Adv. Space Res. Vol.1, pp.185—191. 0273—H77/81/0301—0185$05.OO/O

~COSPAR, 1981. Printed in Great Britain.

ULTRAVIOLET OBSERVATIONS OFJUPITER FROM EARTH-ORBITINGSATELLITES

Vivien Moore

Departmentof Physicsand Astronomy,

UniversityCollegeLondon, London, UK

ABSTRACT

The International Ultraviolet Explorer (IUE) has provided both improved spectralresolution and some spatial resolution for UV observations of Jupiter. Previoussatellite observations have produced albedo curves for Jupiter showing theinfluence of Rayleigh scattering, and of some absorber(s) shortward of

2530gon the UV spectrum. Constraints on the abundance of several minor constituents ofthe Jovian atmosphere were derived from the OAO—2data. The IUE low dispersiondata has a resolution of 8~, making it possible to detect individual molecularfeatures. A series of C H absorptions in the 1750A region have been identified,and indications of NH3 a~s~rptions are present in the l95O~ region.

INTRODUCTION

The study of the ultraviolet spectrum of Jupiter provides another viewpoint on thecomposition and structure of the atmosphere. As we are able to look in greaterdetail at the Jovian atmosphere, it appears increasingly complex and dynamic. Thissituation also applies to the UV spectral region observations of Jupiter, as isshown in this review.

By the early 1970’s, it had been established that the spectrum of Jupiter, forwavelengths shortward of approximately 34O0~, was diagnostic of the upper part ofthe atmosphere, from the level of the thermal inversion (Greenspan and Owen(i] ).It was also evident that the UV albedo of Jupiter was considerably reduced fromthat expected for a pure Ha Rayleigh scattering atmosphere (Anderson et al [21Axel (3) ). In this upper part of the atmosphere, photochemical reaction productscould be present in sufficient quantities to produce observable effects in the UVspectrum; however, no molecular influences had been unambiguously detected,principally due to the low resolution of the available data (Owen and Sagan [4]).

There are additional problems associated with the acquisition and reduction ofplanetary UV data beyond the general problem for UV astronomy of the absorption ofthis portion of the spectrum by the Earth’s atmosphere. The planetary flux isdecreasing rapidly towards shorter wavelengths. This is due to the decline of thesolar flux through the UV, and the presence for the outer solar system objects ofan upper atmospheric absorber(s) which additionally darkens them. This steepdrop in the flux makes it difficult to obtain a good signal over a wide wavelengthspan with a single observation. As an example, exposure times ranging from 2secs

185

186 V. Moore

to 90 mins are required with the IUE to achieve a good signal to noise ratio from32OO~to l5OO~ at a resolution of bA.

In the derivation of planetary albedos, an estimate of the incident solar flux isrequired. A direct measure is rarely, if ever, possible with the same instrumentthat made the planetary observation. Various approaches to this problem havebeen considered. Published solar data can be used as a reference, although thereare discrepancies in the published solar fluxes which may be indicative of realtemporal variations in the solar UV output. The spectra of two physically dis-similar solar system objects can be compared to search for narrowplanetaryfeatures (for examples, see Anderson et al [2] and Wallace et al [5]). The use ofsolar system objects for calibration can provide a more nearly contemporaneouscomparision than published solar fluxes. Observations of solar type stars havebeen used (see Savage and Caldwell [6]). If the comparision observations aremade with the same instrument as the primary planetary observation, some instru-mental effects can be accounted for.

Jupiter has been observed at UV wavelengths below 3200A from several soundingrocket flights and Earth-orbiting satellites. Ultraviolet data for the planetand its environment were obtained from the Pioneer and Voyager space missions tothe outer solar system. (Pioneer results are reviewed by Carison and Judge [71.and the Voyager results are discussed by Broadfoot et al (8a], Sandel et al [8b]and Hord et al [9]). Ultraviolet observations of Jupiter from sounding rocketflights have been reported by Stecher fbi, Evans et al (11), Moos et al [12),Jenkins et al [131, Anderson et al [~], Kondo [141, Rottman et al [15] andGiles et al (16]. Early results produced some spurious features that were notsubsequently observed and were not readily explained, such as an apparentabsorption at 2600A. Results from the more recent flights are discussed laterin a comparison with satellite data

As well as observations of the continuum UV spectrum of Jupiter, a second aim,particularly of much of the rocket work, was to determine the intensity of thehydrogen Lyman-a emission from Jupiter. The Copernicus satellite has contributedto the Lyman-a study, and the results from this satellite, the TUE and therocket flights are reviewed by Cochran and Barker (17], who find that the JovianLyman-~emission appears to be correlated with solar activity, as expressed bythe Zurich sunspot number. This emission appears to be generalised over thedisc, and not necessarily associated with the auroral zones (Giles et al [16]).

Spectroscopic and photometric observations of Jupiter over the wavelength rangefrom Lyman-i to the overlap with ground based measurements at 3400A have beenmade from several Earth-orbiting satellites.

Prior to the IUE two satellites had made UV observations of Jupiter. The OAO-2obtained both photometric and spectroscopic data for wavelengths bongward of l900A.No planetary signal was detectable by its instruments at shorter wavelengths.The European satellite TD1A also acquired spectroscopic data for the wavelengthspan 1350A to 2600A.

The IUE was launched into geosynchronous orbit in January 1978 and is run as anobservatory satellite by the three agencies NASA/ESA/SRC(UK). The scientificinstrumentation consists of a two camera, two spectrograph system described byBoggess et al [18]. The cameras cover the wavelength ranges lbOOA to 2000A and1800A to 3200A. A low dispersion mode provides a resolution of 6A to 1OAdepending on the target/aperture characteristics for a particular observation.The high dispersion echelle grating spectrograph has an approximate resolution ofO.3A. These improvements in resolution with the TUE are sufficient for thedetection of individual molecular features in planetary spectra (Moos and

U.V. Observations of Jupiter 187

Clarke [19], Owen et al [21~). The IUE cameras use two apertures, a circular 3arc sec diameter aperture, and an approximately lOx2O arc sec oval. These make itpossible to undertake spatially resolved studies of Jupiter and Saturn.

Jupiter was successfully observed with the TD1As S2/68 telescope, which had aresolution of 36A for wavelengths from 1350A to 2600A, with a photometric datum440A wide centred at 2800A. Jupiter was too bright for the S59 experiment whichwould have provided higher resolution for three wavelength bands from 2100A to2800A. Three spectra of Jupiter were obtained in March 1972. From these data,an albedo curve was derived using as a solar reference the data of Broadfoot forwavelengths greater than 21SOA (Broadfoot[21J), and the solar spectrum ofDetwiler et al [22]for shorter wavelengths (Duysinx and Henrist [23]).

The OAO-2 observed Jupiter several times from 1969 to 1971 and obtained bothspectroscopic and photometric data. The latter were restricted to the twophotometer channels below 3000A at 1910A and 2450A due to the brightness of theplanet at longer wavelengths. A detailed description of the method used inreducing the data to an albedo for Jupiter is given by Savage and Caldwell [63.It is noted here that for calibrating the photometer measurements, they usedOAO-2 data on a variety of G-star UV fluxes, instead of a solar reference spectrum.

A series of observations of Jupiter were made in the OAO-2 photometric mode,spread over several months, to provide an initial study of the variability of theUV albedo. As might be expected considering the degree of change that has beenseen in the visible cloud cover of Jupiter, the UV albedos provided an indicationof variations occurring over several months. In addition, a series of measure-ments were made for slightly longer than a single rotation period. A specificfeature over this time scale was a reduction in the albedo at the times consistentwith the central meridian crossings of the Great Red Spot (Savage and Caldwell[6J).The OAO-2 photometer had a 10 arc minute circular field of view, so no spatiallyresolved photometry was possible.

The OAO-2 spectrometer data, covering the wavelengths from 2bOO~to ~4OOA,havea resolution of approximately 25A so they show more detail than the photometricdata alone. The spectrometer observations were reduced using a combination ofOAO-2 observations of~Her and the solar spectrum of Broadfoot [21] , thenscaling to ground based photometry (Wallace, Caldwell and Savage [5]).

The Jovian abbedo derived from the OAO-2 data exhibits a broad minimum at 2900A,rising to a maximum at 2500A, followed by a further decline towards shorterwavelengths. The quality of the data deteriorates rapidly below 2300A. Thistype of curve is consistent with those derived from rocket data. There is someindication that part of the 2500A feature is dependent on the solar referenceused in the derivation of the albedo; see the comments in Wallace, Caldwell andSavage [5]. The albedo derivedfrom the TD1A data has an absolute level lyingbelow that of the OAO-2 curve; however, they are similar in shape in the regionwhere they overlap. The TD1A data show a broad absorption around 2000A, andthe onset of a further decline at l800A.(Duysinx and Henrist 1231).

The general structure of the Jovian albedo from infrared through to ultravioletwavelengths was examined by Axel [3]. He postulated the inclusion in the upperatmosphere of Jupiter of a small (i.e less than a wavelength in size) particulateabsorber with an inverse wavelength absorption dependency to provide the reductionin flux towards the shorter wavelengths. Eventually, the H~Rayleigh scatteringwhich has a A~dependence for the cros section for scatterin9 begins todominate. This is seen in the rise in the albedo from approximately 2900A.A variety of other observations are consistent with the presence of a blue/UVcontinuum absorber in the upper atmosphere; for example, ground based

188 V. Moore

spectroscopy to 3200A (Cochran and Slavsky f~), Pioneer 10 large angle photo-metry (Tomasko et al [25]) and methane band photometry (West and Tomasko [26]).For Jupiter (and Saturn) an additional absorber is required to produce the declinein albedo below 2300A which is common to both planets.

Ammonia has been a popular candidate for the cause of the 2300A albedo decline asit is known to be present in the Jovian atmosphere and it has an absorption seriesin this spectral region (Thompson et al (27)). Duysinx and Henrist (23] computeda low resolution absorption coefficient for ammonia from laboratory data whichshowed a qualitative agreement with the shape of the minimum in the TD1A albedocurve at 2000A. Several analyses of rocket data have considered ammonia as alikely absorber in this spectral region (e.g. Greenspan and Owen [1], Tomasko [28]and Giles et al [16)), and the possible influence of solid ammonia on the UVreflectivity was studied by Anderson and Pipes [29). Caldwell [30] argued thatthe similarity of the Saturnian and Jovian albedos, as observed by the OAO-2, wasevidence that ammonia was not responsible for the decline in albedo shortward of2300A seen for both planets. The ammonia in the atmosphere of Jupiter is mainlyconfined to the regions below the temperature minimum. This provides an effectivecold trap for the amonia so only small amount are expected in the upper atmo-sphere; however, the strengths of the absorptions in the 1650A to 225OA range aresufficient to make the detection of small quantities of ammonia feasible. As theatmosphere of Saturn is colder, no ammonia is expected to be in the levels probedby the ultraviolet for this planet.

The OAO-2 spectrometer data were examined for evidence of molecular absorptions byOwen and Sagan [4]. Although no molecular features were unambiguously indentified,upper limits were established for a variety of possible absorbers. Owen and Saganemphasised that the low resolution available only permitted the setting of limitson the abundances of candidate absorbers, as several plausible constituents havefeatures in the 1900A to 2300A range. Both phosphine and hydrogen sulphide havestrong absorptions in this spectral region (Halmann [31), Watanabe and Jursa [32]).The broad absorption in the spectrum of phosphine around 1900A is so strong thatthis molecule must be depleted by photodissociation, compared to the mixing ratioobserved in the IR (Tokunaga et al [33)). If not, Jupiter would be darker as thedepth of penetration by the UV would be less, so reducing the amount of theRayleigh scattering.

The improvement in resolution given by the IUE has made it possible to detectindividual molecular features (Owen et al [20]). If the Jovian spectrum froml7O~Ato 2000A is compared to the solar spectrum from Kjeldseth Moe et al [34]several non-solar features are seen in the planetary spectrum, some of which maybe attributable to ammonia (fig. 1). This permits the determination of an upperlimit for the ammonia mixing ratio in this part of the Jovian atmosphere ofNH3/Ha .~ 0.5 x l0’ cm-am, (using 2.7 km-am of H2 at 2000A, from Tomasko (283).As would be expected from previous discussions, this is reduced in comparison withthe ammonia mixing ratio determined from ground based IR spectroscopy. When thehigh dispersion TUE data are examined, it is possible to see the distinct doublepeaked structure of the ammonia absorptions around 2000A.

In the l57OA to l63OA region, there is a feature which can be considered either asan absorption at l590A or as emission features clustered either side of thiswavelength. No molecular candidate has been suggested for a l59O~absorption;however, there is an H S feature at 1580A (Watanabe and Jursa [32]). If this werean HaS absorption,?C’ scaling of the H2 scattering optical8depth from 2000A tothis wavelength gives an upper limit of H2S/H~ 0.5 x lO . There is no dis-crepancy with the 2000A region where H2S also absorbs, as this upper limit iscompatible with the limits from OAO-2 data (Caldwell [30] , Owen and Sagan [4) , andfrom IR results (Owen et al [35]). This low value is consistent with photochemical

IJ.V. Observations of Jupiter 189

I • ‘I I i I-JU- JUPITER

~2.0

SUN

0.0 I • I1600 1700 1800 1900 2000WAVELENGTH (A)

Figure 1

The spectrum of Jupiter from l600A to 2000A is plotted with the solar spectrumof Kjeldseth-Moe et al D4] for comparison, from Owen et al 120] . The TUEspectrum is the sum of 3 separate 30 minute exposures with the short wave-length camera small aperture of the centre of the Jovian disc from January 1979.The positions of the acetylene and ammonia absorptions are marked by thevertical bars. The full lines mark the four acetylene absorptions that aredefinitely observed, while the broken lines indicate the positions of molecularabsorptions that are less clearly distinguished. The open circles flag theposition of reseaux marks which can distort the data locally. These spectrawere affected by the photometric error in the IUE data extraction proceduresand have since been corrected (using the •Three agency file-4 method’ IUE ESANewsletter no. 5). In this particular case the corrections to the figure aboveare marginal and do not affect the discussion of this spectrum presented byOwen et al [20]

190 V. Moore

models for Jupiter, as HaS is readily photodissociated (Lewis and Prinn [36]). The1590A feature can be explained alternately and probably more convincingly asemissions at 1575A and l6lOA. This latter peak may contribute to the apparenttrace of an absorption at l625A. A similar set of features observed in a rocketflight is discussed by Giles et al [16].

The TUE short wavelength spectra show a series of acetylene absorptions (Owen etal [20]). There are four features from 1700A to 1800A, while a feature at l68OAis barely discernible, and weaker members of this series may contribute to theJovian spectrum above 1800A. A broad absorption at 152OA is also due to C2H~(Nakayama and Watanabe [373). These acetylene features provide a further demon-stration of the strong dependence of the depth of the penetration by the UV onwavelength. The mixing ratio for the 1750A feature, scaling the 2.7 km—amHLabundance at 2000A, and using the absorption coefficients of Nakayama and Watanabe1371, is approximately 2.2 x lOs. This is comparable to the value derived fromIR observations at 760 cni’, which also indicate that the CzH2.is restricted tothe region above the tem2erature minimum (Orton and Aumann [38)). The absorptioncoefficient for the 1520A feature is 500 times greater than that of the individual1700A to l800A features, which have nearly equal coefficients. On this basis also,the l680A absorption would be expected to be stronger (Nakayama and Watanabe [3~).The relative strength of the absorptions above 1700A indicates that the depth ofpenetration by the UV is reduced for the shorter wavelengths. The 2~ Rayleighscattering cross section dependence is not sufficient to account for this, sothat some other continuum absorber must be present. It is not possible to deter-mine the influence of PH in this shorter wavelength region as there is no suitablelaboratory data on its absorption throughout this range.

The IUE provides an oppurtunity to study the UV spectrum for different regions onthe disc of Jupiter. As an example of this capability, a reduction of approxi-mately 50% in the overall flux level is seen between the central region and thesouth pole for wavelengths of 2lOOA to 26OO~. The relative distribution of fluxwith wavelength also shows variations with position.

While the TUE has extended our knowledge of the UV spectrum of Jupiter, it hasalso provided a pointer to possibilities to be pursued in future work. There isstill a requirement for further laboratory studies of the simple molecules likelyto be influencing the UV spectrum of the planets. And, as always, there is agreat deal to be done in relating the observational data to an understanding ofthe vertical and horizontal structure of the Jovian atmosphere.

ACKNOWLEDGEMENTS

The author would like to thank Drs. J. Caldwell, T. Owen, and G. Hunt for manyinformative and interesting discussions during the preparation of this review.The author is supported by an SRC(UK) studentship.

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