thermal batteries to store heat during the Lunar day and then keep things warm during the Lunar night. The Apollo Lunar Rover had wax tanks that were used to store excess heat generated by electronics. As the electronics generated heat, it melted the wax, which stayed at a relatively constant temperature until it was all melted. Water and concrete are other examples of substances that can be used to store and release heat.

Experiments

For students who want to do experiments related to terrariums to support plant life in the lunar environment, they might be able to simulate the environment on Earth. They could do a science experiment that demonstrates what happens to different plants when grown in a Lunar soil simulant which simulates the mineral composition of the Moon. Note that Lunar soil simulant can be a bit expensive and hard to find. They could also do a science experiments that try to simulate light and temperature effects on plants or the terrarium in which they would grow. Here are some ideas for science experiments:

Science Questions (concerning growing plants in the lunar environment)
Question What is Tested? What Stays Constant? Data Collected
How does exposure to sunlight affect the strength of plastic? (simulates the longevity of a domed terrarium) Plastic bags exposed to varying amounts of sunlight (1 day up to 14 days) Thickness of plastic bag, size of plastic bag Amount of mass the bag can hold without breaking
How does a container's size and shape affect evaporation rates? (simulates evaporation in a domed terrarium) Containers of varying size and shape (e.g. short, tall, wide, or narrow, etc.) Initial amount of water, heat source Amount of evaporation over time
What type of sun shade evens out temperature the best? (simulates sun shade for a domed terrarium) Shape and material sun shade is made from Ambient air temperature, distance below surface of soil temperature reading Temperature after time of light exposure
What happens to soil temperature when exposed to various durations of light? Length of time exposed to continuous light (12 hours up to 14 days) Ambient air temperature, distance below surface of soil temperature reading Temperature after time of light exposure
Does radiation exposure affect seeds? Length of time exposed to microwave Type of plant seeds, amount of water, sunlight, and temperature Time until seeds sprout and how big plant is each day after sprouting
What plants grow best in soil with no microorganisms? (which simulates the lack of microorganisms on the Moon) Type of plant seeds used Amount of water, sunlight, and temperature Time until seeds sprout and how big plant is each day after sprouting
What plants grow best in Lunar soil simulant? (which simulates the mineral composition of the Moon) Type of plant seeds used Amount of water, sunlight, and temperature Time until seeds sprout and how big plant is each day after sprouting
How long can a plant grow while in continuous light (simulates 14 days of sunlight of the Moon)? Length of time exposed to continuous light (12 hours up to 14 days) Type of plants, amount of water, and temperature How big plant is each day and how healthy it looks
continuous light plant experiment regular light plant experiment

These pictures are examples of lettuce seeds in a small terrarium after 14 days. The picture on the left came from seeds under continuous 24 hour a day artificial light to simulate the sunlight on the Moon. The picture on the right came from seeds under approximately 12 hour a day natural sunlight as is found on the Earth. The seeds under continuous light tended to sprout earlier than the seeds under natural sunlight. However, the sprouts under natural sunlight grew larger leaves which might be due to the artificial light not being as intense or of the same spectrum as the Sun. A more natural continuous light source might have lead to overheating of the plants environment.

Working Example

For students who want to experiment with a working terrarium, you can build one right here on Earth although you won't have as many issues to deal with as you would on the Moon. Before building anything it's a good idea to know what you're trying to accomplish. For example, if you want to build a terrarium that would ultimately work on the Moon, are you building it to be completely sealed, or do you want it to take advantage of local soil nutrients, or potentially local frozen water or carbon dioxide sources, or using local material as a radiation shield? That will tell you something about what mechanics you might need in order to take advantage of local resources on the Moon.

The job of a terrarium is to allow plants exported from one place to grow in another location even though the environment might be different or even hostile to the plants.

If you want to build your own automated terrarium, you might need to start from scratch.

Build from Separate Components

For this you will need:

You will also probably want to have some environment sensors and environment manipulators. Useful environment sensors for a terrarium would give an indication of available water (H20) like a humidity sensor, available carbon dioxide (CO2) like a CO2 sensor, an atmospheric pressure sensor, a light sensor, and a temperature sensor.

Some retailers for electronics and other parts are:

By knowing the state of these characteristics, you or an automated terrarium could take action to adjust them if they were out of the range that would support plant life. Instead of building your own environment sensor system from scratch, you could use environment sensors built into many Android smartphones. To measure the environment of your terrarium, download the Android Telemetry app to an Android smartphone that is equipped with appropriate environment sensors.

Link to Telemetry for Android smartphones

Run Telemetry and tap the Refresh button to display your smartphones environment sensor values.

For a sealed terrarium, before sealing your terrarium, you should have an estimate of how large your plant will be able to grow. The size a plant with grow depends on the water (H20) and carbon dioxide (CO2) available in the environment. The light available also affects plant growth. The overall chemical equation for the photosynthesis that occurs in plants is:
6 CO2 + 6 H2O + light -> C6H12O6 + 6 O2
The product from photosynthesis is sugar (C6H12O6). A waste product from photosynthesis is oxygen (O2). To estimate the size a plant might grow in a sealed terrarium, download the Android ChemCalc app to your Android smartphone.

Link to ChemCalc for Android smartphones

Run ChemCalc and select the ChemCalc menu item. Calculate the molar mass for each of water, carbon dioxide, and sugar. For example to get the molar mass of water:
     H2O
you can tap on:
     ( H * 2 ) + O =
Or to get the molar mass of carbon dioxide:
     CO2
you can tap on:
     C + ( O * 2 ) =
By knowing the ratios of the molar masses, and knowing the mass of the water and carbon dioxide available in the terrarium, you can estimate the mass that the plant could grow to since plants are primarily composed of sugar based molecules.

Here are some exaples of autonomous terrariums built from various parts.


See also:

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