NIR and Lunar Impact Melts

November 7, 2017

A pretty quite blog today. The only research related update is that I have been continuing with my manuscript and getting closer to finishing a draft. I expect to have a draft completed by the start of December!

 

However, I can't leave this blog empty so I decided to talk about a paper that have been brought to my attention. The first paper is titled "Near-Infrared Spectroscopy of Probable Impact Melt from Three Large Lunar Highland Craters" by Smrekar and Pieters (1985). The paper discusses the anomalous and uniform spectra of impact melts from three craters at different locations on the Moon. The craters compositions were analyzed using Earth-based telescopic near-infrared data, taken from the Mauna Kea Observatory in Hawaii. The spectrometer was attached to the telescope (McCord et al. 1981) to obtain compositional data of the crystalline and impact generated material at Aristillus, Copernicus, and Tycho crater (Images are below, after the Mauna Kea image. Top left is Copernicus, top right is Tycho, and bottom is Aristillus). Details of the spectrometer used are:

  • Spectral resolution - 1.5% 

  • Channels - 120 recorded 0.65 and 2.55 um

The data had to be corrected to account for atmospheric, geometric, and instrumental effects (such as noise) before analysis proceeded. The paper mentions the measurements are corrected to equal air mass; the most efficient path length through the Earth's atmosphere.

 The spectrum data collected from the three craters identify the presence of low and high Ca-pyroxene, intermediate Ca-pyroxene, Fe-feldspar, olivine, Fe-bearing glass, and Fe-oxides. Craters in the Mare regions have a strong high Ca-pyroxene signature (0.97-1 um), while highland craters spectra has a stronger low Ca-pyroxene signature (0.90-0.93 um). The ejecta blankets, walls, and central peaks of the craters (they are all complex craters) spectrum show the crystalline material is different at each crater. For example, Tycho is the only crater associated with Fe-bearing feldspar as it is has the correct spectra band. The central peaks of the craters have the most mineral bands, which makes sense as it is most likely where the most excavated material is located in a complex crater. Copernicus central peak has an olivine ban, and its crater walls have a low Ca-pyroxene band. Aristillus central peak has both a low and high Ca-pyroxene spectrum band. Another interpretation of the spectrum would be the central peak contains an intermediate Ca-pyroxene mineral; solid-solution before solidification of the crystalline rock. Going back to Tycho, Smrekar and Pieters (1985) mention how detecting a strong Fe-bearing feldspar spectra is unusual as feldspar is not normally readable after shock metamorphism (especially above 250 kbars) as the crystal structure of feldspars become disorganized and breakdown. Another reason is feldspar spectrums are usually masked by stronger pyroxene spectrums (Nash and Conel, 1974; Crown and Pieters, 1985). According to Adams and Goullaud (1978), feldspar crystals would require at least 0.2% Fe2+ to be noticeable in NIR spectrums.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

All three of these craters have an anomalous area, which due to its low-albedo signal it is interpreted to be impact melt. The NIR spectrum of the impact melt was collected and soon discovered to be similar. This is strange as impact melt composition should vary slightly as the craters crystalline target rocks are different. The spectra graphs identified pyroxene and Fe-bearing feldspars in the impact melts. Opaque minerals are also interpreted to be present as the anomalous areas have a low-albedo. The opaque minerals probably comprise magnetite, ulvospinel, and other Fe-oxides. 

There was a point during the discussion that the authors were unsure if pyroxene was present with Fe-bearing glass or olivine as they both create broad, 1um bands in the anomalous spectra. It was later concluded that Fe-bearing glass is the candidate with pyroxene as olivine minerals would produce a higher albedo effect. If you look at the figure below, you will be able to see how similar the NIR spectrum from the three craters appears. 

Figure by Smrekar and Pieters (1985)

 

The end of the discussion talked about what form feldspar is present as in the impact melt. Feldspar in terrestrial impact melts exists as either devitrified glass, recrystallized melt, or lithic clasts. NIR spectra can measure the composition and identify minerals, but it cannot determine its crystal structure. Arguments were against devitrified glass in lunar impact melt as Nash and Conel (1973) explained the low concentrations of water on the Moon would have prevented the devitrification of feldspars. Recrystallized and lithic clast feldspars are the structures feldspars must exhibit in lunar impact melts. 

 

I really enjoyed reading this paper as it falls under using terrestrial analogous to under lunar material. It would be an interesting study to figure out why these impact melts have similar NIR spectra despite the crystalline target rocks at the craters comprising different compositions.

 

References:

  • ADAMS, J. B., AND L. H. GOULLAUD (1978). Plagioclase feldspars: Visible and near-infrared diffuse reflectance spectra as applied to remote sensing. Proc. Lunar Planet. Sci. Conf. 9th, 2901-2909.

  • CROWN, D. A., AND C. M. PIETERS (1985). Spectral properties of plagioclase and pyroxene mixtures, Lunar Planet. Sci. XVI, 158-159.

  • MCCORD, T. B., R. N. CLARK, B. R. HAWKE, L. A. MCFADDEN, P. D. OWENSBY, C. M. PIETERS, AND J. B. ADAMS (1981). Moon: Near-infrared spectral reflectance--A first good look. J. Geophys. Res. 86, 4829-4836.

  • NASH, D. B., AND J. E. CONEL (1973). Vitrification darkening of rock powders: Implications for optical properties of the lunar surface. Moon 8, 346-364.

  • NASH, D. B., AND J. E. CONEL (1974). Spectral reflectance systematics for mixtures of powdered hypersthene, labradorite, and illmenite. J. Geophys. Re,~. 79, 1615-1621.

 

 

 

 

 

 

 

 

 

 

terrestrial blocky lava flows. Further work into the surface roughness of impact melts focused on what could generate high CPR ratios? 

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