A lot of scientists and engineers who study Mars worry about planetary protection, the concern that biospheres on other worlds might be contaminated by microbes from Earth. It’s a bit like Star Trek’s prime directive, and NASA and other space agencies take pains to clean their robotic spacecraft of Earth-based life before launching them to other planets.
The discovery of periodic, briny water on the surface of Mars earlier this year reignited concerns about planetary protection, including whether the Curiosity rover was free enough of Earth-based microbes to investigate these features, known as recurring slope lineae (RSL). The problem becomes even worse when humans are thrown into the mix.
Therefore, some in the scientific community believe astronauts should remain off Mars until rovers and other probes have thoroughly studied the question of life on Mars. After the confirmation of present-day water on Mars, for example, The Planetary Society’s Emily Lakdawalla, wrote, “If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars—and obfuscation of the answer to the question of whether we're alone in the solar system—for a little while longer. Maybe just long enough for robots to taste Martian water or discover Martian life.”