Continuously Habitable Zone

According to astronomers and exobiologists, the continuously habitable zone (CHZ) is the region around a star in which one or several (Earth-like) planets can maintain conditions that are appropriate for the emergence and sustained existence of life. One important characteristic of a planet in the CHZ is that its environmental conditions support the retention of significant amounts of liquid water on the planet's surface. This potentially life-supporting region around a star is sometimes called the Goldilocks zone, and any Earth-like planet in the CHZ is often referred to as a Goldilocks planet. Displaying a fine sense of humor, scientists borrowed this unusual but appropriately descriptive nomenclature from the famous children's fairy tale, "Goldilocks and the Three Bears."

Within the context of contemporary astronomy and exobiology, an Earth-like planet is an extrasolar planet that is located in an ecosphere and has planetary environmental conditions that resemble the terrestrial biosphere—espe-cially a suitable atmosphere, a temperature range that permits the retention of large quantities of liquid water on the planet's surface, and a sufficient quantity of radiant energy that strikes the planet's surface from the parent star. These suitable environmental conditions could permit the chemical evolution of carbon-based life, as scientists know it on Earth. The Earth-like planet should also have a mass somewhat greater than 0.4 Earth masses (to permit the production and retention of a breathable atmosphere) but less than about 2.4 Earth masses (to avoid excessive surface gravity conditions that could discourage the development of advanced life-forms—at least as known and understood here on Earth).

Perhaps an even more demanding question is: Does alien life (once started) develop to a level of intelligence? If exobiologists speculate that alien life does evolve to some level of intelligence, then they must also ask: Do intelligent alien life-forms acquire advanced technologies and learn to live with these vast powers over nature?

In sharp contrast to the vision of a universe full of emerging intelligent creatures, other scientists suggest that life itself is a very rare phenomenon and that human beings here on Earth are the only life-forms anywhere in the galaxy to have acquired a high-level of conscious intelligence and to have developed (potentially self-destructive) advanced technologies. Just think, for a moment, about the powerful implications of this latter conjecture. Are humans the best the universe has been able to produce in more than 14 billion years or so of cosmic evolution? If so, every human being is a very special living creature, not only on Earth but in the entire universe.

The preliminary scientific search on the Moon and on Mars for existing (or even extinct) extraterrestrial life has to date been unsuccessful and negative. However, as previously mentioned in chapter 1, the recent detailed study of a Martian meteorite by NASA scientists has renewed excitement and speculation about the possibility of microbial life on Mars—past or perhaps now precariously clinging to existence in some protected, subsurface biological niche.

Further encouraging the contemporary quest for microbial life beyond Earth is the recent discovery and recognition of extremophiles right here on humans' home planet. Terrestrial extremophiles are found in acid-rich hot springs, alkaline-rich soda lakes, and saturated salt beds. Additionally, examples of very hardy microbial life have been found in the Antarctic, living in rocks and at the bottoms of perennially ice-covered lakes. Life, including hardy microbial organisms, is also found in deep-sea hydrothermal vents at temperatures of up to 248°F (120°C). Bacteria have even been discovered in deep (0.6-mile [1-km] or deeper) subsurface ecosystems that derive their energy from basalt weathering. Some of these extremophiles can survive ultraviolet radiation or large doses of nuclear (ionizing) radiation, while others can tolerate extreme starvation, low nutrient levels, and low water activity. Remarkably, spore-forming bacteria have been revived from the stomachs of wasps, entombed in amber that was between 25 and 40 million years old. Clearly, life—at least as present on Earth—is diverse, tenacious, and

Exobiologists postulate that under suitable environmental conditions, life, including intelligent life, should arise on planets around alien stars. This is an artist's rendering of an intelligent reptilelike creature. Some scientists suggest that on Earth, this type of very smart warm-blooded dinosaur might have evolved eventually had not a massive extinction event occurred some 65 million years ago, displacing the dinosaurs and allowing mammals, including Homo sapiens, to emerge. (Department of Energy/Los Alamos National Laboratory)

Exobiologists postulate that under suitable environmental conditions, life, including intelligent life, should arise on planets around alien stars. This is an artist's rendering of an intelligent reptilelike creature. Some scientists suggest that on Earth, this type of very smart warm-blooded dinosaur might have evolved eventually had not a massive extinction event occurred some 65 million years ago, displacing the dinosaurs and allowing mammals, including Homo sapiens, to emerge. (Department of Energy/Los Alamos National Laboratory)

adaptable to extreme environments.

Results from the biology experiments onboard NASA's Viking 1 and 2 lander spacecraft suggest that extant life is absent in surface environments on Mars. (Chapter 3 provides more details about NASA's amazing Viking Project.) However, life could be present in deep subsurface environments where liquid water may exist. Furthermore, although the present surface of Mars appears very inhospitable to life as we know it, recent space missions to the Red Planet have provided scientists with good evidence that the Martian surface environment was more Earth-like early in its history

EXTREMOPHILES

Extremophiles are hardy microorganisms that can exist under extreme physiochemical environmental conditions here on Earth, such as in frigid polar regions or boiling hot springs. Scientists generally characterize extremophiles according to the physical characteristics of the harsh environment in which they live and survive. For example, barophiles are microorganisms that thrive under high hydrostatic pressure conditions, as found typically in deep marine environments; while thermophiles are microorganisms that can live and grow in high temperature environments. Invoking the principle of mediocrity, some exobiologists have speculated that similar hardy (extraterrestrial) microorganisms might exist elsewhere in this solar system, perhaps within subsurface biological niches on Mars or in the suspected liquid water ocean beneath the frozen surface of Europa.

This artist's rendering depicts extremely hostile and diverse environments that might be found on extrasolar planets or companion moons. Biologists have discovered very hardy microorganisms, called extremophiles, surviving and even thriving in extreme environments here on Earth. So exobiologists suggest that similar hardy microorganisms might arise and survive in the harsh environments of extrasolar planetary systems around alien stars. (NASA; artist, Pat Rawlings)

This artist's rendering depicts extremely hostile and diverse environments that might be found on extrasolar planets or companion moons. Biologists have discovered very hardy microorganisms, called extremophiles, surviving and even thriving in extreme environments here on Earth. So exobiologists suggest that similar hardy microorganisms might arise and survive in the harsh environments of extrasolar planetary systems around alien stars. (NASA; artist, Pat Rawlings)

(some 3.5 to 4.0 billion years ago), with a warmer climate and liquid water at or near the surface. Scientists know that life originated very quickly on the early Earth (perhaps within a few hundred million years), and so it seems quite reasonable to assume that life could have emerged on Mars during a similar early window of opportunity when liquid water was present at the surface.

Of course, the final verdict concerning life on Mars (past or present) will not be resolved properly until more detailed investigations of the Red Planet takes place. Perhaps later this century, a terrestrial explorer (robot or human) will stumble upon a remote exobiological niche in some deep Martian canyon, or possibly a team of astronaut-miners, searching for certain ores on Mars, will uncover the fossilized remains of a tiny ancient creature that roamed the surface of the Red Planet in more hospitable environmental eras. Speculation, yes—but not without reason. (Chapter 4 provides more discussion about the search for life on Mars.)

The giant outer planets and their constellations of intriguing moons also present some tantalizing possibilities for extraterrestrial life. Which exobiologist cannot become excited about the possible existence of an ocean of liquid water beneath the frozen surface of the Jovian moon Europa and the (remote) chance that this alien-world ocean might contain communities of extraterrestrial life-forms clustered around hydrothermal vents? Furthermore, data from NASA's Galileo spacecraft suggest that two other Jovian moons, Callisto and Ganymede, may also have liquid water deposits beneath their icy surfaces. (See chapter 5.)

All scientists can say at this point with any degree of certainty is that their overall understanding of the cosmic prevalence of life will be significantly influenced by the exobiological discoveries (pro and con) that will occur in the next few decades both on planetary bodies in the solar system and concerning extrasolar planets beyond.

Recent discoveries show that comets could represent a unique repository of information about chemical evolution and organic synthesis at the very outset of the solar system. After reviewing comet Halley encounter data, space scientists suggested that comets have remained unchanged since the formation of the solar system. Exobiologists now have evidence that the organic molecules considered to be the molecular precursors to those essential for life are prevalent in comets. These discoveries have provided further support for the hypothesis that the chemical evolution of life has occurred and is now occurring widely throughout the Milky Way galaxy. Some scientists even suggest that comets have played a significant role in the chemical evolution of life on Earth. They hypothesize that significant quantities of important life-precursor molecules could have been deposited in an ancient terrestrial atmosphere by cometary collisions.

Meteoroids are solid chunks of extraterrestrial matter. As such, they represent another source of interesting information about the occurrence of prebiotic chemistry beyond the Earth. In 1969, meteorite analysis provided the first convincing proof of the existence of extraterrestrial amino acids. Amino acids are a group of molecules necessary for life. Since that time, a large amount of information has been gathered that shows that many more of the molecules that are considered necessary for life are also present in meteorites. As a result of this line of investigation, it now seems clear to exobiologists that the chemistry of life may not and should

Evidence is now mounting that early in its history, the planet Mars possessed flowing water on its surface—a condition that could have led to the development of life. This is an artist's rendering of what an ancient Mars might have looked like with liquid water on its surface. Detailed exploration of Mars in the first two decades of this century will help confirm this hypothesis and possibly uncover indisputable signs of life—extinct or perhaps even extant in some sheltered subsurface biological niche. (NASA/JPL)

Evidence is now mounting that early in its history, the planet Mars possessed flowing water on its surface—a condition that could have led to the development of life. This is an artist's rendering of what an ancient Mars might have looked like with liquid water on its surface. Detailed exploration of Mars in the first two decades of this century will help confirm this hypothesis and possibly uncover indisputable signs of life—extinct or perhaps even extant in some sheltered subsurface biological niche. (NASA/JPL)

not be unique to Earth. Future work in this area will greatly help scientists develop a better understanding of the conditions and processes that existed during the formation of the solar system. These studies should also provide clues concerning the relations between the origin of the solar system and the origin of life.

The basic question—"Is life—especially intelligent life—unique to the Earth?"—lies at the very core of our concept of self and where humans fit in the cosmic scheme of things. If life is extremely rare, then all members of the human race have a truly serious collective obligation to the entire (as yet "unborn") universe. As an intelligent species, we must preserve carefully the precious biological heritage that has taken more than four billion years to evolve on this tiny planet. If, on the other hand, life (including intelligent life) is abundant throughout the galaxy, then human beings should eagerly seek to learn of its existence and ultimately should become part of a galactic family of conscious, intelligent creatures. Fermi's famous paradoxical question "Where are they?" takes on special significance this century. (See chapter 8.)

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