100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 45
Astrobiology
Christopher P. McKay
and
Wanda L. Davis
NASA Ames Research Center
Moffett Field, California- Introduction 7. How to Search for Life on Mars,
- What is Life? Europa, or Enceladus
- The History of Life on Earth 8. Life about Other Stars
- The Origin of Life 9. Conclusion
- Limits to Life Bibliography
- Life in the Solar System
1. Introduction
Life on Earth is widespread and appears to have been
present on the planet since early in its history. Biochemi-
cally all life on Earth is similar and seems to share a common
origin. Throughout geological history, life has significantly
altered the environment of the Earth while at the same time
adapting to this environment. It would not be possible to
understand the Earth as a planet without the consideration
of life. Thus life is a planetary phenomenon that is arguably
the most interesting phenomenon observed on planetary
surfaces.
Everything we know about life is based on the example
of life on Earth. Generalization to other areas or alien
forms of life must proceed with this caveat. Although we
remain uncertain of the process or the time of its origin,
the advent of life on Earth was established within one
billion years after the formation of the planet. While life
also requires energy and nutrients, liquid water is the
single-most defining ecological requirement for life on
Earth. Thus a liquid water environment is currently the
best indicator of where to search for extraterrestrial life. We
do not expect to discover liquid water environments on any
of the recently discovered large extrasolar planets because
they are too close to their stars. Looking out into the
Solar System, however, we see evidence for liquid water.
Europa appears to have a liquid water ocean underneath a
global ice surface—the evidence is indirect but persuasive.
Enceladus has geysers erupting from its South Polar area
presumably powered by subsurface liquid water. There
are several lines of evidence that suggest that liquid water
existed on Mars in the past. Direct images from orbiting
spacecraft show fluvial features on the surface of Mars.
Orbital infrared spectrometers have found local regions
that show minerals formed in liquid water environments.
The Mars Exploration Rovers also have found evidence
for past aqueous activity at their landing sites on Mars.
Our understanding of life, albeit limited to one example
and one planet, would suggest that life is possible on other
planets whenever conditions allow for environments like
those on Earth—energy, nutrients, and most critically
liquid water. This suggests the possibility of early microbial
life on Mars and forms the basis for a search for Earth-like
planets orbiting other stars. Studies of a second example
of life—a second genesis—to which we can compare and
contrast terrestrial biochemistry will be the beginning of
a more general understanding of life as a process in the
universe. This implies a search for not just fossils but a
search for the biochemical remains of organisms, dead or
alive.