A view of extraterrestrial soils

The nature of soils on celestial bodies other than Earth is a growing area of research in planetary geology. However, disagreement over the significance of these deposits arises in part due to the lack of a unified concept and definition of soil in the literature. The pragmatic definition "medium for plant growth" is taken by some to imply the necessity of biota for soil to exist, and has been commonly adopted in the planetary science community. In contrast, a more complex and informative definition bases on scientific theory: soil is the (bio)geochemically/physically altered material at the surface of a planetary body that encompasses surficial extraterrestrial telluric deposits. This definition emphasizes that soil is a body that retains information about its environmental history and that does not need the presence of life to form. Four decades of missions have gathered geochemical information of the surface of planets and bodies within the Solar System, and more information is quickly increasing. Reviewing the current knowledge on properties of extraterrestrial regoliths, we conclude that the surficial deposits of Venus, Mars, and our moon should be considered soils in a pedological sense, and that Mercury and some large asteroids are mantled by possible soil candidates. A key environmental distinction between Earth and other Solar System bodies is the presence of life, and due to this dissimilarity in soil forming processes, it is plausible to distinguish these (presently) abiotic soils as Astrosols. Attempts to provide detailed classifications of extraterrestrial soils are premature given our poor current knowledge of the Universe, but they highlight the fact that Earth possesses nearly abiotic environments that lend themselves to understanding more about telluric bodies of the Solar System. He found himself in the neighbourhood of the asteroids 325, 326, 327, 328, 329, and 330. He began, therefore, by visiting them, in order to add to his knowledge. (Excerpted from the "The Little Prince" by Antoine de Saint-Exupéry)