NASA just made a major announcement that years of research by the landers, rovers, and especially by the orbiters suggest that there is liquid water on Mars.
While there is likely fresh water on Mars as well, especially near the poles, this announcement relates to salt water in several places elsewhere on Mars, mostly around hills and mountains where they can see it flowing down and then evaporating. The water is a brine with perchlorate salts, which are very water-soluble. The fact that it is salty means it can survive in a liquid form from -70 to about 25 Celsius, rather than from 0 to 100 Celsius on Earth. Fresh water is much less stable in a liquid form on Mars because the current atmosphere is thin, and is only able to stay liquid from about 0 to 10 Celsius.
The Mars rovers and landers have discovered that Mars is actually quite wet; there is a lot of moisture in the soil and a lot more humidity in the air than we thought there was before visiting the planet in depth. At night, the atmosphere cools and sometimes results in ice crystals actually forming on the Curiosity Rover. Along with the heating and cooling of the humid air, the seasonal water flows down Mars's hills show that the planet has an active hydrological cycle.
The applications of this news is that:
1. Mars is habitable for water-based life, including humans who visit the planet (which will be happening soon, likely in the 2020's), who could use the water to make breathable air, drinkable water, and liquid rocket fuel as well as solid booster fuel.
2. Mars probably has life (but we just don't know for sure yet!). It might be microbial, and it might mostly live under the surface, but it's looking more and more likely that life can easily be supported as all the basic ingredients to support it are there, and considering Mars's Earth-like past, it's very possible the planet is alive with some very old life. It's possible Mars's theoretical life came from Earth rocks with living microbes, it's possible Earth's life came from ancient Mars rocks with living microbes, and it's possible both planets independently had life start upon them.
3. We now know where to visit when we next go to Mars. Where do we look for the life? Near the water. Mars 2020 is the next scheduled rover mission, and will specifically aim to look for life in the wet areas of Mars.
If you'd like to know how I know this, go watch the NASATV press broadcast that just happened minutes ago today. Here is a sample if you don't want to watch the whole thing:
