Last year, NASA provided evidence that water may still exist on the surface of Mars. This was based on an analysis of dark streaks on the Red Planet called recurring slope lineae (RSL).
At the time, it looked like there may actually have been flowing salty water on these hillsides. NASA was pretty excited about it, and even told The Martian director Ridley Scott ahead of time. With more time to look at the RSL, NASA still thinks there’s water, but much less of it.
NASA has been using the Mars Odyssey satellite to analyze the water content of the RSL. Odyssey has a handy instrument for this called the Thermal Emission Imaging System (THEMIS).
The presence of water in the Martian soil affects how quickly the ground heats up during the day and cools at night. NASA analyzed several years of THEMIS readings from RSL in Valles Marineris canyon, finding that the likely water content by weight is 3%.
That’s about the same water content as Earth’s Atacama Desert or the Antarctic Dry Valleys. Those are the driest places on the planet.
The low water content of the soil makes it unlikely that any water is flowing down these Martian hillsides as originally hoped. The 2015 announcement was based on the presence of hydrated salts in the soil, and NASA scientists believe that’s still true.
You can have hydrated salts without there being enough water to flow across the surface. The most likely explanation for the RSL is that salts in the soil are pulling water vapor out of the atmosphere. Thus, there’s no evidence for a large underground reservoir of water.
There was hope following the initial announcement last year that there might be enough water on Mars to make human exploration more feasible.
The alternative is bringing all the water the crew needs from Earth, which is expensive. For the time being, that looks like the best option. We’ve still got a few decades before NASA is ready to launch a manned Mars mission. A lot can happen in that time.