Hydroponics is a method of growing plants without soil, instead dissolving nutrients in water and feeding that solution directly to the plant roots. In this post we will discuss the technical challenges presented when designing a hydroponics system for use on Mars. By the end of this post, we will have decided which type of hydroponics system currently in use on Earth is best suited for Mars.
What's so different about Mars?
- Mars gravity is only 38% of Earth's
- Mars gets only 44% as much sunlight as Earth
- Water is limited
- Electricity is limited
- Fertilizer/nutrients are limited
- Planetary protection is an issue
Let's start with the good news! Lower gravity is a huge plus for hydroponics system design. Not only do the physical structures have to support less water weight, but the pump systems do not have to work as hard to overcome pressure head. Most water pumps are rated for a certain vertical distance that they can pump water, which is constrained by the water pressure the pump provides. On Earth, water causes 0.443 psi per vertical foot, so to pump water up 6 feet you would need a pump capable of more than 2.66 psi. On Mars, you would only need 38% of that power, or just 1.0 psi. For consumer-grade pumps, this would reduce the current draw from 230mA to 60mA per pump.
This might not seem like a huge advantage, but we need to design the maximum power usage to be less than or equal to that supplied by the emergency power system, because even a temporary absence of water could dry out the roots and kill the plants. An alternative would be to design the system in a fail-safe way so that in the event of a power failure the roots become flooded with stagnant water. This prevents immediate root death due to desiccation, but deprives the roots of oxygen and fresh nutrients. This would be workable for short emergency power failures, but not long periods of reduced-power operation.
Mizuna lettuce on the International Space Station
We don't know how partial gravity affects plant vegetative growth. You can see in the above image that plants grown aboard the International Space Station appear pretty normal, but I am not sure how strong they are. Would plants grown on Mars be strong enough to remain upright? Tall, spindly plants may fall over as they begin producing fruit. The plants could be too fragile to handle during growth, making maintenance difficult or making it impossible to efficiently utilize space by placing plants close together when small but manually moving them to be farther apart when large. Another danger is that the plants may be capable of growing so tall that they quickly reach the height of the LED's and present a fire hazard. We need to know more about how the specific plants we plan to grow behave in partial gravity.
The bad news is that instead of receiving 44% as much light as on Earth, the reality is that the actual effective natural sunlight that the plants receive is likely to be 0% because the gardens will be inside a radiation-protected, opaque greenhouse accessible to the crew. So 100% of the light energy provided to the plants must be provided by the habitat subsystems. Even if we were able to somehow pipe in light from outside, a dust storm could reduce that light dramatically for weeks or months or even years. The plants will need full LED lighting to ensure survival.
I probably run about 20 watts of LED lighting per square foot for my indoor plants. If you assume that the hydroponic system will fill approximately 2/3 the floor space of an average Manhattan studio apartment, then you end up with 500 sqft 2/3 20 W/sqft = 6.67 kW. This is about as much as the maximum continuous power that a single shuttle fuel cell could provide.
Unlike water, a temporary interruption in light to the plants will not kill them. In the case of an emergency or a system failure, the lights can be left off.
Hydroponics systems consume massive amounts of water, but some consume more than others. Here's a quick run-down from most water to least water:
- Aquaponics consumes the most water, literally dangling roots into a pond
- Deep Water Culture is second, dangling the roots into a shallow water trough which requires changing somewhat frequently
- Kratky Method is the same as deep water culture, but you do not need to change the water
- Ebb and Flow requires enough water to fully submerge the grow media, so it probably requires half the reservoir volume of deep water culture
- Nutrient Film Technique trickles water over roots lying in a trough, and requires the least amount of water
Unfortunately I do not have time to perform any calculations comparing gallons of water used for each method, but I think that no matter what we will be forced to use NFT due to its relatively low water requirements per square foot.
The first missions to Mars will likely have to bring all the water they need from Earth, which would exclude the possibility of any hydroponics system larger than a single basil plant. Future missions may be able to harvest water on Mars, but note that this water will still be in high demand. Besides the normal uses as drinking water and for personal hygiene, harvested water on Mars will likely be converted into rocket fuel. We would have to be able to harvest thousands of gallons of water in order to satisfy all these needs.
I think this has been a sufficient discussion for the time being. Hopefully as we get closer to the 2030's, more research will have been completed and we will know more about the problems we will encounter with hydroponics on Mars.
The post header image is a screenshot of Planetbase, which you should totally play. http://planetbase.madrugaworks.com/