When NASA began to explore Mars, they attempted to sterilize their spacecraft. The Viking missions to Mars (in the late 1970s), however, indicated that the planet was too harsh to even support life, even microbes.
According to an earlier article from New Scientist (“Earth life could invade Mars on contaminated craft”, July 28, 2005), it was stated by David Paige, a planetary scientist at the University of California in Los Angeles, that, beginning in the late 1990s, it was decided that "Mars is wetter, warmer, more interesting and more diverse than we thought.”
A panel with the U.S. National Research Council conducted a study on Earth contamination on Mars and published a report in 2005 that suggested many recommendations on how to prevent Earth contamination on robotic and manned missions to Mars.
The report urged NASA to use sterilization techniques on its spacecraft, which are already in use by Earth-based industries. One such technique uses a bath of toxic gases (including ethylene oxide) to disinfect and sterilize spacecraft.
In all, seventeen distinct recommendations were made by the council members.
The report was called “The 2005 National Research Council report on Preventing the Forward Contamination of Mars.” Read further for quotes from the report.