Exobiology

Introduction

In 1633, the Vatican's Collegio Romano placed Galileo's treatises Assayer and Dialogue on the Index, a list of heretical works banned from publication. This led to one of mankind's most important scientific paradigm shifts. For centuries the doctrine of geocentricism, the Earth as the center of the universe, had been accepted despite attempts by religious and academic scholars to suggest otherwise. This change to the intellectual milieu seemed to have heightened appreciation of the importance of other planets in the universe. However, it was not until 1960 that Lederberg coined the term exobiology to describe the search for life beyond Earth [1]. From the very outset it was clear that studies of Earth's own prebiotic evolution and exobiology were intimately linked. By understanding more of our own prebiotic to biotic evolution, further light could be shed upon the possibility of extraterrestrial life.

Exobiology

Much of exobiology's theories are based on studies of Earth's biota. How life started on Earth is uncertain. Evidence favors that the prebiotic atmosphere was a reducing one (methane and nitrogen, ammonia and water vapor or carbon dioxide, nitrogen and hydrogen), but even this is by no means certain. In their comprehensive review of this subject, Lazcano and Miller discussed the vast number of possible scenarios that could lead to life, and the paucity of hard evidence to really support or refute each one [2]. All current theories of the origins of life--the Standard Model (Oparin-Haldane), in which production of organic molecules on early Earth over 3.8 giga years ago, followed by a fast increasing complexity phase, leads to life; the Panspermia Model, in which life is carried to Earth from another planet; or the Inorganic Model (Clay-Lattice), in which the initial energy source is chemical energy or sunlight--require liquid water [3]. Irrespective of theory, it appears that the accumulation of biological precursors in the prebiotic soup may have been the rate-limiting step to the emergence of life. The Warawoona microfossils provide good evidence for cyanobacteria-like communities on Earth 3.5 billion years ago, and recently, paleontologists have focused on silicified coastal sediments where these complex microbial communities, fueled by photoautotrophy, may have existed. Hydrothermal systems also could have been favorable areas for local emergence of life, as redox reactions between water and certain minerals would establish a potential for organic synthesis in and around such vents [4, 5]. As yet, however, no studies of this hypothesis have been carried out.

The uncertainty over the modus operandi of biotic development on Earth has been a mixed blessing to exobiological studies: while it has allowed the generation of numerous theories, it has been very difficult to establish what significance should be attached to each one. However, exobiology initially began with far greater aspirations than the identification of extraterrestrial microbes--it originated in the search for extraterrestrial intelligence (SETI). An intellectual fascination with extraterrestrial life flourished faster than the technology was available to search for direct evidence, thereby fueling Earth-based searches.

When NASA launched its High-Resolution Microwave Survey (HRMS) to identify main sequence stars within 50 light years that could have Earth-like planets, it was laying the foundations for the targeted search survey (TSS) for intelligent extraterrestrial life. Project Phoenix, using just such a TSS, exists to find evidence of cosmic radio messages by looking at a huge number of frequencies, as well as by using logical guesses of probable frequencies that an intelligent civilization would use to communicate e.g. that of positronium 1(3S1)*1(3S0) transition. The foundations for TSS were laid in 1960 by Frank Drake, who launched Project Ozma. Using a one-channel receiver tuned to 1420 MHz (21cm), the resonant frequency of neutral hydrogen, Drake observed the two solar-type stars, Tau Ceti (11.9 light years away) and Epsilon Eridani (10.7 light years) for signs of intelligent life. His work, as well as that of Cocconi-Morrison, set the stage for the SETI project...