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).
Instrument
LWS01 , LWS02 , LWS04 , PHT40 , SWS02 , SWS06
Temporal Coverage
1997-01-10T11:18:16Z/1997-12-27T19:42:32Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, Coustenis et al., 1999, 'Titan: a study of the composition and temperature of the atmosphere. ', 1.0, European Space Agency, https://doi.org/10.5270/esa-bpo5ivx
