Panspermia Hypothesis: History of a Hypothesis and a Review of the Past, Present, and Future Planned Missions to Test This Hypothesis

Abstract

Speculations about the origins of life on Earth have existed since the dawn of civilization. The Greek philosopher Anaxagoras (500–428 BCE) asserted that the seeds of life are present everywhere in the universe (Nicholson, Trends Microbiol 17:243−250, 2009). He coined the term panspermia to describe the concept as life traveling between planets as seed. The other Greek philosophers, Anaximander (588–524 BCE) and Thales (624–548 BCE), mentioned philosophical point of panspermia theory. Many famous nineteenth-century scientists also wrote about this theory. Among others, Svante Arrhenius posited that microscopic spores are transferred through interplanetary space by means of radiation pressure from the sun, in 1903. In the modern formulation, there are three stages envisioned in this hypothesis: escape (from a planet), transit (through interplanetary space), and landing (on a recipient planet). Each stage has since been investigated, lending some credence to the hypothesis. For example, the possibility of microbial spores escaping a planet has been supported by the capture of radioresistant microbes from high altitudes on Earth. From the space experiments conducted in Earth orbiters and on the International Space Station (ISS), microbes have been found to survive at low Earth orbits (LEO) under some protection from intense solar UV radiation, which could well be available for spores embedded within meteorites. Heating up in the atmosphere due to friction is the main problem during reentry to the planet with atmosphere. However, because the time spent under intense friction is generally in the order of only a few tens of seconds, the amount of heat generated may not be sufficient to kill all the spores, especially if hitching a ride within meteorites. The panspermia hypothesis has been modified and revived since its original proposal and has given a new perspective to the explorations on Mars or the icy moons of Jupiter and Saturn. The hypothesis, its modifications, and past and ongoing research are reviewed in this chapter.