Something more than boulders: A geological comment on the nomenclature of megaclasts on extraterrestrial bodies

Large clasts are common on extraterrestrial bodies, and these are traditionally termed “blocks” and “boulders”. These two terms can easily raise confusion, however, because they are used in a sense that differs from geological definitions. Several classifications of large clasts are currently in use in the Earth sciences, and they differ only in detail. They restrict the size of boulders to 1-4 m; larger particles are called "megaclasts". The analysis of the published information on large clasts on planet satellites, asteroids, and comets imply that the particles often described as "boulders" actually are megaclasts; boulders, as the term is used in the Earth sciences, are too small to be detected given the limited resolution of most images obtained. It were therefore scientifically preferable if the established geological literature were applied in the modern planetary and space research. It appears sensible to distinguish boulders from megaclasts; the latter comprise bodies that might be subdivided granulometrically into blocks, megablocks, and superblocks. It is also shown that the abundance of megaclasts on extraterrestrial bodies may itself be beneficial for our understanding of such particles, which are rare on Earth.