From Scientific American:

During the Mars Society annual meeting on Sept. 22-25, held in Washington, D.C., Jennifer Eigenbrode, a biogeochemist and geologist at NASA Goddard Space Flight Center in Maryland, outlined the constraints on whether living things could have existed on the Red Planet and what obstacles they would have faced, as well as whether it’s possible that any life could still be there. 

It’s important for scientists to make sure they recognize life when they see it, not just to learn more about the universe, but also because such life-forms could potentially be hazardous to humans, she said. 

“The [Mars Science Laboratory on the Curiosity rover] was never prepared for looking for signs of life,” she said. “The Curiosity rover’s instruments are designed so that scientists could answer the questions of whether it’s possible for life to have existed in the past and whether any traces would be detectable. That’s the first thing one has to know before one looks for signatures of living things.” [Amazing Mars Rover Curiosity’s Latest Photos]

The challenges for living things on Mars started billions of years ago, when the planet, for whatever reason, lost its magnetic field, Eigenbrode said. That left Mars with nothing to block the solar wind, which slowly bled off the planet’s atmosphere. 

“This also exposed the planet surface to all of the radiation” from the sun, Eigenbrode said. “The atmosphere was being blown away. This complicates the evolution of a biosphere.” 

As the atmosphere thinned, more ionizing radiation could reach the ground. That type of radiation tends to break up organic molecules, which are molecules containing carbon, Eigenbrode said. She added that in lab experiments, exposure to radiation at levels similar to those on Mars’ surface destroys up to 90 percent of large carbon molecules.

If life appeared on Mars in the past, when …

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