Path 9: Life in Non-liquid Media (Gas-Phase and Solid-Phase Life)

Rationale: Conventional wisdom says life needs a liquid to mediate chemistry. But is it absolutely required? This path explores extreme alternatives: could life exist as organized reactions in a gas (e.g., floating life in gas giant atmospheres) or within solid matrices (e.g., life in ice or rock without free liquid)? The NRC (2007) report briefly considered “life in the solid phase” and “life in the gas phase” as exotic scenarios. The rationale is to push the boundary of environments – Jupiter’s clouds, for example, have layers of complex chemistry and internal energy sources; some have envisioned aerial biospheres of microbial “dust” or even larger creatures (Sagan’s hypothetical Jovian “floaters” and “sinkers”). In solids, radiation or geological energy could drive ultra-slow life processes in ice or mineral matrices (imagine organisms frozen in permafrost that metabolize only a few times a millennium via cosmic ray stimulation). Additionally, plasma-based life or life on the surface of neutron stars (“Iron life” or those cosmic string organisms hypothesized) have been speculated in theoretical papers. While these seem far-out, considering them prevents our concept of life from becoming too narrow and informs what weird physics-chemistry coupling might still allow complexity and evolution. If life could exist in long-lived atmospheric vortices or charged dust, it would greatly broaden habitability criteria (imagine life in the atmospheres of cold brown dwarfs or free-floating in space).

Prerequisites: For gas-phase life, a key prerequisite is understanding aerosol chemistry and stability. Gas life likely requires suspended droplets or particles to concentrate reagents (pure gas is too diffuse for complex reactions). So, study of something like organic aerosols (e.g., in Venus’ atmosphere or Jupiter’s cloud layers) is needed. We must identify if any plausible autocatalytic cycles can occur in micro-droplets or at gas-liquid interfaces. Another prerequisite is a source of free energy in those media – e.g., vertical mixing creating disequilibria that organisms could tap (like reactions between sunlight and atmospheric chemicals on Jupiter, or lightning providing energy). We’d need atmospheric models to show a stable niche where “cells” can form and persist (e.g., an atmospheric pressure/temperature sweet spot where complex molecules neither evaporate nor get ripped apart). For solid-phase life, prerequisites include evidence that diffusion in solids can allow necessary molecular transport (NRC noted diffusion is slow but over geological time might suffice). Also needed: demonstration of chemical reactions at solid interfaces that can be cyclic and self-propagating. Experiments simulating interstellar ices have produced complex organics under UV – a prerequisite would be extending that to something like replication or metabolism in ice. Tools like microfluidic reactors that mimic porous solid networks could test if a metabolic cycle can occur with reactants diffusing through a solid matrix. For plasma or exotic life, prerequisites are largely theoretical physics input – e.g., Tsytovich et al. (2007) proposed dust-plasma life by showing complex plasma can form stable helical structures that grow and reproduce in simulations. Reproducing such phenomena in lab plasma chambers would be a prerequisite to taking it seriously. In general, exploring these requires interdisciplinary knowledge: atmospheric science, surface chemistry, physics of diffusion, etc.

Dependencies: This path is somewhat standalone conceptually, but it draws on outcomes of others. For instance, Path 2 (life in non-polar solvents) has overlap with gas-giant life: Jupiter’s atmosphere might have ammonium hydrosulfide clouds (a polar medium) or hydrocarbon aerosols (non-polar), so results from Path 1 and 2 about non-water chemistry apply. Solid-phase life scenarios depend on Path 1’s cryosolvent insights too – life in ice might actually be life in thin liquid layers on ice grain boundaries (so back to requiring some liquid film). So if Path 1 finds, for example, that supercritical fluids or eutectic brines can act as solvents at ultra-low temperature, that feeds into solid-phase life viability. There’s also synergy with Path 8 (shadow biosphere): Earth’s high atmosphere (stratosphere ~50 km) or deep crust could harbor borderline cases (e.g., spores or radiation-fed microbes) that are stepping stones towards fully gas/solid life. If unusual microorganisms are found in Earth’s upper atmosphere, that would inform gas-phase prospects on other planets. Dependencies on planetary exploration are strong: data from Venus (hints of possible aerial microbial signatures in cloud chemistry) or from Jupiter/Saturn (like the organic-rich “red spots” or other anomalies in composition) can trigger targeted research. For solid-phase, dependence on astrobiology missions like those to comets or icy moons appears – if some self-organizing chemistry is found in subsurface ice or on comets, that suggests something life-like could exist in solid matrices.

Signs of Progress: Currently, evidence for gas-phase life is speculative. One sign of progress was the detection of unexplained chemical imbalances in atmospheres: e.g., Venus has a mysterious depletion of sulfur dioxide at cloud mid-levels and possibly the presence of small amounts of phosphine (PH₃) recently reported – one hypothesis is aerial microbial life consuming SO₂ and producing PH₃. If further studies support that PH₃ is real and no abiotic mechanism suffices, that could be a signature of gas-phase metabolism (phosphine on Venus being analogous to methane on Earth as a biogas). On Jupiter, Sagan’s imaginative ideas aside, a concrete sign might be finding complex organic hazes that regenerate (suggesting a cycle) or local hotspots of certain gases (like a region with excess NH₃ consumption and organic output that moves like a colony). For solid-phase life, extremophile discoveries mark progress: e.g., finding microbes surviving inside rocks at very low metabolic rates (some bacteria in deep rocks have doubling times of centuries; pushing that further to quasi-“paused” life shows the edge of solid-life). The NRC committee speculated even comets’ ices could harbor extremely slow life forms using cosmic rays as energy. A sign of progress would be experiments demonstrating self-replication in an ice matrix (even a very slow, rudimentary replication of a chemical pattern in ice under radiation would be a proof of principle). Tsytovich’s plasma life idea had a sign: simulations showing dust particles in plasma can self-organize into DNA-like strings and even divide. If an experiment confirmed these plasma structures in the lab and they exhibited any Darwinian selection (say different plasma “species” competing), that would be sensational progress. Another sign: if meteorite or sample-return analysis from, say, subsurface Mars or Europa’s ice shows complex organic structures in patterns (maybe microscopic filamentous structures in ice that look organized but without any connection to Earth microbes), that could hint at solid matrix life. Each of these is a long shot, but even partial data – like persistence of chemistry far from equilibrium in a solid with periodic structure – counts as progress. Essentially, any credible hint that non-liquid environments can support stable, increasing complexity akin to life will move this path forward.

Base Camp 9A: Simulating Jupiter Cloud Chemistry

Scope: Create laboratory analogs of gas giant atmosphere regions to test if complex organic or prebiotic chemistry can occur and self-organize in aerosols. Sagan and Salpeter’s classic idea of Jovian balloon organisms is the inspiration. This camp uses planetary simulation chambers with mixtures of H₂, He, CH₄, NH₃ at appropriate temperature (~ -100°C to +20°C depending on pressure ~1 bar) with energy input (UV or electric discharge) to see if any stable complex structures form.

Stepping Stones: (i) Conduct a “honeycomb” experiment akin to Tsytovich’s plasma dust study but with neutral aerosols: e.g., produce fine particles of PAHs (polycyclic aromatic hydrocarbons) or other organics in a chamber with Jupiter gas mix and observe via laser scattering if they aggregate or form membranes around droplets. (ii) Investigate if any catalytic reaction cycles arise: e.g., monitor gas composition for any steady-state deviations when organics present (like continuous production of excess some gas as if catalyzed). (iii) Use microfluidic levitation traps or acoustic traps to hold single droplets in mid-air and analyze if they concentrate organics or show any “proto-cell” behavior (like growth by coalescence, splitting by Rayleigh instability). (iv) In a theoretical model, incorporate known Jovian data: e.g., oxygen scarcity and abundant NH₃ might allow schlubberlike life. Write models for nutrient cycling in a hypothetical aerial ecosystem, to identify key signatures.

• Sagan & Salpeter (1976) – not provided but widely cited (they called Jovian imaginary life forms “sinkers, floaters, hunters”). Schulze-Makuch & Irwin mention alternative evolutionary possibilities like that in Ch.14.1. They likely describe a bit of what those creatures might need chemically (like phototrophy in upper clouds, sinking to get nutrients).
• Tsytovich et al. (2007) (Dust plasma life) – we know from wiki they proposed dust grains in plasma can self-organize and perhaps evolve. For gas-phase in Jupiter, not plasma, but conceptually if complex structures can spontaneously appear in non-solid, non-liquid environment, that was a proof-of-concept in plasma. It’s at least evidence that physical systems outside typical life can show emergent complexity. Use that as optimistic rationale.
• Meadows et al. (2020) – touches on hypothetical life in Venus clouds (though that’s a mixed H₂SO₄ liquid scenario). If referencing gas giants, Lunine’s chapter in that volume likely discusses Titan and giant planets.

Base Camp 9B: Radiation-Driven Solid-State Chemistry

Scope: Investigate the possibility of life or life-like chemical systems in solid environments such as ice or minerals, energized by radiation (cosmic, UV, radioactive decay). Allamandola & Hudgins (1999) and others found photochemistry in interstellar ices can form complex organics. This base-camp asks: can such processes be organized into something self-sustaining? Possibly ultra-slow “metabolism” in deep freeze as NRC speculated.

Stepping Stones: (i) Perform radiation experiments on ice mixtures containing simple molecules (H₂O, CH₃OH, NH₃, etc.) at ~10–50 K. Look for formation of repeating structures or trapped reaction products that could accumulate. Next, see if any feedback or autocatalysis occurs. (ii) Examine mobility in solids: probably extremely low, but at grain boundaries or if periodic melting events occur, molecules can migrate. Simulate partial annealing of ice and see if organics concentrate in certain regions. (iii) In a lab long-term, leave a radioactive source embedded in a block of organic-rich ice at constant temperature for months, then analyze if complex polymers are forming. (iv) If any organized filamentary or cellular microstructures appear in irradiated solid, examine them with spectroscopy to see if they have distinct chemistry from random mixture.

• NRC (2007), Ch.6.3.2 Life in the Solid Phase – they mention diffusion is very slow but cosmic rays could occasionally drive processes and “a weird life form might reside within solids in the Oort cloud living in deeply frozen water, doing few metabolic transformations per millennium”.
• Allamandola & Hudgins (2003 summary by NRC) – summarizing that UV in ices yields prebiotic molecules, which maybe used on early Earth when delivered by comets.
• Wikipedia (Hypothetical Biochem, Nonplanetary life section: dusty plasma & cosmic string life) – shows that scientists are at least discussing life in unusual physical states.

Bibliography (Path 9)

• Sagan, C. & Salpeter, E. (1976). “Particles, environments, and possible ecologies in the Jovian atmosphere.” Astrophys. J. Suppl. 32: 737–755. – Imaginative but physically reasoned speculation of floating ecosystems on Jupiter. Describes “sinkers, floaters, hunters” concept; while dated, it’s the foundation for gas-phase life discussions.
• Tsytovich, V. et al. (2007). “From plasma crystals and helical structures towards inorganic living matter.” New J. Phys. 9: 263. – Simulation study indicating dust in plasma can self-organize, replicate, and evolve (within simulation).
• Allamandola, L. & Hudgins, D. (2003). “From interstellar polycyclic aromatic hydrocarbons and ice to astrobiology.” Astrobiology 3(1): 71–86. – Summarizes experiments showing UV irradiation of PAHs in ice yields complex organics (like amino acids). Suggests potential for prebiotic chemistry in ices.
• Wharton, R. et al. (1985). “Anhydrous ammonia–water eutectic solutions as possible solvents for biogenic activity on Titan.” J. Geophys. Res. 90: C689–C694. – This considers ammonia-water eutectics as liquid below 0°C for life on Titan or elsewhere. Bridges Path 1 & 9.