Accretion, loss, and fractionation of martian water

A recently published model for accretion of water by Mars from asteroids and comets beyond 2.5 AU calls for Mars to have accreted 6–27% of Earthʹs ocean (equivalent to 600–2700 meters depth on Marsʹ surface) with a D/H ratio 1.2–1.6 times Standard Mean Ocean Water. This model is shown to provide enough outgassed water to account for the subsequent evolution of D/H because of loss of hydrogen and the D/H ratio in water of hydration found in martian meteoric minerals, but enhanced rate loss of hydrogen from early Mars is necessary to permit adequate fractionation to occur during the lifetime of the planet.