Transport of water in a transient impact-generated lunar atmosphere

Water delivery to the Moon by asteroidal and cometary impacts

“…Comets may contain a water ice fraction near 50% by mass, but some estimates for 2P/Encke and C/Hale Bopp comets give only 10% fraction of water (see Jewitt, 2004). Nevertheless, in numerical simulations comets are typically treated as bodies made from pure water ice (e.g., cited above Ong et al (2010), Stewart et al (2011), andPrem et al (2015)). As was mentioned above, after impacts of icy comets with typical velocities 20-50 km/s the major portion of cometary water (95 -99.9%) escapes the Moon (Ong et al, 2010).…”

“…Retained water can migrate over the lunar surface and be captured by cold traps at the lunar poles. Formation of a transient atmosphere and transport of water to cold traps after a cometary impact at 30 km/s were modeled by Stewart et al (2011) andPrem et al (2015). The model of Stewart et al (2011) shows that after the impact of a 2-km-diameter comet at latitude 45°S about 0.1% of the initial comet mass can reach the cold traps in several months, and ~1 mm layer of ice can be accumulated at the floors of the shadowed craters.…”

“…Stewart et al (2010), using Ong et al's (2010 cometary flux and size distribution and taking into account loss mechanisms during water migrations and gardening in the cold traps, estimated that about 5.0×10 10 -1.6×10 11 kg of water should be present inside the cold traps after 1 Ga; these deposits are equivalent to an ice layer thickness of 8 -30 mm. Despite the deposition of ice is nonuniform (Prem et al, 2015), it seems that the deposition by the process of water migration from numerous cometary impacts cannot explain why similar nearby craters near the south pole have noticeably different hydrogen concentrations in the regolith and why hydrogen was detected in sunlit areas. Various reasons, e.g., crater age and impact gardening, could potentially explain variations in hydrogen abundance that have been observed through remote sensing, but we also can suggest that high concentration of hydrogen may be caused by remnants of impactor material remaining in a crater.…”

DSMC simulation of Europa water vapor plumes

“…This code has been applied to a variety of flow problems including simulating Io's volcanic plumes ( Zhang et al, 2004 ) and atmosphere ( Moore et al, 2009;Walker et al, 2010 ), comet impacts on the moon ( Stewart et al, 2011;Prem et al, 2015 ), and the water plumes on Enceladus ( Yeoh et al, 2015 ). Code features not relevent to Europa have been disabled.…”

“…Every time-step, molecules above the surface of Europa are assumed to be in sunlight and part of an optically thin plume. Each has a probability of photodissociation given by P photo = 1 − e −r photo t following the model in Prem et al (2015) . Here r photo is the active sun photodestruction rate coefficient of 6.51 ×10 -7 s -1 (at 5.2 AU) for the H 2 O + ν → OH + H reaction ( Huebner et al, 1992 ) and t is time-step size.…”