The following document lists the file abstract/ACOUSTEN_TITAN.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
SCIENTIFIC ABSTRACT
Totally covered by thick hydrocarbon clouds, Titan has parsimoniously
revealed some of its aspects through the Voyager 1 mission in 1980 and
through some recent infrared and mm ground-based observations. The data
point to a highly complex organic chemistry of N2-CH4 reactions in the
dense nitrogen-dominated atmosphere. Many of the expected organics
remain to be detected (water, isotopes, hydrocarbons and nitriles) in
both their gaseous and solid phases. The nature of the surface and the
structure of the satellite's lower atmosphere are yet to be determined.
Until future space missions to Titan bring back in situ measurements, ISO
observations, free of interference from the Earth's atmosphere, is the best
opportunity offered to study Titan.
OBSERVATION SUMMARY
We will use both LWS and SWS spectrometers in the grating mode to explore
the Titan full spectrum (S/N of 70-500).
LWS/GR will cover Titan's continuum and lines seen in emission or absorption
in the submm range (45-180 micron), which has never been investigated before,
with a S/N of about 200 in 1.4 hours. A higher S/N (about 400) will be
attempted in 0.62 hrs for two dedicated concatenated
observations of the weak absorptions
(flux of about 60 in the continuum) of two dimers: CH4-CH4 at 61.8 micron
and N2-N2 at 178 micron. This programme has special pointing requirements.
In the case of LWS01 and LWS02 AOTs, allowance has been made for an
"off-source" observation for each "on-source" observation. For example,
observations 2,5 and 6 are actually "off-source" points. The ideal off-source
position is the same position with respect to Saturn, but observed at a
different time (several days before or after the Titan observation), when
Titan is not in the beam. This observing mode does not appear to be consistent
with the current restrictions. In fact, even simple raster observations
of solar system targets are not allowed. At the moment, therefore, specifying
sensible off-source positions for solar system targets is not possible.
This will need attention if a number of key observations in the solar
system programme are to be made feasible.
The submm range is rich in rotational lines of CH4, H2O, HCN and CO, which,
although intrinsically weak, can be observed on Titan due to the high
abundances of these species. These lines will be measured for the first
time at a spectral resolution of about 0.5 wavenumbers during the LWS/GR scan.
Emphasis with the LWS/FP will be given to H2O and CH4 measurements. H2O is
expected on Titan to account for the oxygen content, but has not yet been
detected. We will attempt the detection of water by searching for two of its
rotational lines near 66.7 and 145 micron (expected line fluxes of 175 and
80 Jy respectively) with a S/N of 7-10 on the line. A total of 2.29 hrs will
be allocated to these observations. In addition, the LWS/FP will be used during
1.4 hrs to study two CH4 lines with S/N of 10 on the line: one near 87 micron
and the other near 120 micron (expected line flux 165-170 Jy for both).
The study of these two lines will allow the retrieval of the methane vertical
content and of the temperature structure in Titan's lower stratosphere (60-120
km). Combined with SWS measurements in the other methane band near 7.7 micron
(diagnostic of the higher stratosphere), the methane abundance and the
temperature on Titan will become known from 60 to 500 km of altitude.
The far-infrared range (7-45 micron) will be covered with the SWS/GR at a
spectral resolution 10 times higher than previous measurements. A full scan
in this region will yield information on the vertical abundances of numerous
species with emission signatures. Combined (and concatenated in the case of (a)
and (b)) observations will be performed to cover simultaneously:
(a) the 7-12 and 29-45 micron in 2.9 hrs (S/N of 20-50 in the first
region and about 300-500 in the second for an expected flux
level of about 3 Jy in the continuum); (b) the 12-16 and 2.3-3.0 micron
region (S/N of 150 in the first region and 20 near 2.7 micron, for a
continuum flux of about 10 Jy); (c) the 16-29 and the 3.0-4.0 micron region
with S/N of about 35 near 3 micron, increasing up to 300 near 29 micron
for a flux of around 25 Jy. The time dedicated to these observations is
about 4.2 hrs.
Finally, 2 hrs will be allocated to spectrophotometry in single pointing
using the PHT-S sub-instrument simultaneously in both 2.5-5 micron
(expected S/N of 40-100) and 6-12 micron (expected S/N of about 16 at 12
micron and increasing at shorter wavelengths).