Life may unfold in various forms throughout the universe, constrained by universal physical laws and consistent principles of organic evolution, but propelled to great variety in detail by local conditions and the specifics of planetary history. What is known of the chemical and physical conditions of any planetary environment and its history enables us to make educated and plausible speculations about the nature and history of life on that world.
Within our Solar System, there is an enormous diversity of planetary environments. On Earth, life evolved on a geologically complex, water-rich world, which today has an oxidizing atmosphere, although this was not always the case. On Mars, the surface is bitterly cold and dry, and the atmosphere very thin. Whether or not life ever existed on the Red Planet is a matter for speculation, but we do know that early in its history, Mars was a warmer, wetter world. Today Venus is a planet with an incredibly hot surface and a dense choking atmosphere, and it seems unlikely, although not impossible, that life could ever evolve here. On the gas giant planets, such as Jupiter and Saturn, it is possible that life might exist in the dense atmospheres of these cloud covered worlds, and might even have evolved on some of their exotic moons such as the sulphur-rich, volcanic world Io, Icy Europa with its possible sub-surface ocean, or Titan with its lakes of liquid petroleum gas on the surface.
Discussions of the great variety of life forms that could evolve in these diverse environments have become particularly relevant in recent years with the discovery of around 300 exoplanets in orbit around other stars and the possibilities for the existence of life in these planetary systems.
About the Author: As a neurobiologist, Louis Neal Irwin has been a student of evolution, complexity, and behavior over a 40 year career of academic teaching and research. Irwin has published close to 60 original research articles, literature and book reviews, encyclopedia entries, and commentaries on the brain, behavior, and evolution, including one book ("Scotophobin") on the early development of neuroscience.
Ten years ago, Irwin became a Solar System Educator for NASA, originally in conjunction with the launch of the Cassini-Huygens Mission to Saturn but later as representative for all the robotic exploratory missions managed by the Jet Propulsion Lab. In that capacity he became familiar with the details of space exploration for the purpose of conducting teacher workshops. Soon thereafter, he also began a collaboration with Dirk Schulze-Makuch on research into the definition of life and the plausibility of searching for and finding life on other worlds. As NASA turned its attention to the emerging field of astrobiology, Schulze-Makuch and Irwin began to publish their research in that area, culminating in the joint authorship of "Life in the Universe: Expectations and Constraints," which many regard as the definitive work in the field of astrobiology for the technical specialist.
Dirk Schulze-Makuch
As a trained hydrogeologist Dirk Schulze-Makuch entered the field of astrobiology by studing extremophilic organisms in hot springs. Propelled by a major NASA grant Dirk then joined the Europa Focus Group and some time later the Titan Focus Group of the NASA Astrobiology Institute. Recent interests include nearly all aspects of astrobiology including mission-aligned efforts to detect life on Mars and the search for extraterrestrial intelligence.
