The extraterrestrial hunt is on. Expectations of finding alien life are rising, for reasonable building blocks are out there, scattered all over our solar system–liquid water and/or ice, sulfur, carbon, nitrogen, hydrogen, oxygen and organic compounds.
In 2010 and 2011 two books summarized in extraordinary detail how life may have evolved and may be living still on the planets and moons of our solar system. No chemical possibility is left unexamined in We Are Not Alone: Why We Have Already Found Extraterrestrial Life by Dirk Schulze-Makuch and David Darling and Cosmic Biology: How Life Could Evolve on Other Worlds, by Louis Neal Irwin and Dirk Schulze-Makuch. Interesting comparisons are made between possible life forms elsewhere and those on Earth since 1977, the extremophiles that live in either very hot (Yellowstone and hydrothermal vents deep in the ocean) or very cold (Antarctic) environments.
The authors of these books consider anything biochemically possible. They review energy reactions available on each likely moon or planet in the solar system and relate the energy source to how any living thing would have to be designed to survive and multiply in specific environments discovered so far. Though carbon is favored for its “chemical versatility” and “talent for making large stable molecules” (quoted from Wolf Vishniac, the first microbiologist to be funded by NASA), silicon is also considered as a building block in some places.
Water is still considered the most likely solvent for energetic biological metabolism, but hydrogen peroxide and water together may provide a solvent for critters on Mars. Hydrogen peroxide can help shield life from the cold and radiation there and could conceivably be involved in a photosynthetic reaction, like carbon dioxide reacting with water to produce methane and hydrogen peroxide.
In the book We Are Not Alone: Why We Have Already Found Extraterrestrial Life, Dirk Schulze-Makuch and David Darling make the case for such a hydrogen peroxide solvent. They postulate that Viking experiments may have missed finding the life on Mars because their procedures killed microbes in the soil samples. The data, the authors claim, are consistent with the assumption that Martian lives depend on hydrogen peroxide, not water, as a solvent. Only in non-sterilized Martian soil did experiments release an initial rush of oxygen or carbon dioxide that lasted some hours then slowed way down–as the Martian microbes died, according to the authors. Their detailed explanation makes sense and fun reading.
The authors also talk about the unexplained continuous release of methane on Mars, as well as larger amounts of formaldehyde. They consider the atmosphere of Venus in terms of an unusual presence of both oxidation and reduction reactions with sulfur. The book also summarizes the candidacy of life on the moons of Saturn and Jupiter.
In their 2011 book, Cosmic Biology: Irwin and Schulze-Makuch systematically list moons and planets in our solar system that could support life, how the changing environments and energy sources could have driven the evolution of different kinds of life, and how those energy sources either encouraged or limited their mobility, their size, even their shape.
The European Space Agency will launch a deep-space mission to explore the icy moons of Jupiter in 2022 the authors say. They finish their book with a look at biocomplexity and the long-term future. Cosmic Biology was written before the exciting discovery of watery plumes jetting from Tiger Stripes on the South Pole of Enceladus, a small moon of Saturn. For a detailed update on the discovery of under-ice water lakes there, go to the article “Watery Enceladus” by John Spencer in Physics Today November 2011, page 38-44.