Astrobiology

From Freepedia

Astrobiology is an interdisciplinary field, combining aspects of astronomy, biology and geology, which considers the question of whether extraterrestrial life exists and if so, then what are its origin, distribution, and evolution. It comes from the Greek words astron = star, bios = life and logos = word/science), and is also known as exobiology (Greek: exo = out) or xenobiology (Greek: xenos = foreign).

Some major astrobiological research topics include:

  • What is life?
  • How did life arise on Earth?
  • What kind of environments can life tolerate?
  • Can we detect life on other planets?

Exobiology and xenobiology are terms also found in science fiction, although typically such terms refer to the speculative biology of an extraterrestrial. A xenobiologist is usually a human doctor or biologist who is expert on the physiology of alien organisms and life forms.

Contents

Overview

Although astrobiology is an emerging field, the presence of life in the rest of the universe is a verifiable hypothesis and thus a valid line of scientific enquiry; Astrobiology assumes the validity of core scientific precepts such as evolution.

A particular focus of current astrobiology research is the search for life on Mars. There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface; water is considered to be an essential precursor to the development of life, although this has not been conclusively proven.

Missions specifically designed to search for life include the Viking and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been canceled.

Research Outcomes

As of 2005, there is no definite evidence of extraterrestrial life. However examination of meteorites from Antarctica which are thought to have originated from the planet Mars have provided what some scientists believe to be microfossils of extraterrestrial life, although that interpretation of the evidence is still controversial. In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Methane has a relatively short half-life in the Martian atmosphere, so there must be a recent source of it. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life.

Missions to other planets (such as Beagle 2: Evolution to Mars and Cassini to Saturn's moon Titan) hope to further explore the possibilities of life on other planets in our solar system.

Methodology

Narrowing the task

Main article: Planetary habitability

When looking for life in other planets, some simplifying assumptions are useful to reduce the size of the task of astrobiologists. One is to assume that the vast majority of life-forms in our galaxy are based on carbon chemistries, as are all life-forms on Earth. While it is possible that non carbon-based life exists, carbon is well known for the unusually wide variety of molecules that can be formed around it.

The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.

A third assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star, with one side of the planet perpetually baked and the other perpetually frozen.

About 10% of the stars in our galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. However, we only know of one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct.

The sciences of Astrobiology

Astronomy

Most astronomy-related astrobiological research falls into the category of extrasolar planet (exoplanet) detection, the theory being that if life arose on Earth then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect 'Earth-like' exoplanets are under development, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs. A number of less ambitious ground-based efforts are also underway (see exoplanet).

One estimate for the number of planets with (intelligent) extraterrestrial life can be gleaned from the Drake equation if and when we ascertain the values of its variables. However uncertainties in the term of the equation make it impossible to predict whether life is rare or common. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the universe then there should be obvious signs of it. This is the purpose of project like SETI, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations.

Biology

Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include biota able to survive kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments. Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the universe.

Geology

The fossil record provides the oldest known evidence for life on Earth. By examining this evidence, geologists are able to better understand the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia are also considered to be geological analogs to regions of Mars and as such might be able to provide clues to possible Martian life.

Criticisms

Like any emerging scientific field, astrobiology has had to struggle for respectability. While many scientists are enthusiastic about the broad questions involved, there remains concern as to whether astrobiology is distinct enough from its parent disciplines (particularly biology) to constitute a true science. Astrobiology is at present a largely speculative extrapolation of Earth conditions into off-Earth environments. Extremophiles, for example, may reveal how life has evolved in exotic locales through out the universe, yet this remains unverified and the study of extremophiles is adequately covered under the rubric of biology. This ambivalent status is reflected by academic acceptance: those interested may specialize in astrobiology as a subset of an established science. Although some think that a formal degree program in astrobiology is unlikely[1], the University of Glamorgan, UK, have started just such a degree this year. [2]

Perhaps a more serious criticism of astrobiology is whether the very act of extrapolating from a single data source is unscientific. At best, this creates tautologies ("Earth-like life will evolve under circumstances which are sufficiently Earth-like for a sufficient period of time") and at worst it may abrogate critical aspects of the scientific method. Characterization of non-Earth life is extraordinarily unsettled; hypotheses and predictions as to its existence and origin vary wildly; true astrobiological experiments (with modest exceptions such as the ALH84001 meteorite) simply cannot occur at present. Finally, astrobiology has been criticized for being unimaginative in the tacit assumption that Earth-like life presents the most likely template for life elsewhere.

See also

External links

General subfields within biology
Anatomy | Astrobiology | Biochemistry | Bioinformatics | Botany | Cell biology | Ecology | Developmental biology | Evolutionary biology | Genetics | Genomics | Marine biology | Human biology | Microbiology | Molecular biology | Origin of life | Paleontology | Parasitology | Physiology | Taxonomy | Zoology
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