Chapter 3

The Elements of life

Synopsis

The materials that form all of the diverse chemicals and structures of life, the mountains, deserts, and oceans on Earth, and the various bodies in the Solar System, all started as nothing but hydrogen and helium burning in stars. Over billions of years, heavier elements such as carbon, oxygen, silicon and iron were formed and spread throughout space through the processes of nucleosynthesis in stellar cores and supernovae. Stars form from the collapse of an initially diffuse mass of dust and gas, and surrounding material is then pulled into a protoplanetary disk from which the planets, moons, asteroids and comets accrete. During the accretion of the solar system bodies, turbulent mixing can expose the material to a variety of different thermal and radiative environments. The processing of material in the protoplanetary disk can lead to the formation of new and more complex chemical species, and radiation interactions with simple organic molecules during this phase may have led to the origins of homochirality in life. Although the initial period of atomic and molecular evolution were fundamental to the formation of the building blocks necessary for life to form and evolve, known exoplanet solar systems which formed following the same physics and from the same starting material show a surprising diversity.    

Both the dynamical and the chemical evolution of our Solar System played roles in determining the composition of the early planetary bodies, and impacts between growing bodies led to further mixing, heating, and chemical changes. After the orbits of the planets had settled into their current configuration, impacts between large, proto-planetary sized objects became rare and the composition of bodies became largely fixed. Further evolution of the chemical and physical environments of the planets - the geosphere, hydrosphere, and atmosphere - then became dependent on their local geochemistry, their atmospheric interactions with solar radiation, and the continued delivery of materials through cometary and meteorite impacts. Photochemistry in the atmosphere can lead to the formation of new molecular species, while interactions between the hydrosphere and geosphere can lead to the development of chemical disequilibria and provide sources of chemical and thermal energy. It is the interplay of all of these factors which go into the formation of a habitable planet, although their presence does not imply that a planet is habitable.