A global network model of abiotic phosphorus cycling on Earth through time.

Phosphorus (P) is a crucial structural component of living systems and central to modern bioenergetics. P cycles through terrestrial geochemical reservoirs via complex physical and chemical processes. Terrestrial life has altered these fluxes between reservoirs as it evolved, which is why it is of interest to explore planetary P flux evolution in the absence of biology. This is especially true, since environmental P availability affects life's ability to alter other geochemical cycles, which could then be an example of niche construction. Understanding how P reservoir transport affects environmental P availability helps parameterize how the evolution of P reservoirs influenced the emergence of life on Earth, and potentially other planetary bodies. Geochemical P fluxes likely change as planets evolve, and element cycling models that take those changes into account can provide insights on how P fluxes evolve abiotically. There is considerable uncertainty in many aspects of modern and historical global P cycling, including Earth's initial P endowment and distribution after core formation and how terrestrial P interactions between reservoirs and fluxes and their rates have evolved over time. We present here a dynamical box model for Earth's abiological P reservoir and flux evolution. This model suggests that in the absence of biology, long term planetary geochemical cycling on planets similar to Earth with respect to geodynamism tends to bring P to surface reservoirs, and biology, including human civilization, tends to move P to subductable marine reservoirs.

Habitability Models for Astrobiology.

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.