  Motility is a ubiquitous feature of microbial life on Earth, and is widely
regarded as a promising biosignature candidate. In the search for motile
organisms, it is therefore valuable to have rough estimates for the number of
such microbes that one may expect to find in a given area or volume. In this
work, we explore this question by employing a simple theoretical model that
takes into account the amount of free energy available in a given environment
and the energetic cost of motility. We present heuristic upper bounds for the
average biomass density and the number density of motile lifeforms for the
Martian subsurface and the ocean of Enceladus by presuming that the motile
microbes in question derive their energy from methanogenesis. We consequently
demonstrate that the resultant densities of motile organisms might be
potentially comparable to, or much lower than, the total microbial densities
documented in various extreme environments on Earth.
