Motility is a ubiquitous feature of microbial life on Earth and is widely regarded as a promising biosignature candidate. In the search for URLanisms, it is therefore valuable to have rough estimates for the number of such microbes that one might 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  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  might be potentially comparable to, or much lower than, those documented in various extreme environments on Earth.
Motility is a ubiquitous feature of microbial life on Earth and is widely regarded as a promising biosignature candidate. In the search for URLanisms, 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 URLanisms might be potentially comparable to, or much lower than, the total microbial densities documented in various extreme environments on Earth.
R
might
may
A
average
A
of URLanisms
R
those
the total microbial densities