A graduate student at the University of Sydney, Daniel Irving, has developed the first detailed classification of Martian soils and a global map of their distribution. The work was published in the journal CATENA. According to the authors, the system will help scientists compare different regions of the Red Planet and choose sites for future research, habitation and possible crop cultivation.
For Martian soils the researchers proposed a new term - "areths", from Ares, the Greek name for Mars. The classification, called PPACS (Provisional Polythetic Areth Classification System), divides soils into 10 major "kingdoms", 100 "associations" and 2,000 detailed "forms". It is based on properties such as texture, mineralogy, acidity (pH) and profile thickness.
The system relies on orbital observations and models. With its help the researchers assigned almost 800 million points on the Martian surface to classes, and the map covers 92% of the planet's surface. "Previously scientists mapped the geology, mineralogy and chemistry of Mars, but our system combines many properties into a single framework," the university's press release quoted Irving as saying.
Separately, the authors assessed all 2,000 classes for suitability for crop cultivation - by water-holding capacity, acidity, profile thickness, and clay and gravel content. The best was the class Nohibareth, with a score of 0.93 on a scale from zero to one. Its modelled average profile includes pH 6.8, 72% clay, minimal sand and gravel, and a water-retention rating of 202 millimetres per metre. On the map this class occurs at mid-latitudes, in the regions of Pharsis, central Arabia Terra and the Elysium Planitia, but occupies only 0.14% of the Martian surface.
The researchers stress that these are candidates for closer study, not selected sites for greenhouses. In 73% of the forms studied, the limiting factor turned out to be precisely water-holding capacity. Classes from the Fareth-Mareth "kingdoms" scored less than 0.3 points.
The authors caution that the results do not mean plants can be grown directly in Martian soil. Future greenhouses will still need a controlled environment, water, nutrients and treatment of potentially harmful substances. In addition, the current map does not take into account perchlorate toxicity, nitrogen availability and deep profiles, and many properties are derived from orbital data and terrestrial analogues. As direct measurements become available, the classification will have to be refined.
The next step is experiments on Earth: scientists can create mixtures simulating Martian classes and test them under controlled conditions with plants, additives and managed irrigation. In time, rovers may be able to confirm whether promising classes really lie where the models show.
Although Martian soils contain no organic matter, the work may also help soil science on Earth - for example, in studying the Atacama Desert, the Pilbara region and polar areas. "People often think soil needs a lot of organic matter, but on Earth there are soil types with very little of it, especially in arid regions. The results from Mars make one think more deeply about such soils," said Irving's supervisor, Professor Alex McBratney.