LINKING TISSINT SHOCK GLASS TO IMPACT MELT ON MARS

Impact melts and glasses are predicted to be prevalent on the martian surface. However, their nature and abundance have not been thoroughly investigated with orbital remote sensing or landed missions, in part due to a lack of proper reference material studied in the laboratory. Here we isolate a nearly pure 2.5 mm impact melt pocket in the Tissint meteorite and for the first time report on visible near-infrared (VNIR, 300-2600 nm) and mid-infrared (MIR, 400-2000 cm^-1) reflectance spectra of black shock glass in a Mars rock. Tissint is an olivine-phyric shergottite and was observed to fall in 2011 in Morocco; as such there is very little terrestrial contamination or alteration present.

Pyroxene and olivine present in the sample show spectral features consistent with previous studies of shergottites such as EETA 79001A. Tissint pyroxene spectra are characterized by strong Fe²⁺ crystal field absorptions at 0.97 and 2.08 μm indicating a mixture of high-Ca and low-Ca or intermediate pigeonite compositions. Olivine phenocrysts are spectrally similar to other martian olivines. Importantly the melt pocket is spectrally unique compared to all other phases, with reduced spectral contrast, low overall reflectance values and distorted absorptions at 0.99 and 2.08 μm due to Fe²⁺ crystal field splitting in the glass. At MIR wavelengths the Tissint shock melt shows broad flat reflectance plateaus from 440-655 cm^-1 and 880-1100 cm^-1 rather than sharp 'peaks' visible in the surrounding crystalline phases and in other martian meteorite spectra.

We are currently using hyperspectral data from the CRISM and TES instruments to investigate the link between laboratory spectra of Tissint shock glass and possible impact melt on the martian surface. Terrestrial obsidian, synthetic K-rich glasses and phyllosilicate spectra have been used to accommodate aspects of thermal infrared spectra in linear spectral models of low-albedo terrains known as 'Surface Type II' with the results interpreted as representing andesitic compositions, silica coatings, or aqueous alteration. Our initial findings suggest that new spectral modeling for these regions including Tissint impact glass as an endmember results in a significant fraction of impact melt material, offering an alternative explanation for the spectral properties of some regions on Mars.