Eriita Jones

An Extensive Phase Space for the Potential Martian Biosphere

We present a comprehensive model of martian pressure-temperature (P-T) phase space and compare it with that of Earth. Martian P-T conditions compatible with liquid water extend to a depth of ∼310 km. We use our phase space model of Mars and of terrestrial life to estimate the depths and extent of the water on Mars that is habitable for terrestrial life. We find an extensive overlap between inhabited terrestrial phase space and martian phase space. The lower martian surface temperatures and shallower martian geotherm suggest that, if there is a hot deep biosphere on Mars, it could extend 7 times deeper than the ∼5 km depth of the hot deep terrestrial biosphere in the crust inhabited by hyperthermophilic chemolithotrophs. This corresponds to ∼3.2% of the volume of present-day Mars being potentially habitable for terrestrial-like life.

To What Extent Does Terrestrial Life “Follow The Water”?

Terrestrial life is known to require liquid water, but not all terrestrial water is inhabited. Thus, liquid water is a necessary, but not sufficient, condition for life. To quantify the terrestrial limits on the habitability of water and help identify the factors that make some terrestrial water uninhabited, we present empirical pressure-temperature (P-T) phase diagrams of water, Earth, and terrestrial life. Eighty-eight percent of the volume of Earth where liquid water exists is not known to host life. This potentially uninhabited terrestrial liquid water includes (i) hot and deep regions of Earth where some combination of high temperature (T > 122 degrees C) and restrictions on pore space, nutrients, and energy is the limiting factor and (ii) cold and near-surface regions of Earth, such as brine inclusions and thin films in ice and permafrost (depths less than approximately 1 km), where low temperatures (T < -40 degrees C), low water activity (a(w) < 0.6), or both are the limiting factors. If the known limits of terrestrial life do not change significantly, these limits represent important constraints on our biosphere and, potentially, on others, since approximately 4 billion years of evolution have not allowed life to adapt to a large fraction of the volume of Earth where liquid water exists.

Using the phase diagram of liquid water to search for life

The correlation between liquid water and life may be our most reliable tool in the search for extraterrestrial life. To help develop this tool, we explore the complex relationship between liquid water, partial pressure, and solute freezing point depression on Earth and Mars and discuss the conditions under which liquid water is metastable on Mars. We establish the physical conditions for the existence of saline aqueous solutions in the pores of the martian near surface substratum. We find that thin films of near subsurface liquid water on Mars at ∼–20°C could provide a viable niche for terrestrial psychrophilic halophiles. Since some martian salts can suppress the freezing point of aqueous solutions with minimal suppression of the water activity, some martian liquid water environments with a water activity above ∼0.6 may also be able to support terrestrial life at temperatures as low as −30°C, ∼10°C lower than the limit of terrestrial life.