Visible/Near-infrared Reflectance Spectra of Drill Tailings in the Glen Torridon and Greenheugh Pediment areas, Gale Crater, Mars

Abstract:

The Curiosity rover analyzed 3 drill samples during its exploration of the southern clay mineral-bearing Glen Torridon (GT) region and ascent onto the Greenheugh Pediment (GP) sandstone. ChemCam reflectance spectra (400-840 nm) of the drill tailings were acquired between Sols 2669 and 2755 (Feb.-May, 2020). Spectra of the target Hutton (HU, south of Tower Butte below GP) were unique among drill tailings observed by Curiosity, with ~flat near-infrared spectra and a negative ~535 nm absorption band depth, consistent with little crystalline Fe³⁺. Once the rover ascended GP, the spectra of drill tailings from the target Edinburgh (EB) were very similar to HU, albeit slightly darker. The ~flat near-infrared slopes of the HU and EB spectra were reminiscent of drill tailings for relatively unaltered sandstone targets Big Sky and Okoruso from Marias Pass within the Stimson formation (Sols 1062-1339) [1]. However, the HU and EB spectra exhibited steeper downturns in the <600 nm region, which—combined with the lack of a ~535 nm absorption—suggested greater influence of Fe²⁺-bearing materials (e.g., pyroxene). Once the rover drove back onto the fractured intermediate unit of GT between Tower and Western Buttes, the Glasgow (GG) target was drilled. The GG spectra were unique among Curiosity tailings, with a peak reflectance near 675 nm, a steep near-infrared slope, and a ~535 nm absorption band. Initial spectral interpretations were that the tailings included either olivine or hydrated ferric sulfate plus minor magnetite. However, subsequent Mastcam spectra showed the near-IR slope extended ~linearly to 1000 nm (inconsistent with hydrated ferric sulfates) and CheMin analyses revealed that crystalline olivine, magnetite, and ferric sulfates were below detection limits [2]. Among the 3 tailings, CheMin data showed that GG contained the most hematite (likely nanophase, given the lack of red hematite spectral features), the largest phyllosilicate, Ca-sulfate, and high amorphous contents. Although lab spectra of some smectite-bearing samples exhibit similar features to GG, the GG spectra were also likely influenced by contributions from the amorphous materials, which is subject to ongoing investigations.

[1] Johnson, J. et al., LPSC, 1155, 2016; AGU P11B-1856, 2016; [2] Thorpe, M. et al., AGU, 2020; Rampe. E., et al., AGU, 2020.