To grow food on Mars, scientists first need to have an in-depth understanding of the different kinds of soil found on the planet. Mars has been mapped extensively for its geology, minerals and chemistry, yet researchers have lacked a unified system to classify its soils by multiple properties. Now, researchers at the University of Sydney have classified Martian soils into 2,000 types, mapping where they occur across the planet. The work could help scientists figure out which areas are worth investigating for future missions and whether any Martian soil could support crops in a greenhouse.The system also identifies one soil class, called Nohibareth, as a promising candidate for supporting plant growth under controlled conditions. “A future greenhouse on Mars would need more than air, water and light. Its builders would also need to know what lies beneath to understand better starting points for growing plants,” Alex McBratney, one of the researchers and a soil scientist at the University of Sydney, said in their study. Giving Martian soils a common languageThe new research work introduces the term ‘areths’ for Martian soil classes, drawing on Ares, the Greek name for Mars.The classification is called the Provisional Polythetic Areth Classification System (PPACS). It organizes soils into three levels: 10 major realms, 100 assemblages and 2,000 detailed forms. Rather than assigning a class based on a single characteristic, the system considers combinations of properties, including texture, mineralogy, pH and profile thickness — the depth of the soil layer.To build the framework, the researchers combined orbital observations of Mars with models to estimate soil properties across nearly 800 million locations. They then classified these locations and produced a map covering 92% of the Martian surface. The result gives scientists a consistent way to compare different areas, rather than treating each soil property as a separate piece of information.“This is the first time a classification system of this scale has been developed for Martian soils. Scientists have mapped Mars’ geology, mineralogy and chemistry before, but our system brings together multiple properties into a single framework, helping us understand how different Martian soils relate to one another and where they’re found,” Daniel Irving, co-study author and a PhD candidate at the University of Sydney, said.Could any of these soils support crops?The researchers tested the system’s practical value by assessing all 2,000 soil classes for five characteristics relevant to plant production: water-holding capacity, pH, profile thickness, clay content and gravel content. Previous research into plants that could potentially thrive on Mars has also explored which crops might tolerate simulated Martian conditions.Nohibareth emerged as the most promising class, scoring 0.93 on a scale from zero to one. Its modeled average profile has a pH of 6.8, contains 72% clay and has little sand or gravel. Its estimated water-holding capacity is 202 millimeters per meter of soil.The class occurs across just 0.14% of Mars, particularly in the midlatitudes around Tharsis, central Arabia Terra and Elysium Planitia. These regions could warrant closer investigation, but the assessment does not establish that any is suitable for a future greenhouse.Water emerged as a major constraint: water-holding capacity was the limiting factor in 73% of the soil forms assessed. All classes in the Fareth to Mareth realms scored below 0.3 in the initial plant-production assessment.Even the highest-scoring class would need further testing. Martian soil lacks organic matter, and the current classification does not account for important factors such as perchlorate toxicity, nitrogen availability or detailed soil layers. Many of its properties are inferred from orbital observations and relationships established using terrestrial soils, rather than direct measurements at every mapped location.What happens before anything grows on Mars?The next step is to test Mars-like soil mixtures on Earth. Researchers could recreate materials representing different mapped classes and grow crops in controlled environments, comparing plant growth, water use and the treatment needed to make each mixture safe. Such experiments could build on previous research into growing crops in simulated Martian soil. Future rover measurements and samples from Mars could then help verify whether the predicted soil properties match conditions on the ground.The framework could also prove useful beyond Mars. Some of Earth’s arid and polar environments contain soils with little organic matter, making them relevant comparisons for understanding how such materials behave. Research into turning Martian dust into material that supports plant growth highlights how much work remains to make barren planetary materials useful for agriculture.“People often think soil needs lots of organic matter to function, but there are soil types on Earth, particularly in arid regions, that contain very little organic matter. These results from Mars actually make us think more deeply about this kind of soil on Earth and what we might do with it,” McBratney said. However,for now, PPACS is a provisional framework. It provides an initial assessment of what Martian soils might offer, but there are many obstacles that scientists need to overcome before they can actually grow crops on Mars..The study is published in the journal CATENA.