Enceladus, a small moon in orbit around Saturn, has been full of surprises since it was first identified in 1789. The discovery of Enceladus is credited in part to William Herschel, who was fortuitously observing Saturn at equinox. During equinox, the Sun and Earth are aligned edge-on with the rings of Saturn, reducing the observed reflection of sunlight off the rings (ringshine), which is usually bright enough to mask Enceladus from most telescopes. Enceladus is named after one of the Titans, giants in Greek mythology, and contrary to its small size (504 km or 313 miles in diameter), it has…
A vast diversity of environments exists within and beyond Earth. To the extent life depends on and interacts materially with its environment, it can be expected that life-hosting potential among these environments varies as much as the physical and chemical properties that define them. To understand this potential demands that we first understand lifes requirements in some detail. Discussion of these requirements is frequently undertaken in considering habitability. Cockell et al. (2016) argue that the term habitability is inherently binary: An environment either can or cannot sustain life. This is an important starting point in evaluating the life-hosting potential of…
Aggregates of archaea and sulfate-reducing bacteria (SRB) recently discovered in methane-seep sediments are widely assumed to engage in anaerobic methane oxidation (AMO), but the reaction mechanism remains poorly understood. We used a spherical diffusion-reaction model that incorporates thermodynamic controls, realistic aggregate morphology, and essential elements of cell structure to quantify maximum reaction rates and energy yields for competing mechanisms, to determine how cellular energy yields are affected by aggregate size and morphology, and to investigate the impact of organic-matter remineralization on archaea and SRB in the aggregate.
Habitability can be formulated as a balance between the biological demand for energy and the corresponding potential for meeting that demand by transduction of energy from the environment into biological process. The biological demand for energy is manifest in two requirements, analogous to the voltage and power requirements of an electrical device, which must both be met if life is to be supported. These requirements exhibit discrete (non-zero) minima whose magnitude is set by the biochemistry in question, and they are increased in quantifiable fashion by (i) deviations from biochemically optimal physical and chemical conditions and (ii) energy-expending solutions to problems of resource limitation. The possible rate of energy transduction is constrained by (i) the availability of usable free energy sources in the environment, (ii) limitations on transport of those sources into the cell, (iii) upper limits on the rate at which energy can be stored, transported, and subsequently liberated by biochemical mechanisms (e.g., enzyme saturation effects), and (iv) upper limits imposed by an inability to use “power” and “voltage” at levels that cause material breakdown. A system is habitable when the realized rate of energy transduction equals or exceeds the biological demand for energy. For systems in which water availability is considered a key aspect of habitability (e.g., Mars), the energy balance construct imposes additional, quantitative constraints that may help to prioritize targets in search-for-life missions. Because the biological need for energy is universal, the energy balance construct also helps to constrain habitability in systems (e.g., those envisioned to use solvents other than water) for which little constraint currently exists.