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- Title
An Examination of Several Discrete Lunar Nearside Photometric Anomalies Observed in Lyman‐α Maps.
- Authors
Cahill, Joshua T. S.; Wirth, Anna A.; Hendrix, Amanda R.; Retherford, Kurt D.; Greathouse, Thomas K.; Mandt, Kathleen E.; Liu, Yang; Greenhagen, Benjamin T.; Denevi, Brett W.; Stickle, Angela M.; Hurley, Dana M.
- Abstract
The Lyman Alpha Mapping Project has detected five discrete low‐albedo anomalies in Lyman‐α (Ly‐α; 121.6 nm) nighttime reflectance maps. These anomalies reside on the nearside of the Moon within the southeastern Oceanus Procellarum and northwestern Mare Nubium, coincident with regions that have been observed to be photometrically anomalous at visible wavelengths. Some of the spectral properties of these regions within near ultraviolet (NUV) to visible (VIS) wavelengths are consistent with lunar swirls, and they have been reported as "probable swirls" in at least one of these studies. However, while these regions have low Ly‐α normal albedo values relative to nearby highlands regions and are consistent with Ly‐α albedo observations of other swirls, they do not spectrally redden at wavelengths >160 nm as other swirls have been shown to do and they are not associated with regions of higher magnetic intensity. Interestingly, while these anomalies are not easily discerned in NUV single‐band images, the anomalies can be discerned in NUV color ratio composites of the Lunar Reconnaissance Orbiter Camera Wide‐Angle Camera. At least one anomaly observed in the far ultraviolet is not easily observed in the NUV. Additional analyses of their thermal emission Christiansen Feature values, empirically corrected for maturity, suggest that regolith maturity is what differentiates them from their surroundings. While these anomalies may not be swirls, they may represent an "endmember" to swirls minus the influence magnetic materials, which have been hypothesized to provide solar wind shielding. If true, they may provide additional insight into the generation of lunar swirls. Plain Language Summary: Global maps of the Moon derived by the Lyman Alpha Mapping Project are providing new and unique views of the surface through far ultraviolet (FUV) eyes. Enigmatic lunar surface features, previously only detected with Earth‐based telescopes collecting visible observations at multiple ratioed lighting geometries (which led to these features being labeled "photometric anomalies"), are readily discerned in nighttime observations at the FUV wavelength of 121.6 nm (i.e., Lyman‐alpha). Their dark albedo characteristics in this wavelength make it tempting to relate them to other enigmatic features called "lunar swirls" (also dark at this wavelength). However, these anomalies lack sinuous geomorphology and proximity to magnetic materials as is typical of lunar swirls. While they are not swirls, they may somehow be related. The lack of proximity to magnetic materials may play a part in differences observed between these two enigmatic lunar surface features. Additional analyses of Lyman Alpha Mapping Project FUV, Lunar Reconnaissance Orbiter Camera Wide‐Angle Camera near ultraviolet, Diviner thermal infrared, and Kaguya Multispectral Imager data products suggest that these anomalies have been less exposed to space weathering relative to their surroundings and are therefore categorized as "less mature." The process that created these unusual surface features remains unknown. Key Points: Nighttime Lyman‐a maps confirm five low‐albedo regions present on the lunar nearside, in low magnetic intensity areas unlike swirlsWhile most readily viewed in Lyman‐a, we note detection in NUV ratio, color composite, and TIR Christensen Feature data productsUV and TIR characteristics are consistent with a shallow/small‐scale change in maturity, porosity, or roughness; however, origin is unknown
- Subjects
OBSERVATIONS of the Moon; ASTRONOMICAL photometry; LUNAR maps; REFLECTANCE; ALBEDO; CAMERAS
- Publication
Journal of Geophysical Research. Planets, 2019, Vol 124, Issue 2, p294
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2018JE005754