Joseph W. Boardman

The Moon Mineralogy Mapper (M3) imaging spectrometer for lunar science: Instrument description, calibration, on‐orbit measurements, science data calibration and on‐orbit validation

[1]xa0The NASA Discovery Moon Mineralogy Mapper imaging spectrometer was selected to pursue a wide range of science objectives requiring measurement of composition at fine spatial scales over the full lunar surface. To pursue these objectives, a broad spectral range imaging spectrometer with high uniformity and high signal-to-noise ratio capable of measuring compositionally diagnostic spectral absorption features from a wide variety of known and possible lunar materials was required. For this purpose the Moon Mineralogy Mapper imaging spectrometer was designed and developed that measures the spectral range from 430 to 3000 nm with 10 nm spectral sampling through a 24 degree field of view with 0.7 milliradian spatial sampling. The instrument has a signal-to-noise ratio of greater than 400 for the specified equatorial reference radiance and greater than 100 for the polar reference radiance. The spectral cross-track uniformity is >90% and spectral instantaneous field-of-view uniformity is >90%. The Moon Mineralogy Mapper was launched on Chandrayaan-1 on the 22nd of October. On the 18th of November 2008 the Moon Mineralogy Mapper was turned on and collected a first light data set within 24 h. During this early checkout period and throughout the mission the spacecraft thermal environment and orbital parameters varied more than expected and placed operational and data quality constraints on the measurements. On the 29th of August 2009, spacecraft communication was lost. Over the course of the flight mission 1542 downlinked data sets were acquired that provide coverage of more than 95% of the lunar surface. An end-to-end science data calibration system was developed and all measurements have been passed through this system and delivered to the Planetary Data System (PDS.NASA.GOV). An extensive effort has been undertaken by the science team to validate the Moon Mineralogy Mapper science measurements in the context of the mission objectives. A focused spectral, radiometric, spatial, and uniformity validation effort has been pursued with selected data sets including an Earth-view data set. With this effort an initial validation of the on-orbit performance of the imaging spectrometer has been achieved, including validation of the cross-track spectral uniformity and spectral instantaneous field of view uniformity. The Moon Mineralogy Mapper is the first imaging spectrometer to measure a data set of this kind at the Moon. These calibrated science measurements are being used to address the full set of science goals and objectives for this mission.

Thermal removal from near-infrared imaging spectroscopy data of the Moon

[1]xa0In the near-infrared from about 2 μm to beyond 3 μm, the light from the Moon is a combination of reflected sunlight and emitted thermal emission. There are multiple complexities in separating the two signals, including knowledge of the local solar incidence angle due to topography, phase angle dependencies, emissivity, and instrument calibration. Thermal emission adds to apparent reflectance, and because the emissions contribution increases over the reflected sunlight with increasing wavelength, absorption bands in the lunar reflectance spectra can be modified. In particular, the shape of the 2 μm pyroxene band can be distorted by thermal emission, changing spectrally determined pyroxene composition and abundance. Because of the thermal emission contribution, water and hydroxyl absorptions are reduced in strength, lowering apparent abundances. It is important to quantify and remove the thermal emission for these reasons. We developed a method for deriving the temperature and emissivity from spectra of the lunar surface and removing the thermal emission in the near infrared. The method is fast enough that it can be applied to imaging spectroscopy data on the Moon.

The mineralogy of late stage lunar volcanism as observed by the Moon Mineralogy Mapper on Chandrayaan‐1

[1]xa0The last major phases of lunar volcanism produced spectrally unique high-titanium basalts on the western nearside of the Moon. The Moon Mineralogy Mapper (M3) on Chandrayaan-1 has provided detailed measurements of these basalts at spatial and spectral resolutions necessary for mineralogical interpretation and mapping of distinct compositional units. The M3 imaging spectrometer acquired data in 85 spectral bands from ∼430 to 3000 nm at 140 to 280 m/pixel in its global mapping mode during the first half of 2009. Reflectance data of several key sites in the western maria were also acquired at higher spatial and spectral resolutions using M3s target mode, prior to the end of the Chandrayaan-1 mission. These new observations confirm that both fresh craters and mare soils within the western high-Ti basalts display strong 1 μm and weak 2 μm absorptions consistent with olivine-rich basaltic compositions. The inferred abundance of olivine is observed to correlate with stratigraphic sequence across different mare regions and absolute ages. The apparent stratigraphic evolution and Fe-rich compositions of these basalts as a whole suggest an origin from evolved residual melts rather than through the assimilation of more primitive olivine-rich sources. Mare deposits with spectral properties similar to these late stage high-Ti basalts appear to be very limited outside the Procellarum-Imbrium region of the Moon and, where present, appear to occur as small areas of late stage regional volcanism. Detailed analyses of these new data and supporting measurements are in progress to provide further constraints on the mineralogy, olivine abundance, and compositions of these final products of lunar volcanism and the nature and evolution of their source regions.

Compositional diversity at Theophilus Crater: Understanding the geological context of Mg-spinel bearing central peaks

[1]xa0Analysis of high resolution Moon Mineralogy Mapper (M3) data reveals the presence of a prominent Mg-spinel-rich lithology in the central peaks of Theophilus crater on the lunar nearside. Other peak-associated lithologies are comprised of plagioclase, olivine, and pyroxene-bearing materials. A consistent spatial association of Mg-spinel with mafic-free anorthosite is recognized. Documentation of Theophilus central peaks brings the global inventory of Mg-spinel-rich lithology to two widely separated occurrences, namely Theophilus on the lunar nearside and Moscoviense basin on the farside. The Theophilus crater target region lies on one of the inner rings of the Nectaris basin, indicating that the Mg-spinel-bearing lithology source was deep in the lunar crust.

A photometric function for analysis of lunar images in the visual and infrared based on Moon Mineralogy Mapper observations

[1]xa0Changes in observed photometric intensity on a planetary surface are caused by variations in local viewing geometry defined by the radiance incidence, emission, and solar phase angle coupled with a wavelength-dependent surface phase function f(α, λ) which is specific for a given terrain. In this paper we provide preliminary empirical models, based on data acquired inflight, which enable the correction of Moon Mineralogy Mapper (M3) spectral images to a standard geometry with the effects of viewing geometry removed. Over the solar phase angle range for which the M3 data were acquired our models are accurate to a few percent, particularly where thermal emission is not significant. Our models are expected to improve as additional refinements to the calibrations occur, including improvements to the flatfield calibration; improved scattered and stray light corrections; improved thermal model corrections; and the computation of more accurate local incident and emission angles based on surface topography.

A wavelength‐dependent visible and infrared spectrophotometric function for the Moon based on ROLO data

[1]xa0The USGSs Robotic Lunar Observatory (ROLO) dedicated ground-based lunar calibration project obtained photometric observations of the Moon over the spectral range attainable from Earth (0.347–2.39 μm) and over solar phase angles of 1.55°–97°. From these observations, we derived empirical lunar surface solar phase functions for both the highlands and maria that can be used for a wide range of applications. The functions can be used to correct for the effects of viewing geometry to produce lunar mosaics, spectra, and quick-look products for future lunar missions and ground-based observations. Our methodology can be used for a wide range of objects for which multiply scattered radiation is not significant, including all but the very brightest asteroids and moons.

Goldschmidt crater and the Moon's north polar region: Results from the Moon Mineralogy Mapper (M3)

[1]xa0Soils within the impact crater Goldschmidt have been identified as spectrally distinct from the local highland material. High spatial and spectral resolution data from the Moon Mineralogy Mapper (M3) on the Chandrayaan-1 orbiter are used to examine the character of Goldschmidt crater in detail. Spectral parameters applied to a north polar mosaic of M3 data are used to discern large-scale compositional trends at the northern high latitudes, and spectra from three widely separated regions are compared to spectra from Goldschmidt. The results highlight the compositional diversity of the lunar nearside, in particular, where feldspathic soils with a low-Ca pyroxene component are pervasive, but exclusively feldspathic regions and small areas of basaltic composition are also observed. Additionally, we find that the relative strengths of the diagnostic OH/H2O absorption feature near 3000 nm are correlated with the mineralogy of the host material. On both global and local scales, the strongest hydrous absorptions occur on the more feldspathic surfaces. Thus, M3 data suggest that while the feldspathic soils within Goldschmidt crater are enhanced in OH/H2O compared to the relatively mafic nearside polar highlands, their hydration signatures are similar to those observed in the feldspathic highlands on the farside.

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

[1]xa0High 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.