Do soils exist outside Earth?

On Earth, soils form thanks to the combined action of at least five factors: parent rock, climate, topography, biota, and time. However, the necessity of biota as unavoidable soil forming factor is debated, as important parts of our planet experiencing extreme climates host virtually life-free soils with advanced horizonation. Now that space exploration has greatly expanded our understanding of the Solar System, providing consistent evidences that the loose, unconsolidated "skin" of some nearby rocky bodies is lifeless, it is time to establish if the latter can be considered to be soil in a pedological sense. Our feeling is that, since the concept of soil chiefly bases on the occurrence of weathering and internal differentiation – both induced by biotic and/or abiotic processes – in an incoherent mass of mineral matter, the clearly altered surfaces of Venus, Mars, and our moon deserve the rank of soils. In case of farming in planetary bases, the soils of Mars seem to have the potential to be fertile substrata, eventually only requiring to be designed in terms of pore-size distribution and pore connectivity and to be freed from excess salts and toxic compounds.