Variability of Hydration Across the Southern Hemisphere of the Moon as Observed by Deep Impact. Issue 8 (8th August 2022)

• Record Type: Journal Article
• Authors: Laferriere, K. L.; Sunshine, J. M.; Feaga, L. M.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-08
• Subjects: Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2022JE007361
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Measurements of the 3 μm absorption feature, associated with the presence of hydroxyl and potentially molecular water, were first observed in 2009 by three separate spacecraft' observations. Subsequent observations have revealed widespread but variable hydration over the sunlit regions of the Moon. The variability can help to disentangle the individual contributions of OH and H2 O to the 3 μm absorption feature and provide insight into the mechanism of production and loss of OH/H2 O on the lunar surface. We investigate the spatial and diurnal variations of hydration on the southern hemisphere of the Moon as observed by the Deep Impact spacecraft during the lunar flybys in 2009 at spatial scales of 30–70 km/pixel. For a subset of observations of across the lunar south polar region (∼2% of the lunar surface), repeat coverage includes three different times spanning half a lunar day, allowing for exploration of diurnal variability. We determine that OH/H2 O is widespread but variable across the lunar south pole. At all but the lowest temperatures observed, highland regions have stronger hydration absorption features than the maria. Changes in band strength demonstrate variable loss rates controlled by surface temperatures with H2 O lost quicker at higher temperatures. Observed variability in the band shape strongly suggests higher H2 O abundance at low temperatures. These observations are strong evidence that the unique shape of the 3 μm band is due to both OH and H2 O. The rapid diurnal evolution of the absorption feature implies that migration of these constituents occurs locally over short distances driven by temperature changes. Plain Language Summary: We observed the Moon's surface over the south polar region at scales of 30–70 km/pix and detected the presence of OH/H2 O at 3 μm. This hydration feature was first found in 2009 in observations from three different spacecraft. In the time since, the origin of this feature and its variations has been explored and debated. Additional observations can help us understand the source of the OH/H2 O on the Moon and mechanisms driving the variability. In this paper, we investigate the distribution of the hydration on the southern hemisphere of the Moon, and how it changes from morning to evening. Consistent with previous work, we find that this feature is widespread across the lunar south polar region with variations in strength largely dependent on composition and temperature. We observe a trend of more water at lower temperatures. Highland regions tend to have stronger total absorption features, except at the lowest temperature regions observed. We also see systematic changes in the shape of the absorption feature, which suggest variations in the amount of H2 O as compared to OH. The rapid changes in hydration imply that the migration of these constituents occurs locally over short distances driven by daytime temperature changes. Key Points: We provide evidence for compositional differences in hydration strength and loss rates between mare and highland deposits Variability in band shape, depth, and loss rates with temperature suggests that OH and H2 O are present, lost at different rates and temperatures Daytime variations over repeated observations reveal that the hydration feature is lost toward local noon and fully replenished by evening