The title is somewhat misleading, as we have not (yet) found extraterrestrial life but only some, hotly disputed, evidence suggestive of the existence of microorganisms on the planet Mars. The search for life on Mars began in August 1960 when NASA authorised the Jet Propulsion Laboratory in Pasadena, California to work on a plan to land a capsule equipped with instruments and experiments for detecting signs of life in the soil of the planet.
In March 1959, NASA had handed "the princely sum of $4,485" to microbiologist Wolf Vishniac, to develop "a prototype instrument for the remote detection of microorganisms on other planets".
One problem was that not only were many biologists convinced that life must be unique to planet Earth, but others could not agree what forms extraterrestrial life might take. Would it be similar to Earth life, totally different, or somewhere in between?
Another problem was that it was possible to fail to detect microorganisms by making false assumptions about them. For example, some biologists interested in Mars explored the Dry Valleys of Antarctica, because conditions there were more like those on Mars than anywhere else on Earth, being very cold and very dry. One biologist, Norman Horowitz, used standard techniques to search for microbial life in the soil there, but drew a blank. However, Wolf Vishniac said that Horowitz had used a broth that was too rich in nutrients to cultivate the organisms and this had killed them off. Vishniac used a weaker brew, more suited to organisms used to a nutrient-starved environment and was successful in detecting life.
As is well known, the experiments carried out on Mars to detect life produced results which were inconclusive. Were they caused by the chemical properties of the soil, or by biological processes?
Another fierce controversy about possible Martian life erupted in 1996 when researchers at the Johnson Space Center announced that they had discovered fossilised microscopic organisms in a meteorite from Mars, referred to as ALH 84001, found in Antarctica. From the description of the controversy, it seems that both sides had sound technical and scientific justifications for their opinions, and there is still no agreed, definite answer to the question of Martian life.
The rest of the book discusses the possibility of life on other planets and satellites in the solar system, and the development of techniques for the possible detection of life on planets orbiting other stars.
This work is a good introduction to astrobiology. -- Reviewed by John Harney