What are the physical, chemical, and physiological limits of alien life?

Astrobiologist Dirk Schulze-Makuch just published an intriguing paper(Opens in a new window) in the journal Life where he explores the possibilities and limits of alien life forms. In particular, he considers two potential new genera evolving within two unique planetary niches: a Mars-style planet hosting creatures that subsist using a hydrogen peroxide-based solvent, and a Titan-type planetary body with esoteric liquid hydrocarbon solvents.

If we presume that life need be energetic enough to persist and propagate itself, but not so energetic that it destroys itself in the process, then a generalized power cycle consisting of the movements of ions across selectively permeable membranes, down their electrochemical gradient, has proven to be fairly universal. This so-called 'chemiosmosis' is the root powerplant of virtually all life here on Earth, from bacteria to the mitochondria and photosynthesizing chloroplasts of multicellular organisms. The 'limits' of life then, are those extremes under which non-dormant creatures can continue to metabolize and produce energy.

In order to make this general concept clearer, consider the jet turbine designer tasked with extracting more power from the same basic thermodynamic cycle. By employing ever more refined engineering tricks (precision air and magnetic bearings or laser initiated combustion), and exploiting a base of ever more robust materials (temperature-refractory ceramics with increased ductility and machinability), the same Brayton cycle can be made more efficient. For example, by running it in a higher temperature regime. The question we therefore have before us, is if there is a fundamental power cycle -- a platonic from of life in the universe that is independent of environment -- how will we recognize it in vastly different locales?

In taking account of the potential ways a planet might animate itself, it has been noted that any sufficiently advanced chemiosmotic geochemistry should be indistinguishable from life. Similar to the evolution of jet engines, as life evolved (at least here on Earth), it developed new tricks and synthesized new materials to compact the same energizing function into higher efficiency. In considering life on a moon like Titan, Dirk Schulze-Makuch notes that with a surface being partly covered by nonpolar liquids like methane and ethane, instead of a polar water solvent, the membrane biochemistry of life would likely be inverted: cells would necessarily expose nonpolar headgroups both to the solvent they floated in, and to their own internal solvent.

We previously discussed the possibility of creatures with reverse-topology membranes built from acrylonitrile that might swim in the ultracold methane seas of Titan. By contrast, Dirk has envisioned membranes built using silanes that could have been synthesized by so-called serpentization reactions(Opens in a new window), reactions similar to those that synthesized the first protostructures to harness the energetic gradients used by life evolving in our own deep sea hydrothermal vents.

But that's just the beginning of Titan-esque living. Others goodies including various cyanide-nitrogen radical metabolisms potentially abound. Acetylene, the stuff we use to turn scrapped aircraft carries into skyscrapers, is produced by solar UV radiation in Titan's stratosphere. Thereafter it would condense and fall, transporting energetic products and raw materials to the surface in the process.

As for a Mars-like geosphere, forms even more fantastic are imagined. Massive bombardier beetle organisms powered by a potent mix of hydrogen peroxide would be capable of jetting 300 meters into the atmosphere on lower gravity planets. Impossible? Consider that our own beetles figured out that if they squirt hydroquinone at a 25% mass concentration and hydrogen peroxide at a 10% mass concentration into a chitinous combustion chamber, where catalase and peroxidase enzyme catalysts are waiting, things would rapidly go critical and boil forcefully out their posterior. Now imagine what organisms could come up with if their very cytoplasm was made from hydrogen peroxide from the beginning. Data from the recent Viking Lander Biology Experiments have led several researchers to suggest such a peroxiplasm would be par for the course in conditions like those on Mars.