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How much water did Mars lose over timequestionMark Can it be estimated by looking at the present value of the D-H ratio at MarsquestionMark The current picture of martian water escape estimates that Mars has lost virgul140 m of global equivalent layer (GEL) of water over its evolution history based on the enhanced D-H ratio of virgul4-8 times the value at Earth. Recently completed analysis of D and H Lyman alpha observations obtained by the Mars Atmosphere Volatile Evolution (MAVEN) mission show that the D and H exospheric densities undergo large changes in a short timescale of weeks. The present escape process that would facilitate such fast depletion of the column of D and H is insufficient to account for the magnitude of change observed comma raising the likelihood of the presence of an energetic population of D and H created by non-thermal processes. The existence of such a population of H has been recently discovered with HST. The proposed 3-orbit observations with STIS is to discover energetic D atoms in the exosphere of Mars. The discovery and characterization of such a population is key towards determining the total water lost by Mars comma and would increase the current estimates by a notable amount. The results from this observation program will also have significant effects towards our understanding of using the D-H metric in determining the total amount of water lost not only from Mars comma but from other planetary atmospheres as well. This proposal supports the HST UV initiative.