Dynamics and Population of OH/H₂O on the Lunar Surface

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The discovery of OH/H₂O absorptions on the lunar surface (Pieters et al., 2009; Sunshine et al., 2009; Clark, 2009) sparked interest in the hypothesis that bombardment of lunar soil with solar-wind protons might form hydroxyl (OH) and perhaps H₂O. Both a broad 3 μm absorption (at the edge of the M3 range) and a narrow, better-resolved 2.8 μm feature were detected. The 2.8 μm feature is assigned to the OH molecular group and has a sinusoidal dependence on latitude; the broader 3 μm absorption also could be due to OH but could also at least partly be due to H₂O. It is largely absent below ± 45° latitude, and strongly concentrated at high latitudes (McCord et al. 2011).

Ichimura et al. (2012) tested the solar wind hypothesis using Apollo 16 (highlands) and Apollo17 (mare) soils. Pre-dried soils were bombarded with 1.1 keV H+ or D+, and provide unambiguous evidence for the formation of OH or OD in both samples. However, in no case was any spectral evidence of H₂O observed.

If solar wind is responsible for H₂O formation, one would expect OH to accumulate first, followed by H₂O as the samples approach saturation. The saturation load of H+, assuming 10 nm penetration, has been estimated by Starukhina (2001) and McCord et al. (2011) to be ~ 10¹⁷ cm^-2. The ion fluence to the lunar samples in the Ichimura et al. experiments was ~ 10¹⁷ cm^-2, so proton saturation and at least some H2O would be expected. Further evidence that the experimental samples were at least near saturation is that D+ bombardment of H-saturated samples resulted in simultaneous loss of OH and formation of OD, suggesting that the original population of OH was a product of dynamic equilibrium between implantation and sputtering loss.

The ability of solar wind ions to mobilize pre-existing OH might explain the apparent diurnal variability of the 3 μm band reported by Sunshine et al. (2009). While these trends naturally point to a temperature dependence, direct comparison of band depth with simultaneously derived temperature (for T > 250K) show no strong T dependence, and thermal corrections based on Diviner observations found a weak increase in the weakest absorptions with temperature, but no significant change with the strongest bands (McCord et al. 2011)

To compare these observations to plausible mechanisms and global patterns, we calculate a time-marching model of [OH] in soil grains. The initial [OH] is set to zero, and at each step we adjust [OH] according to the incident flux, and a characteristic lifetime that is temperature dependent. We run the model until the population is stable as a function of latitude and hour angle. Because the proton flux increases as the cosine of the hour angle, but the escape flux is an exponential function of temperature, as well as being linearly proportional to the surface population, as one moves from the dawn terminator to noon, the escape flux increases much more quickly than the incident flux. The diurnal maximum surface population will occur where j_up = j_down. In most model runs, that occurs on the morning side of the disk, and in general produces an am/pm asymmetry that appears consistent with the qualitative asymmetry described by Sunshine et al., (2009).

Note however that models that reproduce that pattern do not begin to approach saturation of surface OH, but are perhaps 5 orders of magnitude lower, on the order of 10¹² – 10¹³ cm^-2 at the equator. This makes sense, because the solar flux can’t replenishing OH between local noon and sunset with more than its integrated flux over ¼ of a lunar day, (~10¹⁴ cm^-2).

Thus, this model cannot simultaneously match the observed dynamics even qualitatively, while coming within 5 orders of magnitude of saturation, or of the abundances reported by DesMerais (1974). Taken at face value, this suggests most of the H lost from near the subsolar point must be re-accumulated at locations with higher solar incidence angles. However, OH remobilized by sputtering will exist only as a surface species on re-impact, and should show an even more extreme dependence on surface temperature.

Additional hypotheses will be quantitatively tested and discussed during the presentation.

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Pieters, C. M., et al., Science, 326, 568, 2009
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