🏆 A Biologist’s Guide to Mars Dirt
Just add water
💧Introduction
If you put Mars dirt and water in a greenhouse, could anything grow? It’s a tricky question with enormous implications for humanity’s future in space. On one hand, the soil (or regolith) on Mars is known to have toxins like perchlorate that would kill most living organisms, as we’ve written about before. On the other hand, Martian regolith is surprisingly rich in nutrients like nitrates — it comes pre-fertilized! Are there, on the whole, enough nutrients in Mars dirt that living things could overcome the toxins?
Ideally, to answer these questions we would take direct measurements of actual Martian regolith. Unfortunately, those measurements are rare — the only direct one is from 2008, when the Wet Chemistry Laboratory (WCL) on the Phoenix Lander measured soluble Mars dirt chemistry. Until Mars Sample Return is completed in the 2030s (if it doesn’t get delayed again), we won’t be getting more data on what’s soluble in the dirt.
We are interested in this question because we want to engineer microbes for Mars that can generate valuable supplies for the first astronauts like food, clean water and air, and bioplastic building materials. Transporting materials from Earth is expensive, so the more we can make use of local resources the better. In space science, this is called in situ resource utilization (ISRU), which refers to using the materials (i.e. soil, atmosphere, and water) found at the destination instead of paying to rocket all of that matter out of Earth’s atmosphere and on a half-year trip to Mars.
Below, we outline our current recipe for a soluble Mars Regolith Media (sMRM-Phx). With this recipe, we can determine whether or not microbes could rely on Mars dirt for key nutrients like nitrogen and phosphorous. We dive into where the data for this recipe comes from and what that implies for how certain we are of this recipe. Having a defined recipe will improve standardization across the astro-microbiology community and allow us to 🏆 benchmark organisms for Mars-readiness.
🧪 Soluble Mars Regolith Media Recipe
Past Mars simulants have focused on physical characteristics, like the bulk mineral composition and texture. These are important for structural and robotic applications, such as testing rover tires 🛞, but not as useful for plant or microbial ISRU 🌱.
Our sMRM-Phx is an analog specifically made for biological assays to properly simulate the presence of the essential nutrients — nitrogen and phosphorus — that would be present after mixing 40 grams of regolith with a liter of water, then straining out anything insoluble.
Actually making media with this composition is tricky! Below is our recipe for sMRM-Phx media formulation. This required careful consideration of the acceptable range of ion concentrations and their overall charge contribution at ~pH 7.7. This recipe balances the ions using the salts, acids, and bases listed:
sMRM-Phx: mix 40 grams of Mars regolith with a liter of water, then strain out anything insoluble.
Briefly, we composed sMRM-Phx by taking the average of values from Phoenix lander measurements1 and two additional Mars regolith chemical stimulants2. See comparison of the concentrations of 11 key species below. The confidence intervals on sMRM-Phx represent the minimum and maximum values from the reported values in the papers.
What’s different?
There are four main differences between our sMRM-Phx and previous chemical simulants that make it better for biological ISRU applications.
NH4+ | While ammonium was measured and reported because it was expected contamination from the lander’s rocket fuel in the original Phoenix WCL studies, it was found to be below the limits of detection at all three of the sampling locations. It is not included in our simulant.
NO3- | Since there have been no in situ measurements of soluble nitrates, not all previous simulants take this anion into account. Due to research on the presence of nitrate-containing minerals that are thought to be soluble and the measurement of soluble nitrates in synthetic leaches, we have decided to keep these in the mix.
PO43- | Previous Mars simulants notably do not model the presence of soluble phosphates. This is important, since regolith is is the only possible in situ phosphorus source for living organisms. We chose to incorporate this due to promising data on the solubility of apatite minerals that are also found in Mars regolith.
HCO3- | Due to the difficulty of incorporating soluble bicarbonate into our media formulation atom and charge balance calculations, and the sparse data on soluble carbonates, we decided not to include it in our recipe.
🪨 How do we know what’s in Mars regolith?
We have limited data about what’s in the regolith on Mars. There’s global spectroscopy data taken from orbit, lander and rover spectrometer measurements from the midlatitude Northern Hemisphere, and one soluble chemistry measurement from the Phoenix Lander in the polar region of the Northern Hemisphere. That’s it. In addition, scientists look to terrestrial analogues to learn more about the likely chemical composition of Martian regolith, and also to manufacture simulants. Here are categories of Mars regolith analogs that have been produced for different uses.
Of previously published soluble Mars regolith analogs, the Mars Simulant that was chemically mixed to mimic the Phoenix WCL soluble chemistry and the Mars Analog Regolith Extract (MARE) that is a leachate of a Phoenix Lander site mineral mimic are the most accurate. Our recipe above is a higher fidelity and more consistent version of these that averages across Phoenix WCL reports in the literature and supplements the things Phoenix didn’t measure but should be present based on other modeling and analysis.
Numerically, here is a breakdown of the composition of all of these various types of Mars regolith analogues:
⚛️ How we decided on our recipe
In more detail, here's a breakdown of how we decided on the concentrations of each of the major species in sMRM-Phx.
🍔 Nutrients
The 2008 Phoenix WCL experiment measured pH, conductivity, and various cations and anions. But it didn’t attempt to measure biologically relevant anions like sulfates, nitrates, phosphates, and carbonates, which would be the source for the biologically essential elements of sulfur, nitrogen, phosphorus, and carbon. We chose to include other modeled and terrestrially measured data in our recipe average. Earlier we discussed deviations between our recipe and the Phoenix WCL measurements, which overlaps with the nutrient sources described below.
Sulfates | After the initial preliminary reports of the Phoenix WCL data, later analyses used equilibrium models to explain the observed in situ cation and anion concentrations. In brief, the measured concentration of magnesium can be explained by the presence of magnesium sulfate minerals, and thus also soluble sulfate anions. Leachates of synthetic mineral mixes were also found to contain sulfates, supporting the retrospective analysis of the in situ data.
Nitrates | While the Phoenix WCL didn’t look for nitrates, we know from the Curiosity Rover Sample Analysis at Mars (SAM) data that there are nitrates in the regolith, and these would be highly soluble. In the same reports as soluble sulfates, leachates of synthetic minerals identify soluble nitrates supporting their presence.
Phosphates | We know there are abundant phosphate minerals on Mars from alpha particle X-ray spectrometry data. The question remains of how many of them are locked up in the rocks or are soluble. A 2013 study performed dissolution experiments on representative apatite minerals and measured soluble phosphate. This study is the basis of our sMRM-Phx phosphate concentration.
Carbonates | Similarly to sulfates, bicarbonate shows up in some, but not all equilibrium models of the Phoenix WCL data. This combined with the fact that it is not very soluble and that we do not plan to rely on these as a carbon source resulted in us not including carbonates in our sMRM-Phx recipe.
☠️ Toxins
In addition to Mars regolith providing essential elements for life, it also harbors soluble compounds and conditions that can be toxic.
Perchlorates | The main novel takeaway from the Phoenix WCL experiment was the presence of perchlorates in Mars regolith. Perchlorates are bleach-like ions that, if consumed, cause short-term thyroid problems and long-term fatality for humans, as well as stunted growth for plants, and microbial death.
Metals | While soluble metals have never been measured on Mars, we know from meteorites that Mars rock contains many trace metals, similar to Earth soil. For this first sMRM recipe, we have decided not to add trace metals; however, those will be important to include in future formulations as some metals are essential for microbial growth and others are cytotoxic.
pH | The Phoenix WCL experiment measured the soluble regolith to be slightly alkaline and quite close to physiological pH. However, location-specific variation could result in both alkaline and acidic compositions.
💨 Homogeneity
Soil on Mars will vary across the planet just like it does on Earth, but probably to a lesser extant. There should be more homogeneity in the “fines” — or essentially the topsoil — of the regolith because they are small enough to be weathered and mixed by the wind on a planetary scale. These wind-affected “fines” are what the Phoenix WCL measured. We think our recipe accurately measures the result of mixing 40 grams of these homogenous “fines” with one liter of water, then straining out everything that doesn’t dissolve.
🌱 What’s next?
Now that we have our liquid Mars media, we want to test to see if anything grows in it. To do so, we can measure an IC50 in sMRM-Phx to compare strains. This lets us determine whether wild-type and engineered strains can grow from entirely in situ-derived nitrogen and phosphorus.
You may have noticed that there’s no carbon source in our sMRM-Phx recipe! In the short-term, we will supplement with a carbon source that could be brought from Earth (i.e. glucose). Next, we’ll experiment with carbon sources that could be locally sourced from Mars’ 95% CO2 atmosphere — either photosynthetically or electrochemically through feedstocks like acetate and formate.
💬 Do you have opinions?
We want to get feedback on our work faster than traditional scientific publishing will allow. That’s why we’re posting our science updates here. Please do comment below if you have thoughts, on this post or the attached lab notebooks, and sign up to get future updates!
In particular, we would love to hear your thoughts on:
If you’re a biology researcher and interested in using our sMRM-Phx media, is there anything we can do to make it easier for you? Would it be helpful to onboard this recipe at Teknova?
We would love to discuss with geochemists who have insight into how rock composition may vary across the planet. We just defined an ‘average’ Mars topsoil recipe, along with bounds on each biologically-important component. How do we now define a a ‘grid’ of possible media recipes that span the reasonable bounds?
Would it be helpful for us to publish this in a traditional journal?
Acknowledgements
We would like to thank Edwin Kite, Alfonso Davila, Mohit Melwani Daswani, Sam Kounaves, Suniti Karunatillake, and Susanne Schwenzer for helpful discussions and emails about the geology and geochemistry of Mars; and the whole Pioneer Labs team for their input on both this substack and media formulation.
Some of the soluble ionic species (SO42-, NO3-, PO42-, and HCO3-) reported in PHX WCL AVG were not measured during the Phoenix Lander WCL experiment, but were instead derived from equilibrium models taking into account the Phoenix WCL data or measured from synthetic mineral mixes.
The Phoenix WCL average was calculated using data from Hecht 2009, Kounaves 2010a, Kounaves 2010b, Adcock 2013, Stroble 2013, Toner 2014, and Naz 2023.
Mars Simulant is a chemically defined media from the Kounaves group (Stroble 2013). Mars Analog Regolith Extract (MARE) is leachate from a mineral mimetic from the Schuerger group (Nicholson 2012).
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Given the limited Mars soil data, is there any use in following your same approach for soil that you have access to? like if you generated or found the same limited data for potting soil from home depot and then made media using the same logic as your Mars media exploration, and tested microbes, would the data from your home depot lab soil be predictive of the actual soil?
If sterile production of chemical such as bioplastics, fuels etc. Supplementing the media to high concentration NaCl (abundant on Mars according to John Hopkins) could allow for non-sterile chemical production in a halophilic microbial host!