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'O

(NAs?,-cR-175381) SCIENITFIC ,PROGRAM IN N84-19263

PL*(I ` `•TAR Y ATMOSPHER-LC STUDIES Final

Te:.,,r rd !cal Report, 1 May 1980 - 31 Dec. 1983

+ .,..;:coriiia Univ.) 21 p HC A02/MF AO 1 Unclas

C:SC1 03B G3/91 12400

Final Technical Report to the

National Aeronautics and Space Administration

SCIENTIFIC PROGRAMS IN

PLANETARY ATMOSPIIERIC STUDIES

WASA Grant NAGW-62

May 1, 1980 through December 31, 1983

Submitted by

A. Lyle Broadfoot, Principal Investigator

Former affiliation: Center for Space Sciences 4'

University of Southern California

Los Angeles, California 90089 s

Present affiliation: Lunar and Planetary Laboratory

University of Arizona

3625 E. Ajo Way

Tucson, Arizona 85713
I. INTRODUCTION

The Voyager encounters with Jupiter led to two main areas of

investigation: 1) the definition of the structure and composition of the

upper atmosphere and the interaction of the tnagnetoshpere and atmosphere, and

2) the study of the plasma ton,,. using the EUV (Extreme Ultraviolet) data in

conjuncCl,l on with ground—based and in— situ measurements. In the course of

these invvastigations, the atmosphere studies were extended to a comparative

study with the bound atmospheres of Saturn and Titan; and the torus study

expanded to include the extended atmospheres of Titan ( the H torus) and the

rings of Saturn. References listed in the appendix were partially supported

by this grant.

II. IO ' S PLASMA TORUS

1. Introduction

The discovery of Io's warm plasma torus by the Voyager Ultraviolet.

Spectrometer ( UVS) has opened new fields of research in the topics of space

plasmas and energizatiou mechanisms. Interpretations of the EUV spectrum of

the torus in terms of a plasma of singly and doubly ionized sulfur vInd oxygen

atoms with an electron temperature near 8 x 10 4 K were largely verified by in

situ measurements made during the Voyager encounter period. Since then,

continuing interpret a tion of in situ and remote (EUV) measurements made by`

Voyager has been supplemented and extended by ground-^based observers and using

the International Ultraviolet Explorer satellite.

This program was initiated as a result of the Voyager encounters at

Jupiter to support reduction, analysis, and interpretation of data acquired by

the Voyager Ultraviolet Spectrometers and bearing on the questions of the

plasma torus. The basic goal was to use our EUV observations of the torus

emissions, combined when appropriate with information from ground based remote

sensing and in situ measurements, to improve our understanding of the physios

of the plasma torus. These data bear directly on such key questions as the

composition and the energy and mass budgets of the torus.

At their Saturn encounters, the Voyager UVS instruments provided the

first definitive evidence for the existence of an extensive cloud of atomic

hydrogen predicted on theoretical grounds to be associated with Titan. The

scope of this program wasp subsequently expanded to include an investigation of

this phenomenon in collaboration with D.M. Hunten of the University of Arizona

and a graduate student, D. Hilton, who intends to base his PhD thesis on his

investigation of Titan's neutral hydrogen torus. This cloud of H extends

between 8 and 25 R in the plane of Titan's orbit and about 7 B S above and

below the plane. The H density and the lifetime of neutral H against

ionization are large enough that collisions probably play an important role in

determining the distribution of tha H. Collisional redistribution of H

velocities takes on even greater importance if the H is accompanied by larger

amounts of Ha, as expected on theoretical grounds.

_ _ „ .:. ^ ..: ,,. saw.. -r -_.^ ..__a — — „r► ` , .,...,.. ,^, ,,, t-.;s.
The majol, thrust of the effort was directed toward an improved

understanding of Io's plasma torus. In the following section, progress made

so far is summarized.

2. Summary„ of Progress

Analysis of the spectra of the EUV emission of the torus have led to a

fairly complete understanding of its ion composition and the temperature of

the electron gas, The plasma consists primarily of a singly, doubly, and

triply ionized sulfur ( S II, S III, and S IV) and singly ionized oxygen atoms

(0 II) (Shemansky, 1980a). These atoms a re collisionally excited by the

-8 x 10 4 K electron gas of the plasma and emit bright EUV radiation. About

2 x 10 13 W is emitted in the EUV; this is the dominant mechanism for energy

loss from the torus. The ion concentrations inferred from spectral analysis

(Shemansky 1980u , b; Shemansky and Smith, 1981) and the spatial distributions

of the ions inferred from the EUV morphology ( Sandel and Broadfoot, 1982a) are

in general, although not precise, agreement witk in situ measurements.

Indications are that including the details of the radial variation in the

plasma electron temperature derived from the EUV observations in models of the

excitation process removes at: P-pparent discrepancy ( Sandel and Broadfoot,

1982a) between in situ and remote (UVS) determinations of ion density near the

outer edge of the torus.

The spectral analysis of torus emissions has been expanded to large—scale

morphological studies of the torus. Building on information derived from

spectral analysis, these morphological studies have .led to new insight into

the mechanisms that supply energy to the electrons of the pla • :: — a crucial

link in our understanding of the physics of the torus. Althot I it is likely

that the ultimate source is the rotational energy of Jupiter, the details of

the energy transfer process are not well known. Sandel and Bro#dfoot ( 1982a)

showed that the volume emission rate of the dominant S II1 685 A feature in

the torus spectrum is 5095 greater near the dusk meridian than near dawn. They

demonstrated that the brightness difference was the result of a difference in

the plasma electron temperature rather than ion density, and therefore tied

directly to the energy budget of the torus.

The dawn—dusk brightness asymmetry was interpreted by Shemansky and

Sandel ( 1982) as evidence for localization of torus energy input, In this

interpretation, most of the energy was supplied to ^,he sunward half of the

torus, centered near local noon, and cooling of the b lectron gas was by

radiation only. Later Barbosa and Kivelson ( 1983) and: Ip and Goertz (1983)

independently proposed an alternative explanation based on the possibility of

a dawn—dusk electric field shifting the drift orbits of charged particles by

about 0.2 RX ( Barboss and Kivelson) toward dawn. Adiabatic compression of the

electron gas in the dawn— to—dusk half of the orbit would then provide the

observed increase in temperature from dawn to dusk. Thas the heating is of

much the same form as described by Shemansky and Sandel. However, in the

alternative models, adiabatic cooling of the plasma in the dusk—to—dawn

portion of the orbit relieves the requirement imposed by assuming only

radiative cooling, and permita the distribution of energy supply- to be more

uniform in azimuth than demanded by Shemansky and Sandel. In Barbosa and

Kivelson ' s view, the electric field is attributed to the escape of plasma down

the tail of Jupiter ' s magnetosphere. Through this theory, the UVS
y

observations are related to large — scale magnetospheric phenomena, and may

serve as a useful diagnostic in this field of research, if the relationship

can be verified.

A correlation between the EUV brightness of the torus and the rosition^of

Io has led to a direct observation of energy flow into the torus p lectron gas

(Sandel sad} Broadfoot, 1982b). The plasma downstream from Io is brighter in

S III 6fij A emission, reflecting a higher electron temperature there. The

electron tomperatuice is raised by a mechanism operating with 45° of the

azimuthal position of Io in its orbit; the interaction region may extend over

a much smaller range of azimuth. Differences in electron temperature inferred

from spectral analysis account for the observed difference in brightness,

implying that no change in the composition or density of the warm plasma

occur. The lo—correlated source provides a time-averaged power of 8 x 10 -14

erg cm- 3 sec-1 (4 x 10 11 W) or about 20% of the power radiated by the torus.

A persistent puzzle has been the difficulty in reconciling ground—baked

measurements of azimuthal variations in ion concentrations with the picture of

near—uniform distribution of S III in azimuth implied by the EUV observations

(Sandel and Broadfoot, 1982a). In contrast ground based measurements of S II

(e.g., Pileher and Strobel, 1981) and more recently S III (Morgan et al.,

1983) show strong azimuthal variations in the concentrations of both ions with

a peak near k II N 180°, Recently Hill (1983) has suggested a resolution

based on thla fact that the ion velocity falls slightly below co—rotation

velocity: With the proper relationship among the ionization lifetimes and the

System III drift period, a strongly asymmetrical distribution of S IT would

not be carried over to the S III distribution, However, Shemansky (private

communication, 1984) has pointed out that certain critical ionization rates

used by hill are probably incorrect; using better estimates of these values

destroys the relationships necessary for the theory to work. The resolution

of this dilemma is not yet known it therefore seems likely that it is related

to a fundamental property of the plasma torus not yet understood.

Anallsis of the EUV observations has recently revealed a weak periodicity

of 10 14 in the brightness of the eastern and western ansae of the torus

(Sandel, 1983). This period (see Figure 1) exceeds the System III rotation

period by about 3%, and is generally consistent with brightness variations in

visible wavelengths observed by Roesler et al. 1984 and with ion velocity

measurements reported by Brown (1983). These latter measurements were

interpreted in terms of corotation lag near 6 RJ , but this is probably nct the

proper interpretation of the E1N observations. This is because spectral

analysis indicates that the EUV brightness variations are driven by a

difference in electron temperature, rather than an azimuthal variation in ion

concentration. This periodicity may be another direct manifestation of energy

flowing into the torus. A particularly notable aspect of thisdiscovery is

the fact that the presence of the modulation at the eastern ansa depends on

the position of Io, as shown in Figure 2, but the modulation is always present

in the west. The picture is complicated by a period very near that of System

III that appears in the east, bu •. not in the west. We are left with a complex.

set of related signatures, as yet only partly understood and certainly not

within the capability of current theories and models of the physics of the

plasma torus.
I

A,

I,, 1

EAST I

0.10

ORIGINAL PAIGE 19

OF POOR QUALITY

3a

0.20

z

z 0.30

2

C0

Q

M1

0.20

T\^

WEST

0.10 

0.95 1.00 1.04 1.08

PERIOD FACTOR

Figure I

The discriminant is a measure of local scatter of the brightness

vs. phase curve, with smaller values representing less scatter. This

quantity is plotted in terms of the assumed period, expressed as a ratio

to the System III rotation period. The pronounced dip at 1.031 at bath

e$st and west ansae indicates that the EUV brightness have pomponen'ts

thiA vary with a period of 1.031 x (System III period) = 10 14 m . In the

east, the modulation at this period is much weaker than in the west, and

the east also shows modulation very near the System III period.

'".._ ' _ .. ^. .- _. .^. -- .,rte r .a'F i y I•„..4s..— — -
IZ 

<t 

Z 

~ 

0:: 

o 

CI) 

Cl 

OR!GINAL PAGE is 

OF POOR QUALITY 

0.20 r---Ilr---~'----~J---r-'--~'----~I----~'---

EAST 10 in front 

-

I I I I I I I 

0.20 I 

0.10 

" ;':-'~"·'''.L ,"~' : "-.'. '. . .... ,;,.,.i ... , ~ ". h:' .';1';;' • "-6 ~',.."'~~'" ~ '~,'l.."",:" .. /I..!' :.:".'~~l'''''' •..• '~" -.~ I'~~r. ,;u.·.··"· ...... ~I •• / ',',:,'. ,'1,." ,;.~( "'~~-.. T.~f' .'I'~,I I .,,,~" _,,'. • • , 'f ,It , '. I' ':" '.: 't' \"'-. ,if;;1 • 

f t' " .. ,/" 

' .. , 1-'; 

" 

r 

EAST 10 behind 

0.96 1.00 1.04 1.08 

PERIOD FACTOR 

Figur~ 2 

3b 

Data from the eastern ansa divided into two cate8orie~ - thOle for 

which 10 was -in the front half of its orbit- of lupite~ (90 (. ( 

270), where 410 is the orbital phase of 10, and thOle for which l¥ was 

in the "back lialf- ('r < 90 or 'I >270). The featur~ at 1.03 is 

present only when 10 1~ in the frogt half of its Qrbit. This clearly 

demonstrates that 10 exercises some kind of influence over the production 

of this modulation. 
4

III. THE RINGS pr, SATURN: 111L'IR A1110SPHERE AND STRUCTURE

1. The Natur of the, Ring Observations and Progress to Date

During the Voyager 1 and 2 lncounters with Saturn the ultraviolet

sractromcters made fundamental observations of Saturn's rings which can be

grouper; into three broad categories. first were the occultations of bright

stars by the rings; three such occultations were observed. Second was a

remarkable, long integration, spectrum of the rings in reflected sunlight

obtained by Voyager, 2. Third Was the extensive set of observations of the

rings in HT Lyman a (La, 1216 A) obtained by both spacecraft. This latter set

of observations was obtained over a wide range of observing geometries and

constitutes the best avPilable data base on the ring atmosphere. The primary

focus of the study was the analysis of the ring atmosphere observations. As

will be seen, however, both the occultation and vefl.eetance observations Lre

closely related to the analysis of the ring atmosphere data. In the following

sections the progress which has been made in each of the areas is reviewed.

2. Stellar Occultations

Since the Saturn encounters a great deal of effort has gone into the

reduction and interpretation of the stellar occultation data. As mentioned

previously, three separate such events were observed; two extending from inner

to mid C ring and one providing a continuous record of ring opacity wcross the

entire ring system at — 3 km resolution. In all of these observations the UVS

was effectively converted into a, single channel photometer, since at UVS

wavelengths the attenuation of light by the rings appear4 to have no spectral

dependence (Sandel, et al., 1982). A large number of important results has

come from the UVS occultations, mainly in the area of ring dynamics. Nrmerous

features associated wit: satellite resonances have been identified (Holberg,

Forrester and Lissauer, 1982; Holberg, 1982). Analysis of density waves

associated with strong resonances in the A ring (Holberg, Forrester and

Lissauer, 1982) prov.,ded the first surface mass densities in the A ring and

the first reliable determlua"iou of the total mass of th. ring system. Other

features found to be associated with resonance phenomena in the rings include

the outer edge of the A rs.nV which was discovered to be coincident with the

strong 1:6 resonance with Janus (Holberg, Forrester and Lissauer, 1982) and

singular sharp edged featur,^s associated with the five strongest resonances in

the C ring (Holberg and Forrester, 1983).

This phase of the analysis of the UVS ring occultation data is now

complete and a review of the results will apear in Esposito et al. (1983x)

along with other Voyager related results on ring dynamics. Work in this area

is now primarily in the area of collaborative studies involving other Voyager

experimenters and instruments. A multi —disciplinary study of the eccentric

ringlet in the Maxwell Gap (1.45 Rg

) involving UVS radio, photomultiplier and

imaging data has recently been published (Esposito eA al., 1983b). Two

extensive collaborative studies of additional eccentric ringlets (Porco et

I., 1983a) and the eccentric outer edges of the A and B rings (Porco et Al.,

1983b) have recently been completed and submitted for publication. Finally,

novel joint analysis of UVS and radio occultation data has led to a

substantial improvement in the locati , n of Saturn's rotational pole (Simpson,

Tyler and Holberg, 1983). Such collaborative efforts continue.

R y

_. ,. i.. -T. ^ ^ .. ..t

_.'.

."T"..^n•• `-: iw-^f v^.s ..... ^ r.*. ^ .^^!^ !/ ^^ije` ^r'6 `; \
3. Refleatanee Spec

Several days prior to the Voyager 2 encounter with Saturn a considerable

amount of time was devoted to imaging 'spoke' activity in the B ring. This

provided the UVS with a unique opportunity to obtain a very long (^40,000 a)

integration spectrum of the rings in reflected sunlight. This rint; spectrum

is sbovn in Figure 3 along with a comparison spectrum of the sun obt.ailled

through the UVS occultation port. Far from being merely a curiosity, the ring

spectrum contains information on the refleotivity of the ring particles at

wavelengths well shortward of any previous observation. A preliminary

analysis of this spectrum (Sandel et al,, 1982) indicates a relatively flat

reflectance spectrum between 600 and 1100 A and reflectances on the order of

5% between 1300 and 1400 A. The l,ater o result is consistent with measurements

of ring reflectivity at 1650 and 1700 A obtained by Wieser rnd Moos (1978)

from a rocket flight.

4. The Ring Atmosphere

71e realization that the ring particles are composed largely of water ice

naturally leads to the expected presence, at some level, of a ring

'atmosphere' of 1I10 and its dissociation products. Such an atmosphere would

represent an equilibrium between those processes (sources) which produce or

free molecules from ice surfaces in the rings, and those processes whioh lead

to either the re —adsorption of. gas molecules or their final loss from the

rings due to ionization or neutral escape. Initial source estimates

(Dennefeld, 1974 and Blamont, 1974) involving such mechanisms as thermal

sublimation, meteoritic impact, and solar and interstellar wind bombardments

all lead to low production rates and atmospheres so tenuous as to be

undetectable.

The most sensitive method for detecting a t1ing ,atmosphere is through

observation of solar scattered Lyman a (La) emission from neutral hydrogen.

Such hydrogen, presumably formed by photodissociation of H 2 0, OR and ti t , would

form a bright La cloud similar in some respects to those observed to develop

around comets. The first detection of La emission from Saturn's rings came

from a sounding rocket observation by Weiser et al. (1977) who measured 200

100 Ravleighs (R) of La emission from the generr, l ticinity of Saturn's rings.

This measurement was followed by observations of higher spatial resolution

with Copernicus ( Barker et al., 1980), and ME ( Clarke It el., 1981) both of

which produced upper limits to the La brightness of the rings less than that

of Weiser et al. During the Pioneer 11 encounter with Saturn the long

wavelength UV photometer observed an enhanced UV emission ( presumably La)

while the spacecraft was beneath the B ring ( Judge et &1., 1980). The current

observational and theoretical status of the ring atmosphere has been reviewed

by Holberg in Cuzzi et Al, ( 1983).

The mos+ extensive UV observations of the rings are those of the Voyager

1 and 2 WS, which made numerous scans of the rings from widely differing

observing geometries ( Broadfoot —tt Al., 1981b). The preliminary picture of

ring associated La emission whic h emerged from these observations contains

significant differences from those discussed previously. Most significantly

Voyager found the optically thick B ring to be the weakest source of La

i

5

t

^S
600 600 1000 1200 1400 Isc

WAVELENGTH (ANGSTROMS)

Figure 4

OR^GI'NA^ PAGE

 ^,ITY

Sa

0, POOR

CHANNEL.

20 40 60 90 100 120

a

VOYAGER 2

RINGS

-1

b

VOYAGER 2

SUN

Mr

n!t

_ lr^

0

Figure 3

The specrr;um of the rtng s observed with

the Voyager 2 UVS (a) compered with the solar

spectrum (b) observed through the Voyager 2

occultation pert. The ring spectrum, with the

exception of La is entirely due to reflected

sunlight. p rominent features include the Lyman

continuum peaking at 912 A and a blend of lie I

504 A. Mg V and 0 V at 600 X,

2400

Nz0

0

Q

W 0

H

a

J

W

X

2400

La brightness across the illuminated

side of the rings observed by the Voyager 1

WS. The rings were scanned from mid-C rin5

to mid-A ring with the UVS slit (01 RS,

radial resolution) oriented approximately

perpendicular to the radial dimension of

the rings. The signal is composed primarily

of interplanetary background (UBC) In

transmitted through the rings and the ring

atmosphere. Since the signal is anticorrelated with the visual brightness of the

rings, it is concluded that La reflected from

the rings contributes only marginally to the

observed brightness. The minimum signal,

1-360 R, in the B ring is assumed to be primarily

due to the foreground ring atmosphere since

the B ring is opaque. Similar brightnesses

were observed from the unilluminated side of

the rings.

C B A IDaG

1, 2

0.8

•III width

kR r

0.4

^Lr

0
the B ring. The UVS observations also found that La emission apparently does

not extend significantly beyond the outer edge of the A sing in contrast to

Pioneer it which indicated an emission component several Saturn radii (RS)

beyond the A ring. As previously mentioned Weiser 11 &1,, observed excess La

emission from the 'vicinity' of the rings while Copernicus and IUB observed

only upper limits, at a lower level. The Copnp^icus and IUB upper limits are

relatively easy to re gion 0 le with the Voyager observations since they were

obtained either when the rings were viewed edge — on (IUR) or at an orientation

such that the outer A rings only filled a small fraction of the field of view

(Copernicus). As such both the existing IUE and Copernicus observations refer

mainly to emission extending above the ring plane UUE) or beyond the edge of

the A ring (Copernicus),, The Weiser et Al. results were obtained when the

rings were relatively open and are more difficult to interpret. The reason

for this is that as viewed from Voyager the rings actually appear dark in La

when seen against the bright background of the sky. This sky background is

due to resonantly scattered solar La from interstellar neutral hydrogen

entering the solar system and constitutes an important complicating factor in

the interpretation of La emission from the rings. A Voyager based prediction

for the Weiser et #I. observation would therefore be a defier of La from the

vicinity of the rings.

The observed distribution of La emission across the rings is illustrated

in Figure 4 whieb shows a Voyager 1 UVS radial scan over the illuminated

rings. Several significant features are present in Figure 4. First, the La

brightness of the rings is anti —correlated with optical depth; most of this

effect can be explained by La sky background transmitted through the optically

thinner portions of the rings. The unattenuated sky background level is

indicated at the left of Figure 4, Second, the minimum brightness (-360 R) is

seen in mid B ring. This residual B ring brightness cannot be due to

transmitted La sky background since measured B ring optical depths are too

high to permit any significant transmission and the bulk of the La muot

originate on the sunlit side of the rings. Surprisingly, corresponding views

of the unilluminated side of the rings show similar brightnesses. The

simplest, least contrived, explanation for these observations is that tho bulk

of the 360 R minimum observed on both sides of the B ring is due to H

associated with the rings. Since the sun was at very low elevation angles

during both Voyager encounters, any ring atmosphere having an appreciable

scale height 00 .1 RS ) would display approximately equal illu' % inated columns

on both sides of the rings.

In characterizing the ring atmosphere Broadfoot et al. ( 1981b) assumed

411 360 R of La observed by Voyager 1 from the B ring was due to the ring

atmosphere. This then led to an H column of 10 12 cm—s . If this H were

distributed in a spherical shell having the radial extent of the zings, the

corresponding cloud would contain S x 10 52 atoms with a density of 600 cm-0

and an implied production rate of 1 x 10 38 s- 1 . These are surprisingly large

numbers in view of the initial investigations of source mechanisms (Dennefeld,

1974; Blamont, 1974). More recent work has led to a significant revision of

the expected production rate due to meteroid impact. Morfill e1 al , . (1983),

using improved meteroid fluxes and neutral vapor production rates, obtain

estimates of the neutral hydrogen within + 1 Rgg of the ring plane of 200 —

1800 cm- 1 . While the production of B due to m3crometeroid impact is

undoubtably larger than previously realized, the implied erosion rates of ring

i.
undoabtably larger that previously realized, the imp lied oros,ion rates of ring

particles due to this mechanism are also very large. A mochanism which relies

on a source of lI external to the rings is suggested by INS observations of

excited 1I = in Saturn's atmosphere. Shemansky and Smith (1982) propose that

the dissociation of H s by electrons (Shemansky and Ajello, 1983) at the

exobase of Saturn's atmosphere would provide a source of 'hot' If with

sufficient energy (>3 eV) to escape from the planet' yi exosphere. The rings in

this view would then constitute a 'cold trap' for a fraction of the escaping

H. This source, which is estimated to produce a total of 1010 H a-1

(Shemansky and Smith, 1982) from Saturn's atmosphere, must alo be considered

as a potential, or even major, contributor to the observed B in the Titan

torus at 8 to 25 RS.

One major current uncertainty is t1te extent of the ring atmosphere.

Voyager observations indicate that it does not appear to extend beyond the

outer edge of the A ring. Beyond this, little can be said at present. The

ring atmosphere could be confined relatively closely to the ring plane. This

would decrease the total content of the assumed cloud but would also increase

the density since the obser ,cTed column of 11 does not depend critically on the

assumed geometry.

IV. 71111 NEUTRAL ATMOSPHERE AND IONOSPHERE OF JUPITER, SATURN AND TITAN

The successful Voyager encounters with the Jovian and Saturnian systems

have returned extensive data on the structure and composition of the

atmospheres of Jupiter, Saturn and Titan. The UV solar occultation proved to

be a very powerful new technique for probing the composition and temperature

of the neutral upper atmospheres. The optical depth of the atmospheres was

measured from the exospheric region down to the menopause and somewhat lower

in the case of Titan. Atmospheric models were used to infer the atmospheric

structure, and exospheric constituents were measured through planet —wide UVS.

airglow measurements. Airglow observations were modeled to determine the

nature of the exospheres. Below the exosphere the altitude distribution of

chemical and photochemical reactions have been constrained by the optical

depth measurements down to the mesopause. It has takan some time to define

the analysis technique and demonstrate convincingly that the atmospheric

models were in fact constrained by the optical depth determination. Much work

remains to be done on the eight occultation measurements acquired to date on

the Voyager mission.

Early analyses, which inferred the neutral upper atmospheric structure

and temperature from the radio science electron density profiles, were in

error. Voyager and Pioneer radio occultation experiments measured the

electron :dlensity profiles for Jupiter and Saturn. On the Voyager mission to

the outer planets, the neutral upper atmosphere was measured directly by the

UVS solar and stellar occultation experiments. Figure 5 shows the situation

at Jupiter. Curves 1, 2, 3, and 4 are the electron density profiles from the

Radio Science experiment. The Atreya et al (1979) point indicates the

concentration and altitude of the neutral atmosphere based on the measured

electron density profiles. Tho curves U. and H were the measured neutral

upper atmospheric constituent profiles from the UV solar occultation

t
RSS/UVI) OCCULTATION RESULTS

Concontrat1on

ORIGINAL PAGE IS

OF POOR QUALITY

7a

a

J

4 00

E

w

a

<2

2 o o o

,-8

Q

Figure 5
8

which the solar energy deposition would generate the electron peak in the

neutral atmosphere. Although this figure is 'dated', it describes the

situation found at both Saturn and Jupiter. A common feature of all of the

observed electron density profiles is that the altitudes of the peak electron

densities occur at least several hundred kilometers above the region where

photoionization is expected to produce peak electron densities. In addition

peak electron densities are an order of magnitude less than expected. For

Jupiter our ionospheric models indicate that strong moridional winds (or

electric fields) are needed in order to move the plasma to high altitudes and

that vibrationally excited 11, reactions (McElroy, 1973) are needed to reduce

the peak electron densities.

From the occultation do we have derived the following picture of the

neutral atmosphere of these bodies. For Jupiter Cho thermospheric temperature

is about 1000 K with an estimated column amount of H down to the homopause

region of -1.0 x 3,0 17 em—s , Vor Saturn the temperature is —420 K in the

the-,josphere (2500 km above 1 bar level) and decreases to -120 K near the

homopausc at 1000 km (Smith of al, 1983). The density profiles of H s and C11+

indicate an eddy diffusion coefficient near the CH,, homopause of -5 x 106

CM-2 8- 1 . The 11 density profile implies a downward flux of -1.8 x lO s cm-3

s —x which indicates a considerable production of H in the upper regions of the

atmosphere. The solar occultation experiment on Titan probed considerably

deeper into the atmosphere than was possible on Jupiter or Saturn due to the

smaller Chapman function for Titan. Due to this and a programed gain change,

we were able +, iptrobo to several hundred kilometers below the homopause which

resultee, in t)ke, 44iscovery of what we have interpreted as a severe mixing rati N

change w4 la ^Ititude of acetylene. The thermosphoric temperature for Titan is

—180 K. We have definitely determined C11 + to be present with a mixing ratio

of -8% at 3700 km and C.H. with —1% down to -.1% in the region 3500 km to 3300

km. A simple photochemical model indicates that the eddy diffusion

coefficient is N1 x 10 6 em- 1 s- 1 near the homopause and decreases down to —1 x

10 3 cm-3 s- 1 in the Lower stratosphere ( Smith of al, 1982).

The airglow observations have yielded extensive information concerning

the excited species in the upper atmgspheres of these planets in addition to

resonantly scattered Lya and IIe 584 A emissions. For Jupiter and Saturn the

UVS spectra are dominated primarily by H z band emissions, Lya and He 584 A

(Broadfoot et al, 1981x; Broadfoot et al, 1981b; Sandel et al, 1982). The H1

band emissions are excited by electrons but appear only on the day side and

very high in the atmosphere near the exobase ( Broadfoot et al, 1981s; Sandel

et al, 1982; Shemansky and Ajello, 1983). The Titan spectra are dominated

entirely by excited N. and ionized N emissions ( Strobel and Shemansky, 198;.

The emission also appears to come only from the day side; however, for Titan

part of the day side is facing into Saturn ' s co—rotating plasma (Bridge et al,

1981). Upper limits for tue mixing ratios of NeI, ArI, CO, H s , and HI at 3900

km have been set. at 0.01, 0 .06, 0.05, 0.06 and 0.1, respectively.

Analysis of the equatorial airglow emission data and the bright limb

drifts ( Broadfoot et al, 1981a; Broadfoot et al, 1981b; Shemansky and Ajello,

1983) have shown that the electron excited emissions originate near the

exobase regions on Jupiter and Saturn and that the energy of the exciting

electrons is on the order of —30 eV ( Smith et al, 1983; Shemansky and Ajello,

1983). In conjunction with this it definitely appears that solar photons act
9

as a catalyst for this emissio.. (see Broadfoot et 1981x) since the

emission only appears on the day side and, in the case of Saturn, the

omissions noarly disappear within the shadow of the rings. On Titan we have

the emissions cowing only from the day side; however, there is some question 

yet about the actual location of this emission in altitude (Strobel and

Shemanshy. 1982).

.^

^, ..... ^ • . »... _ «, •^, ♦ yry. '.^ ! Ate' ^ Y ^'••i^^^+C >
10

V. REFERENCES

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Lane, A.L., Pomphrey, R.B., Terrile, R.J., Lissauer, J.J., Marouf, E.A.,

and Tyler, G.L., "The Eccentric Ringlet in the Maxwell Gap at 1.45 Saturn 

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Ring," A. J . , §1, 1416, 1982.

Holberg, J,B., and Forrester, W.T., "Voyager UVS Observations of Stellar

Occultations by the Rings of Saturn", IAU Colloquium No. 75, Planetary

Rings, A. Brahic, ed., CNRS, Toulouse, in press, 1983.

Holberg, T.B., Forrester, W.T., and Lissauer, 7.J., "Identification of 

Resonance Features within the Rings of Saturn," Nature, 297, 115, 1982. 

Holberg, J.B., D.E. Shemansky, B.R. Sandel, and A.L. Broadfoot, "High spatial

resolution observations of the lo plasma torus from the Voyager 2

ultraviolet spectrometer," Bull. Am. Astron. Soc,, 12, 673, 1980.

Ip, W.H. and C .K. Goertz, "An interpretation of the dawn—dusk asymmetry of the

EUV emission from the Io plasma torus," Nature, 302, 232, 1983.

Judge, D.L., Wu, F.M., and Carlson, R.W. ► "Ultraviolet Photometer Observations

of the Saturnian System," Science, 2W, 432, 1980.

McConnell, J.C., J.B. Holberg, G.R. Smith, B.R. Sandel, D.E. Ghemansky, and

A.L. Broadfoot, "A new look at the ionosphere of Jupiter in light-of the

UVS occultation results," Planet. Space Sci_., 30, 151, 1982.

McElroy, M.B.; "The ionospheres of the major planets," Space Sci ,,, Rev., 14,

460, 1973.

Morfill, G.E., Fechtig, H., Grun, E., and Goertz, C.H,, "Some Consequences of

Meteroid Impacts on Saturn's Rings," Icarus, 55, 439, 1983. 

...^

IL
Morgan, J.H., C.B. Pilcher, and J.H. Fertel, "1953 CCD Images of the Io Torus:

[S II], [S III], and Na I," Bull,, Am. Astron, Soc., 11, 811, 1983.

Mount, G.H., Rothman, G.J., and Timothy, J.G., "The Solar Spectral Irradiance

1200 - 255 A at Solar Maxim," J. Geophys. Res., M. 4271, 1980.

Pilcher, C.B., and D.F. Strobel, "Emissions from neutrals and ions in the

Jovian magnetosphere," in The Satellites of Jupiter, edited by D.

Morrison, University of Arizona Press, Tucson, 1981.

Porco, C.C., Nicholson, P.D., Borderies, N., Danielson, G.E., Goldreich, P.,

Holberg, J.B., Lane, A.L., and Zebker, H., "The Eccentric Saturnian

Ringlets at 1.29 RS and 1.45 RS ," Icarus (submitted), 1983a.

Porco, C.C., Danielson, G.E., Goldreich, P., Holberg, J.B., and Lane, A.L..

"Saturn's Non—Axisymmetric Ming Edge at 1.95 RS and 2.27 RS ," Carus

(submitted). 1983b.

Roesler, F.L., F. Scherb, and R.J. Oliversen, "Periodic intensity variation in

[S III] 9531a emission from the Jupiter plasma torus," Geophys. Res,

Lett., 11, 128, 1984.

Sandel, B.R., "Corotation lag in the Io plasma torus — evidence from Voyager

EUV observations," Bull. Am. Astron, Soc., 15, 810, 1983.

Sandel, B.R., and A.L. Broadfoot, "Io's hot plasma torus — a synoptic view

from Voyager," J,_ Geophys. Res., ?1, 212, 1982a.

Sandel, B.R., and A.L. Broadfoot, "Discovery of an Io—correlated energy source

for Io's hot plasma torus," J. Geophys, Res., §1, 2231, 1982b.

Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R.,

McConnell, J.C., Strobel, D.F., Atreya, S.S., Donahue, T.M., Moos, H,W.,

Bunten, D.M., Pomphrey, R.B., and Linick, S., "Extreme Ultraviolet

Observations from the Voyager 2 Encounter with Saturn," Science, 215,

548, 1982.

Shemansky, D.E., "Radiative cooling efficiencies and predicted spectra of

species of the Io plasma torus," An. J., 236, 1043, 1980a.

Shemansky, D.E., "Mass loading and the diffusive loss rate of the Io plasma

torus," Ap. J., 1980b

Shemansky, D.E., and Ajello, J.M., "The Saturn Spectrum in the EUV — Electron

Excited Hydrogen," T Geophys, Res., 88, 4S9, 1983.

Shemansky, D.E., and B.R. San=d, "The injection of energy to the Io plasma

torus," J. Geophys. Res., 87, 219, 1982.

Shemansky, D.E., and G.R. Smith, "The Voyager 1 EUV spectrum of the Io plasma

torus,' J. Gees, Res., 06, 9179, 1981.

P j n , ► z
13

Shemansky, D.E., and Smith, G.R., "Whence Comes the 'Titan' Hydrogen Torus,"

AM, !a, 1019, 1982.

Simpson, R.A., Tyler, G.L., and Holberg, J.B., "Saturn's Pole: Geometric

Correction Based on Voyager UVS and Radio Occultations," A. J., $8_, 1531,

1983.

Smith, G.R., McConnell, J.C., Shemansky, D.E., Sandel, B.R., Broadfoot, A.L.,

"Results from the Voyager 1 Ultraviolet Spectrometer Solar Occul{cation

Experiment," Bull, Am. Astron. Soo., 12, 687, 1980.

Smith, G.R., D.F. Strobel, A.L. Broadfoot, B.R. Sandel, D.E. Shemansky, and

J.B. Holberg, "Titan's upper atmosphere: composi',ion and temperature

from the EUV solar occultation," J. Geonhys. Res., ?1, 1351, 1982.

Smith, G.R., D.E. Shemansky, J.B. Holberg, A.L. Broadfoot, B.R. Sandel, and

J.C. McConnell, "Saturn's upper atmosphere from the Voyager 2 EUV Kolar

and stellar occultations," So. Gieoybys,. Res., November, 1983.

Smyth, W.H., and D.E. Shemansky, "Escape and ionization of atomic oxygen from

Io," Afl, J., ,2 71, 865, 1983.

Strobel, D.F., and D.E. Shemansky, "EUV emission from Titan's upper

atmosphere: Voyager 1 encounter," J. Geonhys,. Res., jj, No. A3,

1361-1368, 1982.

Ctrobel, D.F., and G.R. Smith, "On the temperature of the Jovian

thermosphere," J,, Atmos. Sci., 30, 718, 1973.

Weiser, H., Vitz, R.C., and Moos, H.W., Detection of Lyman a Emission from

the Saturnian Dirk and from the Ring System", Schnee, M. 775, 1977.

Weiser, H., and Moos, H.W., "A Rocket Observation of the Far—Ultraviolet

Spectrum of Saturn", Ap,, Jam, 222, 365, 1978.
14

i

Appendix ,.

RECENT PUBLICATIONS

Publications partially supported by this grant are listed below.

Atreya, S.Y., Donahue, T.M „ and Waite, J.H., Jr., "An interpretation of the

Voyager measurement of 3ovian electron density profiles," Natur , 28A,

795, 1979.

Bridge, U.S., Belcher, J.W., Lazarus, A.J., Olbert, S., Sullivan, J.D.,

Bagenal, F., Gazis, P.R., Bartle, R.E., Ogilvie, Y.W., Scudder, J.D.,

Sittler, E.C., Eviatar, A., Siscoe, G.L., Goertz, C.K., Vasyliunas, V.N., 

"Plasma observations near Saturn: initial results from Voyager 1,"

Science, 212, 217, 1981.

Broadfoot, A.L., B.R. Sandel, D.E. Shemansky, J.C. McConnell, G.R. Smith, J.B. 

Holberg, S.Y. Atreya, T.M. Donahue, D.F. Strobel, and J.L. Bertauz,

"Overview of Voyager Ultraviolet Spectrometry results through Jupiter 

encounter," J., Geouhys, Res., M, 8259, 1981.

Broadfoot, A.L., Sandel, B.R., Shemansky, D.E., Holberg, J.B., Smith, G.R.,

Strobel, D.F., McConnell, J.C., Kumar, S., Bunten, D.M., Atreya, S.R.,

Donahue, T.M., Moos, H.W., Bertaua, J.L., Blamont, J.E., Pomphrey, R.B.,

and Linick, S., "Extreme Ultraviolet Observations from Voyager 1

Encounter with Saturn," Science, 22, 206, 1981.

Brown, R.A., D.E. Shemansky, and R.E. Johnson, "A deficiency of 0 III in the

Io plasma torus," An. J., 264, 309, 1983.

Clarke, J.T., Moos, H,W., Atreya, S.K., and Lane, A.L., "IUE Detection of

Bursts of H Lya Emission from Saturn," Nature, 2,20, 226, 1981.

Cuzzi, J.N., Lissauer, J.J., Espositro, L.W., Holberg, T.B., Marouf, E.A.,

Tyler, G.L., and Boishot, A., "Saturn's Rings: Properties and

Processes," a sahap ter in Planetary Rings, Brahic and Greenburg, ads.,

University of Arizona Press.

Esposito, L.W„ Cuzzi, J.N., Holberg, J.B., Marouf, E.A., Tyler, G.L., and

Porco, C.C., "Saturn's Rings: Structure, Dynamics and Particle

Properties," (1983a), Chapter 12 of they book Saturn to be published by

University of Arizona Press.

Esposito, L.W., Borderies, N., Goldreich, P., Cuzzi, J.N., Holberg, J.B.,

Lane, A.L., Pomphrey, R.B., Terrile, R.J., Lissauer, J.J., Marouf, E.A.,

and Tyler, G.L., "The Eccentric Ringlet in the Maxwell Gap at 1.45 Saturn

Radii: Multi—Instrument Voyager Observations," Sc gnce - . 22u, 57, 1983b.

Friedson, A.J „ and Y.L. Yung, "The thermosphere of Titan," in press, 1983.

Holberg, J.B., Forrester, W.T., and Lissauer, J.J., "Identification of

Resonance Features within the Rings of Saturn," Naturs, 297, 115, 1982.

t
15

Ilolberg, J.B., "Identification of 198OS27 end 1980S26 Resonances in Saturn's A

Ring," A. J,, PI, 1416, 1982. 

Holberg, J.A., and Forrester, W.T., "Vov-er UVS Observations of Stellar 

Occultations by the Rings of Saturn," IAU Colloquium No. 75, Planetary

Rings, A. Brahic, ed., CNES, Toulouse, in press, 1983.

Holberg, J.A., D.E. Shemansky, B.R. Sandel, and A . L. Broadfoot, OHigh spatial

resolution observations of the Io plasma torus from the Voyager 2

ultraviolet spectrometer," Bull. Amy Astron. Sac., 12, 673, 1980.

Judge, D.L., Wu, F.H., and Carlson, R.W., "Ultraviolet Photometer Observations 

of the Saturnian System," Science, IE, 432, 1980.

McConnell, J.C., J.B. Ilolberg, G.R. Smith, B ,R. Sandel, D.E. Shemansky, and

A.L. Broadfoot, "Anew look at the ionosphere of Jupiter in light of the

UVS occultation results," Planet. Space Sci., 30, 151, 1982.

Pilcher, C.B., and A.F. Strobel, "Emissions from neutrals and ions in the

Jovian ma netos here," in The Satellites cf Ju 

g p ^ n3;^, edited by D. 

Morrison, University of Arizona Press, Tucson, 1981.

Porco, C . C., Nicholson, P.D., Bordpries, N., Danielson, G.E., Goldreich, P.,

Solberg, J.A., Lane, A.L., and Zebker, H., "The Eccentric Saturnian

Ringlets at 1.29 RS and 1 . 45 RS ," Icarus ( submitted), 1983a.

Porco, C.C., Danielson, G.E., Goldreich, P., Holberg, J.B., and Lane, A.L., 

"Saturn ' s Non—Axisymmetric Ring Le dge at 1.95 RS and 2 . 27 RS ," Icarus

(submitted), 1983b.

Sanded, B.R., and A.L. Broadfoot, "Io's hot plasma torus — a synoptic view

from Voyager," J. Geophys, Res., 87, 212, 1982a.

Sandel, B.R., and A.L. Broadfoot, "Discovery of an Io —correlated energy source

for Io's hot plasma torus," 1 Geophys. Res., §1, 2231, 1982b.

Sandel, B . R., "Corotation lag in the Io plasma torus — evidence from Voyager

EUV observations," Bull. Am. Astron. Sec,, 15, 810, 1983.

Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R.,

McConnell, J.C., Strogel,, D.F., Atreya, S.K., Donahue, T.M., Moos, H.W.,

Hunten, D.M., Pomphrey, R.B., and Linick, S., "Extreme Ultraviolet

Observations from the Voyager 2 Encounter with Saturn," Science, 215,

548, 1982.

Shemansky, D.E., "Radiative cooling efficiencies and p;.edicted spectra of

species of the Io plasma torus," Ap. J., 236, 1043, 1980.

Shemansky, D.E., "Mass loading and the diffusive loss rate of the Io plasma

torus," An. J., 1980b.

„.^ . .... o-v'^^'siv.7t`.'-,f ^W: .,,^ ^o^^ _' ^ ..I^^' i ai I...i-_ "-_ '_' ^ ft—-._._....

---'.e.°'^âMd^ii`^,
16

Shemansky, D.E., and Ajello, J.M., "The Saturn Spectrum in the EUV — Electron

Excited Ilydro$en." J. Goophyrs. r`s., 88, 459, 1983.

Shemansky, D.E., and Smith, G.R., "Whence Comes the 'Titan' hydrogen Torus,"

AM, 6a, 1019, 1982.

Shemansky, D.E., and G.R. Smith, Mic Voyager 1 EUV spectrum of the lo plasma

torus," J. Geopbys. Res,, , 86, 9179, 1981..

Shemansky, D.E., and B.R. Sandal, "The injection of energy to the Io plasma

torus," J. Geophya. Res., $7, 219, 1982.

Simpson, R.A., Tyler, G.L., and Holborg, J.B., "Saturn's Pole: Geometric

Correction Based on Voyager UVS and Radio Occultations", A. J., $8, 1531,

1983.

Smith, G.R., McConnell, J.C., Shemansky, D,E., Sandel, B.R., Broadfoot, A.L.,

"Results from the Voyager 1 Ultraviolet Spectrometer Solar Occultation

Experiment," Bull. Am. Astron. Soc._, 12, 687, 1980. 

1 

Smith, G.R., D.F. Strobel, A.L. Broadfoot, B.R. Sandcl, D.E. Shemansky, and

J.B. Holberg, "Titan's upper atmosphere: composition and temperature

from the EUV solar occultation," J. GeoRhvs. Res., §J, 1351, 1952.

Smith, G.R., D.E. Shemansky, J.B. Holberg, A.L. Broadfoot, B.R. Sandal, and

J.C. McConnell, "Saturn's upper atmosphere from the Voyager 2 EUV solar 

and stellar occultations," J. Geophys„ Res., November, 1983.

Smyth, W.H., and D.E. Shemansky, "Escape and ionization of atomic oxygen from

Io," A . J., 271, 865, 1983.

Weiser, H., Vitz, R.C., and Moos, H.W., "Detection of Lyman a Emission from

the Saturnian Disk and from the Ring System", Science, 197, 775, 1977.

Weiser, H., and Moos, 1€.W., "A Rocket Observation of the Far—Ultraviolet

a. Spectrum of Saturn", AR. J., 222, 365, 1978.

Strobel, D.F., and G.R. Smith, "On the temperature of the Jovian

thermosphere," J, Atmos,. - Sci., 30, 718, 1973.

r

r