Yasuhiro Yokota

Preflight and In-Flight Calibration of the Spectral Profiler on Board SELENE (Kaguya)

The Spectral Profiler (SP) is a visible-near infrared spectrometer on board the Japanese Selenological and Engineering Explorer, which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5-1.0 μm ), NIR 1 (0.9-1.7 μm), and NIR 2 (1.7-2.6 μm). In this paper, we characterize the radiometric and spectral properties of VIS and NIR 1 using in-flight observational data as well as preflight data derived in laboratory experiments using a calibrated integrating sphere. We also proposed new methods for radiometric calibration, specifically methods for nonlinearity correction, wavelength correction, and the correction of the radiometric calibration coefficients affected by the water vapor. After all the corrections, including the photometric correction, we obtained the reflectance spectra for the lunar surface. Finally, we examined the stability of the SP using the SP data near the Apollo 16 landing site observed at four different times. The difference in reflectance among these four observations was less than ~ ±1% for most of the bands, suggesting that the degradation of the SP is not significant over the mission period.

Calibration of NIR 2 of Spectral Profiler Onboard Kaguya/SELENE

The Spectral Profiler (SP) is a visible-near infrared spectrometer onboard the Japanese Selenological and Engineering Explorer (SELENE), which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5-1.0 μm), NIR 1 (0.9- 1.7 μm), and NIR 2 (1.7-2.6 μm). In this paper, we propose a new method for radiometric calibration of NIR 2, specifically for the dark output (background) estimate, which is different from the previous method used for VIS and NIR 1. We show that the reflectance spectra of NIR 2 derived from the new radiometric calibration show less noise than those of the previous method. Based on an analysis of the reflectance spectra at exposure sites of the end-member minerals on the lunar surface, we demonstrated that the spectral features of the 2-μm band in the NIR 2 spectra are consistent with those expected from the minerals inferred from the features of the 1-μm band in the VIS and NIR 1 spectra. Finally, we examined the repeatability of the radiometric calibration of NIR 2 using the SP data near the Apollo 16 landing site observed at four different times. The typical difference in the reflectance at wavelengths <;~2.1 μm was a few percent, which is within the uncertainty due to the error in the background estimate, suggesting that there was no significant change in the sensitivity of NIR 2 over the mission period.

Evidence of impact melt sheet differentiation of the lunar South Pole-Aitken basin

The South Pole-Aitken (SPA) basin is the largest basin on the Moon. The basin-forming impact likely melted the upper part of the mantle and formed an impact melt sheet. Impact melt of large terrestrial craters differentiated following a general magmatic differentiation sequence. However, it is still debated whether or not the SPA melt sheet underwent differentiation. To address this, we investigated the vertical and lateral variations in mineral composition of the SPA impact melt sheet area by analyzing the surface mineralogy including 277 post-SPA craters using remote sensing reflectance data. We identified a 7xa0km thick, high-Ca pyroxene-rich layer and below that, axa0>8xa0km thick, low-Ca pyroxene-rich layer. Both filled the entire impact melt sheet area (630xa0km in diameter). Considering the vast distribution of these two layers, their thicknesses, their FeO abundances (around 13.5xa0wtxa0%), which are lower than most of the lunar mare basalt but within the estimated range of the differentiated SPA impact melt, and that the observed lithologies are consistent with a differentiation model, we conclude that the SPA impact melt was differentiated. Moreover, based on the lunar magma ocean differentiation model, Al2O3 and CaO abundances of the upper part of the mantle would be much lower after the mantle overturn. If the SPA impact occurred after overturn, these elemental abundances of the initial impact melt would be much lower than that of the before-overturn impact case and result in different solidified layers. Our observation matches only the after-overturn case, which may imply that the SPA impact occurred after overturn.

Featureless spectra on the Moon as evidence of residual lunar primordial crust

We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane, while there are no FL sites in the Procellarum regions nor the South Pole-Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

Validation of Frame-Transfer Correction of SELENE/LISM/MI

Since the visible detector of the Multiband Imager (MI) of the Lunar Imager/Spectrometer mounted on the Selenological and Engineering Explorer operates in a frame-transfer mode of a 2-D charge-coupled device, signals suffer from additional light exposure during the charge transfer, causing complicated artifacts. In this paper, we developed an iterative image-correction algorithm based on a model of multiband artifact phenomena. Using lunar images acquired while in orbit, we validated the correction performance based on a cross-comparison with near-infrared images of the MI obtained at a similar wavelength. Our correction algorithm corrected the additional electric charge of up to 10% of the obtained signal at the Apollo 16 site. Photometric and terrain corrections that include the effect of the observation angle were found to be important in precise cross-calibration.

Usability of lunar reflectance model based on SELENE/SP for planned HISUI radiometric calibration

We have developed a method for radiometric calibration of HISUIs hyper and multi-spectral sensors using a lunar reflectance model developed from SELENE SP data, which involves lunar surface reflectance and photometric properties. For evaluating the utilization of the model, we simulated a lunar observation conducted by ASTER of its three visible and infrared bands and confirmed the model describes lunar surface photometric properties correctly because correlation coefficients of observed and modeled radiance exceed 0.99 for all bands. Although absolute radiance shows some discrepancy between the observed and the simulated Moon in visible band, the model is, at least, useful to evaluate relative degradation of sensors.