The 3-D rotation-vibration band of carbon monoxide was identified in the near-infrared spectrum of Titan. A preliminary mixing ratio of CO/N2 = 0.00006 was determined. This result supports the probable detection of CO2 by Samuelson et al and strengthens possible analogies between the atmosphere of Titan and conditions on the primitive Earth

Debergh, C.

Lutz, B. L.

Owen, T.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19830005768 2020-03-21T05:17:03+00:00Z
r•1-
Ir
(rasi-ca- 169Se0) 21t6: DISCOTUT Of CIDHOY	 v63-14039
UOSOIIDS IP its &T80SPRUZ (Lowell
Obserratorl) 12 P HC 102111! L01	 CSCL 038	 Oaclas
63/91 02076
TITAN: DISCOVERY OF CARBON MONOXIDE IN ITS ATMOSPHERE
Barry L. Lutz*
Planetary Research Center
Lowell Observatory
Catherine de Bergh*
Laboratoire d'Astronoote Infrarouge
Observatoire de Neudon
and
Tobias Oren*
Department of Earth and Space Sciences
State University of New York at Stour Brook
*Visiting astronaoers, Kitt Peak National Observatory, which is operated
by the Association of universities for Research in Astrono1w. Inc..
under contract with the National Science Foundation.
4L
	 t
&s_&Mct Me have identified the 3-0 rotation-vibration band of
carbon monoxide in the near-infrared spectrum of Titan and have
determined a preliminary mixing ratio CO/Ns = 6 x 10's. This result
supports the probable detection of COs by Samuelson et 1. (7) and
strengthens possible analogies betwen the atmosphere of Titan and
conditions on the primitive Earth.
Saturn's satellite Titan has been known to have an atmosphere
since methane was discovered in its spectrum In 1944 (1) . However,
the large mass and marvelous complexity of its nitrogen-dominated
atmosphere was not realized until the successful Voyaber I flyby in
1980 (2, 3) . The list  of atamos pheri c constituents has gram steadily
with subsequent analyses of the Vgyager data.
These spectros sopi c identifications, when combined with the mean
molecular weight deduced for the atmosphere. sees to indicate that argon
may be among those constituents which are present in large quantities (4).
This possibility further suggests a model for Titan in which the entire
atmosphere is formed as a result of degassing of hydrates frozen as
clathrates in the mantle during formation (5). In this model, much
of the N= we now see in the atmosphere could have been trapped in
this way, provided that the proto-Saturnian nebula contained nitrogen
in this diatomic form, rather than in ammonia. This assumption carries
with it the prediction that some CO would be trapped as well. This
association of CO with Na in the nebula (6) provided the original
z
i
-3-
motivation for a search for CO am Titan. The subsequent discovery
of C0& in Titan's atmosphere (7) gave additional impetus to this
searchs, since the existence of COg in such a reducing environment
strongly implies the presence of CO as a precursor.
Here we report the successful result of our search and derive
a preliminary estimate of the CWNs mixing ratio.
During the week of 1-5 June Me, we recorded a series of spectre
of Titan in the 1.6 1a atmospheric winder at a resolution of 1.2 co-'
with the coudi Fourier Trensfam Spectrometer and the 4-meter Neyai l
telescope at Kitt Peak National Observatory. The signal-to-noise
ratio of the co-added spectra (from 5600 to 7200 co"') is approxi-
mately 40:1. reached with a total integration  time of 7.5 hours.
Figure 1 shows a portion of this spectrum in the region from 6300 to
6380 a'', along with spectre of Uranus recorded under similar con-
ditions during the same observing run. A comparison of One two
shows that the spectra of Titan conWas strong absorptions not
present in the spectrum of Uranus. These additional Titanian absorp-
tions are readily identified as the P- and R-branches of the 3-0
rotation-vibration band of 00. The predicted line positions are
also indicated in Figure li and indeed * two of the lines. P(3) and
P(4)q occur in a region uncontaminated by either solar lines or
methane absorptions.
Since the atmosphere of Uranus contains rho appreciable CO and
its spectrum is dominated by methane in the 1.6 pm window (with
f-4
the exception of CNgD absorption near 6425 cm'' (8)). we used our
observations of Uranus to estimate the contribution of mathane
absorption to the Titan spectra. Although the atmospheric structures
	 f
of the two obJects are very differents there is a striking similarity 	 J 'q
between the overall shapes of the methane absorptions, with those
of Uranus being somewhat deeper. Consequentlye after correcting for 	
f
the relative Doppler shifts and scaling the absorption depthse we
used Uranus as a reference to eliminate the maJor effects of the
methane absorption as well ass the solar and telluric lines in the
Titanian spectrum. The resulting residual spectrum, shown in figure 1
for illustrative purposes only, is at best a first-order approximati^n
to the low-tewperature behavior of methane in this spectral region,
but we consider it sufficient for the preliminary analysis reported
here.
Although CO is unequivocally present in Titan's atmosphere; and
we can trace its contribution to the spectrum by P- and R-branch lines
up to J n 99 it is not a simple matter to determine its mixing ratio
from these observations because of the presence of aerosols and possibly
clouds. However, for this preliminary report we have used a simple
reflecting layer model (RLM) to establish the order of magnitude of
the CO abundance.
For the RLN we assumed that at these wavelengths we are "seeing*
to the satellite's surface or to a dense cloud layer just above it.
This hypothesis is the basic premise of the RLN and in this case gains
- 5 -
credence from the observation that weak methane bands in this spectral
region correspond to a column abundance that is sow ton times larger
than abundances derived from bands with comparable strengths at visible
wavelengths (9910).
He computed synthetic spectra for all P(J) and R(J) lines with
J < 10, shown in Figure I t
 and derived a best fit to the residual
spectrum with a total line-of--sight column abundance of raa - 15 m-amagats.
where n - 3 for the ; ametry► of our observations. Using a surface
pressure of 1.6 bars as determined by the Voyager radio occultation
team (2). and assuring that Ns constitutes 82% of the bulk atmosphere
as proposed by the Voyager IRIS team (3). we derived a mixing ratio of
CO/Ne - 6 x 10"5.
This RLN is in fact a gross simplification of the radiative
transfer in the atmosphere of Titan. First, our RLN estimate of
the CO/N,, mixing ratio could be significantly less than the actual
amount if the "reflecting layer" lies much higher than the radio
occultation surface at 1.6 bars. In view of the large column density
estimated for CH. in this spectral region, this circumstance seems
unlikely, but nevertheless is a potential source of uncertainty in
the RLN.
Secondly. the RLN ignores the enhancing effects of scattering
on absorption b; Teak lines such as these * the result of which is to
overestimate the abundance and to fail to account for observed line
shapes. This tatter i nabi 1 i ty of the RLM is evident in Figure 1
,4 -z«
M 6 •
where the sharpness of the CO lines in the synthetic spectrum fait$
to match the observed shapes in the residual spectra.
No encountered this sew type of mismatch in our ALN analysis
of CHA in the spectra of Titan near 6426 cm'' and found that the
Introduction of a scattering hate is required to account for the
observed line shapes of CH4 (8). In the 6426 ca" regions where we
have been able to estimate the temperature dependence of CH4
 absorp-
tion (8). a model with a thin hue layer overlying clear qas above a
thick cloud provides rather satisfactory agreement with the observed
spectrut. This two-cloud model (TCN) yields a CH4 column density of
na s 2.2 km•aaagats, which again suggests that we are in fact seeing
very deep. If the dense cloud corresponds to the radio occultation
surface at 1.6 baro the model further yields a CH 4/N: mixing ratio
near 10-2,, In agreement with the Voyager estimates.
Conseq#Antlys we have ased the seme model for the CO analysis.
The preliminary results are encouraging: although not yet entirely
satisfactory. this TCN provides an improved fit to the observed band
structure over that for the RLN. We obtain from this model a pre-
liminary CO column abundance of na a 12 m•amagats above the dense
cloud and a CO/Na mixing ratio of 6 x 10' s if the dense cloud is at
1.6 bar.
A considerable amount of additional work has yet to be done to
establish our final estimate of the amount of CO in Titan's atoo-
sphere. Proper account o? the temperature dependence of methane
3-7-
absorpti ce in the 6900 cm" region is required to obtain an improved
residual spectrum. Similarly, a refined atmospheric model appropriate
to this region is needed to improve the fit of the synthetic line pro-
files with the observations; and perhaps better observations above
our present signal-to-noise level of 40 may be necessary. We are
continuing our research efforts along these lines; but in the mean-
while, we believe that we have established the abundance of CO in
Titan's atmosphere to within a factor of two.
The detection of CO in this amount requires same additional
discussion in light of the large CD abundance predicted by the photo-
chemical model used to explain the COs detection (7). If the influx
of Hs0 (assumed to be the source of the OH radical required in the
w, reactions producing COs) into the satellite's atmosphere is about
one-tenth the value associated with the meteoritic infall on the Earth,
the observed COs abundance implies CO/Na n 1.4 x 10'' (7). Our result
is well below this prediction, requiring a very large influx of Hs0 if
the proposed reaction scheme is correct, greater than that on Earth.
Thus, our abundance determination implies either that a large local
source of icy grains exists in th6 vicinity of Titan (e.g., the E ring,
impact debris from outer satellites) or other reaction pathways to
convert CO to COs must be found. In either case, the discovery of
CO supports the probable identification of CO. reported by Samuelson
at al. (7) 9 but it does not necessarily follow that the CO we see now
was captured from the nebula as a clathrate by the forming satellite
and is not the result of photochemical reactions. An unambiguous
detection of ssAr remains the best evidence for clathrate capture
process.
The addition of some 50 ppm of CO to Titan's atmosphere, with
the concomitant implicatson of active OH, wakes this atmosphere even
more interesting as a natural laboratory for testing our ideas about
chemistry on the pre-biological Earth. Recent thinking has suggested
that the early Earth mar have had an atmosphere that was less reduc-
ing than the traditional Miller-Urey model (11) . Titan departs most
drastically from this revised Earth model in being far too cold to
permit the existence of liquid water. Nevertheless, atmospheric
chemical reactions taking place on Titan today my resemble some of
those that occurred on the early Earth. Of particular interest is
the issue of preferential pathways: are some routes towards chemical
synthesis favored over others? The 93 K surface will serve as a cold
trap for arty codex organic aerosols that precipitate from the atmo-
sphere; and with the presence of both HCN and CO established, these
compounds should include many of biological interest (12).
:j
I .	
- 9 -
REFERENCES -AND NOTES
1. G. P. Kuiper, Astrophys. J. 1%. 378 (1944).
2. R. Hanel, B. Conroth, F. M. Flasar, V. Kunde, W. Maguire, J. Pearl,
J. Pirraglia, R. Samuelson, L. Herath, M. Allison, D. Cruikshank,
D. Gautier, P. Gierasch, L. Horn, R. Koppagy, C. Ponnamperuma,
ScitgAc 21,x?, 192 (1981); G. L. Tyler, V. R. EsKam, J. D. Anderson,
G. S. Levy, G. F. Tindal, G. E. Wood, T. A. Croft, i_bid.s p. 201;
A. L. Broadfoot, B. R. Sandel, D. E. Shemansky, J. B. Holberg,
G. R. Smith, D. F. Strobe'- J. C. McConnell, S. Kumar, D. M. Hunten,
S. K. Atroya, T. M. Donahue, H. W. Moos, J. L. Bertaux, J. E. Blaeant,
R. B. Pomphrey, S. Linick, ibid., p. 206.
3. V. G. Kunde. A. C. Aikin, R. A. Hanel, D. E. Jennings, W. C. Maguire,
R. E. Samuelson, Science 29 . 686 ( 1.981); W. C. Maguire, R. A. Hanels,
D. E. Jennings. V. G. Kunde. R. E. Samuelson, ibid., p. 683.
4. R. E. Samuelson, R. A. Hanel, C G. Kunde, W. C. Maguire, ibid..
P. 688.
5. T. Owen, J. Mol. Ev. 18. 150 (1982); Planet. S pace Sci. 30. 833
(1982); cf. S. L. Miller, Proc. Nat. Aced. Sci. 47, 1798 (1961).
6. R. G. Prinn and B. Fegley, Jr., Astrofts. J. 24„x, 308 (1981).
7. R. E. Samuelson, W. C. Maguire. R. A. Hanel. V. G. Kunde, D. E.
Jennings. Y. L. Yung, A. C. Aikin, Icarus (in press).
8. B. L. Lutz, C. de Bergh, J.-P. Maillard, T. Owen, J. Brault,
Astrofts. J. 248, L141 (1981). These new spectra of Titan confirm
our previous tentative detection of CH,D. A revised abundance
1T
•10-
estimate wi:l be published separately.
9. U. Fink and H. P. Larson, Astro hm,_,J. 2339 1021 (1979).
10. B. L. Lutz, T. Owen, R. D. Coss, AstMphys. J. 20-3, 541 (1976),
i_ bi d. 2%, 686 (1982) .
11. S. L. Miller and L. E. Orge1. The Ori gins of Life on the Earth
(Prentice Hall, Englewood Cliffs, 1971). Chap. 4; J. C. G. Walker,
Evolution of the Atmosphere (Naomi l l an. Ni4 York, 1977) . Chap. 6.
12. S. L. Ni11er and L. E. Orge1, ibid., p. 47; J. Oro. personal com-
munication, paper in press.
M We thank R. E. Samuelson and Y. Yung for supplying material prior
to publication. This research was supported by NASA throw
grants NSG-7499 and NGR 33015141.
h.
-•:
FIGURE CAPTION
Figure 1. Spectra of Uranus (a) and Titan (b) in the 6950 co"'
region at a resolution of 1.2 co". obtained with the Fourier
Transform Spectrometer a-rd the 4-meter telescope at Kitt Peak
National Observatory. Residual Titan spectra (c) obtained after
using the Uranus spectrum to correct for the major effects of methanes
tellurics and solar absorption (see text for details). Synthetic
spectrum of CO (6-1 u zaputed for a reflecting layer model of Titan.
At top are the rotational assignments and position for the CO 3-0
rotation-vibration band.
a
t
IR'
i
I
r
03
1	 ^)
i	 .
I
i
1
E
i
'
Y
i.
p
Iii
i
i
O
ordarM Pia
OF POOR QUALITY
a
 E EE
to,
u
«
< zo%=
<=
^Oo C^c j aIL <4 2 Z4 E!. 2N ~ Y ^. Y
a _
A
8
R
c	 o	 0
r
r	 '^


Tita: discovery of carbon monoxide in its atmosphere

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server