Several studies have linked the Tagish Lake ungrouped carbonaceous chondrite meteorite to Phobos based on their similar spectral properties in the visible/near-infrared (VNIR) wavelength region. At these wavelengths, both Phobos and Tagish Lake are dark, spectrally featureless (or nearly so), and display red slopes.
For this work, we acquired thermal IR (TIR; ~5-25 µm) spectra of a portion of Tagish Lake under ambient and simulated asteroid environment (SAE) conditions using the Planetary and Asteroid Regolith Spectroscopy Environment Chamber (PARSEC) in the Center for Planetary Exploration at Stony Brook University. PARSEC is able to create thermal gradients like those present in airless body regolith by placing samples in a vacuum (≤ 10-6 mbar) under an LN2-cooled cold shield, heating samples from below, and heating samples from above using a 75 W quartz halogen lamp. Sample spectra are calibrated using an internal blackbody target.
A ~0.1 g portion of Tagish Lake was loaned to us by the American Museum of Natural History. We gently ground the sample using an agate mortar and pestle and acquired spectra with PARSEC. The fine particle size of the sample and its extremely low albedo lead to a strong thermal gradient that makes it difficult to calibrate the SAE spectra. Nevertheless, we are able to determine the position of the silicate emissivity maximum (Christiansen feature; CF) and identify key absorption features between 5 and 25 µm. We then compare these data with thermal emission spectra of Phobos acquired by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES).
The ambient spectrum of Tagish Lake displays a CF at 1109 cm-1 (9.01 µm) and strong absorption features at 1011, 856, and 426 cm-1 (9.89, 11.68, and 23.47 µm). A strong feature near 1600 cm-1 clearly indicates the presence of adsorbed or bound water. In contrast, the preliminary SAE spectrum displays a very narrow CF at 1240 cm-1 (8.06 µm) with weak absorption bands at 1094, 1024, 673, and 546 cm-1 (9.14, 9.77, 14.86, and 18.31 µm). A weak feature near 1600 cm-1 may be due to remnant bound water in the sample.
The differences between the ambient and SAE spectra are greater than is typically seen with pure mineral samples. We hypothesize that this is due to fine-grained and low-albedo nature of the sample, which creates a stronger thermal gradient under SAE conditions than we get for minerals. The key indicators of this behavior are the large shift (> 1 µm) in the CF position and the narrow shape of the CF.
TES spectra of Phobos display a relatively broad CF centered at ~1140 cm-1 (8.77 µm) with minor emissivity minima at 1015, 825, and 460 cm-1 (9.85, 12.12, and 21.7 µm). The spectra are more similar to the ambient spectrum of Tagish Lake than the SAE spectrum, but neither is a particularly good fit. Results of this study show that thermal-IR spectroscopy can provide new constraints on airless body compositions, especially when VNIR spectra are featureless.