Ambient and cold-temperature infrared spectra of ammoniated phyllosilicates and carbonaceous chondrite meteorites relevant to Ceres and other solar system bodies


Ehlmann BL1, Hodyss RP2, Bristow TF3, Rossman GR1, Ammannito E4, de Sanetis MC5, Raymond CA6

1. Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA

3. Exobiology Branch, NASA Ames Research Center, Moffett Field, California 94035, U.S.A.

4. Department of Earth Planetary and Space Sciences, University of California, Los Angeles, 90095-1567, USA.

5. Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, 00133 Roma, Italy.


Abstract

Ammoniated phases are key constituents in Mg-phyllosilicate-bearing dark surface materials on the dwarf planet Ceres. We ammoniated phyllosilicate minerals and meteoric materials, which may be the NH4-carrier phase(s) on Ceres, to compare their spectral properties with the infrared data acquired by Dawn’s Visible and Infrared (VIR) imaging spectrometer. We treated Mg-, Fe-, Al-smectite clays, Mg-serpentines, Mg-chlorites, and a suite of carbonaceous meteorites with NH4-acetate to simulate exchange of ammonium in fluids with silicate phases likely present on Ceres. Serpentines and chlorites showed no evidence for ammoniation, as expected due to their lack of exchangeable interlayer sites. Most smectites showed evidence for ammoniation by incorporation of NH4+ into their interlayers, resulting in the appearance of absorptions from 3.02-3.06 µm. Meteorite samples tested showed little evidence for ammoniation, likely due to the high proportion of serpentine relative to expandable smectite phases; only Cold Bokkeveld showed changes in an absorption between 3.0-3.1 µm, but it is not unambiguously due to NH4+ rather than H2O. The wavelength position of the NH4 absorption feature in most smectites showed little to no variation between IR spectra acquired under dry-air purge at 25°C and under vacuum at 25°C to -180°C. Collectively, data from the smectite samples show that the precise center wavelength of the characteristic 3.05 µm v3 absorption in NH4 is variable and is likely related to the degree of hydrogen bonding of NH4-H2O complexes. Comparison with Dawn VIR spectra indicates that the hypothesis that Mg-saponite is the ammonium carrier phase is simplest explanation to explain the observed data.