Image copyright 1998 Keith Cowing All rights reserved.

We have been exploring the universe for millennia - all the while speculating about life elsewhere. During this quest we had to be content to explore without leaving Earth. We could only venture outward in our minds or wait for the universe to drop rocks out of the sky. Fifty years ago we began to throw things back at the universe - but they always fell back to Earth within a few minutes. Forty years ago, we sent things out there - this time to stay. We followed shortly thereafter for short visits. Now we're also in space to stay.

So where do we stand in this quest? We've completed an initial reconnaissance of our solar system and have begun to identify others. We have peered backward in time across great distances to the earliest days of our Universe while simulating the events of those early times in laboratories here on Earth. We know how to live in space for years at a time, and understand both the norms and extremes wherein life thrives on Earth. The preparatory work has been completed - we know how to find life elsewhere - and where to look. Welcome to Astrobiology.

Theorizing and model building are one thing; it is another to go out and get data that will support science and the acquisition of new knowledge. For this purpose, NASA has instituted its Astrobiology Program to study the origin, evolution, distribution, and destiny of life in the universe. Existing programs and new endeavors will be brought together in a multidisciplinary fashion to tackle the questions surrounding life's place in the organization of the universe.

In so doing, NASA has adopted six canonical questions to use as guideposts as its programs are developed. What follows is a description of what each these questions--and a new one, that cannot be ignored--and what answers to these questions portend for astrobiology.

(1) "How do habitable worlds form and how do they evolve?"

Just what is Habitable?

At its most basic, habitable means suitable for life to exist and, probably, to evolve. Given terrestrial examples, we know that life can be found at extremes of temperature, pressure, salinity, and radiation of several types, including ionizing radiation. These extreme environments are far from the relatively temperate realm in which we humans dwell. What, then, is a habitable world? How did it get to be that way? How long has it been habitable? How long will it remain that way?

Since we do not know of any biochemistries other than that found within Earth life, our own example will have to suffice as the metric against which to define habitability--for now.

First and foremost, our world is habitable because its located at a distance from its central star (within the "habitable zone") that permits water to exist in all three of its phases, with an emphasis on the liquid phase. While the Sun's output varies with time, it has not ranged far from the range where liquid water is possible on the Earth.

Our planet also has an atmosphere (whose composition is modulated by life) that blocks some harmful radiation from the Sun, and a magnetic field that has similar protective properties. Our planet has active tectonic processes that drive an interchange between internal and external planetary layers so as to recycle geo- and biological materials. These phenomena (and many more) contribute to the habitability of our planet.

How Did the Earth Become Habitable?

The Earth was not always habitable. Early in its history conditions were likely inimical to life owing, in no small part, to heavy bombardment from planetesimals early in our Solar System's history. Indeed, the large impact which is thought to have caused the formation of our moon could have easily sterilized the entire planet.

However, all indications are that almost as soon as conditions were supportive of life, it established a toehold on this planet. While we cannot be sure that life did not arise and fail and arise again (and again!), it is likely that life has been found on the Earth for some 3.8 billion years. Whereas early life was single-celled and anaerobic, life became much more complex: it became multicellular and aerobic and, in the process, changed the atmosphere of the planet. There are even signs that humans--a relatively late addition to the panoply of life forms on the Earth--may be deleteriously affecting that same atmosphere by their industrial activities.

Habitable Worlds elsewhere?

Our own solar system has several candidate habitable worlds, although they do not exhibit the full breadth of phenomena that we find on the Earth. Liquid water once flowed on Mars; Europa, a moon of Jupiter, is though to have a subsurface liquid water ocean, covered by a thick layer of ice that shows signs of being modified over time. Callisto, another moon of Jupiter, may also have liquid water present. The search continues for signs of liquid water elsewhere in our Solar System.

We are not limited to exploring our own Solar System, however. Using recently developed technologies we can use tools like the Hubble Space Telescope to look for other stars, and to see if those stars have planets in orbit about them. With all the stars out there, how do we determine which ones to examine?

In looking for habitable worlds, we must find stars at the right stage of their life cycle, and of suitable spectral type. Recent searches for extrasolar planets have found large Jupiter-sized planets far more easily than Earth-sized worlds. These easy-to-find Jupiter-equivalents are closer to their parent star than terrestrial life forms would find comfortable. But it does give further support to theories that say that planetary formation is a routine part of the activities of the cosmos. If planets exist in habitable zones around other stars, there may be many more habitable (and, possibly inhabited) planets than the one we are sure of.

(2) "How do living systems emerge?"

How Can We Define Life?

What is "life?" Would you know life from somewhere else it if you saw it? Indeed, would it recognize you?

Somewhere along the cosmic continuum that began at the Big Bang billions of years before our sun was born and leading up the present, conditions led to a phenomenon we now define as "life." But, agreeing on a definition that will satisfy everyone has proven all but intractable. How, then, do we design instruments, sensors, probes, and missions to seek out and characterize this phenomenon, if we cannot even define it to everyone's satisfaction?

Many aspects of what we call "life" seem to arise from the inherent tendencies of matter to organize itself, part of a continuum that begins with protostellar organization and ending with planets populated with life. Indeed, there seems to be evidence that points to life as a cosmic imperative, i.e., the universe seems to favor the development of complex systems, of which life may be a particular endpoint.

Based upon the plethora of examples on Earth, there are several characteristics that we can agree upon: Life stores and uses energy; life engenders more life in an exquisite ballet of time and molecules; life responds to external stimuli, and so on. Further, life as we know it is carbon-based, requires aqueous solution for its chemistry, and transfers genetic information from one complex linear polymer (DNA) to another complex linear polymer, the protein. Despite differences in preferred environment or complexity of body architecture, all life on this planet adheres to these basic principles and, as far as we can tell, this has been the case for billions of years.

How Did Life Begin on the Earth?

For much of the past century, life's origin on Earth has been described as having originated in a warm little pond, with lightning and other environmental energy sources providing the energy to drive the process. The conditions under which life arose were thought to be random, seemingly benign, and took a long, long time to occur.

Over the past few decades, however, this view has changed. Earth life has been shown to be far more robust and adaptive that previously imagined, and, as a result, it has been found in places where no one had thought to look for it previously. Further, microbial fossils have been found in the some of the oldest rocks on Earth, and isotopic fossils indicative of past life have been found in even older rocks. Taken together, these observations point to life's appearance at virtually the instant the Earth's surface cooled enough for life to exist.

The raw materials from which life arose are now known to be relatively common throughout the cosmos. Comets, for example, appear to be thoroughly impregnated with these materials. An early influx of comets and other bodies is thought to have played a significant role in delivering the organic compounds required for life's synthesis. It is of significance that these compounds have been shown to come from a far larger and ancient process, one which operates in vast clouds from which the stars themselves are formed. Life, then, appears to be a natural outgrowth of the universe's basic structure and organization.

Is All Biochemistry the Same?

Since it is dangerous to generalize from one data point, we must ask, Might there be other basic premises of life, chemical or purely physical, that have arisen under conditions other than those found on the Earth? Are there other chemistries that would support the vast array of functionality we find in terrestrial life forms?

Even before we can decide on a chemical lingua franca, we must establish the most basic principles of organization and complexity that would lead a collection of organic molecules to assemble into the earliest ancestors of contemporary cells. In so doing we need to consider other chemistries than that which comprises Earth life. Until the range of alternate possibilities is fully explored, we're going to have to rely upon the composition of Earth life as a guide for our search.

(3) "How can other biospheres be recognized?"

What Is a Biosphere?

A biosphere is that portion of a planet that comprises life, and the environmental parameters that affect it. On Earth, the biosphere exists in the region within and above Earth's outer crust, and interacts with Earth's lithosphere, hydrosphere, cryosphere, and atmosphere. In the past few decades, as we have discovered life in ever more locations, the biosphere has come to encompass some of the remotest regions and most extreme environments on Earth.

Over at least the past 3.85 billion years Earth's biosphere has been actively maintained by the input of materials and energy from deep within the Earth's mantle, by materials impacting the Earth, and by energy from our local star. In exchange, the biosphere itself--and the regions it overlaps--have been reshaped and regulated to a large extent by the life within it.

Current theories suggest that life arose on Earth from materials that were at hand at the time, or from those brought to the Earth in the form of comets and other debris left over from the formation of the solar system. We have reason to believe that life arose almost as soon as the Earth was cool enough for it to do so. We might expect life to arise on other worlds under similar conditions, using materials that are common.

But not all worlds are like Earth.

How do we Identify Other Biospheres?

Looking for biospheres similar to our own is straightforward. We understand the biochemistry, physiology, and much of the way life interacts with its non-living environment. But what happens when a biosphere is underneath an impenetrable layer of material such as ice or rock? Could we find it? What happens if the local biochemistry is wholly different? Would we recognize it?

For example, looking at the early history of Mars we see that it likely not only resembled early Earth, but that Mars probably enjoyed environmental conditions more conducive to life's origin than the Earth. Mars has since dried out, lost most of its atmosphere, and gotten much colder. If Mars once had a biosphere, did it contract into environments where some subset of its components could still function, such hydrothermal deposits, or caves, or other niches that we may not be prepared to find, much less recognize?

There are other possibilities within our Solar System. Recent data give tantalizing signs that there may be a liquid, salty water ocean underneath Europa's outer, icy crust. Tidal heating caused by Europa's orbit about Jupiter may pump energy into the oceans in a manner akin to deep sea hydrothermal vents on Earth. A thin oxygen atmosphere and tantalizing readings of ice composition make the possibility of life ever more enticing. More recent data about another Jovian satellite, Callisto, indicates that it, too, may have reserves of liquid water, and may, therefore, provide an abode for life.

Extrasolar Biospheres

Had we instituted a search for life on a planet called Earth, looking only where we expected to find it barely a few decades ago, we never would have found any of these life forms. Instead we stumbled upon them. We need to keep this in mind as we search for life elsewhere.

In the near term, our efforts will be limited to extremely remote sensing. Sending probes into interstellar space is more or less pointless until we have away to get them to their destination at a reasonable speed. Large space-based telescopes that could infer the existence of -- and perhaps actually image -- extrasolar planets are being designed. Regardless of whether we stay here or go there, a simple question remains: What do we look for?

One approach to take is to look for atmospheres that have a chemical imbalance or disequilibrium which is not likely to occur without life's intervention. Earth's original atmosphere has been substantially altered by life into the composition we see today. As far as we understand life, such disequilibria are only maintained by living systems.

Searching for similar atmospheric disequilibria would be a prime means of detecting the possibility that something of biological origin might be going on within the atmosphere of another world. Add a search for the presence of atmospheric contaminants of artificial origin say, chloroflurocarbons, and it might even be possible to infer the presence of a technological civilization.

NASA's current administrator has even challenged those who would seek to search for life elsewhere to examine anomalous stellar output, perhaps from a Dyson sphere or some other deliberate manipulation. Listening for deliberate transmissions, an approach currently being pursued by several private scientific organizations, is another way to detect advanced civilizations.

(4) "How have the Earth and its Biosphere Influenced each other over time?"

Life has Changed Earth

Earth and the environment at its surface have undergone continuous change since the accretion of Earth began.

As the planet evolved, the mechanisms whereby its surface and atmospheric composition became increasingly complex led to a particularly complex organization of matter, life. Earth's biosphere became very active in the processes of further shaping the Earth.

The presence of life on Earth has had a profound impact upon this planet's composition. The free oxygen in Earth's atmosphere is a by-product of life's presence. This current assortment of plant life on Earth serves a pivotal role in the sequestering of carbon dioxide from the atmosphere. Since carbon dioxide is a greenhouse gas, life modulates the ability for Earth to shed heat in the form of radiation to space. In addition, the distribution of plant life across the surface plays a profound role in redistributing water vapor back into the atmosphere.

Earth has molded Life

Biological evolution is thought to be the product of long-term changes in populations arising from natural selection operating at the level of the individual. Selection involves responses to the physical and biological aspects of the environment, as ecosystem level processes interact with environmental change. Environmental factors can change slowly over time (such as day length or continental drift) leading to subtle evolutionary changes.

Changes in the environment can also have nearly instantaneous effects on the evolutionary process. Volcanoes can cauterize large areas while planetary impacts or stellar flares can sterilize the entire planetary surface, leaving organisms residing in protective niches to repopulate the devastated areas. Life itself can cause environmental changes that can lead to a feedback loop wherein these changes act as additional selective factors.

Interactions and Feedback

How do we go about understanding the interrelationship between a planet and its biosphere? Going back to the onset of the relationship would be a good start. Earth's earliest biosphere was dominated by microbes which appeared 3.85 billion years ago and remained the acme of biological sophistication for the next 3 billion years. Complex, multicelled organisms only appeared after profound atmospheric changes and a series of large planetary impacts had occurred.

As life evolved so did the complexity of interactions within the biosphere and between the biosphere and other Earth systems. Identification and dating of key branching points that occurred in the evolution of "the tree of life" must be coupled with increased knowledge of the Earth's biogeochemical systems.

Understanding how to maintain a healthy biosphere is of concern for the possibility of long-term human presence in space--whether in low-Earth orbit, or on interplanetary trajectories. Knowing how various natural ecosystems have evolved and how they function would help closed ecological life support system designers.

(5) "How Do Rapid Changes in the Environment Affect Emergent Ecosystem Properties and their Evolution?"

The biosphere of an inhabited, habitable planet, if it is to develop and maintain life over billions of years, must be capable of dynamic modulation by an interplay of large and small events, long term trends and short term bursts, as well as indigenous recycling and extraterrestrial surprises. When discussing biological evolution, there is often a tendency to think of long time spans, and an almost smooth, flowing continuum of events.

But, the closer you look, the more granular the picture becomes.

What Constitutes "Rapid Change"?

There have been many catastrophic and short-term events in the Earth's history that have had far-reaching consequences. Indeed, there is a whole continuum of dramatic events with aftereffects of various durations. The effects of the recent El Niño event, the recent volcanic eruption of Mt. Pinatubo, and human deforestation around the globe all happen over geologically short time spans, yet all have effects that could endure for far longer periods.

There is, of course, a danger from not taking a distant look at the Earth as a system. Some processes, such as glaciation cycles, orbital oscillations, and patterns of continental drift, simply do not manifest themselves in a noticeable way at time intervals of less than a few millions of years. Yet these events can have effects just as profound upon Earth's biosphere as does the immediacy of an asteroid impact.

What Kinds of Ecosystem Responses Might We Find?

Rapid changes in Earth's environment must be viewed in terms of how organisms and ecosystems respond to properties of oscillations, and what responses there are to impulse functions and their aftereffects. Given that life still exists here on Earth after billions of years fraught with regular catastrophes, there is obviously some sort protective mechanism at work.

The structure and function of the various ecosystems that reside within Earth's biosphere are mainly adaptations to climate, atmospheric chemistry, gravity, and biogeochemical patterns. These ecosystems are perpetuated through a number of emergent properties, including vegetation-soil cycling of moisture and geochemicals, biomass content and structure, plant seed banks, and co-dependent plant, animal, and microbial population genetics.

Fires have been shown to be a critical factor in maintaining the overall health of a forest. We might speculate that planetary impacts or large terrestrially induced disasters such as volcanisms might serve similar roles. Indeed, we may find that the very course of evolution on Earth has been driven by the ability of life to respond again and again to such incursions upon ecosystem stability.

(6) "What is the potential for survival and biological evolution beyond the planet of origin?"

Can Earth life--and, by extension, life from any world--adapt and thrive within environments beyond the one within which it evolved? Discussions are underway in several communities that address the possibility of intentionally seeding other planets--such as Mars--with terrestrial life in an effort to convert the conditions on that planet into those that are consistent with terrestrial life, i.e., to terraform the planet.

Seeding the Universe with Life

Recent discoveries within the ALH84001 meteorite, a piece of Mars blasted off by a large impact and thrown to Earth, suggest that there may be a naturally occurring mechanism for exchanging material between planets, one that could transfer life forms. While the ALH84001 results are still hotly disputed, hoary concepts such as panspermia are now being revisited.

Human Exploration of Space

In the process of moving off this planet and into space, humans will be taking other terrestrial life forms with them. Most of this effort is going into the design and implementation of closed ecological life support systems (CELSS). These systems will use micro-ecologies to provide potable water, breathable air, and food for extended human presence in space.

Looking at the physical extremes ("extreme" from a human standpoint) to which life has adapted--and thrived--here on this planet, we note many obvious similarities to environments known or suspected to exist on nearby worlds. But Earth-based capabilities cannot adapt to every extraterrestrial environment. Therefore, we will have to either take our environment (or at least the important parts) with us, modify ourselves to more easily function in that environment, or change the extraterrestrial habitat to suit our needs. The likely outcome will involve a mixture of all three approaches.

(7) How will Astrobiology affect and interact with human societies and cultures?

During discussions at the Astrobiology Roadmap Workshop at NASA's Ames Research Center in July 1998 , a seventh question pertinent to astrobiology activities arose. While it has not been formally woven into NASA's programs, many astrobiologists believe that the underlying principle should be considered throughout astrobiology research activities.

Finding life elsewhere in the universe will have an effect upon how we view ourselves. Finding extant life elsewhere would mean that we are not alone in the universe. Specifically, if life can be found elsewhere in our own solar system, then chances are at least good that it is rather common in the cosmos.

What if we find evidence of technology, perhaps derived from the analysis of another civilization's smog? Such discoveries will cause people to reconsider various aspects of their lives, from culture to religion. Would the discovery of life elsewhere spur a surge in the zest to explore, or would we rush through the excitement only to be bored a few weeks later? Or, might it keep us from exploring altogether?

If life is indeed abundant in the universe, it would seem to be a given that one world's life will eventually interact with another's - be it in the form of a microbe randomly blasted into space on a rock or a piloted spaceship. If physical meeting is not practicable, then perhaps we could seek to exchange messages across interstellar distances.

However the interaction occurs, the consequences of the search should be considered hand in hand with the process of searching itself.