Nanoscale Mid-IR to UV of Lunar Regolith Constituents Through Vibrational Electron Energy Loss Spectroscopy (vibEELS)

The anorthite-dominated highlands and the basalt-dominated mare have been bombarded by solar radiation, cosmic rays, charged particles, comets, meteorites, and micrometeorites for over 4.5 billion years, resulting in space weathering at many scales and the collection of interplanetary matter. A majority of the finest fraction (<75 microns) of lunar regolith are endogenous materials: micro- to nano-sized crystalline fragments of the original materials, minerals shocked and amorphized by solar and cosmic radiation, vapor-deposited glass, impact splash, volcanic glass spheres. The exogenous materials include: micro- and nano-scale meteorites and extraterrestrial particles. Despite this conventional microscopy-derived knowledge of the nanoscale, the components of finest fractions of lunar regolith have always been challenging to study with IR and UV spectroscopy due to small grain size, and thermal and space weathering effects that often confound bulk spectra. In fact, until recently, spectroscopy on the individual components at scales of causality (nanometer level) was intractable.

 

We have applied vibrational electron energy-loss spectroscopy to six Apollo samples (three from highlands, three from mare) from four missions, each with differing space weathering maturities (Is/FeO). For highland samples: Apollo 62231 is mature (Is/FeO=91), 61141 sub-mature (56), 61221 immature (8.2). All mare samples are mature: 14259 (85), 15041 (95), and 79221 (80). Identified components in the regoliths include crystalline anorthite, amorphous CaAl₂Si₂O₈(maskelynite) rims with/without iron nanoparticles (FeNPs), olivine, pyroxene, ilmenite, micrometeorites, and glass spheres. This method is employed by a special dedicated scanning transmission electron microscope that generates a monochromated ultrahigh energy resolution electron beam allowing Mid/near IR (MNIR) ‘aloof’ spectral analysis, akin to IR, albeit with a slightly poorer energy resolution, but a much higher spatial localization thanks to the sub-nm electron probe used here. Crystalline anorthite spectra reproduce positions of the five clusters of MNIR absorption peaks (217, 363, 548, 750, 976-1049 cm^-1) at slightly lower resolution than FTIR. Loss of crystalline structure causes a split peak at ~1100 cm^-1 to broaden, merge, and decrease in intensity. Also, the peak at ~550 cm^-1 drops dramatically in intensity in more highly weathered samples. The addition of FeNPs within the amorphous material flattens, or attenuates, the spectra, leaving only the 1100 cm^-1 peak. The MIR Christiansen Feature position appears to be affected by crystallinity, glass composition, and abundance of FeNPs at this scale. In the Visible and UV range, "impact" vibEELS collects spectra that document the color absorption changes associated with space weathering as the amount of FeNPs and vitrification increases. The shift towards a reddened slope observed in remote near-IR and UV of bulk samples, is also observed in individual particles that have more FeNPs. The vibEELS data also allows for the determination of the band gap, and therefore, the estimation of the dielectric constant of the weathered surface of regolith particles, which can be used to calculate lunar regolith properties relevant to interpretation of radar wavelengths. 

VibEELS is exquisitely well suited for examination of lunar finest fraction and brings planetary events and materials mixed into this fraction into new focus and perspective.