Can we grow plants on Mars?

All the recent talk about the Mars One project got me thinking; what are these guys and gals going to eat? According to the website they’re planning on growing a lot of food within the habitation pods, but just how easy is it to cultivate plants on Mars?

First, the basics.

Can plants grow on this icy world? Image credit: NASA

An obvious problem is that Mars is very cold. It’s further than Earth from the Sun and it has a very thin atmosphere. A summer’s day at the Martian equator can reach 20°C but at night the temperature would fall to -73°C, killing even the most cold-tolerant of arctic plants.

Plants on Mars will have to be grown in heated greenhouses, with additional lighting to combat the lower light levels, which are around 50% of Earth’s light intensity. Liquid water can be obtained by melting the ice found in Martian soil, and carbon dioxide for photosynthesis makes up most of the Martian atmosphere (although the atmospheric pressure would have to be increased for both plant and human survival).

Yes yes, the basic necessities of plants are covered on Mars. BORING!

What about the nitty gritty details of growing plants on another planet? There are a lot of big unknowns, but we are slowly finding the answers to some of these questions.

Are plants able to grow on Martian soil?

The dusty soil on the Red Planet lacks reactive nitrogen, which plants need to grow. Image credit: NASA

Shipping tonnes of compost to the red planet would be expensive, so it’s better to grow plants on native Martian soil. Samples taken by Mars Pathfinder showed that the soil on Mars is sand-like, but does contain nearly all the essential nutrients that plants need to grow, with the exception of reactive nitrogen compounds like nitrate (NO₃^–) and ammonium ions (NH₄⁺), which are vital for plants to make proteins and DNA. On Earth, the majority of reactive nitrogen comes from rotting organic matter from dead plants and other organisms. Since there is no life on Mars, it makes sense that the soil is lacking these nutrients.

So is it possible to grow plants in this soil? Scientists from the Netherlands used simulated ‘Martian’ soil to find out. (Check out the paper here – open access).

NASA produced this Mars simulant soil using volcanic ash from Hawaii, which has little organic material, and treated it until it chemically and structurally resembled Martian soil. The researchers then used this soil to try and grow plants.They used 14 different species with small seeds so that the plants couldn’t rely on the nutrients stored inside the seed for growth.

All the plants germinated on the Mars soil simulant, and the researchers were amazed to find that despite the lack of reactive nitrogen, many of the plants grew better than those on nutrient poor Earth soil taken from a riverbed. The four crop plants in the study, tomato, rye, carrot and cress, survived well on the Mars soil simulant, which bodes well for growing plants on Mars in the future.

The simulant soil was a very close replica of Martian soil, but it did contain trace amounts of organic material because it came from Earth. Still, a colony on Mars could fertilise their soil with -ahem- human waste, so it is a promising start.

The problem of gravity

This is about to get a little Physics-y, but don’t worry, it’s simple enough even for me to understand!

The gravity on Mars is only around 38% of that which we experience on Earth. This means you weigh 62% less on Mars! (Click here to explore your weight around the solar system, but if you’re dieting, avoid travelling to the Sun…).

Science writer Mary Roach experiences weightlessness in a parabolic flight. Image credit: NASA

Gravity is really important for the movement of heat and gases. Cold air is denser than hot air, meaning a given volume of hot air weighs less than cooler air. The hot air rises and is replaced by dense cooler air, causing the air to move and circulate in a process known as convection.

At zero gravity everything weighs the same (nothing), so as Martian gravity is lower than Earth’s then the weights of hot and cold air will be more similar to each other. Convection on Mars will be reduced and the air will not circulate as much as on Earth.

This will limit the plant’s ability to take in CO₂ for photosynthesis and release water through their leaves in the process of transpiration. Transpiration is important because it cools leaf surfaces much like sweating cools us down, but it is also the driving force behind the uptake of nutrients from the soil; as the water travels from the roots to the shoots it carries dissolved nutrients with it in solution.

To find out more, a daredevil research team from Osaka took to the skies to investigate the effect of gravity on plants. They strapped strawberry plants, lights and sensors into an aeroplane and took 28 parabolic reduced gravity flights used to train astronauts, measuring transpiration and photosynthesis rates in both doubled gravity (2g) while the plane ascended and at near zero gravity during the periods of weightlessness. (The paper can be found here behind a paywall).

In zero gravity plant transpiration decreased by 46% and there was 20% less photosynthesis than there was under normal gravity conditions. Interestingly, the opposite was true of the high gravity periods; plants increased their transpiration by 32% and their photosynthesis by 7%. This shows that gravity plays a major role in regulating plant transpiration and photosynthesis, so the reduced gravity on Mars may have a negative impact on plants, limiting their growth, nutrient uptake and temperature regulation.

Is it safe?

According to a recent MIT study (freely available here), growing crops on Mars isn’t as idyllic as it sounds. In fact, oxygen levels will shoot up as the plants mature and increase photosynthesis, posing a huge risk for fires. Oxygen venting systems are not currently well enough developed to keep the concentration at a safe level without venting nitrogen, which is needed to maintain the air pressure needed to breathe, so it would seem the choice is either suffocate or possibly burn to death.

Add to that the other unknowns described above, and it looks like a Mars colonisation attempt would need a lot of new technology and further research before anyone blasts off.