We humans have begun our exploration of the universe right here on Earth. Over time, we have developed a collection of theories of how things worked generally, based largely on local observations. Our earliest theories were often geocentric--even the stars were thought to be a manifestation of a universe dominated by Earth. As time progressed, we came to see that Earth was one of many planets orbiting a star, and that our star was just one of a vast collection reaching across distances far beyond human experience.

Despite our progressive appreciation of a huge universe--one wherein Earth is just a speck--we must remember that we have observed all these wonders and drawn conclusions from the vantage point of the Earth.

Almost in parallel with--but seemingly separate from--cosmological discoveries have come a myriad of observations and conclusions about another fascinating phenomenon that may or may not have universal consequences: biology.

Everything we know about life in the universe comes from the study of living systems on, in, and around Earth. Even without consideration of biological phenomena, Earth is a planet, worthy of study in its own right. Add in the phenomena of biology, and Earth becomes additionally worthy of study from a broad, astrobiological point of view.

First and foremost, it is the only planet we know of that harbors life. It is the only planet we know of (at least, so far) that bears water in all three of its physical phases. Finally, it is the only world we can observe with equal sophistication from above as well as upon the planet, providing valuable calibration or "ground truth" for measurements taken from space.

Life isn't what it used to be

Recent findings demonstrate that life--usually in relatively simple form, but increasingly in complex assemblages--is found at extremes of environmental conditions. Life has been found at regions of high temperature, salinity, acidity, and alkalinity, and ionizing radiation. Life forms have been found deep in rock, embedded in ice, beneath the surfaces of rocks, within nuclear reactors, and at ocean depths far from any energy-transferring light sources, long thought to be a requirement for life. Based on our own, somewhat parochial point of view, we refer to these newly discovered forms of life as being "extremeophiles."

The result of these studies is that life on Earth is now thought to be capable of existing and thriving wherever there is liquid water. This new approach to understanding where (on Earth) life can exist--and thrive--has stunning implications for the possibilities of life's existence beyond the Earth. The old catch phrase, "life as we know it," has now expanded its horizons to encompass the search for life--as we know it--in environments long thought to be inimical to life. Indeed, rather than limiting our searches to terrestrial conditions, we now are looking across the Solar System, and finding an ever-increasing number of potential habitats for life.

Warning: Exploding paradigms!

While Earth is the only example of life we have at hand, we need to be certain that we are not overtly biased by our proximity to it, or the assumptions we may therefore be tempted to make based on such proximity.

While certain aspects of Earth can be used as models for environments elsewhere, they also have their limitations. Indeed, we have found ourselves to be victims of our own observations on Earth. Until the 1970s and 1980s no one thought that life could exist at deep ocean thermal vents, inside Antarctic rocks, or deep within the earth's crust. So we didn't look for it in these locations. And then, relatively suddenly, we stumbled upon life where none was thought to exist. As a result, we've taken fresh approach to considering where life might be found elsewhere.

For example, the search for life on Mars must take an approach different from those applied thus far. The 1976 Viking landers just barely scratched the surface of their target areas on Mars. Those sites were chosen because they were conducive to a safe landing, and not necessarily because life (or its signatures) might be found there, such as at (or near) what are now thought to be river beds. Further, the chemical tests used to detect life had a clear terrestrial bias which, while understandable and justifiable given the state of knowledge at the time, didn't take into account the basic chemical characteristics of the Martian surface--and what could lie underneath. Similarly, despite the recent geological and technical success of the recent mission of the Mars Pathfinder lander and its Sojourner rover, neither were able to look into rocks, where we now know life can exist--at least, on Earth. For all we know there is a thriving ecology literally right under the spacecraft we have landed on Mars.

Earth-based implications for extraterrestrial biology

As alluded to earlier, microorganismal communities have been found living inside rocks found in the dry valleys of Antarctica. These cryptoendolithic communities may provide a perfect model system for Martian life. Analogously, deep oceanic hydrothermal communities here on Earth may be good models for ecosystems in the putative oceans on Europa and Callisto, and life forms and communities and found deep within the Earth's crust may provide models for life existing deep within Mars.

And, while there are no data to support the possibility of liquid water on the Jovian satellite, Io, the presence of the bacterium Deinococcus radiodurans within food-containing cans that have been irradiated in nuclear reactors here on Earth at least allows us to consider the possibility of life on worlds bombarded with high levels of radiation.

In short, we must now consider all sites in the Solar System (and beyond) that have (or may have, or may have had) liquid water as possible homes for biological systems.

Earth as an interactive life support system

Studying life forms here on Earth can do more than just indicate where life may exist on other planets in our Solar System. Earth can also be considered as a model system to develop technologies and analytical tools to search for and characterize the ability of extra-solar planets to support life.

The oxygen we require to live is a major constituent of Earth's atmosphere, present in large quantities only because of biological activity. Without such activity we'd expect to find an atmosphere rich in carbon dioxide. Other atmospheric gases, which exist in trace quantities, are also present in concentrations that are far out of the equilibrium one would expect from non-biological conditions. These, too, are present because of biological activity. Taken together, these abundant and trace gases are considered signatures of the presence of biology, and may be used by remote sensing instruments to look for life on other planets, in this Solar System and beyond. Terrestrial-based calibration of remote sensing instruments that detect the presence and sources of these gases--their production via geological, biological and solar-driven processes, and recycling on Earth--provides a clearer understanding of what might occur elsewhere. It also helps us decide what to look for, and how to look for it.

Recent efforts around our planet to understand Earth's myriad systems and their interactions will similarly support new awareness of how habitable planets form, the nature of the interactions between planetary systems and life, and what interactive feedback is provided between them. Further, by understanding how off-planet phenomena, such as changes in Solar luminosity, affect biological systems on our planet, we can begin to understand many aspects of planetary formation, life's origins, and its distribution and destiny throughout the cosmos.

Space-rating humans

To date, humans have spent their time off Earth inside spacecraft orbiting our home world. With the exception of six short sorties to the Moon, we really haven't done much exploring ourselves; we've left that to our robots. That will soon change as human missions back to the moon and for the first time to Mars are implemented. With lunar exploration, a trip home can take place over the short span of a few days; however, this will not be the case with travel to Mars and other remote locations. These lengthy voyages necessitate solving problems that will arise beyond the engineering and scientific challenges: we must address the human factors that accompany such prolonged missions with the same fervor we do the technical challenges. Addressing these problems here on Earth--before long missions are launched--is both cost- and time-effective.

Antarctic and the Arctic research activities have often been cited as perfect analogs for lengthy interplanetary missions. Indeed, there is a long history of studying human performance in these and other isolated environments and applying the results to space mission architectures. Recently, the Mars Society set a goal of establishing a Mars base analog in the Canadian Arctic. This would be one the first time that a base was established on Earth specifically to study how we might explore using a base on Mars.

As we expand the scope of our search for life into more varied extraterrestrial environments, additional Earth-based research analogs become obvious sources of operational experience. These analogs include studies done within deep sea submersibles, desert microbiology and paleontology expeditions, assays of nuclear reactors and accidental contamination, and surveys of repopulation of areas devastated by fire and volcanic eruptions. Each time we send humans into a remote, hostile--and interesting!--environment on Earth, we should be able to learn lessons with applicability to extraterrestrial environments and voyages to them.

Terraforming

It is clear that--just as the environment affects biology--biology affects the environment.

Indeed, humans have been conducting an experiment in transforming our home planet for millennia--an experiment that began as soon as trees were felled or fires were built, as both of these activities (and more) have long-term environmental effects. Only within the past few decades have we begun to understand the ramifications of such an ad hoc experiment, and what will be needed to lessen its potentially deleterious consequences.

The well-established phenomenon of ozone holes appearing over Antarctica, the Arctic, and now, it seems, over mid-latitude regions of Earth, is one such experiment. The use of chlorofluorocarbons as coolants has led to this potentially life-threatening phenomenon. The chemistry is well known, and there is no doubt that use of this industrial gas--in concert with specific environmental conditions--is the cause of the ozone hole. Other "experiments," such as the building of cities, paving previously vegetation-covered land, the use of irrigation to grow crops in otherwise arid regions, and the dumping of pollutants into the atmosphere and oceans are now beginning to make their way into the database of ways in which humans--specifically--affect their environment.

This kind of planetary engineering may indicate our ability to transform--potentially for benefit--other worlds, to allow them to support terrestrial life forms. Terraforming, however, is not to be undertaken lightly, as the consequences of changes may not be felt for many years. Until we have a full understanding of how planetary systems--atmospheric, hydrological, lithospheric, and biospheric--interact, we would do well to keep a tight rein on our "creative" tendencies.

Learning lessons and recognizing bias

While we are able to send robots to other the planets in our solar system, and, in some cases, actually land upon them, we're still only stumbling along the path towards becoming a spacefaring species. As a result, for the near term Earth is the only planet we can study "up close and personal."

While we certainly strive to design spacecraft to operate in the environments they traverse, we do so based upon the theories, models, and simulations we develop here on Earth.

For better or for worse, Earth is our prime testbed for the exploration of the Solar System. As we move outward from our home planet we must be imaginative and open minded as we apply lessons learned from the planet Earth, and we must be equally wary of the biases that come from such endeavors based, as they are, on one data point.