Ethane in Titan’s Stratosphere from Cassini CIRS Far- and Mid-Infrared Spectra. (arXiv:1908.01926v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lombardo_N/0/1/0/all/0/1">Nicholas A Lombardo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nixon_C/0/1/0/all/0/1">Conor A Nixon</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sylvestre_M/0/1/0/all/0/1">Melody Sylvestre</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jennings_D/0/1/0/all/0/1">Donald E Jennings</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Teanby_N/0/1/0/all/0/1">Nicholas Teanby</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Irwin_P/0/1/0/all/0/1">Patrick G J Irwin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Flasar_F/0/1/0/all/0/1">F Michael Flasar</a>
The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission
in the far- and mid-infrared (from 10 cm$^{-1}$ to 1500 cm$^{-1}$), enabling
spatiotemporal studies of ethane on Titan across the span of the Cassini
mission from 2004 through 2017. Many previous measurements of ethane on Titan
have relied on modeling the molecule’s mid-infrared $nu_{12}$ band, centered
on 822 cm$^{-1}$. Other bands of ethane at shorter and longer wavelengths were
seen, but have not been modeled to measure ethane abundance. Spectral line
lists of the far-infrared $nu_{4}$ torsional band at 289 cm$^{-1}$ and the
mid-infrared $nu_{8}$ band centered ay 1468 cm$^{-1}$ have recently been
studied in the laboratory. We model CIRS observations of each of these bands
(along with the $nu_{12}$ band) separately and compare retrieved mixing ratios
from each spectral region. Nadir observations of of the $nu_{4}$ band probe
the low stratosphere below 100 km. Our equatorial measurements at 289 cm$^{-1}$
show an abundance of (1.0$pm$0.4) $times$10$^{-5}$ at 88 km, from 2007 to
2017. This mixing ratio is consistent with measurements at higher altitudes, in
contrast to the depletion that many photochemical models predict. Measurements
from the $nu_{12}$ and $nu_{8}$ bands are comparable to each other, with the
$nu_{12}$ band probing an altitude range that extends deeper in the
atmosphere. We suggest future studies of planetary atmospheres may observe the
$nu_{8}$ band, enabling shorter wavelength studies of ethane. There may also
be an advantage to observing both the ethane $nu_{8}$ band and nearby methane
$nu_{4}$ band in the same spectral window.
The Cassini Composite Infrared Spectrometer (CIRS) observed thermal emission
in the far- and mid-infrared (from 10 cm$^{-1}$ to 1500 cm$^{-1}$), enabling
spatiotemporal studies of ethane on Titan across the span of the Cassini
mission from 2004 through 2017. Many previous measurements of ethane on Titan
have relied on modeling the molecule’s mid-infrared $nu_{12}$ band, centered
on 822 cm$^{-1}$. Other bands of ethane at shorter and longer wavelengths were
seen, but have not been modeled to measure ethane abundance. Spectral line
lists of the far-infrared $nu_{4}$ torsional band at 289 cm$^{-1}$ and the
mid-infrared $nu_{8}$ band centered ay 1468 cm$^{-1}$ have recently been
studied in the laboratory. We model CIRS observations of each of these bands
(along with the $nu_{12}$ band) separately and compare retrieved mixing ratios
from each spectral region. Nadir observations of of the $nu_{4}$ band probe
the low stratosphere below 100 km. Our equatorial measurements at 289 cm$^{-1}$
show an abundance of (1.0$pm$0.4) $times$10$^{-5}$ at 88 km, from 2007 to
2017. This mixing ratio is consistent with measurements at higher altitudes, in
contrast to the depletion that many photochemical models predict. Measurements
from the $nu_{12}$ and $nu_{8}$ bands are comparable to each other, with the
$nu_{12}$ band probing an altitude range that extends deeper in the
atmosphere. We suggest future studies of planetary atmospheres may observe the
$nu_{8}$ band, enabling shorter wavelength studies of ethane. There may also
be an advantage to observing both the ethane $nu_{8}$ band and nearby methane
$nu_{4}$ band in the same spectral window.
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