Laboratory Investigations
While previous studies have shown
that carbon monoxide (CO) and
nitrogen (N2) ices exist on Triton,
we decided to investigate their
spectral features — specifically,
we wanted to see how the spectra
changed as a function of the mix-
ing ratio between the CO and N2.
In order to study spectroscopic
telescopic data, one needs to have
an appropriate library of labora-
tory spectra. Most laboratory experiments
collect spectra of thin ice samples of only
microns thick. These experiments are su-
perb at studying intrinsically strong absorp-
tion bands. Thin film experiments are not as
good for studying intrinsically weak absorp-
tion bands. Longer path lengths are needed
to study these bands. In the Astrophysical
Materials Laboratory, we have a unique ex-
perimental setup that enables us to study ice
samples as thick as 2 centimeters. As a result,
we can study very weak absorption bands.
Our thick cell is mounted on top of a cryo-
cooler. Gas enters the cell from above via
a fill tube (Figure 2a). The dotted lines in
Figure 2 represent the spectrometer beam
through the sample. Thermometers (T1 and
T2) and heating elements (H1 and H2) con-
trol the temperature of the sample down to
30 Kelvin (K). Further details concerning the
cell are described in Tegler et al. (2019).
We measured the absorption coefficient of
varying mixtures of CO and N2, and noticed
an unidentified, weak band that wasn’t in ei-
ther pure species. This band was strongest
when the ratio of CO to N2 was at 50:50 (Fig-
ure 3). The spectra shown in Figure 3 are all
taken at 60 K, where the ice mixture is in the
b -phase. A maximum band strength for sam-
ples with nearly equal amounts of CO and
Figure 2.
(a) Schematic diagram of
the Astrophysical Materials
Laboratory thick cell in
cross section as seen from
the side. (b) The optical
train in our experiment
as seen from above. The
spectrometer beam is
represented by dashed lines.
Only the infrared detector
was used in the experiments
described here.
Figure 3.
Spectra of CO/N2 ice
samples with the CO
abundance ranging
from (a) 0% to 40%
and (b) 60% to 100%.
In panel (a), the
spectra show the new
band near 4467 cm -1 .
The new band is not
present in the pure
N2 sample (black line) and increases in strength with increasing CO abundance. The saturated band at 4252 cm -1 is a CO overtone and the
weak, broad band at 4654 cm -1 is N2. The strength of the weak, unidentified band at a CO abundance of 60% in panel (a) is nearly the same
as its strength at 40% in panel (b) and then decreases in strength with increasing CO abundance. The band is not present in the pure CO ice
sample in panel (b). Figure and caption modified from Tegler et al. (2019).
July 2019
GeminiFocus
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