This plot shows the first-ever look at the deuterium-to-hydrogen ratio measured from the surface of Mars, as detected by the Sample Analysis at Mars (SAM) instrument on NASA's Curiosity rover.
Deuterium is a heavier version of the hydrogen atom. Scientists look at the deuterium-to-hydrogen ratios on Mars, along with isotopes of other elements, to understand how the atmosphere has changed over time. Mars, which has less gravity than Earth and lacks a strong enough magnetic field to shield its atmosphere from the sun, is slowly losing its atmosphere. As this process occurs, the lighter hydrogen atoms are preferentially lost compared with the heavier deuterium ones.
SAM measured the D/H ratios in water released when "Rocknest" sand samples were analyzed, and these samples show the water vapor consists of more deuterium than that of Earth's water -- i.e., the water is heavier. This is to be expected since the lighter hydrogen atoms in the Martian atmosphere are escaping faster than the heavier ones.