PX282 - M1 - requirements for life

the definitions of life are notoriously difficult and controversial, but its requirements are better understood through the study of extremophiles, mainly bacteria

eg:

• temperature $-20\to 121\mathrm{°}$C (hydrothermal vents at mid-ocean ridges)
• ionizing radiation upto $>1000\times$ lethal dose for humans
• chemical extremes, like pH $-0.06\to 11$
• pressure $>1100$ bar

the fundamental requirements are:

• energy source, eg: photosynthesis, chemosynthesis
• carbon source, as its valency allows complex organic chemistry. this is also unlikely to be a limiting factor due to its abundancy
• liquid water, which is a solvent by all all life on earth and is relatively rare in the solar system

carbon-based life may not be the sole possibility, but H, O and C are the first, the third and the sixth most common elements

liquid water needs moderate temperatures and pressure $\ge 0.006$ bar

the high temperature limit is set by organic chemistry

hydrothermal vents are favourable for life because nutrients are abundant from heated water/rocks

fossil evidence of life on earth $\sim 3.8$ Gyr ago

from the faint young sun paradox, it implies that liquid water must have been present

this could be possible due to stronger greenhouse effect from higher atmospheric CO${}_2$ abundance