Jovian Surprises

Voyager 1 was launched on 5 September 1977 upon a Titan III launch vehicle with a Centaur escape stage, the most powerful combination available and, in fact, the final vehicle of this type to run off the production line as NASA phased out 'expendable' rockets in favour of the then-favoured Shuttle-only policy.

As Voyager 1 started its interplanetary cruise, it deployed its various booms and activated its scan platform, whose action was impaired by tiny fragments of debris trapped in the mechanism during assembly, but this was progressively ground down and expelled by a series of slewing exercises. Two weeks out, and almost 12 million kilometres away, the spacecraft returned a historic 'departure shot' capturing for the

Voyager Route
A polar projection of Voyager 1's route through the Jovian system.

first time the Earth and its Moon in the same frame, both of which were showing the same crescent phase.

In fact, Voyager 1 had been launched a fortnight behind its mate, but because it pursued a slightly faster trajectory it took the lead on 15 December. Although it is rather arbitrarily defined, Voyager 1 emerged from the far side of the asteroid belt on 8 September 1978. A month later, Voyager 2 did likewise. NASA's record was now four-for-four in this respect. Evidently, earlier fears concerning the risk of debris had been overestimated, and the highway to the outer Solar System was open to regular traffic.

As Voyager 1 closed within 100 million kilometres of Jupiter, the resolution of its narrow-angle imagery was better than that from telescopes. On 4 January 1979, it initiated a month-long campaign to investigate the dynamics of the atmosphere. By mid-February, it was able to expand its activities to remote sensing of the Galilean satellites. The magnetosphere had extended sunward about 100 planetary radii when the Pioneers had visited, but the Sun had been fairly quiescent then, and it was now near the peak of its sunspot cycle and the increased pressure of the solar wind had compressed the magnetosphere. Voyager 1 did not sense the bow shock until 28 February, and it was not until 3 March, after having the shock wave wash back and forth, that it finally crossed the magnetopause and entered the inner domain, by which time the craft was at a distance of only 47 radii, and within hours of crossing the orbit of the outermost of the four large satellites. The moon, however, was not in the vicinity; in fact, the spacecraft had a clear run to Jupiter, during which it was able to chart the magnetosphere and document the atmospheric structures on the sunlit hemisphere of the planet. The imagery was later sequenced as a movie showing a 10hour period as the planet rotated once upon its axis. The high-resolution imagery revealed an astonishing variety of detail and an astoundingly dynamic atmospheric system.12

In 1664 Robert Hooke in England noted a spot spanning one-tenth of Jupiter's diameter, just south of the equator. It was discovered independently in 1665 by J.D. Cassini in Italy, who, after moving to Paris, recorded it intermittently until his death in 1712. 'Hooke's Spot', as it was known, was drawn by H.S. Schwabe in Germany in 1831, by W.R. Dawes in England in 1857, by A.M. Mayer in America in 1870, and repeatedly by the fourth Earl of Rosse in Ireland in the early 1870s. It took on a striking red hue in 1878, prompting the new name of the 'Great Red Spot'. After slowly fading in 1882, it reappeared in 1891. Although it briefly fades on occasion, and is often difficult to detect, it has become a permanent feature of the South Tropical Zone as a 26,000-kilometre-wide oval spanning 20 degrees of longitude at latitude 22 degrees south.13 The Pioneer imagery had hinted at internal structure.14 Voyager 1 revealed this in intricate detail, documenting its 7-day anti-clockwise rotation. It is an anticyclonic vortex with a central upwelling column that yields to subsidence around its periphery. In obstructing the prevailing latitudinal ('zonal') jet stream, the Great Red Spot leaves an extensive system of eddies in its 'wake' - an intricate structure that had never been suspected by the most fortunate of telescopic observers. The latitudinal banding was discovered to extend to higher latitudes than expected. The 'whistlers' detected at radio-frequencies indicated the presence of electrical discharges in the atmosphere. Although the cameras did not capture any lightning flashes on the dark hemisphere, it was concluded that the discharges likely occur just below the cloud deck forming the visible surface, which implied in turn that there is vigorous vertical circulation at work.

The closest point of approach early on 5 March was at a planetocentric range of 4.9 radii, with Voyager 1 passing 270,000 kilometres above the cloud tops, slightly beyond the evening terminator. The slingshot increased Voyager 1's speed by 48,000 kilometres per hour. Six hours earlier, shortly prior to crossing the orbit of Io, the innermost of the Galileans, some long-range images were taken of Amalthea, the moonlet which E.E. Barnard had spotted orbiting close to the planet, showing it to be an irregular body with its long axis aimed towards Jupiter. As the spacecraft flew through the equatorial plane, it secured a long-exposure picture 'over its shoulder' to search for moonlets even closer to the planet, and in doing so it revealed that Jupiter possesses rings. Unlike Saturn's rings, Jupiter's are dark because they comprise small particles of dust which reflect sunlight poorly. They were striking from down-Sun because small particles are efficient at forward-scattering sunlight. This suggested that the Jovian particles are micron-sized specks of dust, rather than ice, the main constituent of Saturn's system of rings.15

All the major moons were best seen from the exit trajectory, and Voyager 1 met them in sequence. The point of closest approach to Jupiter had been well inside Io's orbit, and as the craft caught up with this moon it observed its trailing hemisphere

Jupiter's Great Red Spot is a 26,000-kilometre-wide oval that spans 20 degrees of longitude in the South Tropical Zone. In obstructing the prevailing zonal jet streams, it leaves an extensive system of eddies in its 'wake'. The intricate structure in this contrast-enhanced Voyager 1 image had not been suspected by even the most fortunate of telescopic observers. The image was taken on 25 February 1979 from a range of 10 million kilometers, and resolves details as small as 160 kilometers across.

Jupiter's Great Red Spot is a 26,000-kilometre-wide oval that spans 20 degrees of longitude in the South Tropical Zone. In obstructing the prevailing zonal jet streams, it leaves an extensive system of eddies in its 'wake'. The intricate structure in this contrast-enhanced Voyager 1 image had not been suspected by even the most fortunate of telescopic observers. The image was taken on 25 February 1979 from a range of 10 million kilometers, and resolves details as small as 160 kilometers across.

before passing within 21,000 kilometres of its south pole, in the process documenting much of the hemisphere that permanently faces the planet. Telescopic studies had discovered a cloud of neutral atoms and ionised nuclei in a toroidal form centred on Io's orbit.16 This is so wide that if the Earth were located at the geometric centre, then the Moon's orbit would comfortably fit inside the hole of the torus. The plasma was studied from afar by Voyager 1's Ultraviolet Spectrometer. By passing within Io's orbit, the spacecraft spent several hours inside the torus and the particles and fields instruments made in situ measurements. When skimming Io's south pole, the instruments detected a strong flow of charged particles. Electrons stream to and fro along Jupiter's magnetic field lines, forming a pair of 'flux tubes' that connect the moon to the planet. This 1-million-ampere current is by far the most powerful direct current in the Solar System.

Voyager 1 detected strong ultraviolet emission over Jupiter's sunlit hemisphere. Dubbed 'dayglow', this emission meant that the temperature of the thermosphere is 1,000K, but a haze which absorbs ultraviolet light overlies the polar regions of the atmosphere. On the night side, there is visible and ultraviolet auroral emission. In the case of the Earth, the auroral excitation is due to solar wind particles streaming into the polar cusps of the magnetosphere and interacting with the ionosphere, but Jupiter's magnetic field is strong enough to fend off the worst of the solar wind. The arc-like auroral glows are caused by particles originating in Io's plasma torus. Upon being launched in 1990, the Hubble Space Telescope imaged this excitation extending across Jupiter's dayside, and upon being upgraded with an imaging spectrograph it was able to see the spot-like glows where the ends of the flux tubes connect with the atmosphere. Jupiter rotates more rapidly than Io's travel around its orbit, and these 'footprints' stay at the satellite's longitude, but as the excitation slowly diminishes the spot is stretched out into a 'comma'. In fact, Io, which has been described as the 'beating heart' of the Jovian system, is the powerhouse of the magnetosphere.

However, little was known of Io itself. It orbits only 350,000 kilometres above Jupiter's cloud tops, which is approximately the distance between the Earth and the Moon, but while Io is comparable in size to the Moon, the giant planet is 318 times more massive than the Earth. The tremendous gravitational field will draw in material from interplanetary space and accelerate it, so the expectation was that Io would be heavily cratered. When the early low-resolution Voyager 1 imagery showed vaguely circular albedo features these were taken to be craters, but as the spacecraft closed in, it became evident that these dark features were not impact scars at all. Astonishingly, a thorough survey revealed that Io has no impact craters. As no object can completely escape impacts, the absence of craters indicated that some process was continuously resurfacing the moon, 'removing' its craters. This process was evidently volcanism. Nevertheless, no one seriously expected to see a volcano in the process of erupting. On 8 March, as Voyager 1 departed the Jovian system, it took an extended exposure to show the position of the crescent moon against the stars for navigation purposes to verify that the slingshot had deflected the trajectory for Saturn. When navigation team member Linda A. Morabito 'enhanced' this image by computer, she spotted a faint mushroom-shaped plume projecting 280 kilometres beyond the limb.17 It was an active volcano! An anomalous glow on the terminator was another volcano! Its vent site was in darkness, but its plume of dusty gas was so tall that it caught the rays of the Sun. In fact, further analysis identified nine active vents with plumes rising up to 300 kilometres. The IRIS noted infrared emissions from a large number of isolated 'hot spots' which were not producing plumes. Volcanism was rife.

Just before Voyager 1 ventured into the Jovian system, a paper published in the journal Science had reported the results of modelling the tidal stresses acting on Io.18 Europa's orbital period is twice that of Io, and Ganymede's is twice that of Europa. These resonances make Io's orbit slightly elliptical. The eccentricity is just 0.0041, but it has a significant effect. Io maintains one hemisphere facing Jupiter because it rotates synchronously. A 'tidal bulge' rises and relaxes in response to the cyclically

The Labled Moon

On 8 March 1979, as it withdrew from Jupiter, Voyager 1 snapped an over-exposed image of Io (left) so that JPL's interplanetary navigational engineers could verify the spacecraft's aim for Saturn by measuring the moon's position with reference to the stars. When Linda A. Morabito drew attention to the umbrella-shaped 'plume' on the limb, it was realised that Io is volcanically active. In fact, there was a second plume on the terminator. The limb site (Plume 1) was later named Pele, and the terminator site was named Loki.

On 8 March 1979, as it withdrew from Jupiter, Voyager 1 snapped an over-exposed image of Io (left) so that JPL's interplanetary navigational engineers could verify the spacecraft's aim for Saturn by measuring the moon's position with reference to the stars. When Linda A. Morabito drew attention to the umbrella-shaped 'plume' on the limb, it was realised that Io is volcanically active. In fact, there was a second plume on the terminator. The limb site (Plume 1) was later named Pele, and the terminator site was named Loki.

varying gravitational force. The induced mechanical stress is converted into heat. The analysis found that Io must derive two to three orders of magnitude more heat from this stress than from the decay of its radioactive elements. In addition to implying that the moon's interior should be significantly thermally differentiated, the authors of the paper tentatively predicted that it might be volcanically active. However, they were just as surprised as anyone to find that Io is the most volcanically active object in the Solar System. Terrestrial infrared observations made in 1981 determined that Io's heat flow is 30 times greater than that of the Earth.19 At 3,100 kilometres in diameter, Europa is the smallest of the Galileans. It was not well positioned for viewing, and Voyager 1 approached no closer than 732,000 kilometres. Europa's intriguingly high albedo of 64 per cent made it one of the most reflective bodies in the Solar System. It had been speculated on the basis of maps drawn by telescopic observers that icy caps may have left only a narrow strip of dark rocky terrain along the equator. In fact, even long-range imagery revealed the moon to be completely enshrouded in ice. Ganymede and Callisto were observed too, but the closest approach was in darkness over their anti-Jovian hemispheres, and the highest-resolution imagery was of their dusk terminators on the way in and their dawn terminators on the way out.

sun occultation

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