Life on Earth is found in a wide range of environments as long as the basic requirements of a liquid solvent, a nutrient source, and free energy are met. Previous hypotheses have speculated how extraterrestrial microbial life may function, among them that particle radiation might power living cells indirectly through radiolytic products. On Earth, so-called electrophilic organisms can harness electron flow from an extracellular cathode to build biomolecules. Here, we describe two hypothetical mechanisms, termed "direct electrophy" and "indirect electrophy" or "fluorosynthesis," by which organisms could harness extracellular free electrons to synthesize organic matter, thus expanding the ensemble of potential habitats in which extraterrestrial organisms might be found in the Solar System and beyond. The first mechanism involves the direct flow of secondary electrons from particle radiation to a microbial cell to power the organism. The second involves the indirect utilization of impinging secondary electrons and a fluorescing molecule, either biotic or abiotic in origin, to drive photosynthesis. Both mechanisms involve the attenuation of an incoming particle's energy to create low-energy secondary electrons. The validity of the hypotheses is assessed through simple calculations showing the biomass density attainable from the energy supplied. Also discussed are potential survival strategies that could be used by organisms living in possible habitats with a plentiful supply of secondary electrons, such as near the surface of an icy moon. While we acknowledge that the only definitive test for the hypothesis is to collect specimens, we also describe experiments or terrestrial observations that could support or nullify the hypotheses. Key Words: Radiation-Electrophiles-Subsurface life. Astrobiology 18, 73-85.

Pluto is a large icy body composed of N 2 , CH 4 , and H 2 O ices. In many ways, Pluto can be seen as one large matrix isolation experiment where N 2 is the inert matrix that can act to trap and isolate reactive species. The temperature changes on the dwarf planet induce sublimation of N 2 from the surface. Any previously trapped reactive species could then react with the new ice or neighboring molecules. To see if this process might lead to a significant formation of molecules, Fourier-Transform Infrared (FTIR) Spectroscopy (4 cm −1 resolution) was used to study and monitor the sublimation of ices created from irradiated gas mixtures of 1:1:100 CO+H 2 O+N 2 or 1:1:100 CH 4 +H 2 O+N 2. The gas mixtures were initially prepared and deposited on a cold finger at a temperature of 6 K and a baseline vacuum of about 1 x 10 −7 Torr. Gas mixtures were irradiated using an electric discharge or a microwave discharge before deposition to create the unstable chemical species. To sublimate the matrix, the temperature was brought up step-wise in 5-10 K intervals to 45 K. Slow sublimation (10 min per step) resulted in the new species being trapped in a water ice. In addition to (FTIR) spectroscopy, chemical species were also identified or monitored using ultraviolet-visible (UV-Vis) spectroscopy and a residual gas analyzer (RGA). Carbon suboxide (C 3 O 2), a common component found in meteorites and a potentially important prebiotic molecule, was formed only after the sublimation step. Other products formed included deprotonated versions of products formed in the original matrix ice. C 3 O 2 's potential importance in Pluto's surface chemistry and its overall astrobiological significance will be discussed.

A number of planetary bodies, including Triton and Pluto, and a number of Kuiper Belt objects contain nitrogen ices on their surfaces. Nitrogen ices were also used in laboratory experiments as a matrix isolation material before noble gases could be condensed. Planetary bodies with nitrogen ices then may act as giant matrix isolation experiments, trapping reactive species onto the surface and concentrating them. Upon sublimation, these reactive species are much more likely to encounter each other or another molecule to react with. A pilot study was conducted to test the feasibility of testing the chemistry occurring during the sublimation of nitrogen ices. A high vacuum laboratory setup was used to create ices at ∼6 K (±0.5 K). Ices were deposited under microwave radiation to create radicals to simulate what might be present in the tenuous atmospheres of these planetary bodies. The ices consisted of a mixture of 1:1:100 carbon source + H2O + N2, where the carbon source was either CO or CH4. Reagents and products were primarily identified using FTIR and UV–vis transmission spectroscopy. Once the predominantly N2 ice was characterized with the spectroscopic techniques, the N2 was sublimated to create a H2O ice, and then this ice was characterized using the aforementioned techniques. One completely new product was observed, namely, carbon suboxide (C3O2), and a couple products identified in the nitrogen ice formed various salts. Future work could make use of multiple sublimation steps and other astrochemically relevant matrices and introduce more astrophysically relevant sources of radiation like electron beams or UV irradiation.