Based on recent readings, especially from Christopher McKay in How to Search for Life on Other Worlds, I’ve come to believe that life only needs the right combination of three main requirements: a main solvent to host chemical reactions (ours on Earth being water), a mixture of common building-block elements (Carbon, Hydrogen, Oxygen, Phosphorus, Nitrogen, Sulfur are examples of this), and enough energy to cause chemical reactions. In Astrobiology lecture, we have discussed life being found on many places within our solar system such as Mars and even the moon, but I think any life we do find will be on Titan, Enceladus, or Europa.
First looking at the requirement for solvent: all three moons (Titan, Enceladus, and Europa) are the only other bodies in the solar system that we know to have water. Mars has water in the form of ice on each of its poles, but chemical reactions need a liquid solvent. Enceladus and Europa both have a thick icy crust, but with liquid underneath, whereas Titan is covered almost entirely of liquid water. This makes Titan the only other body in the solar system that has liquid water on its surface.
Energy is the next huge requirement because it’s what drives all chemical reactions, although Everett Shock and Melanie Holland point out that the right amount of power is needed to meet the power demand of the type of life. A flower gains energy through photosynthesis, but if you were to give the flower too much sun or too little sun, it would start to die. My point is that there must be enough energy from the sun and geothermal energy to have liquid water on these moons. They aren’t even close to what is considered to be in the habitable zone, but perhaps the habitable zone is only proposed because of the amount of solar energy provided to the planets within it. In the same journal, Shock and Holland also point out that extremophiles such as penguins living far below freezing temperatures, or bacteria near geothermal vents living at the temperature of boiling water. If life is able to sustain itself in such cold temperatures, I don’t see any reason why life couldn’t live on these moons as long as they have a rich oxygen full of essential elements.
From what we’ve seen on earth, oxygen has been a biosignature to all biotic life and the production of oxygen skyrocketed during the great oxidization event. Other biosignatures have also been considered such as methane and nitrogen. The only moon with an atmosphere heavily compromised of oxygen would be Europa. Europa’s main core is made up of mostly iron, nickel, and silicate rock, and as we have also discovered in class, silicone has been proposed as a possible backbone for extraterrestrial life because of its similarity to carbon (same family = same valence electrons), which makes it easy for other atoms to bond to. The two other moons do not have an oxygen-rich atmosphere, but Titan does have one containing heavy amounts of nitrogen and methane.
There are little odds that we will find life within our solar system, but there is a good chance that it will not be on Mars or our moon. The three moons discussed are near absolute zero on the surface, but they likely reach above freezing closer to the core otherwise there wouldn’t be liquid water on either of them. In the end though, there probably won’t be life on any of these moons!
McKay, Christopher P. “How to Search for Life on Other Worlds.” NASA Ames Research Center
Shock, Everett L, and Melanie E Holland. “Quantitative Habitability.” ASTROBIOLOGY, vol. 7, no. 7, 2007, doi:DOI: 10.1089/ast.2007.0137.