The dark streaks seen on Martian slopes might be an indication of where water sometimes flows, especially since orbiters have also observed salts in the same locations. (Credit: NASA/JPL/University of Arizona)

Mars’ surface is dry and dusty. But researchers know there’s water there. It’s locked up in the polar ice caps, and occasionally it probably seeps to the surface as liquid. And at night, the Red Planet’s plummeting temperatures raise the humidity drastically, possibly to 80 or 100 perc