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Volcanic activity is apparently higher on Io than on any other body in the Solar System. Its volcanic landforms can be compared with features on Earth to indicate the type of volcanism present on Io.
Irregular or fretted scarps on Io are similar to those found on Earth and Mars. A sapping mechanism involving liquid SO2 is proposed to explain these complexly eroded terrains on Io.
Io and Earth are the only planetary bodies known to be volcanically active; the energetics of the eruptive plumes on Io have important structural implications and are closely linked with the presence of sulphur and SO2.
Ballistic and aerodynamic models are proposed to explain the volcanic plumes on Io, with particular reference to Plumes 1 and 3 which seem to have the same origin.
Gaseous SO2 has been identified on Io. The estimated abundance of 0.2 cm atm is consistent with an atmosphere in equilibrium with solid SO2 at the local surface equilibrium temperature. Preliminary upper limits for several gases, including sulphur compounds and other terrestrial volcanic emissions have been derived. Io is apparently depleted in hydrogen, carbon and nitrogen.
Absorption features in Io's reflectance spectrum suggest that frozen SO2 molecules are present on the surface of Io as free frost; this may have important implications for Io's atmosphere.
The reflectance spectrum of Io can be explained by a surface layer consisting of fine-grained particles of sublimated alkali sulphides and sulphur on which H2S and SO2 are adsorbed.
A band of whistler-mode noise identified as auroral hiss has been observed on the inner edge of the Io plasma torus. This noise provides evidence for the existence of aurora-like charged particle beams on magnetic field lines through the inner edge of the torus. These beams probably consist of low-energy electrons and may be associated with field-aligned currents linking the plasma torus to the jovian ionosphere.
The Voyager 1 data yield detailed information on the latitudinal dependence of velocities in the jovian atmosphere. This enhances the possibility of interpreting the time-dependent behaviour of long-lived jovian cloud features. This article summarises pre-Voyager velocity data, stressing gross differences with the Voyager 1 results. Analytic expressions are given for average drift rates and shrinkage rates for the Great Red Spot and white ovals (located at −23° and −34° latitude, respectively) from 1943 to 1979.
Measurements of the latitudinal profile of zonal velocity are used to estimate the absolute vorticity gradient. A classification scheme of small features visible at resolutions of 100 km or better is presented
Remote sensing observations of the atmosphere at wavelengths from the UV to the IR are affected by the presence of haze layers above the visible clouds. Such layers are difficult to detect as they generally contain small particles (⩽1 µm). An imaging observation of high-altitude haze is presented that extends through the jovian stratosphere into the mesosphere.
The geometric reduction of the discovery picture of lightning on the dark side of Jupiter relates the positions of the lightning flashes to visible cloud structure.
The large vertical extent and diurnal variation in the electron density profile of Jupiter observed in radio occultation by Voyager 1 imply a homopause eddy diffusion coefficient of 1–3×105 cm2 s−1 and an exospheric temperature of about 1,300 K.
The first measurements of the wave–particle interactions of Jupiter's bow shock are reported, and some of the wave phenomena detected during the inbound passage are discussed.
Positive ions with various mass per charge values up to ∼160 have been identified within 20 jovian radii of Jupiter from the analysis of data from the plasma science experiment on Voyager 1.
Magnetic field observations of the jovian magnetosphere suggest an extended magnetic tail, which has been formed by solar wind interaction with the planetary field.
