The new cometary hydrogen particle-trajectory model, completed last year, has been used successfully to analyze observations of Comet P/Giacobini-Zinner. The Pioneer Venus Orbiter Ultraviolet Spectrometer observed the comet at 1216 A (hydrogen Lyman-a) on 11 September 1985 when the comet was 1.03 AU from the Sun and 1.09 AU from Venus. The analysis implies a production rate at 1.03 AU 2.3 x 10 to the 28th power/sec of the water molecules which photodissociate to produce the observed hydrogen. An upper limit for the H2O production rate of Comet P/Halley of 5 x 10 to the 28th power/sec at 2.60 AU was also obtained from the Pioneer Venus instrument

Combi, M. R.

Smyth, W. H.

English

The cometary hydrogen particle-trajectory model was used successfully to analyze observations of Comet P/Encke. The Pioneer Venus Orbiter Ultraviolet Spectrometer observed the comet at 1216A (hydrogen Lyman-alpha) on 15 April 1984, when the comet was .58 AU from the Sun and 1.02 AU from Venus. The analysis implies a production rate at .58 AU of 3.3 x 10 to the 28th power/sec of the water molecules which photodissociate to produce the observed hydrogen

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submission. 
 
 
 
 
 
 
 
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Extended Atmospheres of Outer Planet Satellites
and Comets
(NASA-CE- 174338) EYTENDEE AlrWSPHEELS GF	 N85-17916
OUTLB PLANET SATEIL111S 46L CCCETS laterim
Beport, Aucj. - Oct. 1584 (Atacsiberic and
Environmental Research) 7 F EC A02/CF A01
	 Uncias
CSCL 03B 63/91 01409
William H. Smyth and
Michael R. Combi
Atmospheric and Environmental Research, Inc.
840 Memorial Drive
Cambridge, MA 02139
December 1984
Iuterim Report for the Period
August 1, 1984 to October 31, 1984
Prepared for
NASA Headquarters
^^- y^r.^ -^^i.afcsma^--.,z^ .__ __	 ^,.^; ^.s^:.xa^x,. -	 _ ^. 7^+.^^a^S n^t.si^•iv^'•	 f'.46^.r._3a*ev^iF^+..^t	
-"^
TECHNICAL REPORT STANDARD TITLE PAGE
1. Revert Me. 2	 Goverment Accession Me. 3. Recrpent's Catalog Me.
4.	 T.flo and ► 	 rt	 et•
December 1984
Extended Atmospheres of Outer Planet 6
	
Performing Organisation cede 
Satellites and Comets
7. AueMr(s1 s. Performing Orgenisotion Report Ne
William H. Smyth and Michael R. Combi
9. Performing Orgenis•hen Name end Address 10. worli Unit No.
Atmospheric and Environmental Research, Inc.
840 Memorial Drive
II. Con ► ract or Grent Me.
NASW-3966
Cambridge, Massachusetts	 02139 13. Type of Report and Period Covered
Interim Report12. Sponsoring Agency Name and Address
NASA Headquarters
August - October 1984
Headquarters Contract Divisionq 1. Spenserinq Agency Ceo•
Washington, DC	 20546 HW-2
15. Supple mentory Notes
16. Abstrect
The new cometary hydrogen particle-trajectory model, completed
last	 year,	 has	 been	 ;:aed	 successfully	 to	 analyze	 observations	 of
Comet	 P/Encke.	 The Pioneer Venus Orbiter Ultraviolet Spectrometer
observed	 the	 comet	 at	 1216A	 (hydrogen	 Lyman-a)	 on	 15 April	 1984,
when the comet was .58 AU from the Sun and 1.02 AU from Venus. 	 The
analysis implies	 a production	 rate at	 .58 AU of	 3.3 x	 1028 s-1 of
the water molecules which	 photodissociate	 to produce	 the observed
hydrogen.
17. Kay Word. (Selected by Author(&)) IE. Distrilsution Statement
satellites
comets
19. Security Clossif. (of this report) 20. Security Closslf. (of this page) 21. No. of Page% 22. Price'
Unclassified Unclassified 6
*For sale by the Clrarinphouar kw Frdrrel Scirnrific and Technical Information. Sprinpfaeld, Virginia 22151.
1
I. Prograu of Research for the First Quarter
Research activities during the first quarter have concentrated on the
cometary portion of this project. A collaborative effort with A.I.F. Stewart
from LASP of the University of Colorado has been established for analyzing
cometary observations made with the Pioneer Venus Orbiter Ultraviolet
Spectrometer. The AER particle-trajectory model is ideally suited to
analyzing this type of data.
Analysis of Pioneer Venus Observations of Comet P/Encke
The Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUS) observed Comec
P/Encke at 1216 A (hydrogen Lyman-a) on 15 April 1984, when the comet was
0.58 AU from the Sun and 1.02 AU from Venus. The field of view of the instru-
ment made 270 passes across the coma, taking samples every 4.9 x 10 5 km along
a swath whose width was 3.7 x 10 6 km. The path of the swath made an angle of
about 42° with the comet-sun line as projected on the sky plane. The maximum
obsez^,ed signal was 1.30 kk, including an interplanetary background of 0.43 kR.
The distortion of the hydrogen coma due to solar radiation pressure was evi-
dent. The fully calibrated and reduced data were provided to us by A.I.F.
Stewart.
These data have been analyzed with the AER particle-trajectory model
based on the production of H atoms by photodissociation of H 2O and OH. This
model, developed under previous NASA contracts, calculates exact atom trajec-
tories in three dimensions taking into account both variable solar radiation
pressure and solar gravity, and includes the fully time-dependent spatially
extended random ejection directions of photodissocisted atoms and radicals.
Table 1 lists the numerical parameters of the model run appropriate to
the P/Encke observations. The photodecay lifetimes shown in Table 1 are
varied appropriately as the square of the heliocentric distance, and the
initial H2O velocity is given in the model as 0.58 rH 1/2 km s-1 (Delsemme
1982). The shape of the solar Lyman-a profile used both for the radiation
pressure acceleration as well as for the Lyman-a emission rate, both of which
are heliocentric velocity dependent, was taken from Lemaire et ai. (1978).
The absolute solar Lyman-a emission rate at the time of the observation was
provided by Stewart (1984) from Solar Mesospheric Explorer data. The H2O
production rate was essumed to vary as the inverse square of the heliocentric
2
Table 1
NUMERICAL MODEL PARAMETERS
(RH - 0.58 AU)
T(H20)
T(OH)
T(H)
V(H20)
V(OH)
V(H)
V1-"NUS-COMET-SUN ANGLE
=	 2.9 x 104s
=	 9.6 x 104s
3.4 x 105s
0.76 km/s
1.8 km/s
8, 20 km/s
=	 43.8°
	
G-FACTOR
	
3.74 x 10-3 photons/atoms/s
	
Q(H20)
	
RH -2
3
distance of the comet, as is appropriate for comet-sun distances <0.7 AU
(Delsemme 1976).
Figure 1 shows a comparison of the observed Lyman-a scans with the
modeled scan. The model output includes the trapezoidal slit function
appropriate to the PVOUS data. The model analysis implies a water production
rate of 3.3 x 1028 molecules s -1 from Comet P/Encke at the time of the
observation. This value is in very good agreement with the OH production
rates of P/Encke measured and compiled by A'Hearn, Feldman and Stewart
(1984). These results were presented at the meeting of the Division of
Planetary Sciences of the American Astronomical Society in Kona, Hawaii in
Cctober (Combs, Stewart and Smyth 1984).
II. Program of Research for the Second Quarter
Research activities during the second quarter in the area of comets will
concentrate on initial steps toward the development of the carbon and oxygen
models and writing a paper with A.I.F. Stewart on the PVOUS Comet P/Encke
observations. Initial development of the carbon and oxygen models will entail
an evaluation of the photochemistry of CO 2 , CO and H 2O relating to the produc-
tion of C and 0 atoms.
Research activities in the Saturn system will be continued in the second
quarter. The Titan hydrogen torus model, which was improved to include the
spatial lifetime of H atoms in the Saturn magnetosphere last year, will
undergo further testing in preparation for modeling the Lyman-a data acquired
by the Voyager 1 UVS instrument. This Lyman--a data is currently being
properly formatted by D.E. Shemansky, with whom a collaborative effort has
been established, for use in this modeling analysis.
4
TSCAN #2
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100
}
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N
Z
W
I-
Z
SCAN #1
1000 r1--1T
N
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QC 100
}
H
N
Z
W
I-
Z
10
-10 -9 -e -7 -6 -5 -4 -2 -2 -1	 0	 1	 2	 3	 4	 S	 6	 7	 6	 !	 10
DISTANCE ALONG THE SCAN (10**6 KM)
n
,y -10 -5 -• -7 -6 -5 -4 -2 -2 -1	 0	 1	 2	 2	 4	 5	 6	 7	 f	 10
DISTANCE ALONG THE SCAN (10**6 KM)
Figure 1. Model-Data Comparison for Pioneer Venus Observations
of Hydrogen Lyman-a in Comet P/Encke.
The two scans taken six hours apart on 15 April 1984 are shown as the crosses
in the plots above. The solid lines are for the best fit particle-trajectory
model run as discussed in the text.
5
I
v^^^eegap r!.c-- --°^eti^+u.^r^azcir^.rw.n^-^a..^+^sasnr<.e_+fiaua.rG^i s..r^^^.^nnAi^.=^a'e^^^i^,rw^;^•s[rv-'^^,it
References
A'Hearn, M.F., P.A. Feldman and A.I.F. Stewart, 1984, Bull. A.A.S., 16, 638.
Comb, M.R., A.I.F. Stewart and W.H. Smyth, 1984, Bull. A.A.S., 16, 638.
Delsemme, A.H., 1976, "Comet Kc outek" edited by G.A. Gary, NASA SP-355, 195.
Delsemme, A.H., 1982, "Comets" edited by Laurel L. Wilkening, Universi,:y of
Arizona Press, Tucson, Arizona, 85.
Lemaire, P., J. Chanan, A. Jouchous, A. Vidal-Madjar, G.E. Antzner, J.C.
Vidal, R.M. Bonnet and A. Skumanich, 1978, Ap. J. Lett., 2,'3, L55.
tee,
6


Extended atmospheres of outer planet satellites and comets

An analysis of the extended atmospheres of outer planet satellites and comets is made. Primary emphasis is placed on cometary atmospheres because of the return of Comet P/Halley. As part of a collaborative effort with A.I.F. Stewart, observations of the hydrogen coma of Comet P/Giacobini-Zinner obtained from the Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) were successfully analyzed at AER and are reported. In addition, significant pre-modeling and post-modeling activities to support and analyze the PVOUVS observations of Comet P/Halley successfully acquired in late 1985 and early 1986 are also discussed. Progress in model preparation for third-year analysis of the Voyager UVS Lyman-alpha brightness distribution emitted by hydrogen atoms in the Saturn system is also summarized

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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=19860004740 2020-03-20T16:33:38+00:00Z
.Jr
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(NASA-C6-i7bJ y o)	 EXTENDED ATCOSPHEHES OF	 N86-14209
JUTEH PLANET SATELLITES AND CCCETS Interim
6epurt., Au(4. - Cct. 15E5 'AtLcspheric and
Euvirunmeotal hesEarct,)	 10 p HC ?02/MF A01	 Uucias
CSCL 036 G3/90 046b6
A1MOSPH.ERfC AND ENV1RONMENSAI RESEARCH, INC,
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-AMCRiDGE. MA ACHUSETTS
la)4
^% n
1
Extended Atmospheres of Outer Planet Satellites
and Comets
William H. Smyth and
Michael R. Combi
Atmospheric and Environmental Research, Inc.
840 Memorial Drive
Cambridge, MA 02i39
November 1985
Interim Report for the Period
August 1, 1985 to October 31, 1985
Prepared for
NASA Headquarters
,V- ).
^I
.1
(	 t^
TECHNICAL REPORT STANDARD TITLE PAGE
i	 Report No 2	 Go.ernmenr Access.on No 3. Recip­ r's Catalog No_
.t a end S.6t-5T.—
I
3.f eorl	 et•
Extended Atmospheres of Outer Planet November 1985
Satellites and Comets 6	 Performing 0rgo—satren Code
7	 AYther(s) 8. Perbrm- 9 Orgonisotion Report No
William H. Smyth and Michael R. Combi
9. Performing Orgonisarron Nome and Address 10	 worli Unit No
Atmospheric and Environmental Research, Inc.
11	 Cant.av	 Grant He
NASW-966
840 Memorial Drive
Cambridge, MA	 02139
13. Type of Report and P9r^ed Co.er9d
Interim Report.2. Sponsoring Agency Name and Address
NASA Headquarters A•lgust - October 1985
Headquarters Contract Division 4. Sponsor-ng Agency Code
Washington, DC	 20546 HW-2
11. Supplemen10ry Notes
16	 Abstract
The new cometary hydrogen particle-trajectory model, completed
last	 year,	 has	 been	 used	 successfully	 to	 analyze	 observations	 of
Comet	 P/Giacobini-Zinner.	 The	 Pioneer	 Venus	 Orbiter	 Ultraviolet
Spectrometer observed
	
the	 comet	 at	 1216 A,	 (hydrogen Lyman-a)	 on	 11
September	 1985 when the zomet was	 1.03 AU from the Sun and 	 1.09 AU
from Venus.	 The analysis	 implies
	 a production	 rate at	 1.03 AU of
2.3 x 1028 s -1	 of	 the	 water	 molecules	 which	 photodissociate	 to
produce	 the observed hydrogen.	 An upper limit for the H 2O produc-
tion	 rate	 of	 Comet	 P/Halley	 of	 5 x 10 28 s -1	 at	 2.60 AU	 was	 also
obtained from the Pioneer Venus instrument.
17	 Key Words (Selected by Author(s)) 18. Dis+rtbu+ion Statement
satellites
comets
19.	 S•carity Clostif. (of thin	 report) 20. Sect	 ty Clossif. (of this 9099) 21. No. of Page. Price'
Unclassified Unclassified 8
122.
-For salr by thr (__I nnjthr.usr (m Frdr;91 Ccirntrfre and Trchnresl Informstion, Slxinftheld. Virlime 22151.
1
ttt^	
t>^ ia4 ^.
	 V
I. Program of Research for the First Quarter
Research activities during the first quarter have concentrated on the
comet portion of this project. Analysis of successful hydrogen observations
of Comet P/Giacobini-Zinner by the Pioneer Orbiter Ultraviolet Spectrometer
(PVOUS) is discussed below. A paper summarizing these observations and their
analysis was presented in October at the AAS/DPS meeting (Stewart, Combi, and
Smyth 1985). This represents the second step in our Comet Observing and
Analysis Program, a collaborative effort with A.I.F. Stewart, Principal
Investigator of PVOUS. The first step involved the successful observations
and analysis of Comet P/Encke reported earlier (Combi, Stewart, and Smyth
1984). Nearly continuous observations of Comet P/Halley and their subsequent
analysis, to begin in the next quarter, will initiate the third step of this
program. From limited observations on September 11, Stewart derived a 3s
upper limit of 5 x 10 28
 s -1 for the H 2O production rate from a null PVOUS
observation of Lyman-a in P/Halley when it was 2.60 AU from the Sun. This is
consistent with a vectorial model analysis of IUE OH observations on 12.9
September which implied a production rate of 2 x 10 28 s -1 (Feldman et al.
1985).
Analysis of Pioneer Venus Observations of Comet P/Giacobinin-Zinner
The Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUS) observed Comet
P/Giacobini-Zinner at 1216 / (hydrogen Lyman-a) on 11 September 1985, when
the comet was 1.03 AU from the Sun and 1.09 AU from Venus. The field of view
of the instrument made 2500 passes across the coma, taking samples every
4.4 x 10 5 km along a swath whose width was 1.7 x 10 6 km. The path of the
swath made an angle of about 47° with the comet-sun line as projected on
the sky plane. The maximum observed signal was 405 R, including an inter-
planetary background of 261 R. The fully calibrated and reduced data were
provided to us by A.I.F. Stewart.
These data have been analyzed with the AER particle-trajectory model
(PTM) based on the production of H atoms by photodissociation of H 2O and OH.
This model, developed under this and previous NASA contracts, calculates exact
atom trajectories in three dimensions taking into account both variable solar
radiation pressure and solar gravity, and includes the fully time-dependent
spatially extended random ejection directions of photodissociated atoms and
radicals.
2
+0
f -0
The numerical parameters of the model are the same as those used
previously for the Lyman--a P/Encke observations (Combi, Stewart, and Smyth
1984) and are scaled in the PTM appropriately for the orbit of P/Giacobini-
Zinner. Therefore, all time, heliocentric distance, and heliocentric velocity
dependences are automatically taken into account. The H 2O production rate was
assumed to vary as the inverse cube of the heliocentric distance, as is
appropriate for H 2O sublimation near 1 AU.
Figure 1 shows a comparison of the observed Lyman-a scan with the modeled
scan. The model output includes the trapezoidal slit function appropriate
to the PVOUS data. The model analysis implies a water production rate of
2.3 x 10 28
 molecules s -1 from Comet P/Giacobini-Zinner at the time of
the observation. This value is bracketed by the OH production rate of
P/Giacobini-Zinner obtained by 18 cm radio observations by Bockelee-Morvan
et al. (1985) in August (1.8 x 10 28 molecules s -1 ) and October (2.9 x 1022
molecules s -1 ). Contour plots of the model two-dimensional Lyman-a intensity
as seen by the Pioneer Venus Orbiter as well as seen normal to the comet plane
are shown respectively in Figure 2 and Figure 3. Because of the viewing
geometry, the distortion of the hydrogen coma due to solar radiation pressure
is more obvious in Figure 3 than in Figure 2.
1
^ 	 I
i
it
3
J.I
l-'1
{i
II. Program of Research for the Second Quarter
1:
s^
Research activities during the second quarter in the area of comets will
involve pre paring model results to be used for initial interpretation of Comet
P/Halley data to be obtained by the Pioneer Venus Spacecraft by A.I.F. Stewart
starting in late December. Analysis of the PVOUS data will likely be
initiated in January 1986 as it is available from Stewart. Additional effort
in the second quarter will also be directed toward writing a paper to document
the AER particle-trajectory model.
Research activities in the Saturn system will be continued in the second
quarter. Significantly improved values of the electron and ion number
densities and temperatures in the Saturn magnetosphere are expected early in
the second quarter from E.C. Sittler as part of our continuing collaborative
effort with him. This information will be used to refine in our Titan
hydrogen torus model the lifetime description of H atoms in the Saturn
magnetosphere. Exploratory model computations will then follow as we initiate
our analysis of the Lyman-a data acquired by the Voyager 1 and 2 spacecrafts
and recently properly formatted by D.E. Shemansky for this purpose.
It
4	 M
11
5I	 1
1
i
`^^
References
Bockelee-Morvan, D., Bourgois, G., Crovisier, J., and Gdrard, E. (1985) Radio
Ob s ervations of OH in P/Halley and P/Giacobini-Zinner. Poster 13.18
presented at the AAS/DPS meeting Oct. 28 - Nov. 1, 1985, Baltimore, MD.
Combi, M.R., Stewart, A.I.F., and Smyth, W.H. (1984) An Analysis of Pioneer
Venus Hydrogen Lyman-a Observations of Comet P/Encke. BAAS 16, 638.
Delsemme, A.H. (1982), Chemical Composition of Cometary Nuclei, in Comets,
(L. Wilkening, ed.), University of Arizona Press, Tucson, AZ, p. 85.
Feldman, P.D., et al. (1985) IUE Observations of Comet P/Halley in September
1985. BAAS 17 (late paper abstract).
Rottman, G.J. (1985) Private communication.
Stewart, A.I.F., Combi, M.R., and Smyth, W.H. (1985) Lyman-alpha Measurements
of P/Giacobini-Zinner. BAAS 17, 686.
•I
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OBSERVER VIEW INTENSITY (RAYLEIGHS)
.oG
sL 0G^,!tea 	
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0
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O	 _	 O
~	 OO
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^	 O• f
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•O	 4j•
,D O
.0
F— sun
DISTANCE FROM THE NUCLEUS (BOX SIZE= 1.25E+06KM)
Figure 2
A 2-D map of H Lyman-a from the PTM analysis is shown with the Pioneer Venus
Swath. North is down and East is to the left.
1
(-.*I
TOP VIEW INTENSITY (RAYLEIGHS)
5.0
0 S
.0
,00	
^0 p
?p.0
f
g^.0
00n
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J	 ^
O	 0
0	 O
	
50 .0	 .y^.
so_	 \	 10.0	 5.
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5.0
DISTANCE FROM THE NUCLEUS (BOX SIZE= 1.25E+06KM)
Figure 3
An out-of-plane view of the 2-0 H Lyman-a map from the FTM analysis is
shown. This view maximizes the apparent radiation pressure distortion.
Z<^/
	
J4


Collisions between neutral hydrogen atoms in the interstellar medium and those in the so-called Titan hydrogen torus may provide an additional lifetime sink for atoms in the Saturn environment. Progress toward re-sorting the Voyager UVS scans of neutral hydrogen in the Saturn system to enable both a factor of two increase in the amount of data to be analyzed as well as to help identify near-Titan hydrogen is discussed. Progress toward development of the cometary carbon and oxygen models is also discussed and a preliminary model run for the H2O source of cometary oxygen is presented

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=19850016747 2020-03-20T19:27:14+00:00Z
(NASA-CH-175673) EXTENDED ATMOSPHERES OF 	 U95-25058
OUTER PLANLT SAHILISES AND CCCRTS Interim
Report, Feb. - Apr. 1585 (Atmospheric and
Environsental Research) 	 10 p HC A02/MF A01 	 Unclas
CSCL 03R G3/91 14876
Extended Atmospheres of Outer Planet Satellites
and Comets
William H. Smyth and
Michael R. Combi
Atmospheric and Environmental Research, Inc.
840 Memorial Drive
Cambridge, MA 02139
April 1985
Interim Report for the Period
February 1, 1985 to April 30, 1985
Prepared for
NASA Headquarters
n^
4
f1EC11NICAL REPORT STANDARD TITLE PAGE
I. Report No. ?. Go.ernment Acerst.on No. 7. Recipient'. Catalog No.
47 q can	 u 5, Re perl	 ale
Extended Atmospheres of Outer Planet April 19856. Performing Organisation CodeSatellites and Comets
w
7. Author(s) 8. Performing Organisation Report No,
William H. Smyth and Michael R. Combi
9. Performing Organisation Nome and Address 10, Wart Unit No..
Atmospheric and Environmental Research, Inc.
840 Memorial Drive 11. Contract or Grans No,
Cambridge, Massachusetts 	 02139 NASW-3966
17. Type of Re port and period Covered
Interim Report17. Sponsoring Agency Nome and Address
NASA Headquarters February - April 1985
Headquarters Contract Division c• Sponsoring Agency Code 
Washington, DC	 20546
15. Supplementary Notes
i
6. Abstract
Collisions between neutral hydrogen atoms in the interstellar
medium and those in the so-called Titan hydrogen torus may provide
an additional lifetime sink for atoms in the Saturn environment.
Progress toward re-sorting the Voyager UVS scans of neutral hydro-
gen in the Saturn system to.enable both a factor of two increase
in the amount of data to be analyzed as well as to help identify
near-Titan hydrogen is discussed herein. 	 Progress toward develop-
ment of	 the cemetery carbon and oxygen models is also discussed
and a preliminary model rt_a for the H 2O source of cometary oxygen
is presented.
17. Key Words (Selected by Autbor(s)) 19, Distribution Statement
satellites
comets
19. Seaurify Clossll. (of this report) 20. Security ClossiL (of this page) 21. No, ofPages 72. Price-
Unclassified Unclassified 9
-Far .I, try the Clnringhnuce for Federal Scientific and Technical Information. Sr4ingfield. Virt,inis 22151.
r	 I
^i
a
i
•	 r	 ^
I. Program of Research for the Third Quarter
Research activities during the third quarter have concentrated on (1)
piI	early development of models for cometary carbon and oxygen, (2) beginning a
paper which documents the AER particle-trajectory model, (3) identification
of a new possible lifetime sink for neutral hydrogen in the Saturn system, and
(4) initiation of a new processing step at LPL fo; analysis of Voyager
UVS Lyman-a scans of hydrogen in the Saturn system.
1.	 Cometary Atmosphiires (
Work has begun in the area of modeling the extended atomic carbon and
oxygen clouds in comets. 	 The spatial distributions of cometary C and 0 have
f
been observed in the emissions at 1657 A and 1304 A respectively in Comet
tI
Kohoutek. 1973XII (Opal and Carruthers 1977).	 Smith et al.	 (1980) have {
measured the distribution of carbon in Comet West 1976VI at 1657 A and also #j
C('D) at 1931 A.
	
Weaver et al. (1981) have reported IUE observations of C and
0 in a few faint comets, but these were only single nucleus-centered bright-
^1ness values.
As for most cometary observations, all of these data have been analyzed
in terms of !laser's model, which cannot be used to provide reliable physical
information regarding lifetimes, outflow velocities, or exothermic ejection
velocities.	 Using the particle-trajectory method, we can provide a physically
realistic framework in which true source and sink characteristics and their
spatial and temporal dependencies can be understood.
To the best of our knowledge, the sources of the bulk of the observed
carbon and oxygen lie in the photolysis of cometary H2O, OH, CO and CO2
(Delsemme 1982).	 Carbon production from the visibly prominent carbon
radicals, CN, C2 , and C 3 , only accounts for a minor fraction of total carbon
in comets (Feldman 1978, A'Hearn and Feldman 1980). 	 Although the existence of
CO2 ice in comets is strongly suggested by the presence of CO 2+ in ion tails,
Feldman (1978) has put forth some convincing arguments in favor of CO as the
ultimate parent of much of the cometary carbon. 	 With this in mind, we have
constructed photochemical scenarios for the production of carbon and oxygen
with H2O, CO and CO 2 as primary parent molecules and will constrain our
conclusions by the appropriate observations.
2
,Table 1 shows a list of the branching ratios and exothermic velocities
for the production of C and 0 from H 2O, CO2 and CO. The total photolysis
rates for these three parents are 1.2 x 10 -5 s-1 , 2.0 x 10-6 s-1 and
6.5 x 10 7 s-1 , respectively. The data for CO and CO2 are front Huebner and
Carpenter (1979); those for 112 0 are from Festou (1981) to which the 0('D)
branching ratio has been updated (Slanger 1982).
The 1120 source of cemetery oxygen has already been implemented in the
particle-trajectory model and a test run, based on the geometry and H2O
production rate corresponding to the hydrogen observation of Comet P/Encke by
the Pioneer Venus Orbiter, has been made. Similarly to the hydrogen Lyman-a
case, the emission rate factor (g-factor) for the oxygen 1304 A triplet is
highly dependent on heliocentric velocity. The 1304 A emission is excited by 	 1
resonance scattering with the solar oxygen emission as well as by fluorescence
by solar Lyman-S. This velocity dependent g-factor as calculated by Feldman
(1982) has been adopted in the model and is shown in Figure 1. 	 It
The modeled 2-D sky 	 ane view of the 1304 A p	 oxygen emission as viewed
for the Pioneer Venus geometry is shown in Figure 2. The nucleus-centered
slit position of the Pioneer Venus OUVS instrument should have recorded a
slit-averaged intensity of only 1.6 Rayleighs which is unfortunately one order
of magnitude below the background noise limit for the two hour integration
attempted by Pioneer Venus on 15 April 1984.
2.	 Titan Hydrogen Torus
During this past quarter, W. Smyth visited D.E. Shemansky at the Lunar
and Planetary Laboratory of the University of Arizona in Tucson to further our
ongoing collaborative effort to analyze the hydrogen Lyman-a emissions from
the Saturn system obtained by the UV instrument aboard the Voyager
spacecrafts. The Titan hydrogen torus is expected to be an important source
of H atoms for these observed emissions. By comparison of the Titan torus
model calculations, to be performed at AER, with the Voyager data, the Titan
l
source rate will be evaluated and other possible non-Titan sources will also
be investigated. The visit to Tucson was most productive. Two important
aspects that have resulted from this visit are discussed below.
A primary subject of the discussion during the Tucson visit was the
Voyager 1 data set, which consists of ease-west' slit scans of the Saturn
system. The Lyman-a radial bright ,nss profile obtained from about 25 percent
3
4of these scans was reported earlier by Broadfoot at al. (1981). Additional
processing that has been completed in this quarter by Shemansky now makes
available for our analysis more than 50 percent of these scans. This increase
in available information will not only improve the statistics for newly
constructed Lyman-a radial brightness profiles but will also be used to
determine if there is a heightened signal at Titan's location in the east-west
scans. The analysis of the scans for this signature will be completed by
Shemansky early in the fourth quarter and will be utilized in our future
model-data comparisons.
A new lifetime sink for the hydrogen torus was also identified as a
result of the Tucson visit. This sink is the removal of H atoms from the
torus by elastic collisions with the rapid flow (-20 km s -1 ) of the
interstellar medium (mostly H atoms) through the solar system. Most torus H
atoms involved in these collisions can gain sufficient additional energy
(0.3-0.03 ev) so that they are gravitationally lost from the Saturn system.
The new lifetime mechanism for torus atoms thus depends upon the elastic
cross-section between hydrogen atoms for transfer energies that are relatively
small. This cross-section for such small energy transfers is fairly large and
is currently being evaluated by Shemansky. The torus lifetime for this cross-
section is expected to compete with the I1-atom lifetimes produced by the
planetary magnetospheric plasma and the solar wind (z 1 x 108 sec), both
reviewed in the second quarter progress report. The potential impact of this
new lifetime process may be threefold: (1) to reduce the long integration
times for H-atom trajectory orbits that would otherwise be required to model
the torus properly, (2) to play a significant role in regulating the number of
H-atoms in the torus, and (3) to provide a 30 year modulation of the number of
H-atoms in the torus because of orbital motion (-9.6 km/sec) of Saturn about
the Sun. Because of the first of these potential impacts, further testing
originally scheduled this quarter for the Titan hydrogen torus model to
investigate more efficient ways to calculate very long integration time
trajectories was postponed.
1
5II. Program of Research for the Fourth Quarter
Research activites in the Saturn system will involve further evaluation
of the new lifetime sink for the Titan hydrogen torus and its incorporation
into the Titan torus model. Voyager I data for east-west scans of the
hydrogen Lyman-a brightness profile will be obtained from Shemansky and steps
to analyze these data will be initiated.
Research activities in the area of the extended cemetery atmospheres will
involve continued development of the carbon and oxygen models, continued work
on the paper which documents the comet model, and preparation for the
September 1985 Pioneer Venus Orbiter UVS observations of Comets P/Giacobini-
Zinner and P/Halley.
1
I
6References
A'Henrn, M.P. and P.D. Feldman (1980), Astrophys. J. 242 0 L187.
Sroadfoot, A.L. et al. (1981) Science 212, 206.
Delsemme, A.H. (1982) "Chemical Composition of Cemetery Nuclei" in Comets
(ed. L. Wilkening), University of Arizona Press, p. 85.
Feldman, P.D. (1978) Astron. & Astrophys. 70, 547.
Feldman, P.D. (1982) "Ultraviolet Spectroscopy of Comae" in Comets (ed. L.
Wilkening), University of Arizona Press, p. 461.
Festou, M.C. (1981) Astron. Astrophys. 96, 52.
Huebner, W.F. and Carpenter, C.W. (1979) Los Alamos Scientific Lab Report,
LA-8085-MS.
Opal, C.S. and G.R. Carruthers (1977) Ap. J. 211 1 294.
Slanger, T.C. (1982) Private communication.
Smith, A.M., T.P. Stecker and L. Casswell (1980) Ap. J. 242, 402.
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Extended atmospheres of outer planet satellites and comets

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