The Temporal and Geographic Extent of Seasonal Cold Trapping on the Moon

Abstract

We assess the geographic distribution and temporal variability of seasonal shadow at the lunar polar regions and explore its influence on surface water migration and deposition within known permanently shadowed regions (PSRs) in the modern era. At its largest expanse near the winter solstice, seasonally shadowed area more than doubles the permanently shadowed area at both poles. The growth and decay of polar shadow throughout the year enforce distinct seasonal patterns in the poleward migration of water as well as a cyclical variation in the polar surface hydration throughout the year if a continual source of water is assumed. The polar surface water abundance peaks near the hemispheric vernal equinox—significantly offset from the solstice where the seasonal trapping area is most expansive—due to the retention of seasonally trapped water. Owing to their low areal density, lower‐latitude PSRs do not significantly hamper the poleward migration of water, enabling water to reach the high polar latitudes where cold trapping area is densest. We find that northern hemisphere PSRs accumulate more water per unit area than southern hemisphere PSRs and that this disparity is especially prominent beyond 85°. The north/south asymmetry is attributed to differences in the hemispheric PSR size‐frequency distributions; such differences enable unique north/south migration diffusivities, which favor more water reaching the high northern latitudes.

Stratigraphy of Ice and Ejecta Deposits at the Lunar Poles

“…The results from this new analysis (see the supporting information) give 8.1% of H 2 O molecules becoming trapped in the Diviner-defined permanent cold traps (2.7% in the north pole and 5.4% in the south). This is smaller than previous estimates, for example, 20-50% from Butler (1997), 21.5% from Ong et al (2010), 21.7% from Kloos et al (2019), but is in the range of 5.2-13.4% from Schorghofer (2014). In effect, the micro cold traps and seasonal cold traps create a vast peripheral holding pen ( Figure S1) where H 2 O molecules dwell and can be removed by destructive processes (Farrell et al, 2019) before they can reach the large cold traps at higher latitudes.…”

“…Hayne et al (2020) recently calculated the fractional area of cold traps at all size scales as a function of latitude. We used these three sets of values to modify the migration model from Kloos et al (2019), which was previously based on shadowing instead of temperature. The results from this new analysis (see the supporting information) give 8.1% of H 2 O molecules becoming trapped in the Diviner‐defined permanent cold traps (2.7% in the north pole and 5.4% in the south).…”

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

“…Moores (2016) predicted that cold traps farther from the pole accumulate far more water per unit area, while others find the variations to be small (Schorghofer 2014;Prem et al 2018). An updated version of Moores' model (Kloos et al 2019) also predicts nearly uniform infall in the polar region, leading to a consensus among models. Cold traps are too small to have any "rain shadow" effect.…”

“…4). Besides diurnal variability, seasonal trends in the lateral transport of water vapor on the Moon have also been recently examined; model results indicate that the cold traps in the northern hemisphere may accumulate more water per unit area than those in the southern hemisphere (Kloos et al 2019).…”

Accessibility Data Set for Large Permanent Cold Traps at the Lunar Poles

“…Robotic spacecraft and humans will need direct access to cold trap regions at the poles in order to dramatically improve lunar volatile research and economic prospecting going forward. High-resolution and time-resolved spacecraft observations now give a firm understanding of shadowing (Hayne et al, 2020;Kloos et al, 2019;Mazarico et al, 2011) and temperature conditions (Hayne et al, 2020;Williams et al, 2019) at the polar regions, advancing from theoretical predictions by Watson et al (1961) and Arnold (1979). These data tell us were ices could exist.…”