Looking High and Very Low -
Quest for the Origins of Life

Rivers of molten rock and oceans of sulfurous boiling water. Intense solar radiation and a constant hail of comets and asteroids. This was the picture when life on Earth-the only life in the universe that we know of-sparked into existence approximately 3.8 billion years ago.

It seems surprising that life could flourish, let alone take hold, in such an environment. Generally, we imagine that water, air, and sunlight, along with a "livable" climate, are the prerequisites for life. While that may be true for most animals bigger than a bread box, scientists are finding that life, at least among smaller organisms, can sustain itself in conditions never before thought possible.

Part of NASA’s mission is to investigate the presence of life in the universe. How did life begin on Earth? Can it happen on other planets or in different ways? Studying the extremes of life on Earth and the building blocks of life present elsewhere in the solar system brings us closer to understanding how and why life can come into being.

Taking It to the Extreme

In the late 1970s, oceanographers discovered the first communities living around geothermal vents on the ocean floor. Beyond the reach of the sun and under incredible water pressure, these tiny ecosystems thrive without photosynthesis, previously thought to be the energy source for all life on Earth. The entire food chain of these communities, from microorganisms to animals, is based on microbes that produce energy through chemical reactions with minerals spewed from beneath the ocean floor by the vents.

Since that first discovery, scientists have tracked down other microbes and larger organisms in a variety of extreme environments, often where minerals and water provide the sole energy sources. These tough little life forms live, for example, on the bottom of perpetually frozen Antarctic lakes, in acid lakes, and in pools on cave floors that have seen no sunlight for millions of years.

Many microorganisms have found a home more than a mile below the Earth’s surface, where temperatures can exceed 150°C. In fact, some now estimate that microbial life hidden sub-surface may compose at least half of the Earth’s biomass, giving newfound importance to the lives of these poorly understood organisms.

Our comprehension of the extremes life can tolerate is continually expanding. Microbes found in the undersea Japan Trench, nearly 20,000 feet deep, live under water pressures of up to several million tons per square inch. Certain lichens and bacteria can survive at -45°C, a temperature where any surface water, along with most water-based organisms, would turn to ice. Many organisms adapted to extreme environments have no need for oxygen, and may even find it poisonous.

The tenacity of microscopic life has led some scientists to suggest an alternative origin for life on Earth. If the surface world of 3.8 billion years ago was too hostile to support the sunlight-fueled primordial soup of traditional theory, perhaps it formed beneath the crust, where water, geothermal energy and organic compounds were in abundance.

Could It Happen Anywhere?

The discovery of life on Earth in what were previously unthinkable environments has opened up new concepts for life on other planets. If Earth possesses a sub-surface biosphere, other planets may as well. Liquid water, still considered a prerequisite for life as we know it, may exist or have existed on several planets and moons in our solar system. While this liquid water would be or have been present in distinctly un-Earth-like environments, they can no longer be dismissed out of hand as unsuitable for life.

If liquid exists below Europa's frozen surface, this satellite of Jupiter may have the potential to harbor life.

The Jovian moon Europa is thought to have surface ice, organic molecules, and a constant internal energy supply. If water exists below the surface, as is suggested by that moon’s cracked and frosty outer layer, the circumstances could be sufficient to sustain life. The living evidence found in sub-freezing Earth environments brings the idea within the realm of possibility.

Titan, a satellite of Saturn, has the potential to answer many questions about the formation of the building blocks of life. Apart from Earth, it is the only other planetary body in the solar system with a significant atmosphere composed primarily of nitrogen. Too cold to harbor liquid water, it is nevertheless a factory for organic compounds. More than a dozen have been identified. The base compounds that form DNA have been detected in meteorites. Observing organic evolution in process on Titan may give us a clue as to how these compounds were first formed and ejected into space.

Titan’s example begs the question of whether life can spring from a biochemistry different from our own. Is water the only medium that can support life? Must DNA be the genetic code? To be considered alive, a thing must meet two criteria: it must carry the information necessary for reproduction and it must posses the ability to evolve. This definition puts no limits on the materials from which life is made. Still, we have only the one example of life here on Earth.

What are the origins of life? In piecing together this most fundamental of puzzles, scientists will continue to send probes under the Earth’s crust, into the ocean’s depths, and out across the solar system. Along the way, we are discovering as much about our own world as we are about our celestial neighbors. And, after all, as we scan the universe for evidence of life, we have to be sure that we know it when we see it.

Studying our solar system helps us understand the origins of life, just as studying life on Earth helps us understand the potential for life to exist elsewhere.

If liquid exists below Europa's frozen surface, this satellite of Jupiter may have the potential to harbor life.