Visible And Near-infrared Imaging Spectrometer Research Articles

Analysis of a large buried impact crater and vertical mineral composition at the Chang'E-4 landing site by multi-source remote sensing data

Exploring the concealed subsurface structures and materials beneath the lunar surface can reveal significant insights into geological history. This study offers a comprehensive analysis of the stratigraphic interpretation and subsurface material composition at the Chang'E-4 landing site, integrating both in-situ and orbital radar with multispectral datasets. We report the identification of a subsurface structure, which resembles a buried impact crater (∼420 m in diameter) under the Yutu-2 rover's path. This crater could degrade over a period of 0.42 to 0.53 Ga, with an initial diameter of 293 to 323 m and an initial depth of 45.9 to 51.4 m. Surface material above the buried crater, evaluated by the in-situ visible and near-infrared imaging spectrometer (VNIS) detector, shows a higher abundance of clinopyroxene compared to surrounding areas, where a near-equal mix of clinopyroxene and orthopyroxene is observed. Assessment of crater diameters in proximity to the Chang'E-4 landing site, along with the mineral compositions at their epicenters, reveals a decrease in the abundance of clinopyroxene and plagioclase with depth. Conversely, the quantities of orthopyroxene and olivine increase, implying that orthopyroxene-rich Finsen ejecta significantly influenced the Chang'E-4 landing site's geological composition. Two potential stratigraphic boundary depths are identified at 13.5 and 22 m, based on pronounced variations in mineral abundance, offering fresh insights into subsurface delineation beyond radar data. Considering the VNIS and vertical mineral composition, we propose the buried crater's formation resulted from Finsen crater's ejecta. Also, we identify eight potential historical impacts by comparing subsurface relief variations with mineral composition ratios between clinopyroxene and orthopyroxene. The integration of subsurface structure, along with surface and subsurface mineral composition, enables a more robust stratigraphic interpretation, facilitates shallow material source analysis, and allows for historical impact tracing.

Lunar Terrestrial Analog Experiment on the Spectral Interpretations of Rocks Observed by the Yutu-2 Rover

A visible and near-infrared imaging spectrometer (VNIS) loaded by the Chang’e-4 rover is the primary method for detecting the mineral composition of the lunar surface in the landing region. However, different data processing methods yield inconsistent mineral modes in measured lunar soil and rocks. To better constrain the mineral modes of the soil and rocks measured by Chang’e-4 VNIS, a noritic-gabbroic rock with a mineral composition similar to that of the lunar highland rocks is measured by scanning electron microscopy (SEM), the spare flight model of Chang’e-4 VNIS and TerraSpec-4 of ASD. Backscattered electron and energy dispersive spectrometry show that olivine, pyroxene, and plagioclase modal mineral abundances are 12.9, 35.0, and 52.2%, respectively. The estimated results of the spectrum by the Hapke radiative transfer model are 7.5, 39.3, and 53.2% for olivine, pyroxene, and plagioclase, respectively, which is consistent with to those of SEM mapping within error. In contrast, the estimated results of the modified Gaussian model are 29 and 71% for olivine and pyroxene, respectively, indicating the absence of plagioclase. Based on our implemented Hapke model, we decode the data of the two rocks detected by the rover on the 3rd and 26th lunar days of mission operations. The obtained results suggest that both rocks are norite or gabbro with noticeable differences. The first rock, with more olivine and pyroxene, may have been excavated from the Finsen crater. The second rock, with more plagioclase, may have been ejected from the southwestern edge of the Von Kármán crater, indicating the initial lunar crust.

Mafic mineralogy assemblages at the Chang’e-4 landing site: A combined laboratory and lunar in situ spectroscopic study

Context.Chang’e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site.Aims.Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site.Methods.VNIR spectra of nine lunar analogs – prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase – were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer.Results.The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo60−79), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980–1300 °C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.

Dusty Mafic Rocks Along the Path of Chang'E‐4 Rover: Initial Analysis of the Image Cubes of the Onboard Visible and Near‐Infrared Imaging Spectrometer

AbstractThe Yutu‐2 rover of Chang'E‐4 encountered four special rocks along its path. The onboard Visible and Near‐infrared Imaging Spectrometer (VNIS) collected their spectral data at close range. In this work, we focus on the visible/near‐infrared image cubes captured by the VNIS. Simple classification was performed using the K‐means algorithm based on the 750/900 nm spectral ratios to highlight the surface material variations. In the case of narrow measurements of phase angle, the classification results were not significantly affected by viewing geometry. Varying degrees of dust coverage on the rock surfaces were revealed, which may be attributed to ejecta deposition and dust electrostatic levitation. The spectral shapes of the less dust‐covered surfaces indicated that these rocks are dominated by mafic materials, suggesting they may not originate from the lunar upper crust. The influence of dust coverage on spectral measurement of rocks was also confirmed, which significantly attenuates the absorption depths.

In-Flight Calibration of Visible and Near-Infrared Imaging Spectrometer (VNIS) Onboard Chang’E-4 Unmanned Lunar Rover

The Visible and Near-infrared Imaging Spectrometer (VNIS) is one of the scientific payloads onboard the Yutu-2 lunar rover of the Chang’E-4 mission. Based on Acousto-Optic Tunable Filter (AOTF) dispersion, a moving platform and a calibration unit, the VNIS was designed for lunar surface observations of the far side of the Moon for composition exploration and spectral experiments in the wavelength range of 0.45–2.4μm. Large differences in radiance caused by the discrepancy between the laboratory and lunar environment could affect the spectral shape and features. Therefore, in-flight calibration of the VNIS is critical for the interpretation of the acquired spectra. To precisely map the digital output to the spectral radiance, a calibration unit was installed on the VNIS to facilitate radiometric calibration measurements on the lunar surface. After the elimination of stray light and the shadow effect, the measured radiance of the calibration unit was derived by digital multiplication of the pre-flight coefficients. Considering the observation geometry, Moon-Sun distance, and the bi-directional reflectance factor (BRF) of the calibration panel, the theoretical radiance at the entrance pupil of VNIS was calculated. Using linear regression, in-flight calibration of the coefficients was performed and used to correct the spectral data. The performance of the spectrometer was evaluated using 12 lunar-day data. It was determined that the coefficients gradually converged and the variation was less than 1%, indicating that the in-flight calibration was robust. The proposed in-flight calibration method facilitated the acquisition of reliable data for lunar composition interpretation.

Mineralogical and chemical properties inversed from 21-lunar-day VNIS observations taken during the Chang\u2019E-4 mission

We report on the mineralogical and chemical properties of materials investigated by the lunar rover Yutu-2, which landed on the Von Kármán crater in the pre-Nectarian South Pole–Aitken (SPA) basin. Yutu-2 carried several scientific payloads, including the Visible and Near-infrared Imaging Spectrometer (VNIS), which is used for mineral identification, offering insights into lunar evolution. We used 86 valid VNIS data for 21 lunar days, with mineral abundance obtained using the Hapke radiative transfer model and sparse unmixing algorithm and chemical compositions empirically estimated. The mineralogical properties of the materials at the Chang’E-4 (CE-4) site referred to as norite/gabbro, based on findings of mineral abundance, indicate that they may be SPA impact melt components excavated by a surrounding impact crater. We find that CE-4 materials are dominated by plagioclase and pyroxene and feature little olivine, with 50 of 86 observations showing higher LCP than HCP in pyroxene. In view of the effects of space weathering, olivine content may be underestimated, with FeO and TiO2 content estimated using the maturity-corrected method. Estimates of chemical content are 7.42–18.82 wt% FeO and 1.48–2.1 wt% TiO2, with a low-medium Mg number (Mg # ~ 55). Olivine-rich materials are not present at the CE-4 landing site, based on the low-medium Mg #. Multi-origin materials at the CE-4 landing site were analyzed with regard to concentrations of FeO and TiO2 content, supporting our conclusion that the materials at CE-4 do not have a single source but rather are likely a mixture of SPA impact melt components excavated by surrounding impact crater and volcanic product ejecta.

Effect of Lunar Complex Illumination on In Situ Measurements Obtained Using Visible and Near-Infrared Imaging Spectrometer of Chang’E-4

In-situ measurements of the spectral information on the lunar surface are of significance to study the geological evolution of the Moon. China’s Chang’E-4 (CE-4) Yutu-2 rover has conducted several in-situ spectral explorations on the Moon. The visible and near-infrared imaging spectrometer (VNIS) onboard the rover has acquired a series of in-situ spectra of the regolith at the landing site. In general, the mineralogical research of the lunar surface relies on the accuracy of the in-situ data. However, the spectral measurements of the Yutu-2 rover may be affected by shadows and stray illumination. In this study, we analyzed 106 CE-4 VNIS spectra acquired in the first 24 lunar days of the mission and noted that six of these spectra were affected by the shadows of the rover. Therefore, a method was established to correct the effects of the rover shadow on the spectral measurements. After shadow correction, the FeO content in the affected area is corrected to 14.46 wt.%, which was similar to the result calculated in the normal regolith. Furthermore, according to the visible images, certain areas of the explored sites were noted to be unusually bright. Considering the reflectance, geometric information, and shining patterns of the multi-layer insulation (MLI), we examined the influence of the specular reflection of the MLI on the bright spot regionsd, and found that the five sets of data were likely not affected by the specular reflection of the MLI. The results indicated that the complex illumination considerably influences the in situ spectral data. This study can provide a basis to analyze the VNIS scientific data and help enhance the accuracy of interpretation of the composition at CE-4 landing sites.

Overview of the Chang’e-4 Mission: Opening the Frontier of Scientific Exploration of the Lunar Far Side

China’s Chang’e-4 (CE-4) mission is the first human lander/rover mission on the far side of the Moon. Its probe is composed of a lander, rover, and the Queqiao relay satellite. Queqiao was successfully launched on May 21, 2018, and entered the halo orbit of the L2 point on June 14, becoming the first satellite connecting the Earth and the Moon’s far side. The lander carrying Yutu-2 was successfully launched on December 8, 2018, and landed in the Von Karman crater (45.5° S, 177.6° E) at 10:26 (UTC+8) on January 3, 2019. The CE-4 probe carried nine science instruments. Four instruments are on the lander: a landing camera (LCAM), a terrain camera (TCAM), a low-frequency radio spectrometer (LFRS), and a lunar lander neutrons and dosimetry (LND) provided by Germany. Four instruments are on the rover: a panoramic camera (PCAM), a visible and near-infrared imaging spectrometer (VNIS), a lunar penetrating radar (LPR), and an advanced small analyzer for neutrals (ASAN) provided by Sweden. The instrument on the relay satellite is the Netherlands-China Low-Frequency Explorer (NCLE). The scientific objectives of the CE-4 mission include (1) performing low-frequency radio-astronomical observations; (2) investigating the geomorphology, mineral compositions and shallow subsurface structure of the landing and roving sites; and (3) detecting the Earth-Moon space environment at the lunar far side. As of February 1, 2020, CE-4 has completed 14 lunar days of scientific exploration after one year of operation. The components, fight, scientific objectives and investigation of CE-4 are introduced in this paper. We also describe the accessibility of the initial archived science data and their preliminary analysis results.

Photometric Normalization of Chang’e-4 Visible and Near-Infrared Imaging Spectrometer Datasets: A Combined Study of In-Situ and Laboratory Spectral Measurements

Until 29 May 2020, the Visible and Near-Infrared Imaging Spectrometer (VNIS) onboard the Yutu-2 Rover of the Chang’e-4 (CE-4) has acquired 96 high-resolution surface in-situ imaging spectra. These spectra were acquired under different illumination conditions, thus photometric normalization should be conducted to correct the introduced albedo differences before deriving the quantitative mineralogy for accurate geologic interpretations. In this study, a Lommel–Seeliger (LS) model and Hapke radiative transfer (Hapke) model were used and empirical phase functions of the LS model were derived. The values of these derived phase functions exhibit declining trends with the increase in phase angles and the opposition effect and phase reddening effect were observed. Then, we discovered from in-situ and laboratory measurements that the shadows caused by surface roughness have significant impacts on reflectance spectra and proper corrections were introduced. The validations of different phase functions showed that the maximum discrepancy at 1500 nm of spectra corrected by the LS model was less (~3.7%) than that by the Hapke model (~7.4%). This is the first time that empirical phase functions have been derived for a wavelength from 450 to 2395 nm using in-situ visible and near-infrared spectral datasets. Generally, photometrically normalized spectra exhibit smaller spectral slopes, lower FeO contents and larger optical maturity parameter (OMAT) than spectra without correction. In addition, the band centers of the 1 and 2 μm absorption features of spectra after photometric normalization exhibit a more concentrated distribution, indicating the compositional homogeneity of soils at the CE-4 landing site.

IMPLEMENTATION STRATEGY OF VISIBLE AND NEAR-INFRARED IMAGING SPECTROMETER ON YUTU-2 ROVER BASED ON VISION MEASUREMENT TECHNOLOGY

Abstract. The Chang'e-4 successfully landed on the far side of the moon in January 2019. By the 12th lunar day, its Yutu-2 rover had achieved a breakthrough travel distance of greater than 300 m. A visible and near-infrared imaging spectrometer (VNIS), consisting of a visible and near-infrared (VNIR) imaging spectrometer and a shortwave infrared (SWIR) spectrometer was used for detecting mineralogical compositions of lunar-surface materials. Because VNIS is fixed on the front of the rover, and the field-of-view (FOV) of VNIR and SWIR are small (8.5° and 3.6° respectively), approaching and accurately pointing at the specific science target depend completely on the precise control of the moving rover.In this paper, a successful method of VNIS target detection based on vision measurement is proposed. First, the accurate position of the target is calculated via navigation camera imaging. Then, the moving path is planned by considering the terrain environment, illumination, communication condition, and other constraints. After the rover moves to the designed position, the binocular imaging of the hazard-avoidance cameras are activated, the detection direction and forward distance are calculated according to the images, and the FOV trajectory of the VINS is predicted while moving. Finally, by choosing the required moving control parameters, the imaging field of the VINS accurately cover the detected targets visually.These methods have been verified many times, and the results show that they are effective and feasible. The research results based on the VNIS data have successfully revealed the material composition on the far side of the moon and have deepened human understanding of its formation and evolution.

Estimation of Noise in the In Situ Hyperspectral Data Acquired by Chang’E-4 and Its Effects on Spectral Analysis of Regolith

The Chang’E-4 (CE-4) spacecraft landed successfully on the far side of the Moon on 3 January 2019, and the rover Yutu-2 has explored the lunar surface since then. The visible and near-infrared imaging spectrometer (VNIS) onboard the rover has acquired numerous spectra, providing unprecedented insight into the composition of the lunar surface. However, the noise in these spectral data and its effects on spectral interpretation are not yet assessed. Here we analyzed repeated measurements over the same area at the lunar surface to estimate the signal–noise ratio (SNR) of the VNIS spectra. Using the results, we assessed the effects of noise on the estimation of band centers, band depths, FeO content, optical maturity (OMAT), mineral abundances, and submicroscopic metallic iron (SMFe). The data observed at solar altitudes <20° exhibit low SNR (25 dB), whereas the data acquired at 20°–35° exhibit higher SNR (35–37 dB). We found differences in band centers due to noise to be ~6.2 and up to 28.6 nm for 1 and 2 μm absorption, respectively. We also found that mineral abundances derived using the Hapke model are affected by noise, with maximum standard deviations of 6.3%, 2.4%, and 7.0% for plagioclase, pyroxene, and olivine, respectively. Our results suggest that noise has significant impacts on the CE-4 spectra, which should be considered in the spectral analysis and geologic interpretation of lunar exploration data.

Mineralogy of Chang’e-4 landing site: preliminary results of visible and near-infrared imaging spectrometer

The exploration of mafic anomaly in South Pole-Aitken (SPA, the largest confirmed) basin on the Moon provides important insights into lunar interior. The landing of Chang’e-4 (CE-4) and deployment of Yutu-2 rover on the discontinuous ejecta from Finsen crater enabled in-situ measurements of the unusual mineralogy in the central portion of SPA basin with visible and near-infrared imaging spectrometer (VNIS). Here we present detailed processing procedures based on the level 2B data of CE-4 VNIS and preliminary mineralogical results at the exploration area of Yutu-2 rover. A systematic processing pipeline is developed to derive credible reflectance spectra, based on which several spectral and mineral indices are calculated to constrain the mafic mineralogy. The mafic components in the soils and boulder around CE-4 landing site are concluded as clinopyroxene-bearing with intermediate composition and probably dominated by pigeonite although the possibility of mixing orthopyroxen (OPX) and calcic clinopyroxene (CPX) also exists. These mineralogical results are more consistent with a petrogenesis that the CE-4 regolith and rock fragment are derived from rapid-cooling magmatic systems and we interpret that the materials at the CE-4 landing site ejected from Finsen crater are probably recrystallized from impact melt settings.

In Situ Photometric Experiment of Lunar Regolith With Visible and Near‐Infrared Imaging Spectrometer On Board the Yutu‐2 Lunar Rover

AbstractChina's Chang'E‐4 (CE‐4) mission successfully landed in Von Kármán crater within South Pole‐Aitken basin of the Moon. The Visible and Near‐Infrared Imaging Spectrometer (VNIS) on board Yutu‐2 rover investigated the photometric properties of lunar regolith. Seven VNIS measurements were conducted on a small lunar surface with a diameter <5 m by the rover rotating at the center, with the phase angles from 39.6 to 97.1° obtained in the similar observational geometry of solar altitude and observation angle. The phase function, which varies in different wavelength, is derived using a third‐order polynomial fitting, in combination with the calibration and comparison of orbital/in situ VNIS data at the Chang'E‐4 landing site and the same regions. After the photometrical correction of the spectra with the phase function, the derived FeO contents and optical maturity parameters of the regolith reduce much of their deviations, which is consistent with the homogeneity of the regolith and hence demonstrates the significance of the photometric correction on the VNIS spectra.

The subsurface structure and stratigraphy of the Chang’E-4 landing site: orbital evidence from small craters on the Von Kármán crater floor

Chang’E-4 (CE-4) successfully landed on the floor of the Von Kármán crater within the South Pole-Aitken basin (SPA). One of its scientific objectives is to determine the subsurface structure and the thickness of lunar regolith at the landing site and along the traverse route of the Yutu-2 rover. Using orbital data, we employed small craters (diameters <1 km) on the floor of the Von Kármán crater as probes to investigate the subsurface structure and stratigraphy of the CE-4 landing site. In this study, 40 dark-haloed craters that penetrate through the surface Finsen ejecta and excavate underlying mare deposits were identified, and 77 bright ray craters that expose only the underlying fresh materials but do not penetrate through the surface Finsen ejecta were found. The excavation depths of these craters and their distances from the Finsen crater center were calculated, and the thickness distribution of Finsen ejecta on the Von Kármán floor was systematically investigated. The boundary between Finsen ejecta and underlying mare basalt at the CE-4 landing site is constrained to a depth of 18m.We have proposed the stratigraphy for the CE-4 site and interpreted the origins of different layers and the geological history of the Von Kármán crater. These results provide valuable geological background for interpreting data from the Lunar Penetrating Radar (LPR) and Visible and Near-infrared Imaging Spectrometer (VNIS) on the Yutu-2 rover. The CE-4 landing site could provide a reference point for crater ejecta distribution and mixing with local materials, to test and improve ejecta thickness models according to the in situ measurements of the CE-4 LPR.

Correlated compositional and mineralogical investigations at the Chang′e-3 landing site

The chemical compositions of relatively young mare lava flows have implications for the late volcanism on the Moon. Here we report the composition of soil along the rim of a 450-m diameter fresh crater at the Chang′e-3 (CE-3) landing site, investigated by the Yutu rover with in situ APXS (Active Particle-induced X-ray Spectrometer) and VNIS (Visible and Near-infrared Imaging Spectrometer) measurements. Results indicate that this region's composition differs from other mare sample-return sites and is a new type of mare basalt not previously sampled, but consistent with remote sensing. The CE-3 regolith derived from olivine-normative basaltic rocks with high FeO/(FeO+MgO). Deconvolution of the VNIS data indicates abundant high-Ca ferropyroxene (augite and pigeonite) plus Fe-rich olivine. We infer from the regolith composition that the basaltic source rocks formed during late-stage magma-ocean differentiation when dense ferropyroxene-ilmenite cumulates sank and mixed with deeper, relatively ferroan olivine and orthopyroxene in a hybridized mantle source.

Reflectance calibration and shadow effect of VNIS spectra acquired by the Yutu rover

Yutu is the first lunar rover after the Apollo program and Luna missions. One of the payloads on the Yutu rover, the Visible and Near-infrared Imaging Spectrometer (VNIS), has acquired four VIS/NIR images and SWIR spectra near its landing site in Mare Imbrium. The radiance images were reduced through repairing bad lines and bad points, and applying flat field correction, and then were converted into reflectance values based on the solar irradiance and angles of incidence. A significant shadow effect was observed in the VIS/NIR image. The shadowed regions show lower reflectance with a darkening trend compared with illuminated regions. The reflectance increased by up to 24% for entire images and 17% for the VIS/NIR-SWIR overlapping regions after shadow correction. The correction for the shadow effect will remarkably decrease the estimate of FeO content, by up to 4.9 wt.% in this study. The derived FeO contents of CD-005∼008 after shadow correction are around 18.0 wt.%.

Operating principles and detection characteristics of the Visible and Near-Infrared Imaging Spectrometer in the Chang'e-3

The Visible and Near-Infrared Imaging Spectrometer (VNIS), using two acousto-optic tunable filters as dispersive components, consists of a VIS/NIR imaging spectrometer (0.45–0.95 μm), a shortwave IR spectrometer (0.9–2.4 μm) and a calibration unit with dust-proofing functionality. The VNIS was utilized to detect the spectrum of the lunar surface and achieve in-orbit calibration, which satisfied the requirements for scientific detection. Mounted at the front of the Yutu rover, lunar objects that are detected with the VNIS with a 45° visual angle to obtain spectra and geometrical data in order to analyze the mineral composition of the lunar surface. After landing successfully on the Moon, the VNIS performed several explorations and calibrations, and obtained several spectral images and spectral reflectance curves of the lunar soil in the region of Mare Imbrium. This paper describes the working principle and detection characteristics of the VNIS and provides a reference for data processing and scientific applications.

Data processing and preliminary results of the Chang'e-3 VIS/NIR Imaging Spectrometer in-situ analysis

The Chang'e-3 Visible and Near-infrared Imaging Spectrometer (VNIS) is one of the four payloads on the Yutu rover. After traversing the landing site during the first two lunar days, four different areas are detected, and Level 2A and 2B radiance data have been released to the scientific community. The released data have been processed by dark current subtraction, correction for the effect of temperature, radiometric calibration and geometric calibration. We emphasize approaches for reflectance analysis and mineral identification for in-situ analysis with VNIS. Then the preliminary spectral and mineralogical results from the landing site are derived. After comparing spectral data from VNIS with data collected by the M3 instrument and samples of mare that were returned from the Apollo program, all the reflectance data have been found to have similar absorption features near 1000 nm except lunar sample 71061. In addition, there is also a weak absorption feature between 1750∼2400 nm on VNIS, but the slopes of VNIS and M3 reflectance at longer wavelengths are lower than data taken from samples of lunar mare. Spectral parameters such as Band Centers and Integrated Band Depth Ratios are used to analyze mineralogical features. The results show that detection points E and N205 are mixtures of high-Ca pyroxene and olivine, and the composition of olivineat point N205 is higher than that at point E, but the compositions of detection points S3 and N203 are mainly olivine-rich. Since there are no obvious absorption features near 1250 nm, plagioclase is not directly identified at the landing site.

Effects of Spectralon absorption on reflectance spectra of typical planetary surface analog materials.

Acquiring accurate visible and near-infrared (VisNIR) reflectance values of atmosphereless celestial bodies is very important in inferring the physical and geological properties of their surficial materials. When a calibration target with inherent non-trivial absorption features is used, the calibrated reflectance would essentially always contain spurious spectral features and the spectroscopic data may easily be misinterpreted if the artifact is not properly taken care of. We demonstrate with laboratory reflectance measurements that the VisNIR spectra of three typical planetary surface analog materials, lunar simulant JSC-1A, olivine and pyroxene grains, have an artificial peak at 2.1 µm when Spectralon-type plaque made of polytetrafluoroethylene is used as the calibration target in the NIR region. The degree of severity of this artifact is dependent on the strength of the 2.0 µm absorption feature of the mineral. Empirical methods are proposed to remove this artifact to bring the spectra close to that calibrated by a gold mirror which does not have any conspicuous absorption features in the NIR region. The correction methods may be applied to reflectance data acquired by the VisNIR imaging spectrometer onboard the Yutu Rover of the Chinese Chang'E 3 lunar mission which employed an onboard Spectralon-type calibration target.

Visible and near-infrared imaging spectrometer and its preliminary results from the Chang'E 3 project.

In order to conduct lunar surface mineral composition studies and content analysis, the Visible and Near-infrared Imaging Spectrometer (VNIS), one of the scientific payloads of the Chang'E 3 Yutu rover, has been developed to detect lunar surface objects and to obtain their reflectance spectra and geometric images. This is achieved with a 45° visual angle and at a height of 0.69 m. VNIS is equipped with a lunar surface calibration function, and the spectral range is 0.45-2.40 μm with a spectral resolution of 2-12 nm. It is capable of synchronously acquiring the full spectrum of lunar surface objects and in situ calibration. Here, we describe the VNIS and explain the preliminary results of the lunar surface exploration and calibration, which provides valuable information for scientific data processing and applications.