The goal of many Mars missions has been to determine if there is or ever has been water on the red planet. Thanks to the Sample Analysis at Mars (SAM) instrument suite on NASA's Curiosity rover, we now know that the Martian soil is roughly 2% water.
To make this discovery, the rover scooped up the dust, dirt, and fine soil at the Rocknest site it is currently in. Part of the fifth scoop was placed in SAM, which heats samples to 835 ºC to reveal its components. It found water as well as a compound that is likely chlorate or perchlorate, which previously was only found near the north pole; the Rocknest site is equatorial. This suggests the chlorine and oxygen compound is more widespread than previously believed. Due to the dust storms on Mars though, the entire planet is covered in a layer of surface soil that is similar everywhere. This is quite useful as a sample in one place is going to tell you about the entire planet.
Curiosity also looked at the isotopes of hydrogen and carbon in the sample, and found the isotope ratios to be similar to that found in the Martian atmosphere. This implies that as the dust moves around the planet, it is reacting with the atmosphere.