Recycled waste could make the moon or Mars suitable for growing food

Tiny pits, webbing patterns, and a dusting of nanoparticles are not what most people picture when they think about farming. Yet those small scars may end up mattering if you ever try to grow food on the Moon or Mars.

In a study described in ACS Earth and Space Chemistry, researchers tested what happens when a nutrient-rich liquid made from recycled waste meets fake Moon and Mars dirt. The short version is this: the “soil” starts to change. Also, the liquid starts picking up useful elements that plants need.

“In lunar and Martian outposts, organic wastes will be key to generating healthy, productive soils,” said Harrison Coker, the study’s first author from the Department of Soil and Crop Sciences, Texas A&M University and Texas A&M AgriLife. “By weathering simulant soils from the moon and Mars with organic waste streams, it was revealed that many essential plant nutrients can be harvested from surface minerals.”

The Moon and Mars are covered in regolith, a dry mix of dust and broken rock. It is not soil in the Earth sense. Regolith has no biology and no built-in cycle for recycling nutrients. If people want long-term bases there, they will need ways to grow food. Furthermore, they cannot rely on endless shipments of fertilizer from Earth.

Popular culture has played with the idea before. One well-known story about a Mars colony features a botanist using waste from astronauts to help turn regolith into something plants can grow in. Coker and Julie Howe worked with colleagues at NASA. They are trying to put a real-world version of that concept on a more chemical footing.

Their focus is a system NASA is developing at Kennedy Space Center called a bioregenerative life support system, or BLiSS. It is designed to break down waste into a stream of water and dissolved nutrients that could be reused. A high-fidelity prototype at Kennedy Space Center is the Organic Processing Assembly. This prototype uses anaerobic bioreactors and membrane filtration. Next, a phototrophic membrane bioreactor oxidizes nitrogen species.

For this work, the researchers used an artificial sewage feedstock called COPAS, mixed at 50 grams per liter of tap water. The BLiSS system broke it down. The final effluent was filtered at 0.22 micrometers. That effluent, with an unadjusted pH of 7.0, became one of the test liquids.

They compared it against two others: deionized water and a half-strength version of Hoagland’s solution. This is a common inorganic nutrient mix used in plant experiments, adjusted to pH 5.8.

Then came the dirt, or at least close stand-ins. The team used a lunar simulant called JSC-1A and a Martian simulant called MGS-1. In batch experiments, they reacted 0.5 grams of simulant with 25 milliliters of each liquid in centrifuge tubes. The tubes shook at 60 rpm for 24 hours at room temperature, about 22°C.

The point was not to grow plants yet. It was to see how the liquids and minerals interact. Does the liquid dissolve useful nutrients out of the simulant? Do some nutrients stick to mineral surfaces and vanish from the liquid? Does the simulant itself start looking less like jagged dust and more like something you might call soil?

Even before any regolith got involved, the two nutrient liquids looked very different. The effluent had far higher electrical conductivity than the Hoagland’s mix, 2070 microSiemens per centimeter compared with 713. In terms of ions, the effluent carried far more ammonium and nitrite. Meanwhile, the Hoagland’s solution carried far more nitrate and sulfate.

After 24 hours of shaking with simulants, the pH rose in both nutrient liquids. Hoagland’s shifted from 5.8 to about 6.6 to 6.7. The effluent rose from 7.0 to about 7.3 to 7.5. Electrical conductivity climbed too, especially in the Martian simulant tests. That jump suggested more material dissolved out of MGS-1 than out of JSC-1A.

The study describes those post-reaction liquids as slightly saline.

When the team tested how elements behaved across a range of dilutions, three stood out as showing consistent patterns that could be modeled: phosphorus, potassium, and zinc.

Phosphorus was the clearest case. It sorbed, meaning it stuck to the MGS-1 Martian simulant, but it did not show that same behavior with the lunar simulant. The researchers fit phosphorus sorption to a Langmuir model, which can suggest monolayer sorption. They reported a maximum sorption capacity (qmax) on MGS-1 of 7.103 micromoles per gram for Hoagland’s. For effluent the qmax was 17.685 micromoles per gram. The Hoagland’s solution showed a higher binding affinity value (KL) than the effluent.

Potassium sorption showed up only with Hoagland’s solution, on both simulants, and the team said a Freundlich model fit better. Zinc sorption appeared on MGS-1, but only from Hoagland’s, also matching a Freundlich model.

At the same time, many elements moved in the opposite direction. They dissolved out of the simulants into the liquid. The lunar simulant released relatively little compared with the Martian simulant. JSC-1A dissolution was characterized by release of metals such as aluminum, copper, manganese, nickel, and zinc. MGS-1 dissolution produced strong releases of sulfur and major ions like calcium, magnesium, and sodium. It also released some other metals.

Chemical measurements were only half the picture. Under a scanning electron microscope, the lunar simulant showed sharp edges that looked somewhat rounded after treatment. Holes appeared in mineral faces where small particles seemed to have collapsed. Webbing patterns appeared in an anorthosite fraction.

The Martian simulant showed more dramatic changes. The researchers noted what looked like a reduction in particle size in a broader survey. They found nano to microsized particles adhering to MGS-1 after weathering. The manufacturer also reported diatoms in all MGS-1 samples. The team said electrical charge buildup made the Martian simulant harder to image than the lunar one.

They also used X-ray photoelectron spectroscopy to look for changes in bonding on particle surfaces. Phosphorus peaks suggested metal phosphates, and in MGS-1 they saw a calcium peak likely corresponding to calcium phosphate.

The researchers are careful about what these results mean. Simulants are broadly representative, not perfect copies. Real lunar soils are chemically and mineralogically heterogeneous. Their redox state differs sharply from Mars. Agglutinates in lunar regolith are not well represented in JSC-1A. The reactivity of those agglutinates in water remains unknown. The lunar simulant also contains small amounts of nonlunar components such as carbonates, sulfates, nitrates, and clays. These could skew reactivity despite low abundance.

MGS-1 includes soluble components like epsomite and gypsum that dissolve readily, contributing to strong signatures of sulfur, magnesium, and calcium. The study also notes something missing for safety reasons: chlorates and perchlorates, which are widespread on Mars and highly soluble. How BLiSS-like organic streams interact with perchlorates remains an open question in their discussion.

Time is another limitation. The experiment lasted 24 hours, while soil formation is usually slow. The authors argue there is reason to expect weathering would continue with longer reaction times.

If you ever try to farm off Earth, you are going to recycle everything, especially water and waste. This study suggests that the waste-processing stream itself might do double duty. It can help dissolve useful nutrients out of local minerals. In addition, it may possibly dull some of regolith’s sharpness through early weathering.

It also flags a problem. The BLiSS effluent in this work lacked several plant-essential elements compared with the Hoagland’s solution. These include copper, iron, manganese, sulfur, and zinc. The authors suggest those elements may have precipitated or sorbed inside the BLiSS system. This could hurt the effluent’s usefulness for fertilizing crops unless it is fortified.

In other words, the loop is not closed yet. Still, the early chemistry points to a route where local rock and recycled waste could start filling the same role that imported fertilizer plays on Earth.

Research findings are available online in the journal ACS Earth and Space Chemistry.

The original story "Recycled waste could make the moon or Mars suitable for growing food" is published in The Brighter Side of News.

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