Areths and the Provisional Polythetic Areth Classification System (PPACS): a hierarchical and nomenclatural framework for Martian soil classification

The absence of a formally defined and operationally robust classification system for Martian soil material represents a critical gap in planetary science. While substantial compositional and remote sensing data exists, these datasets have not yet been integrated into a coherent, scalable taxonomy comparable in function—though not necessarily in structure—to terrestrial soil classification systems. This paper introduces the concept of the areth as the fundamental unit of Martian soil and presents the Provisional Polythetic Areth Classification System (PPACS) as a structured framework for its classification. PPACS is organised into a three-tier hierarchy comprising Realms, Assemblages, and Forms, with a fixed 10–10–20 structure yielding 2000 terminal areths. The system is explicitly polythetic, defining classes through constellations of properties rather than single diagnostic criteria. This iteration, for instance, incorporated soil mineralogy, texture, pH and thickness derived from remote sensing data. The nomenclature encodes hierarchical position directly into the name of each areth, thereby integrating classification and language. The system is provisional by design, reflecting current limitations in areopedology, particularly the absence of depth-resolved and horizon-based information. This iteration of the methodology reveals clear surface features at the scale of Realms, primarily driven by changes in texture and mineralogy across the mid-latitudes. Further analysis into Assemblages and Forms reveals local variability in soil properties at a higher resolution, while retaining a clear relationship between taxonomic and lexical distance.