Lunar Spectra Research Articles

Utilization of a meteorological satellite as a space telescope: the lunar mid-infrared spectrum as seen by Himawari-8

The Japanese meteorological satellite Himawari-8 has captured the Earth’s atmosphere and stars, planets, and the Moon in its field of view, enabling us to capture their spectroscopy with 16 bands from visible to mid-infrared wavelengths. The nine infrared bands in the Advanced Himawari Imager (AHI) onboard Himawari-8 are unique in spaceborne observations and are potentially useful for lunar science. In addition, infrared bands of AHI cover wavelengths similar to those of other interplanetary instruments and thus are useful for calibrations. However, infrared AHI data have not yet been investigated in planetary science. To confirm the utility of AHI for planetary science, we develop a procedure to retrieve the lunar infrared spectrum and compare it with thermal conduction simulations. Our analysis shows that lunar brightness temperature curves can be obtained in the morning, evening, and nighttime for all nine bands. Particularly at 8.5 μm, they show a good agreement with previous observations by the Diviner radiometer onboard NASA’s Lunar Reconnaissance Orbiter. As pointed out previously, the brightness temperatures differ between the bands, indicating temperature mixing within a pixel. Our simulation suggests that surface roughness as steep as those measured at the Apollo landing sites significantly contributes to the observed brightness temperature differences in the morning and evening; however, nighttime brightness temperatures are greatly affected by rocks with higher thermal inertia than the regolith. The rock abundances are estimated to be 0.18–0.48% and 6.1–10.3% at the equator and within Tycho crater, respectively. Our estimations from AHI data are consistent with those of Diviner. These results support the idea that AHI potentially serves as a space telescope for future lunar and planetary sciences, for example, for constraining water content on the lunar surface.Graphical

Two Simulated Spectral Databases of Lunar Regolith: Method, Validation, and Application

Our simulated lunar regolith spectra database, based on the Hapke AMSA radiative transfer model (RTM), is a large supplement to the limited number of lunar spectra data. By analyzing the multiple solutions and applicable scopes of the Hapke model by means of Newton interpolation and the least square optimization method, an improved method was found for the simulation of spectra, but it remained challenging to use to invert mineral abundance. Then, we simulated the spectra, mineral abundance, particle size and maturity of 57 mare and highland samples of the Lunar Soil Characterization Consortium (LSCC) in size groups of 10 µm, 10–20 µm and 20–45 µm. The simulated and measured spectra fit well with each other, with correlation coefficients greater than 0.99 and root mean square errors at a magnitude of 10-3. The parameters of mineral abundance, particle size and maturity are highly consistent with the measured values. Having confirmed the reliability of our simulation method, we analyzed the mechanism, reliability and applicability of the “spectral characteristic angle parameter” proposed by Lucey using the simulated spectral data of lunar regolith. Lucey’s method is only suitable for macro analysis of the entire moon, and the error is large when it is used for areas with high abundance of forsterite or ilmenite. In the spectral simulation of lunar regolith, olivine was subdivided into forsterite and fayalite, and the two end-members were mixed to approximately estimate the effect of the chemical composition of olivine on the spectra, which has been confirmed to be feasible.

Diurnal variability of stratospheric column NO<sub>2</sub> measured using direct solar and lunar spectra over Table Mountain, California (34.38° N)

Abstract. A full diurnal measurement of stratospheric column NO2 has been made over the Jet Propulsion Laboratory's Table Mountain Facility (TMF) located in the mountains above Los Angeles, California, USA (2.286 km above mean sea level, 34.38∘ N, 117.68∘ W). During a representative week in October 2018, a grating spectrometer measured the telluric NO2 absorptions in direct solar and lunar spectra. The stratospheric column NO2 is retrieved using a modified minimum-amount Langley extrapolation, which enables us to accurately treat the non-constant NO2 diurnal cycle abundance and the effects of tropospheric pollution near the measurement site. The measured 24 h cycle of stratospheric column NO2 on clean days agrees with a 1-D photochemical model calculation, including the monotonic changes during daytime and nighttime due to the exchange with the N2O5 reservoir and the abrupt changes at sunrise and sunset due to the activation or deactivation of the NO2 photodissociation. The observed daytime NO2 increasing rate is (1.34±0.24)×1014 cm−2 h−1. The observed NO2 in one of the afternoons during the measurement period was much higher than the model simulation, implying the influence of urban pollution from nearby counties. A 24 h back-trajectory analysis shows that the wind first came from inland in the northeast and reached southern Los Angeles before it turned northeast and finally arrived at TMF, allowing it to pick up pollutants from Riverside County, Orange County, and downtown Los Angeles.

Uncertainty Introduced by Darkening Agents in the Lunar Regolith: An Unmixing Perspective

On the Moon, in the near infrared wavelength range, spectral diagnostic features such as the 1-μm and 2-μm absorption bands can be used to estimate abundances of the constituent minerals. However, there are several factors that can darken the overall spectrum and dampen the absorption bands. Namely, (1) space weathering, (2) grain size, (3) porosity, and (4) mineral darkening agents such as ilmenite have similar effects on the measured spectrum. This makes spectral unmixing on the Moon a particularly challenging task. Here, we try to model the influence of space weathering and mineral darkening agents and infer the uncertainties introduced by these factors using a Markov Chain Monte Carlo method. Laboratory and synthetic mixtures can successfully be characterized by this approach. We find that the abundance of ilmenite, plagioclase, clino-pyroxenes and olivine cannot be inferred accurately without additional knowledge for very mature spectra. The Bayesian approach to spectral unmixing enables us to include prior knowledge in the problem without imposing hard constraints. Other data sources, such as gamma-ray spectroscopy, can contribute valuable information about the elemental abundances. We here find that setting a prior on TiO2 and Al2O3 can mitigate many of the uncertainties, but large uncertainties still remain for dark mature lunar spectra. This illustrates that spectral unmixing on the Moon is an ill posed problem and that probabilistic methods are important tools that provide information about the uncertainties, that, in turn, help to interpret the results and their reliability.

Development of a nighttime shortwave radiative transfer model for remote sensing of nocturnal aerosols and fires from VIIRS

The launch of the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Sumo-NPP satellite in 2011 ushered in a new era of using visible light and shortwave radiation at night to characterize aerosol and fire distributions from space. In order to exploit the full range of unprecedented observational capabilities of VIIRS, we have developed a nighttime shortwave radiative transfer model capability in the UNified and Linearized Radiative Transfer Model (UNL-VRTM). This capability is based on the use of additional source functions to treat illumination from the Moon, from fires, and from artificial lights. We have applied this model to address fundamental questions associated with the VIIRS sensing of aerosol and fire at night. Detailed description of model developments and validation (either directly with surface measurements of lunar spectra or indirectly through cross validation) are presented. Our analysis reveals that: (a) when convolution with the broad-range (500–900 nm) relative spectral response (RSR) function of the VIIRS Day-Night Band (DNB) is omitted, AOD retrieval from the DNB have uncertainties up to a factor of two in conditions with low or moderate AOD (<0.5 in mid-visible); (b) using a wavelength independent spectrum for the surface illumination source can lead to an AOD bias of −10% over surfaces illuminated by light-emitting diodes and fluorescent lamps, and −30% illuminated by high-pressure sodium lamps; and (c) a DNB-equivalent narrow band for AOD retrieval over the surfaces illuminated by the three types of bulbs studied in this paper is found to be centered at 585 nm at which the look-up table can be generated for AOD retrieval from DNB. Furthermore, while uncertainty in AOD retrievals from the DNB decreases as AOD increases, fire characterization can be affected by AOD; for a smoke-scenario AOD of 2.0, the DNB and SWIR (1.6 μm) radiances can be reduced by 50% depending on the fire area fraction and temperature within VIIRS pixel. DNB is overall more sensitive to smaller and cooler fires than SWIR and can be used to retrieve AOD over bright surfaces. Finally, three-dimensional (3D) radiative transfer effects and the non-collimated nature of most artificial light sources are neglected in this 1D radiative transfer (plane-parallel) model, resulting in possibly large uncertainties (e.g., the inability to reproduce side-illumination of clouds by city lights) that should be studied in future.

OSIRIS‐REx Visible and Near‐Infrared Observations of the Moon

AbstractThe Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer (OSIRIS‐REx) mission observed the Moon during the spacecraft's Earth gravity assist in 2017. From the spacecraft view, the lunar phase was 42°, and the in‐view hemisphere was dominated by anorthositic highlands terrain. Lunar spectra obtained by the OSIRIS‐REx Visible and InfraRed Spectrometer show evidence of several candidate absorption features. We observe the 2.8‐μm hydration band, confirming the spectral results from other missions, but detected in full‐disk spectra. We also tentatively identify weak spectral features near 0.9 and 1.3 μm, consistent with lunar regolith containing a mixture of plagioclase and orthopyroxene minerals, as expected for highlands terrain.

Illumination of the Earth’s ionosphere by the vacuum ultraviolet radiation reflected by the moon

The spectral curve of the flux density of the radiation, which is reflected by the full moon towards the Earth’s ionosphere within a wavelength range of 200–1700 A, has been presented. This curve is obtained by the approximation of space experiment data available in the scientific literature on the lunar spectral albedo and solar spectrum. Estimates of maximum values of spectral densities of fluxes reflected by the full moon in the neighborhood of wavelengths of ionization of basic ionospheric particles (neutral atoms of H I, He I, N I, O I, and Ar I and ions of He II, N II, O II, Ar II, N III, O III, and Ar III, as well as molecules of H2, N2, and O2) are given.

Interference data correction methods for lunar observation with a large-aperture static imaging spectrometer.

The lunar spectrum has been used in radiometric calibration and sensor stability monitoring for spaceborne optical sensors. A ground-based large-aperture static image spectrometer (LASIS) can be used to acquire the lunar spectral image for lunar radiance model improvement when the moon orbits over its viewing field. The lunar orbiting behavior is not consistent with the desired scanning speed and direction of LASIS. To correctly extract interferograms from the obtained data, a translation correction method based on image correlation is proposed. This method registers the frames to a reference frame to reduce accumulative errors. Furthermore, we propose a circle-matching-based approach to achieve even higher accuracy during observation of the full moon. To demonstrate the effectiveness of our approaches, experiments are run on true lunar observation data. The results show that the proposed approaches outperform the state-of-the-art methods.

In situ optical measurements of Chang'E‐3 landing site in Mare Imbrium: 1. Mineral abundances inferred from spectral reflectance

AbstractThe visible and near‐infrared imaging spectrometer on board the Yutu Rover of Chinese Chang'E‐3 mission measured the lunar surface reflectance at a close distance (~1 m) and collected four spectra at four different sites. These in situ lunar spectra have revealed less mature features than that measured remotely by spaceborne sensors such as the Moon Mineralogy Mapper instrument on board the Chandrayaan‐1 mission and the Spectral Profiler on board the Kaguya over the same region. Mineral composition analysis using a spectral lookup table populated with a radiative transfer mixing model has shown that the regolith at the landing site contains high abundance of olivine. The mineral abundance results are consistent with that inferred from the compound measurement made by the on board alpha‐particle X‐ray spectrometer.

THE OLD, SUPER-METAL-RICH OPEN CLUSTER, NGC 6791—ELEMENTAL ABUNDANCES IN TURN-OFF STARS FROM KECK/HIRES SPECTRA

The study of star clusters has advanced our understanding of stellar evolution, Galactic chemical evolution and nucleosynthesis. Here we investigate the composition of turn-off stars in the intriguing open cluster, NGC 6791, which is old, but super-metal-rich with high-resolution (46,000) Keck/HIRES spectra. We find [Fe/H] = +0.30 +/-0.02 from measurements of some 40 unblended, unsaturated lines of both Fe I and Fe II in eight turn-off stars. Our O abundances come from the O I triplet near 7774 A and we do a differential analysis relative to the Sun from our Lunar spectrum also obtained with Keck/HIRES. The O results are corrected for small nLTE effects. We find consistent ratios of [O/Fe]n with a mean of $-$0.06 +/-0.02. This continues the trend of decreasing [O/Fe] with increasing [Fe/H] found in field stars that are also both old and metal-rich. The small range in our oxygen abundances is consistent with a single population of stars. Our results for the alpha elements [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] are near solar and compare well with those of the old, metal-rich field stars. The two Fe-peak elements, Cr and Ni, are consistent with Fe. These turn-off-star abundances provide benchmark abundances to investigate changes in the giants that might arise from nuclear-burning and dredge-up processes. Determinations of upper limits were found for Li by spectrum synthesis and these results support the prediction from standard theory that higher-metallicity stars deplete more Li. Probably no stars in NGC 6791 have retained their initial Li.

Optical maturity variation in lunar spectra as measured by Moon Mineralogy Mapper data

High spectral and spatial resolution data from the Moon Mineralogy Mapper (M3) instrument on Chandrayaan‐1 are used to investigate in detail changes in the optical properties of lunar materials accompanying space weathering. Three spectral parameters were developed and used to quantify spectral effects commonly thought to be associated with increasing optical maturity: an increase in spectral slope (“reddening”), a decrease in albedo (“darkening”), and loss of spectral contrast (decrease in absorption band depth). Small regions of study were defined that sample the ejecta deposits of small fresh craters that contain relatively crystalline (immature) material that grade into local background (mature) soils. Selected craters are small enough that they can be assumed to be of constant composition and thus are useful for evaluating trends in optical maturity. Color composites were also used to identify the most immature material in a region and show that maturity trends can also be identified using regional soil trends. The high resolution M3 data are well suited to quantifying the spectral changes that accompany space weathering and are able to capture subtle spectral variations in maturity trends. However, the spectral changes that occur as a function of maturity were observed to be dependent on local composition. Given the complexity of space weathering processes, this was not unexpected but poses challenges for absolute measures of optical maturity across diverse lunar terrains.

Resolved spectroscopy of Mercury in the near-IR with SpeX/IRTF

We present resolved near-infrared spectra of Mercury scanning 70% of the surface in latitude and longitude from three separate observations, allowing us to perform a compositional investigation of its surface. By scanning the surface we find that all spectra in our sample are remarkably similar suggesting overall compositional homogeneity. We do, however, observe a slope difference between the spectra. These slope changes are most likely due to differences in the emission angle over different parts of the surface. We confirm the presence of a 1.1 μm feature that had been previously detected (Warell, J. et al. [2006]. Icarus 180, 281–291) and attributed to Ca-rich clinopyroxene. Finally, we investigated Mercury’s surface composition by comparing its spectrum with ground-based lunar spectra, lunar soil spectra collected in the laboratory, and analysis with a simple linear mixing model using various minerals as end-members. The result of this compositional investigation reveals that Mercury’s surface composition is likely to be quite different from the Moon’s. While low-Ca iron-rich pyroxenes are main surface components on the Moon (abundance varying from ∼5% to ∼35%), their abundance on Mercury may not exceed 5%. We also find that a Ca-rich clinopyroxene (in the hedenbergite–diopside series) is likely to be a main component of Mercury’s surface whereas this mineral is almost absent on the Moon. Our analysis also suggests the possible presence of olivine. We find that Mercury’s slope is less red than that of the Moon, in agreement with results from MESSENGER (McClintock, W.E., and 12 colleagues [2008]. Science 321, 62–65), and composition rather than variation of space weathering is likely the cause of this difference.

A Hapke model implementation for compositional analysis of VNIR spectra of Mercury

An implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media is presented with special emphasis on the analysis of reflectance spectra of Mercury. The model allows intimate mixing of an arbitrary number of regolith components with varying modal abundances, modal chemistries and grain sizes, matured by microphase iron. Reflectance spectra of suites of silicates of varying grain sizes and chemistries are used to calculate the imaginary coefficient of the complex index of refraction as a function of chemistry, thus limiting the modeling effects of chemically atypical laboratory samples, and allowing controlled modeling of minerals with varying chemical compositions. The performance of the model in the visual to near-infrared wavelength range is evaluated for a range of chemically characterized silicate mixtures of terrestrial powders, meteorite powders, matured lunar return samples, and remotely sensed lunar spectra.

The influence of long tides on ecosystem dynamics in the Barents Sea

Abstract The Barents Sea ecosystem has been associated with large biomass fluctuations. If there is a hidden deterministic process behind the Barents Sea ecosystem, we may forecast the biomass in order to control it. This presentation concludes, for the first time, investigations of a long data series from North Atlantic water and the Barents Sea ecosystem. The analysis is based on a wavelet spectrum analysis from the data series of annual mean Atlantic sea level, North Atlantic water temperature, the Kola section water temperature, and species from the Barents Sea ecosystem. The investigation has identified dominant fluctuations correlated with the 9.3-yr phase tide, the 18.6-yr amplitude tide, and a 74-yr superharmonic cycle in the North Atlantic water, Barents Sea water, and Arctic data series. The correlation between the tidal cycles and dominant Barents Sea ecosystem cycles is estimated to be R=0.6 or better. The long-term mean fluctuations correlate with the 74-yr superharmonic cycle. The wavelets analysis shows that the long-term 74-yr cycle may introduce a phase reversal in the identified 18-yr periods of temperature and salinity. The present analysis suggests that forced vertical and horizontal nodal tides influence the ocean's thermohaline circulation, and that they behave as a coupled non-linear oscillation system. The Barents Sea ecosystem analysis shows that the biomass life cycle and the long-term fluctuations correlate better than R=0.5 to the lunar nodal tide spectrum. Barents Sea capelin has a life cycle related to a third harmonic of the 9.3-yr tide. The life cycles of shrimp, cod, herring, and haddock are related to a third harmonic of the 18.6-yr tide. Biomass growth was synchronized to the lunar nodal tide. The biomass growth of zooplankton and shrimp correlates with the current aspect of lunar nodal tidal inflow to the Barents Sea. The long-term biomass fluctuation of cod and herring is correlated with a cycle period of about 3×18.6=55.8 yr. This analysis suggests that we may understand the Barents Sea ecosystem dynamic as a free-coupled oscillating system to the forced lunar nodal tides. This free-coupled oscillating system has a resonance related to the oscillating long tides and the third harmonic and superharmonic cycles.

Numerical estimation of lunar X-ray emission for X-ray spectrometer onboard SELENE

Abstract We conducted a numerical estimation of lunar X-ray spectra, which is applicable for lunar X-ray fluorescence observations using an X-ray spectrometer (XRS) onboard the SELENE orbiter, with an improved simulation model. We investigated the integration times of measurements for six elements (Mg, Al, Si, Ca, Ti, and Fe) to achieve signal-to-background ratio of over 10 under various solar conditions. The results of these calculations indicate that expected along-the-track spatial resolutions of a single orbital path for Mg, Al and Si will be &lt;90 km and 20 km under normal and active Sun conditions, respectively. Ca, Ti and Fe will be also detectable with a spatial resolution of 20 km during the periods active solar flares over M1 class happen to occur.

The phase dependence of brightness and color of the lunar surface: a study based on integral photometric data

A procedure of an a posteriori correction of the available data on the integral photometry of the Moon is described. This procedure reduces the regular errors of the integral phase curves caused by variations of the libration parameters; the effect due to libration can reach 4%. A method allowing the integral measurements of the Moon to be compared correctly with the photometric measurements of the lunar areas or laboratory samples imitating the lunar soil has been developed. To approximate the phase curves of integral albedo in the phase-angle range from 6° to 120°, we proposed a simple empirical formula A eq(α) = m l e −ρα + m 2 e −0.7α, where α is the phase angle, ρ is the factor of effective roughness, and m 1 + m 2 is the surface albedo at a zero phase angle. An empirical phase dependence of the slope of the lunar spectrum in the 360–1060 nm range has been obtained. The results may be used to test various theoretical models of the light scattering by the lunar surface and to calibrate the data of ground-based and space-borne spectrophotometric observations.

Nitric acid measurements at Eureka obtained in winter 2001–2002 using solar and lunar Fourier transform infrared absorption spectroscopy: Comparisons with observations at Thule and Kiruna and with results from three‐dimensional models

For the first time, vertical column measurements of nitric acid (HNO3) above Eureka (80.1°N, 86.4°W), Canada, have been made during polar night using lunar spectra recorded with a Fourier transform infrared (FTIR) spectrometer, from October 2001 to March 2002. This site is part of the primary Arctic station of the Network for the Detection of Stratospheric Change. These measurements were compared with FTIR measurements at two other Arctic sites: Thule, Greenland (76.5°N, 68.8°W), and Kiruna, Sweden (67.8°N, 20.4°E). Eureka lunar measurements are in good agreement with solar ones made with the same instrument. Eureka and Thule HNO3 columns are consistent within measurement error. Differences between HNO3 columns at Kiruna and those at Eureka and Thule can be explained on the basis of available sunlight hours and location of the polar vortex. The measurements were also compared with results from a chemistry‐climate model, the Canadian Middle Atmosphere Model (CMAM), and from a three‐dimensional chemical transport model, SLIMCAT. This is the first time that CMAM HNO3 columns have been compared with observations in the Arctic. The comparison of CMAM HNO3 columns with Eureka and Kiruna data shows good agreement. The warm 2001–2002 winter with almost no polar stratospheric clouds makes the comparison with this version of CMAM, which has a known warm bias, a good test for CMAM under these conditions. SLIMCAT captures the magnitude of HNO3 columns at Eureka, and the day‐to‐day variability, but generally reports higher values than were measured at Thule and Kiruna.

Using the modified Gaussian model to extract quantitative data from lunar soils

The Lunar Soil Characterization Consortium (LSCC) has examined and characterized a suite of lunar soils with a wide range of compositions and maturities. The purpose of this study is to compare the Vis/NIR spectral properties of these lunar soils with their petrologic and chemical compositions using the modified Gaussian model (MGM) to obtain quantitative data about the character of relatively weak near‐infrared absorption bands. Useful compositional information can be extracted from high‐quality soil spectra using the MGM. The model had some difficulty fitting absorption bands in the 2 μm region of the lunar spectrum, but bands in the 1 and 1.2 μm regions provided physically realistic results. The model was able to distinguish high‐Ca and low‐Ca pyroxenes in the LSCC suite of lunar soils in the appropriate relative abundance. In addition, unexpected insights into the nature and causes of absorption bands in lunar soils were identified. For example, at least two distinct absorption bands are required in the 1.2 μm region of the spectrum, and neither of these bands can be attributed to plagioclase or agglutinates, but are found instead to be largely due to pyroxene.

Application of a radiometric calibration method to lunar Fourier transform IR spectra by using a liquid-nitrogen-cooled high-emissivity blackbody

Since winter 1994/1995 the Moon has been used in addition to the Sun as an IR source of radiation to measure atmospheric absorption spectra with a Bruker IR Fourier transform spectrometer IFS 120M located near Kiruna, Sweden. A two-point radiometric calibration method with blackbody references was applied to lunar spectra in the long-wave detector channel to improve the accuracy of evaluation of the column amounts of different atmospheric trace gases. A new liquid-nitrogen-cooled high-emissivity blackbody without an entrance window is described that is used for this calibration method.

Determination of the isotopic abundances of heavy O3 as observed in Arctic ground‐based FTIR‐spectra

Ground‐based FTIR‐measurements have been performed at the NDSC (Network for Detection of Stratospheric Change) station on Spitsbergen (79°N, 12°E). We have investigated several well isolated absorption lines of isotopically heavy O3 in the 10 µm region that allowed us to quantify their natural abundances in the arctic atmosphere for the period April 1994 to August 1995. On the average, our solar spectra show an excess of 15.4±0.9% in 16O16O18O and 11.2±1.4% in 16O18O16O, compared to the natural isotopic ratio in atmospheric oxygen. The lunar spectra recorded during polar night give slightly lower results. The margins of error do not include the isotopic uncertainties of the spectroscopic database used (HITRAN), which might cause a shift by several percent enrichment.