Physical Constraints on Motility with Applications to Possible Life on Mars and Enceladus

TL;DR

This paper develops a theoretical model to estimate the potential density of motile microbes in extraterrestrial environments like Mars and Enceladus, based on energy constraints and motility costs.

Contribution

It introduces heuristic upper bounds for microbial biomass and motile organism densities in extraterrestrial subsurface environments considering energy limitations.

Findings

Motile organism densities could be comparable to Earth's extreme environments.

Energy constraints significantly limit microbial densities on Mars and Enceladus.

Motility may serve as a detectable biosignature in extraterrestrial life searches.

Abstract

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.