A Planetary View of Soils: An Introduction to Astropedology and Space Agriculture.

The nature of soils on celestial bodies beyond the Earth is a fast growing area of interest. Decades of space exploration have gathered significant information regarding the surfaces of planets, moons, asteroids, and comets in our solar system. It has been argued that many of the surficial deposits of these celestial bodies should be considered as soils or soil candidates in a pedological sense. These celestial bodies have unconsolidated rocky, mineral, and icy materials on their immediate surfaces that have been subjected to and show the effects of the environmental impacts of climate, relief, parent material, and time (i.e., four of the five general soil-forming factors, with the exception of biota). These physically and/or chemically weathered, unconsolidated materials, often with some degree of organization (such as aggregation and horizonation), can be broadly considered as soils. Such a planetary view of soils has the following benefits: 1) Treating soils as recorders of environmental history in celestial bodies so that possible environmental changes in the past may be decoded through the investigation of their soils, along with bedrocks and other environmental components; 2) Assisting space exploration and searching for past or present extraterrestrial life by utilizing terrestrial soil science knowledge, including signs of microbial life in soils and the role of biota in weathering and soil evolution. Soil habitability (a soil’s potential to nurture and sustain life) is an emerging topic in planetary science; and 3) Helping the plan and implementation of possible farming on planetary bases and possible human colonization in the future, including growing plants for food supply, purifying water for reuse, and engineering for waste treatments in space. Hence, space exploration has created a brand-new window of opportunity for the study of extraterrestrial soils – "astropedology" – as well as space agriculture. Exciting frontiers abound, which in return can help to enhance terrestrial soil and plant sciences by placing Earth's soils in a broader planetary science context.