Lunar Cryptomare: New Insights Into the Balmer‐Kapteyn Region

Abstract

Cryptomaria record the early mare volcanisms and put important constraints on the lunar thermal and volcanic history and on the early partial melting of the mantle. This work focuses on the representative cryptomare region of Balmer‐Kapteyn and employs multisource exploration data including Lunar Reconnaissance Orbiter Camera Wide Angle Camera, Lunar Reconnaissance Orbiter Camera Narrow Angle Camera, Lunar Orbiter Laser Altimeter Digital Elevation Model (DEM), GLD 100, Chang'E‐1 Interference Imaging Spectrometer, Moon Mineralogy Mapper, Clementine, and Lunar Prospector Gamma‐Ray and Neutron Spectrometers data to provide new insights into the distribution, geometry, buried depth, space weathering, chemical compositions, lithology, and mineralogy of the cryptomare in the Balmer‐Kapteyn region. Some viewpoints are primarily as follows. (1) Cryptomare may widely occur in the Balmer‐Kapteyn region with various small and separate exposure areas. The lithology in this region has an obvious north‐south dichotomy. Alkali suite may dominate the northern region, responsible for the local Th enhancement, whereas ferroan anorthosite suite and magnesian suite may spread over the southern part. Cryptomare may occur in both the north and the south, with most exposures within the craters dominated by alkali suite or ferroan anorthosite suite. (2) The buried depths of the cryptomare may fall into the range from at least ~33 m to at least ~1,849 m. Thus, the buried thickness may be more than ~1.8 km. (3) The buried mare basalts in this region are member of the high‐alumina type and a previously unsampled new type of high‐alumina mare basalts may be revealed. (4) At least two periods of mare volcanisms or at least an intrusion of basaltic lava may have happened in this region. The erupted or intruded mare basalts in the later period are more abundant in Al but relatively poor in Fe, Ti, Ca, and Mg, and some of them were buried or intruded much shallowly (~33 m) under the surface ejecta.

New view of the Balmer-Kapteyn region: Cryptomare distribution and formation

“…Hawke et al (2005) indicated that some of the buried mare basalts may be high-alumina (Al 2 O 3 > 11 wt.%) mare basalts in the B-K region. Wang & Qiu (2018) further verified that almost all the buried mare basalts in this region may be high-alumina ones through the measurement results of dark-haloed crater (DHC). Whitten & Head (2015a) analyzed the mineral content using the Moon Mineralogy Mapper (M 3 ) in the B-K region, and further concluded that only the mare basalts of the ancient igneous rocks are consistent with the cryptomaria in absorption band centers and FeO and Th concentrations.…”

“…However, this method has some limitations: (1) some exposed mare surrounding DHCs may be pyroclastic material or deposits of low-albedo impact melt (Hawke & Head 1977;Hawke et al 1989Hawke et al , 2005Schultz & Spudis 1979); (2) the image quality may affect DHC recognition, mistaking terrain shadows for DHC (Wang & Qiu 2018); ( 3) not all excavated mare basalts showed complete dark-haloed feature (most dark deposits occur only in or around the crater and do not appear annular). To overcome these problems, in this work, the criteria for cryptomare determination include the following conditions: (1) the presence of a high Fe content area (FeO > 12 wt.%, Northeast Africa 001-B1B is the sample of mare basalt with the lowest content of FeO, 12.21 wt.%; Snape et al 2011) determined from the FeO abundance maps;…”

“…Cryptomaria are considered as a product of early (∼4.01 Ga) mare basalt volcanisms (Terada et al 2007;Whitten & Head 2015b). Therefore, researching cryptomaria can help us understand the thermal and volcanic history of the Moon (Head III & Wilson 1992;Whitten & Head 2015a;Wang & Qiu 2018). However, there are questions related to cryptomaria that require further study, such as their extent, chemical components, mineral composition, and the source of their high-albedo ejecta.…”

“…We selected the Balmer-Kapteyn (B-K) region as the research focus in this work. The B-K region is the oldest cryptomare area on the lunar nearside (Whitten & Head 2015b) and it provides significant clues about the formation and evolution of the cryptomaria (Wang & Qiu 2018). The B-K region located in the southeast of the mare Fecunditatis and southwest of the mare Smythii on the lunar nearside.…”

“…Hawke et al (1989) further studied DHCs and concluded that the thicknesses of the hidden mare basalt may be <0.6 km in most regions, but the hidden mare basalt might reach a thickness of several kilometers in the central region of the Balmer Basin. Wang & Qiu (2018) noted the DHC, and indicated the buried thickness of the cryptomare may be more than ∼1.8 km. One part of exploring the formation processes of the cryptomare in the B-K region includes determining the thickness and source of high-albedo ejecta.…”