Martian soil even more toxic than previously thought.

Researchers exposed the bacterium Bacillus subtilis, a common spacecraft contaminant, to perchlorates and UV radiation at levels similar to those found at and near the Martian surface. (Because Mars' atmosphere is just 1 percent as thick as that of Earth, UV fluxes are much higher on the Red Planet than on Earth.)

The bacterial cells lost viability within minutes in Mars-like conditions, the researchers found. And the results were even more dramatic when Wadsworth and Cockell added iron oxides and hydrogen peroxide, two other common components of Martian regolith, to the mix: Over the course of 60 seconds, the combination of irradiated perchlorates, iron oxides and hydrogen peroxide boosted the B. subtilis death rate by a factor of 10.8 compared to cells exposed to UV radiation alone, the researchers found.

"These data show that the combined effects of at least three components of the Martian surface, activated by surface photochemistry, render the present-day surface more uninhabitable than previously thought and demonstrate the low probability of survival of biological contaminants released from robotic and human exploration missions," Wadsworth and Cockell wrote in the study.

So, yet one more reason establishing cities of "millions," as some have said, on Mars is not very likely to happen.