Chapter 1: An Introduction to Life on Earth
Issues in Biology
How would you determine if life existed on another planet? In the Viking payload was a 15-kilogram apparatus that could carry out three separate tests. In the Labeled Release Experiment, the Lander extended a mechanical arm and scraped up some soil. The soil was placed into a closed container and mixed with water and with radioactively labeled nutrients that had been brought from Earth, and the mixture was incubated at 10oC for 8 days. Then, the air above the moist soil was examined to see if it contained carbon dioxide gas, which could indicate the presence of microbes with a metabolism similar to much of the life on Earths surface. The data that Viking I and II sent back were disappointing: None of the experiments provided strong evidence of life on Mars. However, recent discoveries back on planet Earth indicate that Viking was looking in the wrong place. If you want to find life on an inhospitable planet, you may need to look deep beneath the surface.
What might be wrong in thinking that Mars microbes would be physiologically like Earth microbes? Living things may or may not operate or be constructed in the same way as Earth life. Why would NASA scientists make this assumption? Even scientists are limited to what they know and what they can imagine as possibilities. We only know about Earth life.
What would be the characteristics of life forms on Mars? Many believe they could be similar to members of the Domain Archaea and, thus, are prokaryotic, unicellular organisms as on Earth. In addition to dwelling in the deepest recesses of the planet, Archaea thrive in some places on Earths surface, but in the most inhospitable conditions imaginable. Their habitat ranges from the frozen ice seas of Antarctica to the boiling water geysers of Yellowstone National Park and the black smokers on the floor of the sea. Some live in environments equivalent to household ammonia or vinegar. It isnt just that they can manage to survive in these conditions, but for many, these harsh conditions are actually required for the cell to live and reproduce. This penchant for conditions that would kill most of the rest of us, Bacteria and Eukaryota alike, has earned for Archaea the nickname, "Extremophiles," reflecting their preference (or requirement) for "life on the edge."
Most recently, a British team of exobiologists has pushed forward in the effort to search for life on Mars. Using all of the previous studies on Mars as a guide, the team designed and launched The British Beagle 2 lander in 2003. One of their approaches will be to test for the presence of methane in the atmosphere, as it is produced only by biological processes. They propose that the presence of any methane will finally provide proof of life on the Red Planet.