11:15 AM - 11:30 AM
[PPS09-08] Evidence of indigenous volatiles from lunar surface topography and radar signals indicating subsurface void
Keywords:Subsurface Void, Lunar Radar Sounder (LRS), Indigenous Volatile, Magma Ascent Process, Moon
Lunar indigenous volatiles are important clues to understanding the early thermal environment and the volcanic activity evolution. Recent reanalyses of Apollo samples suggest that lunar indigenous volatiles are considerably more abundant than thought earlier, although these are localized information [e.g. 1]. Furthermore, the global distribution of the lunar surface water is being clarified gradually through spacecraft spectroscopic observations [e.g. 2]. However, much of the lunar surface water is thought to deliver from external sources, and the understanding of indigenous volatile distribution is insufficient. A new indicator of indigenous volatiles is a gas void formed when volatiles within magma exsoluted in a low-pressure environment and concentrate at the magma tip [3]. Mare Tranquillitatis has a thick crust [4] and almost no radioactive elements [5]. In addition, many surface features involving volatiles (IMP [6], RMDS [7]) exist. Therefore, it is strongly suggested that volatiles have been involved in the magma ascent and eruption in the Mare Tranquillitatis.
We use Lunar Radar Sounder (LRS) onboard SELENE to investigate subsurface gas voids in Mare Tranquillitatis. Previous LRS analyzes have rarely interpreted small-scale subsurface structures such as subsurface voids. This is because surface scattering, which could be mistaken for reflections from small-scale subsurface structures, is superimposed on the LRS data [8], making it difficult to distinguish between the two reflections. In this study, we extracted subsurface echo candidate (SECs) within LRS data by calculating surface scattering from lunar elevation data. Furthermore, we investigated the existence of subsurface gas voids in Mare Tranquillitatis.
As a result, we discovered an area where SECs exist for approximately 30 km along the extension of the graben indicating that magma rise to the shallow depth. The SEC depths are approximately 200 m, which are almost the same as the position of the magma tip expected under the graben. Moreover, SECs are detected around surface topography associated with volatiles (IMP, RMDS) and surface topography indicating magma intrusion (graben, depression) throughout Mare Tranquillitatis. The reflection intensity of SECs (> -25dB) suggest voids or high-porosity materials (hereinafter, these two are collectively designated as void structures) from the radar equation analysis.
From the above, it is thought that there are subsurface voids related to magma intrusions in Mare Tranquillitatis. Drainback, which does not necessarily require volatiles, is one of the mechanisms for forming subsurface voids through magma intrusion. However, considering the thick crust, few radioactive elements, and surface topography indicating the involvement of volatiles in Mare Tranquillitatis, these voids are likely to have been formed by a concentration of volatiles at the magma tip. In this presentation, we will also present the results of Mare Serenitatis and Mare Fecunditatis, which exist near Mare Tranquillitatis, and the subsurface void distribution between different ocean regions.
Reference
[1] Saal et al. (2008), Nature. [2] Milliken et al. (2017), Nature Geoscience. [3] Head and Wilson. (2017), Icarus. [4] Ishihara et al. (2009), GRL. [5] Kobayashi et al. (2012), EPSL. [6] Qiao et al. (2018), Meteoritics & Planetary Science. [7] Zhang et al. (2020), JGR [8] Kobayashi et al. (2002), Earth, Planets and Space.
We use Lunar Radar Sounder (LRS) onboard SELENE to investigate subsurface gas voids in Mare Tranquillitatis. Previous LRS analyzes have rarely interpreted small-scale subsurface structures such as subsurface voids. This is because surface scattering, which could be mistaken for reflections from small-scale subsurface structures, is superimposed on the LRS data [8], making it difficult to distinguish between the two reflections. In this study, we extracted subsurface echo candidate (SECs) within LRS data by calculating surface scattering from lunar elevation data. Furthermore, we investigated the existence of subsurface gas voids in Mare Tranquillitatis.
As a result, we discovered an area where SECs exist for approximately 30 km along the extension of the graben indicating that magma rise to the shallow depth. The SEC depths are approximately 200 m, which are almost the same as the position of the magma tip expected under the graben. Moreover, SECs are detected around surface topography associated with volatiles (IMP, RMDS) and surface topography indicating magma intrusion (graben, depression) throughout Mare Tranquillitatis. The reflection intensity of SECs (> -25dB) suggest voids or high-porosity materials (hereinafter, these two are collectively designated as void structures) from the radar equation analysis.
From the above, it is thought that there are subsurface voids related to magma intrusions in Mare Tranquillitatis. Drainback, which does not necessarily require volatiles, is one of the mechanisms for forming subsurface voids through magma intrusion. However, considering the thick crust, few radioactive elements, and surface topography indicating the involvement of volatiles in Mare Tranquillitatis, these voids are likely to have been formed by a concentration of volatiles at the magma tip. In this presentation, we will also present the results of Mare Serenitatis and Mare Fecunditatis, which exist near Mare Tranquillitatis, and the subsurface void distribution between different ocean regions.
Reference
[1] Saal et al. (2008), Nature. [2] Milliken et al. (2017), Nature Geoscience. [3] Head and Wilson. (2017), Icarus. [4] Ishihara et al. (2009), GRL. [5] Kobayashi et al. (2012), EPSL. [6] Qiao et al. (2018), Meteoritics & Planetary Science. [7] Zhang et al. (2020), JGR [8] Kobayashi et al. (2002), Earth, Planets and Space.