Abstract
Understanding the history of water on Mars is important for understanding both its geological and potential biological history. The abundance and physical state of water has evolved through time, from the surface having an early warmer and wetter environment to the present-day colder and drier one. Although multiple lines of evidence support this change, attempts to determine the abundance of water on the planet, the history of water at the surface, and the sequestration into both permanent and exchangeable sinks have yielded a wide range of results. I explore the uncertainties in the processes and interpretation, to understand our ability to quantitatively determine the water inventory and its changes through time. Results indicate that the present state of models and of the data constraining them preclude determining a unique history for water. This uncertainty does not affect the conclusion that significant amounts of water have been lost to space and to other sinks and that these losses are consistent with the changes in climate and surface environment.