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
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.