Garry E. Hunt

The Jupiter System Through the Eyes of Voyager 1

The cameras aboard Voyager 1 have provided a closeup view of the Jupiter system, revealing heretofore unknown characteristics and phenomena associated with the planets atmosphere and the surfaces of its five major satellites. On Jupiter itself, atmospheric motions—the interaction of cloud systems—display complex vorticity. On its dark side, lightning and auroras are observed. A ring was discovered surrounding Jupiter. The satellite surfaces display dramatic differences including extensive active volcanismn on Io, complex tectonism on Ganymnede and possibly Europa, and flattened remnants of enormous impact features on Callisto.

The Galilean Satellites and Jupiter: Voyager 2 Imaging Science Results

Voyager 2, during its encounter with the Jupiter system, provided images that both complement and supplement in important ways the Voyager 1 images. While many changes have been observed in Jupiters visual appearance, few, yet significant, changes have been detected in the principal atmospheric currents. Jupiters ring system is strongly forward scattering at visual wavelengths and consists of a narrow annulus of highest particle density, within which is a broader region in which the density is lower. On Io, changes are observed in eruptive activity, plume structure, and surface albedo patterns. Europas surface retains little or no record of intense meteorite bombardment, but does reveal a complex and, as yet, little-understood system of overlapping bright and dark linear features. Ganymede is found to have at least one unit of heavily cratered terrain on a surface that otherwise suggests widespread tectonism. Except for two large ringed basins, Callistos entire surface is heavily cratered.

Voyager 2 in the Uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70� to -27�), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20�. Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 102 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earths moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Mirandas surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Lightning activity on Jupiter

Abstract Photographic observations of the nightside of Jupiter by the Voyager 1 spacecraft show the presence of extensive lightning activity. Detection of whistlers by the plasma wave analyzer confirms the optical observations and implies that many flashes were not recorded by the Voyager camera because the intensity of the flashes was below the threshold sensitivity of the camera. Measurements of the optical energy radiated per flash indicate that the observed flashes had energies similar to that for terrestrial superbolts. The best estimate of the lightning energy dissipation rate of 0.4 × 10 −3 W/m 2 was derived from a consideration of the optical and radiofrequency measurements. The ratio of the energy dissipated by lightning compared to the convective energy flux is estimated to be between 0.27 × 10 −4 and 0.5 × 10 −4 . The terrestrial value is 1 × 10 −4 .

A determination of the composition of the Saturnian stratosphere using the IUE

Abstract We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 A. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C 2 H 2 and C 2 H 6 with mixing ratios and distributions of (9 ± 3) × 10 −8 in the top 20 mbar of the atmosphere with none below for C 2 H 2 and (6 ± 1) × 10 −6 also in the top 20 mbar with none below for C 2 H 6 . The C 2 H 2 and C 2 H 6 distributions and the C 2 H 6 mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc. 13 , 722 (1981), and private communication]. The Voyager IRIS model also includes PH 3 , which is not consistent with the uv albedo from 1800 to 2400 A. Our model requires a previously unidentified absorber to explain the albedo near 1600 A. After considering several candidates, we find that the best fit to the data is obtained with H 2 O, having a column density of (6 ± 1) × 10 −3 cm-am.

An Examination of the Formation of Linear Lee Waves Generated by Giant Martian Volcanoes

Abstract The vertical wave displacement generated by a Martian volcano, at heights comparable with and above the altitude of the topography, is investigated in this paper. An examination is made of the leakage of this displacement into atmospheric heights well above the topography, and consideration is given to two large Martian volcanoes, Olympus Mons and Ascraeus Mons. Viking Orbiter imagery portraying wave clouds in the vicinity of these volcanoes is compared with theoretical results obtained by considering a simple one-layer atmosphere in which the ratio of the stability factor to the square of the atmospheric velocity profile is deemed to be constant at all vertical levels. It is found that a wave plume phenomenon is predicted by this theory, and by comparing with visual observations, information can be deduced as to the atmospheric mean structure of velocity and stability when these cloud formations occur.

Scattering and absorption properties of CO2 ice spheres in the region 360–4000 cm-1

Abstract We present the results of Mie calculations for CO 2 ice particles of radius a = 0.1, 1 and 10 μm in the region 360–4000 cm -1 . Since CO 2 possesses a very small imaginary refractive index, the larger particles behave as conservative scatters. This is quite different from the equivalent water ice particles. The results presented in this study will be useful for investigation of the Martian atmosphere and surface.

The opacity of some local Martian dust storms observed by the Viking IRTM

Abstract In this paper we analyze some Viking infrared thermal mapping (IRTM) measurements of local Martian dust storms observed in the southern tropical region of the planet between L s =225 and 262°. The derived opacities of these storms show that in the most opaque regions of the cloud, the optical thickness may be ≈6. Away from the individual clouds, the opacity is ≈2, which is still about four times the background level of dustiness in the Martian atmosphere. We find considerable structure in the derived opacity which will create corresponding variations in the atmospheric heating, which in turn may have an important feedback upon the local winds.

Studies of the sensitivity of the components of the earth's radiation balance to changes in cloud properties using a zonally averaged model

Abstract The accuracy of a simple radiative transfer scheme suitable for use in a general circulation model of the atmosphere is assessed by comparing the calculated radiative heat balance of the Earth/atmosphere system with the available observational data. Studies are then performed to determine the sensitivity of the radiative components to changes in the cloud data and cloud parameterisations which constitute the largest potential source of error in the model input data.

ON THE DIURNAL PROPERTIES OF CLOUDS FROM GEOSTATIONARY SATELLITE OBSERVATIONS

The interaction of clouds with the general circulation of the atmosphere forms one of the fundamental problems in atmospheric physics. One of the major limitations that prevents rapid progress with the current generation of numerical models concerns the absence of a suitable cloud climatology. Knowledge of the vertical, spatial and temporal behaviour of clouds, and their interaction with the radiation field, which may be measured in terms of the Earths radiation budget is required. Observations from geostationary satellites are a key factor in future studies. In this paper the temporal behaviour of clouds and the interaction with the radiation field is considered. A discussion is also given of the possible effects of cloud models in general circulation studies.