the CO fundamental and the N2 fundamen-
tal. For this to happen, the CO and N2 mol-
ecules have to be intimately mixed together.
Triton Observations
Figure 4.
A portion of an
80-minute, binned-
spectrum of Triton
taken with the 8.1-
meter Gemini-South
telescope and the
IGRINS spectrometer
(red squares). The
broad absorption in
the Triton spectrum is
consistent with the broad
absorption of the two-
molecule combination
band at 2.239 m m (4466.5
cm -1 ) in our laboratory
transmission spectrum
(blue line). Both broad
bands are inconsistent
with the telluric (black
squares at top of figure)
and the solar (black
squares at bottom of
figure) spectra. Figure and
caption modified from
Tegler, et al. (2019).
N2, and its absence in pure N2 and pure CO,
reinforces the idea that the band is caused
by the CO and N2 molecules being near each
other, and probably interacting.
Molecular Understanding
Individually, carbon monoxide and nitrogen
ices each absorb their own distinct wave-
lengths of infrared light, but the tandem
vibration of an ice mixture absorbs at an ad-
ditional, distinct wavelength. Looking at the
pure species, we are able to identify the fun-
damental vibrational frequencies, as well as
their overtones and combinations. However,
this band (first noted but not identified by
Quirico and Schmitt, 1997) did not align with
any known features. Since the band had
maximum strength in samples with nearly
equal amounts of CO and N2, and was ab-
sent in pure N2 and pure CO, we realized it
must arise from both molecules simultane-
ously. We refer to the band as a two-mole-
cule combination band.
We were able to quantitatively show the
new band was the result of the simultaneous
excitation of adjacent CO and N2 molecules.
Specifically, we found the energy (wave-
number) required to excite the weak, un-
identified band was equal to the sum of the
energies (wavenumbers) required to excite
6
GeminiFocus
One exciting aspect of this work is that if we
detect this band on any astronomical object
we know that carbon monoxide and nitro-
gen must be intimately mixed together at
the molecular level. That excitement rose
as we used the 8-meter Gemini South Tele-
scope in Chile on the night of July 2, 2018, to
explore Triton’s icy surface with IGRINS. The
combination of this large aperture telescope
with the phenomenal throughput of IGRINS
over long exposure times, coupled with the
high spectral resolution gives the ability to
bin to get desired signal-to-noise ratio. All
this was necessary to even have a chance to
detect this weak feature. We summed our
individual Triton spectra to obtain a single
spectrum with a total exposure time of 80
minutes.
Since our objective was to detect the spec-
trally broad CO-N2 combination band at
2.239 microns ( m m) (4466.5 cm -1 ), we used in-
verse variance weighting to bin the spectrum
into blocks of 64 pixels, and thereby improve
the signal-to-noise ratio of the Triton spec-
trum. The binned spectrum had a resolution
of λ/Δλ = 2,500.
As can be seen in Figure 4, there is a broad
feature in the Triton spectrum (red squares)
located at the same position as the band in
our laboratory spectrum of 8% CO and 92%
N2 ice sample at 60 K (blue line). For com-
parison, we show absorption due to Earth's
atmosphere (black squares seen at top) and
reflected sunlight, i.e., Fraunhofer lines (black
squares at bottom of figure). The telluric and
solar spectra are binned to the same resolu-
tion as the binned Triton spectrum, i.e., λ/Δλ
= 2,500. The vertical dotted line marks the
wavelength of maximum absorption by the
July 2019