The Voyager infrared investigation uses a Michelson interferometer (IRIS) covering the spectral range from 200 to 3000 cm/1 (3.3 to 50 micrometers) and a bore sighted radiometer covering the range from 5000 to 25000 cm/1 (0.4 to 2 micrometers). The spectral resolution of the interferometer is 4.3 cm/1 and the field of view is 0.25 deg. Scientific results anticipated from the investigation of the Saturnian system are discussed

Hanel, R. A.

English

NASA Technical Reports Server

IY79-1 6777  
WHAT THE VOYAGER INFRARED 
INVESTIGATORS HOPE TO LEARN 
ABOUT THE SATURN SYSTEM 
Labomtory for Extraterreshial Pbyn'cs, NASA Coddard Space Fligbt Cmtm 
Greeubelt, MmyLrad 20771 
The Vqager infrared investigation uses a Michelson interkromecer (IRIS) 
covering t!e ~pectral range from 200 to MOO cm-' (3.3 to 50 pm) and a 
bore sighted radiometer cowring the range from 5000 a, 25000 an-' 
(0.4 a, 2 pan). The spectal resolution of rhe interferometer is 4.3 an-' 
and the &Id of view is 0.25". Scientitic results anticipated from the in- 
vestigation of the Saturnian system are discussed; these are contrasted 
to those which were expected from the advanced interferometer (MIRIS), 
which was not qualified in time for flight. 
INTRODUCTION 
This paper summarizes what the Voyager Infrared Investigator Team expects 
to learn about the Saturnian system. The discussion addresses Saturn, its rings, 
Titan and the other smaller satellites. The format of this summary is similar to that 
of the team paper published in Space Science Review (Hanel, et d., 1977), hereafter 
called paper one. However, in contrast to paper one the present discussion concen- 
trates on Saturn, and takes account of the fact that the IRIS, rather than the MIRE3 
instrument, is on board Voyager. 
An in-flight IRIS calibration is now available for the 200-2000 om-' range 
which s h m  that the instrumental performance is somewhat below expectation. The 
near infrared part of the spectrum (above 2000 cm-l) will be calibrated shortly by 
https://ntrs.nasa.gov/search.jsp?R=19790008606 2020-03-22T01:37:50+00:00Z
viewing the spacec2aft mounted solar diffusing target. Figure 1 (similar to Figure 9 
of @aper onej shows the actually measured noiseequiwlent-radiance (NER) of the 
Voyager 1 interferometer. The response has not changed since the initial cool down 
after launch is close to that during the final thermal vacuum test. The NER of the 
Voyager 2 interferometer after launch was similar to that of the Voyager 1 instrument, 
but haa apparently not yet etabilized. 
Also shown in Figure 1 are the NER values of the flight qualified MIRIS instru- 
ment as measured in a thermal vacuum chamber. Unfortunately, the instrument was 
qualified too late for the Voyager mission. Figure 2 illustrates the degree of improve- 
ment in zhe NER which can be obtained by averaging of spectra over indicated time 
intervals. Several radiometric data poi& obtained from the ground by Morrison et d. 
(1972) and Gillet et d. (1973) for Titan are s h m  for comparison. The Voyager 
scientific objectives defined in paper one are  definitely affected by the substitution of 
IRIS for MIRE. Objectives based on the analysis of the specbal range between 
1000 cm-I and about 3000 cm-' will suffer in precision: objectives based on data from 
the 300@-7000 om-' range can not be accomplished a t  all. On the other hand the lower 
NER of IRIS compared to MIRE between 200 and 900 cm-' will benefit some objectives 
based on data in this spectral range; the lower spectral resolution of the IRIS instru- 
ment will affect other objectives, however. The discussion below treats the individual 
scientific objectives at  Saturn following the overall format of paper one. 
ATMOSPHERIC GAS COMPOSITION, ELEMENTAL ABUNDANCES 
AND ISOTOWC RATIOS 
Hydrogen to Helium Rstio 
The hydrogen to helium ratio for Saturn will be derived primarily from the far 
infrared ~pectrum (200-800 crn-I). Since the derivation will be made by using a large 
number of spectra the difference in performance between instruments becomes 
insignificant. The limit in the precision of the derived hydrogen to helium ratio is  
expected to be due to uncertainties in the knowledge of gas absorption coefficients and 
aerosol properties, rather than to random instrumental errors in the spectra. Thus, 
the objective of determining the H ~ / H ~  ratios as discussed in paper one seems 
obtainable. 
Frgure I ,hour rmrtttd and m$hted raJtanr~ lnrls 3'-hnnatrrallj /or indicatai 
bvigbtnur remperaturv and al&. h l t a ~ u d  not,~-~qait~lrnt-radratlct u l n a  
( NER a n  ~ h o u a  /or \'n)ager 1 /mrddIt h h c d  lint) and tbe far rnband (upper 
&shed fiwt rnd nwr mnfrared (imw Asbed Irnt) cbanntf~ of MIRIS. 
Ftgure 2 J ~ U U Y  radranrr trrJus uilrr nurnkr ~tnitlar to Frg. 1 ,  hut on an 
expan&d trule. The NER of 1 KIS rr jhouv h) Ashtd runu~ for tndr~,rdual 
Jpectra (48 re,-) ..n uz.w~ge nf 10 ( 8  mrn,, lo0 r I h 20 nrtn) anda /oral 
of 1000 r/wctra ( I3 h 20 nrtn). . . \ / to tlfutrrdted are a f a  ~tvund b m d  
nrrarrrementr o/ Tttrm. 
Methane 
Information on the methane mixing ratios on Saturn and Titan will be based 
primarily on t l ~ e  1306 cm-' methane band in conjunction with temperatures derived 
from pressure induced H2 lines. Instead of using individual MIRIS spectra aL average 
over several hours of IRIS spectra will be required to obtain adequate precision. 
Since one hemisphere of Saturn fills the 0.25" field of view of IRIS about 10 days before 
encounter, 'and Titan fills the field of view about one d8y before closest approach to 
Titan, the original objective seems to be obtainable. However, an independent 
measurement of the mixing ratio in  the upper stratosphere, based on the i019 cm-l 
CH4 band, wil probably not be posoible. 
Ammonia 
Only the 200 to 275 em-' part of the rotation spectrum and the region near 
900 cm-l will be available. The 100 to 200 cm-' range may have been useful to see 
uncondensed NH3 in the lower atmosphere of Titan. However, the higher sensitivity 
of lRlS between 200 and 3"-0 cm-' might compensate for its more restricted spectral 
range. 
Trace Constituents 
The lack of sensitivity in spectral ranges of potential atmospheric windows 
(2000, 3700 and 6200 cm'l) will hamper the search for unknown minor constituents 
severely. The availability of the 700 to 900 cm-' range, on the other hand, may make 
a search for hydrccarbons (C2H2, C2H6) in the atmospheres of Saturn and Titan more 
productive. 
Elemental Abundances 
Without fne prospect of good measurements in spectral ranges of potential 
atmospheric windows it seems probable that only the H2/He and possibly the C/N 
ratios will be measurable with precision, 
botopic Ratios 
12 I., A? observation of the D/H and C /C ratios may be possible by averaging 
all available Saturn spectra (20 days). The 2200 cm" C15D band and the 1306 cm-' 
CHp band may then also provide adequate data for analysis. For Titan the same task 
seems margin~l.  
Clouds and Hazes 
Lack of good near IR coverage will have a strong impact on specifying the 
particle composition and size, and the distribution of clouds and aerosols. Averages 
of carefully sorted groups of spectra (rather than individual measurements) may be 
usable. However, this will only allow parametrization of large areas, such as  belts 
and zones rather than of local phenomena. 
Temperabre Profiies 
Atmospheric temperatures a r e  derived from the pr tssure  induced lines caused 
by collisions between hydrogen molecules o r  between hydrogen and heiium or  methane 
molecules. On Titan collisions between methane molecules may also he important. 
Many of these pressure induced Lines fall within the 200 cm-' to 1000 em-' range 
where IRTS is serisitive. In the upper airnospheric layers (up to the 5 mbar level for 
Saturn ar d ilp to much lowt.2 pressures for Titan) the 1306 crn-' methane band can be 
used. Depending on the actual atmospheric temperatures, spectra averaged over times 
ranging from 10 minutes to an hour will be required to obtain temperature profiles with 
adequate precision. In the 100 to 1000 mbar range individunl spectra trill be suffi- 
cient to yield a precision cif I to 2 K with a vertical resolution of approximately one 
scale height. Thus, the task of deriving ahospher ic  temperatures on Saturn and 
Titan seems to be possible. 
Dynmios and Heat Balance 
Information an atmospheric motiozmr is derived from the temperature field. 
For Sa- gbod temperat;rxre data from the hydrogen lines will allow determination 
of the north-south temperature gradient this will then be used to derive the east-west 
wind field. 
Energy balance calculations require knowledge of both the total reflected and 
emitted energy. The radiometer provides a measure of the f irst  and the interferometer 
of the second component. Derived composftional and temperature data will be used 
to extrapolate over part of the thermal spectrum. In fhia case MIRE would have been 
preferable but an adequate estimate of the local and total heat Wee of Saturn can be 
expected from IRIS. 
The absence of the near infrared spectrum will make the identification of sur- 
face minerab and ices much more difficult; hmtwer, some far infrared features such 
as #e 230 cm" water and the 280 cm-I ammonia lattice modes fall within the IRIS 
range, as do features in the spectrum of silicates. 
The measurement of the surface temperature and thermal inertia of the surface 
material cau be carried out even if the satellite does not fill the field of view. This 
condition will actcar more often with TRlS (0.25") then it would have with BfIRIS (0.159. 
In the absence uf strong spectral features a precise surface temperature can be 
established by fitting a Planck function to the measured spectntm. 
All properties of the rings dc-ived fmm the thermal part of the spectrum and 
its variation with emission and phase angles and time can be accomplished by IRE, 
while tasks based on the near infrared reflectivity will be severely iimited, Particle 
sizes or thermal inertia can be estimated from the cooling curves depadirg on the 
mean size of particles. 
SUMMARY 
The scientific objectives formulated in anticipation of MIRE being on bozurd 
Voyager have to be modified because of the different parameters of the IRIS instrument. 
Tasks which depend on dab in tbe 3000-7000 em-' range cannot be accomplished with 
IRIS. This will curtail but not eliminate the search for minor atmospheric constitrlents, 
the characterization of aerosols, and tbe surface commit ion of satellites and rings. 
Investigations based on data between 1000 and 3000 Em-', will suffer in precision o r  
in spatial resolution due to longer averaging times required to compensate for the 
higher NER of IRIS compared to MIRIS. Finally, tasks based on the interpretalion of 
the 200-1000 cm-' range, such a s  temperature and hydrogen to helium measurements, 
will even gain to some degree by the lower NER of IRE3 compared to BlTRIS in this 
spectral region 
REFERENCES 
Gilh F. C.. Foms~ W. J.. and M l l .  K. N. (1973). 8-13 micron ohvrsrtiom of Tim. AJ~&I.  J. 184. 193-L95. 
Ham1 R.. Cor~mh B.. Gauche D.. G i c d  P.. Kumu S.. K w d c  V.. b m a n  P., htag~rr W.. Pati J.. b y l i r  J.. -pcm- 
ma C.. and Sunuclron R. ( 1972. The Voyager I r . h c d  and bdiancrry Inrrniptioo. S p r  Sri. R e .  21. 129- 157. 
Morrison D.. Cruikslunk D. P.. and M e  R. E. ( 1972). TcmprrnuradTiwn and Galilean SPailitcs ac 20 micmns. A ~ I .  J .  173.
LII3-L!46. 
DISCUSSION 
J. CALDWELL: What spectral range will you use to look for the cloud 
features on Titan? If the clouds are methane ice, r m  skeptical that there will be 
featmis that you can idenm.  Certainly, the solids will have absorption features, a t  
lesrst at 8 pm and to shorter wavelengths, but there the gas which overlies the clouds 
will probably be completely opaque. 
J. POLLACK= Water ice has a strong lattice band at about 45 pm, s o  he has 
some hape. 
Dm MORRISON: Could you say a few words about the capability of the 
instrument as a thermal radiometer for measuring the eclipse cooling and heating 
rates of the rings or the satellites? How iow a temperature can you measure, and 
will your sequences allrw vou to observe dark side temperatures on the satellites o r  
eclipse measurements of the rings? 
R. HANEL: For a black body of 55 K, the radiometric signal to noise ratio is 
about 50 to 1. 
D. hlORRISON: Do you anticipate making measurements of ring eclipses 
o r  of the dark sides of tke satellites? 
R. HANEL: Yes. 


What the Voyager infrared investigators hope to learn about the Saturn system

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