Say you’re an astrobiologist tasked with designing a world that would be able to support life we’d be able to recognize as such. What absolute essentials would you need to include?
Abundant liquid water? Biologists have found algae thriving inside desert rocks, moistened only by dew seeping in through microscopic pores. A comfortable temperature? We’ve found bacteria living in environments as cold as 5ºF and as warm as 250ºF. A solid surface to grow on? Arthur C Clarke (A Meeting with Medusa) wrote about complex life-forms floating in the 3,000-mile-deep atmosphere of Jupiter. OK, that’s science fiction, but Clarke’s the guy who invented geostationary satellites for telecommunications and GPS.
Many scientists would say that the obvious essential is a source of chemical energy. I’d add, “and an efficient mechanism to convert the source energy to a form that can be transported within an organism.” To my knowledge, all life-forms now on Earth have met the second prerequisite by using the ATP molecule for intra-cellular energy transport. But life has been amazingly creative in finding ways to build those ATPs. The tall diagram lists some biologic energy sources in decreasing order of how much energy is released.
All the Biology textbooks tell us that Earth’s energy cycle starts with the Sun. Solar photons energize plant photosynthesis which creates loads of ATP molecules. Some of them power a multistep process which combines CO2 and H2O to release O2 and create carbohydrates (CH2O)x. (Glucose, for instance, is (CH2O)6. Guess where the term “carbohydrate” came from.) Earth’s biologic carbon cycle completes when other life “burns” carbohydrates to exploit the energy stored therein. On this chart, “burn” means “combine with O2” and usually doesn’t involve fire.
Notice that “Make (CH2O)x” is at the bottom of the chart — that process absorbs a lot of energy per carbon atom. Conversely, “Burn (CH2O)x” releases energy which is why we like sugar too much.
In the past couple of decades we’ve learned that’s not the only way, or maybe even the dominant way, that Earth-life makes its ATPs. Microbes have evolved a surprising number of “front ends” to the energy machinery. Here in Colorado we’ve got problems in old mines where microbes build ATPs by oxidizing iron pyrite (FeS) to sludgy rust (Fe2O3) and sulfuric acid (H2SO4). Works great for them, not so good for downstream organisms.
Iron compounds are such a good energy source that many scientists believe (it’s still controversial) that Earth’s hematite and magnetite deposits were laid down half-a-billion years ago by archaea, microorganisms that preceded bacteria.
Way down on the energy-source scale are the methanogens, archaea that use molecular hydrogen to convert CO2 to methane (CH4). They only live in zero-oxygen environments — peat bogs, ocean-bottom hydrothermal vents and subsurface veins that are perilous to mine.
Earthly biology participates in many cyclic processes. The bi-level diagram below highlights two — oxygen cycling between O2 and oxygen compounds, and carbon cycling between CO2 and living tissues (which contain carbohydrates).
If it weren’t for light-driven photosynthesis ( ~~ is a photon), pretty soon all our O2 would be locked up in the ground where it came from. In a sense, Earth uses life and carbon to get oxygen back up into the atmosphere. Astronomers look for O2 in a planetary atmosphere as a sign of life.
Maybe Titan does something similar. Titan’s atmosphere contains methane (CH4) and H2 but the quantities aren’t right. The purple “Lyman α” and blue “Balmer α” lines on the energy chart denote particularly strong solar photons that can break up C-H bonds and generate H2 in Titan’s upper atmosphere. We understand the relevant processes pretty well and can calculate how much methane, acetylene (C2H2) and H2 should be up there.
The calculated quantities pretty much match what astronomers found in Titan’s upper atmosphere. But they’re not what Cassini-Huygens found on the ground. Acetylene just isn’t there, and a (somewhat precarious) computer simulation indicates that there’s much less ground-side H2 than you’d expect from simple diffusion. Dr Chris McKay has put those clues and the energy stack together to suggest that something on Titan inhales acetylene and hydrogen and exhales methane.
Something alive, maybe?
~~ Rich Olcott