Plants will grow in lunar regolith, but they don’t like it

As anyone who has read or watched The Expanse or The Martian knows, growing plants in space has some big advantages. Plants can contribute to the maintenance of a healthy atmosphere, as they recycle water and provide some variety to diets. While they can be grown hydroponically, the process requires a significant amount of water, which might be in short supply. So for missions that will land on a body like the Moon or Mars, growing plants in the local soil might be a better solution.

But local soils on these bodies don't look like the ones we find on Earth, which have a complicated mix of minerals, organic compounds, and microbial life. Can plants adjust to these differences? A group of researchers at the University of Florida—Anna-Lisa Paul, Stephen Elardo, and Robert Ferl—decided to find out, and they used some incredibly rare material: lunar soil returned by the Apollo missions.

In the mix

The lunar soil exists in a form called regolith, which is basically loose, dusty material created by the constant bombardment of lunar rocks by micrometeorites. When the first samples were returned during the Apollo era, studies of the interactions of this regolith with living things focused on the fear of pathogens that could pose a danger to life on Earth. As a result, plants and seeds were briefly exposed to lunar soil and then tested to see if this exposure altered their growth. There were no attempts to grow anything in the soil.

NASA has since developed an Earth-made material, called JSC-1A, that is meant to simulate lunar soil. But there are some significant differences between it and lunar soil. These include chemical differences, with lunar regolith containing higher amounts of titanium and some trace minerals than JSC-1A. Earth's oxidizing environment also creates some differences in the chemical state of some of the metals present, including that of iron, a key component of many enzymes, such as those involved in photosynthesis.

Finally, there are some physical differences between the material and the soil. The rapid melting and cooling caused by micrometeorite impacts on the regolith creates small globs of glassy material. JSC-1A uses volcanic glasses to approximate this process, but there are still physical differences.

So the researchers decided to try working with the real thing, using JSC-1A as a control. And with the help of the Johnson Space Center staff, they obtained three different lunar samples returned by Apollo 11, Apollo 12, and Apollo 17. The samples all came from regions with a volcanic origin but differed in their age, with Apollo 11's material having the longest exposure on the surface and Apollo 17's having the shortest.