The Source of Martian Water

There is much evidence from chemical and dynamical reasons that the source for the terrestrial crustal and surface water originated from planetary embryos, which have their origin in the asteroid belt [4]. This source is consistent with the isotopic record of the water content of the asteroid belt, by the chondrite record [5] and in the role of gas giants in clearing the asteroid belt [6]. The observed D/H ratio is consistent with the isotopic record of D/H in carbonaceous chondrites and terrestrial seawater (TSW) whose principal source is the asteroid belt.

The ratio of D/H isotopes of the water vapour in the current Martian atmosphere is about 5.2 times the terrestrial seawater value [7], where the cause for this difference is commonly assumed to be the isotopic fractionation during atmospheric escape of water, which can exchange with sources of water in the Martian crust and surface [7, 8]. Although the original D/H ratio in Martian water obtained from planetesimals is not well known, there are three known possibilities for the delivery of water to growing terrestrial planets, i.e. planetary embryos from beyond 2.5 AU, small asteroids from beyond 2.5 AU and comets from the Jovian orbital distance and beyond.

A recent study determined how much water on Mars could have been acquired from asteroid and comet populations [9]. By estimating the cumulative collision probability between asteroids and comets with Mars and by assuming that comets consist by about 50 % of water ice with a D/H ratio about twice the terrestrial value and that asteroids have about 10 wt. % water with D/H values comparable to the terrestrial ocean water value based on carbonaceous chondrites, it was found that Mars can acquire an amount of water equivalent to 6-27 % of the terrestrial oceans, with a D/H ratio of about 1.6 and 1.2 times the TSW ratio [9].

This study indicates that Mars actually received more water from small comets and asteroids with semi-major axes greater than 2.5 AU than the Earth, while the Earth may have received the bulk of its water from large embryos. By using this result one finds that the estimations of 6-27 % of the terrestrial oceans on Mars correspond to an equivalent global ocean depth of about 600 to 2700 m in the Martian crustal regolith and on the surface. (The water amount on Mars is often expressed in terms of the depth of a hypothetical ocean covering the entire Mars. Hereafter, we refer this simply to as the "equivalent depth".) This is in agreement with previous studies [10, 11], where an outgassed amount with an equivalent depth of about 500 m on the Martian surface corresponds to a total accreted amount corresponding to 1000 m [9]. The D/H ratio of about 1.2 to 1.6 times the TSW ratio is below the average value of about 2.3 measured in Martian Shergot-tite meteorites [12], while an enrichment of about 1.6 times the TSW value is in agreement with the D/H ratio measured in the 3.9 Ga old Martian meteorite ALH 84001.

The difference between the D/H ratio in the Shergottites and the asteroid-comet collision study can be interpreted as follows: either the D/H ratio in Martian meteorites was derived from magmatic water, which represents a primordial Martian value obtained from accretion of a mixture of asteroidal and cometary water or it was enhanced from the primordial value due to XUV-driven hydrodynamic escape caused by a more active young Sun [9, 12]. See also Chap. 1 by Baker et al. for the discussion of D/H ratios gathered from various Martian meteorites.

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