NASA: New Insights into How Mars Became Uninhabitable

Isotopes are versions of an element with different masses. As water evaporated, light versions of carbon and oxygen were more likely to escape into the atmosphere, while the heavy versions were left behind more often, accumulating into higher abundances and, in this case, eventually being incorporated into the carbonate rocks. Scientists are interested in carbonates because of their proven ability to act as climate records. These minerals can retain signatures of the environments in which they formed, including the temperature and acidity of the water, and the composition of the water and the atmosphere.