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Life as we define it on Earth is based on the following characteristics:

1. Reproduction. Living systems are able to reproduce on their own.
2. Growth. After a living system is “born,” it grows.
3. Metabolism. Living systems share a complex chemistry that includes nutrient cycling (taking in food and giving off waste), energy production, protein synthesis, and DNA replication.

Macroscopic living systems such as animals and plants have so far not been found anywhere off our planet. But it is possible that there are systems off Earth that possess the cellularity, chemical organization, and complexity we equate with being “alive.” Traces of once-living microbes (fossils) have been found on Mars. Also, a meteorite found in Antarctica and thought to be from Mars contains some organic waste products and minerals very similar to those found where dead microorganisms on Earth have been collected. Finally, scientists have found microorganisms living in the Atacama Desert in Chile, which was once thought to be an inhospitable, sterile environment reminiscent of Mars. The microbes apparently are capable of long-term dormancy. When environmental conditions become favorable (rain once every decade), it is thought that these microbes then flourish.

Questions

1. On Earth, viruses are not considered living systems because they cannot reproduce on their own. What could we assume if we found viruses anywhere else in the universe?
2. Life as we know it on Earth is based on carbon. What if other life could be based on another element? How would this impede our search for life off Earth?
3. Finding a living system on Earth in what was once thought to be a sterile environment should enhance the likelihood of finding “life off Earth.” Why?
4. Based upon what we know about energy and the sun, how likely is it that we will find life in the outer limits of our solar system?
5. If we did find life off Earth, how could that affect the perception that humans are unique among living systems and that a divine being is guiding the universe?

Assignments

1. Devise an experiment to test whether any form of “life” as defined on Earth could prosper on Mars.
2. Is it justifiable to spend tax dollars to search for life off Earth? Form two opposing groups to debate the economic, moral, and scientific issues surrounding this question.

References

Dawkins, Richard. 1996. The Blind Watchmaker. Norton, W.W. & Company, Inc.

http://www.astrobio.net

http://nssdc.gsfc.nasa.gov/

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