 SPECTRAL REFLECTANCE OF FORMATES, ACETATES, AND SOME OTHER CARBOXYLATES.  D. M. Applin 1 and E. A. Cloutis 1, 1 Department of Geography, University of Winnipeg, 515 Portage Ave., Winnipeg, MB, Canada R3B 2E9, daniel.m.applin@gmail.com  Introduction:  Small carboxylic acids (COOHs) and their salts are predicted metastable products of meteoritic carbon compound chemistry on the surface of Mars [1]. Results from instrumentation carried on the Mars Science Laboratory (MSL) Curiosity rover have  been consistent with significant concentrations of these compounds [2-4]. It has also been shown that these compounds may be surficially widespread, as results from instrumentation on both the Phoenix Scout lander and the Viking landers are consistent with significant concentrations of  these compounds [4].   In addition to the chemical pathways expected of meteoritic and cometary carbon on the surface of Mars, several endogenous mechanisms for the formation of these compounds exist and have been briefly discussed [4].   The upcoming Mars2020 rover will carry the SuperCam spectroscopy suite, which has spectral reflectance capabilities as long as 2.60 microns in wavelength, with spatial resolution <60 microns [5]. In support of these capabilities, we continue our spectral reflectance survey of carbon compounds relevant to the exploration of the martian surface, and other planetary surfaces within the solar system. Here, we focus on formates (C1 monoCOOH) and acetates (C2 monoCOOH), which form, in particular, during serpentization [6], and a wide variety of other abiotic chemical processes [4]. Experimental procedure: Samples were purchased from Sigma-Aldrich, and all are of >98% purity on a trace metals basis. All samples were crushed with an alumina mortar and pestle, and dry-sieved to obtain <45 microns fractions. Samples were poured into beveled aluminum sample cups, with the surfaces scraped by a glass slide held at 45° to provide a matte surface. Reflectance spectra were acquired from 0.35-2.5 µm using an ASD FieldSpec 4 instrument and measured relative to a calibrated Spectralon disk. Spectra were acquired at i=30º and e=0º; a total of 1000 spectra were collected and averaged to improve the SNR. Results and discussion:  The reflectance spectra of formate salts bear many sharp absorption features in the near-infrared (NIR) (Fig. 1). The existence of sharp feature in the NIR is not unique, as most anhydrous sodium and calcium salts also have sharp features. What appears to be relatively unique is the number and strength of the features, the main minima of which occur near 1.80 and 2.35 microns. These features both exist as doublets or triplets, with several other superimposed overtones/combinations. The positions of these features are offset in the two phases under study, which provides phase discrimination capabilities using reflectance spectroscopy. The reflectance spectra of acetates are simpler and more carbonate-like (Figs 2 and 3). The methyl-related overtones occur as doublets centered near 1.70 microns. Carbonate-like overtones/combination envelopes exist near 2.30 and 2.50 microns, made of partially resolved and overlapping sharp absorption bands. The center of these envelopes appear to be very much carbonate-like, and may appear like carbonates in data of low S/N and/or of low spectral/sampling resolution. The minima of these envelopes are primarily at 2.27 and 2.48 microns. The band depth ratio of these features is reversed versus most carbonates (but not all), i.e. the 2.30 micron feature is stronger than the 2.50 micron feature. This behaviour is the opposite for Mgacetate tetrahydrate, which has shallower 2.30 micron features. This may be due some variation of the resonance between the C=O and H2O fundamental vibrations near 6.00 microns, which is common in carboxylates and other carbonyl-bearing materials [4] Sodium citrate (triCOOH) also has two main overtones at 2.30 and ~2.45 microns - the contrast on the second feature is low due to a series of overlapping, sharp overtones/combinations at 2.50-2.60 microns (Fig. 4). Very similar spectral behaviour is observed in the spectra of the sodium malonate and sodium dodecanoate powders. Sodium citrate and malonate powders exhibit the overtone/combination common in hydratedcarboxylates (carbonates and oxalates; C1 and C2 diCOOH) at ~2.08 microns, and also have a weak feature near ~2.19 microns. As with the other C-H-bearing carboxylates, these exhibit features near 1.70 microns, and sodium dodecanoate also has a weak feature near 1.80 microns.  In the mid-infrared (MIR), hydrated carboxylates exhibit the H2O combination near 4.5 microns, which we have observed in most hydrated salts. Formates and acetates, in general, exhibit the characteristic Fermi-enhanced overtones of COOH salts near 4.0 microns. These compounds also exhibit strong overtones near the position of CO2 absorption at 4.25 microns. The position and shape of these features are variable and provide avenues for phase discrimination. These salts also exhibit strong overtones between 3.60 and 3.80 microns, which is observed in oxalates [4].  Fig 1. Spectral reflectance of calcium and sodium formate powders.   Fig 2. Spectral reflectance of sodium and calcium acetate powders.   Fig 3. Spectral reflectance of magnesium and ferrous acetate powders.   Fig 4. Spectral reflectance of some other carboxylate powders.  Summary and conclusions:  The spectral reflectance of some formates, acetates, and other carboxylates was investigated in support of the search for carbon compounds by the upcoming Mars2020 rover's SuperCam instrument. We find that absorption features exist that enable the discrimination of different salts within a COOH group and between salts of different COOH groups. In particular, we find that acetates exhibit carbonate-like reflectance spectra in the NIR. COOH salts with C-H bonding (COOH salts that are not carbonates, oxalates, or some benzenecarboxylates) can be distinguished by features near 1.70 and 1.80 microns, which are at most only present as very weak overtones in most relevant minerals. Formates and acetates exhibit absorption features between 3.50 and 3.80 microns, which is both rare and found in oxalate salt reflectance spectra. Formates and acetates exhibit relatively strong overtones/ combinations near 4.0 microns and at/near 4.25 microns, where CO2 also absorbs, potentially complications their detection with this spectral region when significant CO2 is present. References: [1] Benner S. A., et al. (1999) PNAS, 97, 2425-2430. [2] Archer P. D., et al. (2015) AGU meeting, 2123. [3] Sutter B., et al. (2016) AGU meeting, P21D-07. [4] Applin D. M., et al. (2016) Icarus v278 p7. [5] Gasnault O., et al. (2015) LPSC 46, Abstract #2990. [6] Lang S. Q., et al. (2010) GCA v74 p941 Acknowledgments: This study was supported by NSERC, CSA, CFI, MRIF, and UWinnipeg.   
