FACT SHEET: VOYAGER JUPITER SCIENCE SUMMARY<\/p>\n
\t\tNASA launched the two Voyager spacecraft to Jupiter,
\nSaturn, Uranus, and Neptune in the late summer of 1977. Voyager
\n1’s closest approach to Jupiter occurred March 5, 1979. Voyager
\n2’s closest approach was July 9, 1979.
\n\t\tPhotography of Jupiter began in January 1979, when
\nimages of the brightly banded planet already exceeded the best
\ntaken from Earth. Voyager 1 completed its Jupiter encounter in
\nearly April, after taking almost 19,000 pictures and many other
\nscientific measurements. Voyager 2 picked up the baton in late
\nApril and its encounter continued into August. They took more
\nthan 33,000 pictures of Jupiter and its five major satellites.
\n\t\tAlthough astronomers had studied Jupiter from Earth for
\nseveral centuries, scientists were surprised by many of Voyager 1
\nand 2’s findings. They now understand that important physical,
\ngeological, and atmospheric processes go on – in the planet, its
\nsatellites, and magnetosphere – that were new to observers.
\n\t\tDiscovery of active volcanism on the satellite Io was
\nprobably the greatest surprise. It was the first time active
\nvolcanoes had been seen on another body in the solar system. It
\nappears that activity on Io affects the entire Jovian system. Io
\nappears to be the primary source of matter that pervades the
\nJovian magnetosphere — the region of space that surrounds the
\nplanet, primarily influenced by the planet’s strong magnetic
\nfield. Sulfur, oxygen, and sodium, apparently erupted by Io’svolcanoes and sputtered off the surface by impact of high-energy
\nparticles, were detected at the outer edge of the magnetosphere.
\n\t\tParticles of the same material are present inside Io’s
\norbit, where they accelerate to more than 10 percent of the speed
\nof light. It is clear to scientists from a comparison of data
\nfrom Pioneers 10 and 11 (which flew past Jupiter in late 1973 and
\n1974) and the Voyagers that something changed in the four and
\none-half years between the Pioneer and Voyager encounters.
\n\t\tIt is not entirely clear just how far-reaching those
\nchanges are, or what brought them about. They may be related to
\nIonian activity. It is difficult to imagine, however, that at
\nleast some of Io’s volcanoes were not erupting when the Pioneers
\nflew past; it is also, the Voyager scientists say, difficult to
\nbelieve the Pioneers’ instruments failed to see magnetospheric
\nconcentrations of sulfur detected by both Voyager spacecraft
\n(Voyager 1 saw greater concentrations than Voyager 2).
\n\t\tHere is a summary of the more important science results
\nfrom the Voyager encounters with Jupiter:
\nJUPITER’S ATMOSPHERE
\n\t\tAtmospheric features of broadly different sizes appear
\nto move with uniform velocities. That suggests that mass motion
\n(movement of material) and not wave motion (movement of energy
\nthrough a relatively stationary mass) was being observed.
\n\t\tRapid brightening of features in the atmosphere was
\nfollowed by spreading of cloud material. That is probably the
\nresult of disturbances that trigger convective (upwelling and<\/p>\n
downwelling) activity.
\n\t\tA pattern of east-to-west winds extends as far poleward
\nas 60 degrees north and south, roughly similar to the pattern
\nseen in more temperate areas where belts and zones are visible.
\nPrevious investigations led scientists to believe the near-polar
\nregions (above 45 degrees latitude) are dominated by convective
\nupwelling and downwelling. Voyager showed they apparently are
\nnot, at least up to 60 degrees latitude, and probably to 75.
\n\t\tMaterial associated with the Great Red Spot, Jupiter’s
\nmost prominent atmospheric feature, moves in a counter-clockwise
\n(anticyclonic) direction. At the outer edge, material appears to
\nrotate in four to six days; near the center, motions are small
\nand nearly random in direction.
\n\t\tSmall spots appear to interact with the Great Red Spot
\nand with each other.
\n\t\tVoyager instruments observed auroral emissions, similar
\nto Earth’s northern lights, in the polar regions, in ultraviolet
\nand visible light. Pioneer 10 and 11 didn’t see the ultraviolet
\nemissions during their encounters. The auroral emissions appear
\nto be related to material from Io that spirals along magnetic
\nfield lines to fall into Jupiter’s atmosphere.
\n\t\tVoyager also saw cloud-top lightning bolts, similar to
\nsuperbolts in Earth’s high atmosphere.
\n\t\tAtmospheric temperature at 5 to 10 millibars (1\/200th
\nto 1\/100th Earth’s surface atmospheric pressure) is about 160
\nKelvins (-170 degrees Fahrenheit). An inversion layer — a warmregion above a cold layer, similar to the phenomenon that traps
\nsmog in the Los Angeles Basin — exists near the 150-millibar
\nlevel. (Pressure at Earth’s surface is about 1,000 millibars.)
\n\t\tThe Voyagers observed ionospheric temperatures that
\nchanged with altitude, reaching about 1,100 Kelvins (1,500
\ndegrees Fahrenheit). That was also not observed by Pioneers 10
\nand 11, and Voyager scientists believe they are witnessing large
\ntemporal or spatial changes in the ionosphere of Jupiter.
\n\t\tThe Voyagers measured helium in the upper atmosphere;
\nits percentage compared to hydrogen is important to understand
\ncomposition and history of the atmosphere — and the primordial
\ncloud of which the Sun and planets formed. Relative abundance of
\nhelium to hydrogen is about 11 percent by volume.
\nSATELLITES AND RING
\n\t\tVoyager 1 identified nine currently active (erupting)
\nvolcanoes on Io, probably driven by tidal heating. Many more are
\nsuspected. Voyager 2 observed eight of the nine; the largest
\nshut down by the time Voyager 2 arrived at Jupiter. Plumes from
\nthe volcanoes reach more than 300 kilometers (190 miles) above
\nthe surface. The material was being ejected at velocities up to
\n1.05 kilometers a second (2,300 miles an hour). By comparison,
\nejection velocities at Mount Etna, one of Earth’s most explosive
\nvolcanoes, hit 50 meters a second (112 miles an hour). Volcanism
\nis associated with heating of Io by tidal pumping. Europa and
\nGanymede, two large satellites nearby, perturb Io in its orbit
\nand Jupiter pulls Io back again. The pumping action causes tidal<\/p>\n
bulging up to 100 meters (330 feet) on Io’s surface, compared
\nwith typical tidal bulges on Earth of one meter (three feet).
\n\t\tVoyager 1 measured the temperature of a large hot spot
\non Io associated with a volcanic feature. While the surrounding
\nterrain has a temperature of about 130 Kelvins (-230 degrees
\nFahrenheit), the hot spot’s temperature is about 290 Kelvins (60
\ndegrees Fahrenheit). Scientists believe the hot spot may be a
\nlava lake, although the temperature indicates the surface is not
\nmolten; it is, at least, reminiscent of lava lakes on Earth.
\n\t\tEuropa displayed a large number of intersecting linear
\nfeatures in the distant, low-resolution photos from Voyager 1.
\nScientists at first believed the features might be deep cracks,
\ncaused by crustal rifting or tectonic processes. Closer, high-
\nresolution photos by Voyager 2, however, left scientists puzzled:
\nThe features were so lacking in topographic relief that they
\n“might have been painted on with a felt marker,” one scientist
\ncommented. There is a possibility that Europa may be internally
\nactive due to tidal heating at a level one-tenth or less that of
\nIo. Models of Europa’s interior show that beneath a thin crust
\n(5 kilometers or 3 miles) of water ice, Europa may have oceans as
\ndeep as 50 kilometers (30 miles) or more.
\n\t\tGanymede turned out to be the largest satellite in the
\nsolar system. Before the Voyager encounters, astronomers thought
\nthat Saturn’s satellite, Titan, was the largest. Ground-based
\nobservations of Titan, of necessity, had included its substantial
\natmosphere. Voyager measurements of Ganymede showed it is largerthan Titan. Ganymede had two distinct terrain types — cratered
\nand grooved, telling scientists that Ganymede’s entire, ice-rich
\ncrust has been under tension from global tectonic processes.
\n\t\tCallisto has an ancient, heavily cratered crust, with
\nremnant rings of enormous impact basins. The largest craters
\napparently were erased when the ice-laden crust flowed during
\ngeologic time; almost no topographic relief is apparent in ghost
\nremnants of the impact basins, identifiable only by their light
\ncolor and surrounding subdued rings of concentric ridges.
\n\t\tAmalthea is elliptical: 270 kilometers (170 miles) by
\n165 kilometers (105 miles) by 150 kilometers (95 miles). It is
\nabout 10 times larger than Mars’ larger satellite, Phobos, and
\nhas 1,000 times the volume.
\n\t\tVoyager discovered a ring around Jupiter. Its outer
\nedge is 129,000 kilometers (80,000 miles) from the center of the
\nplanet, and, though the brightest portion is only about 6,000
\nkilometers (4,000 miles) wide, ring material may extend another
\n50,000 kilometers (30,000 miles) downward to the top of Jupiter’s
\natmosphere. Evidence also suggests that diffuse ring material
\nextends as far out as the orbit of Amalthea. The ring is no more
\nthan 30 kilometers (20 miles) thick. Thus Jupiter joins Saturn,
\nUranus, and Neptune as a ringed planet — although each ring
\nsystem is unique and distinct from the others.
\n\t\tTwo new satellites, Adrastea and Metis, only about 40
\nkilometers (25 miles) in diameter, orbit just outside the ring.
\nA third new satellite, Thebe, diameter about 80 kilometers (50<\/p>\n
miles), was discovered between the orbits of Amalthea and Io.
\nMAGNETOSPHERE
\n\t\tAn electric current of 5 million amperes was detected
\nin the flux tube that flows between Jupiter and Io, five times
\nstronger than predicted. Voyager did not fly through the flux
\ntube, as planned, since the stronger current had twisted the tube
\n7,000 kilometers (4,300 miles) from the predicted location.
\n\t\tThe Voyagers saw ultraviolet emissions from doubly and
\ntriply ionized sulfur and doubly ionized oxygen. Pioneers 10 and
\n11 did not detect them, so hot plasma evidently was not present
\nin 1973 and 74. The sulfur comes from Io’s volcanoes.
\n\t\tPlasma-electron densities in some regions of the Io
\ntorus (an inner-tube-shaped ring of matter in the region of Io’s
\norbit) exceeded 4,500 per cubic centimeter.
\n\t\tA cold plasma, rotating with Jupiter, lies inside six
\nJupiter radii (430,000 kilometers or 270,000 miles) from the
\nplanet. Ions of sulfur, oxygen, and sulfur dioxide were found.
\n\t\tHigh-energy trapped particles were also detected near
\nJupiter, with enhanced abundances of oxygen, sodium, and sulfur.
\n\t\tKilometric radio emissions were coming from Jupiter.
\nThe emissions, in the frequency range from 10 kilohertz to 1
\nmegahertz, may result from plasma oscillations in the Io torus.
\n\t\tPlasma flows in the dayside outer magnetosphere; the
\nplasma rotates with the planet every 10 hours.
\n\t\tVoyager 1 saw evidence of a transition from closed
\nmagnetic field lines to a magnetotail on the antisolar side ofJupiter. Although such a magnetotail was never in serious doubt,
\nits existence had not been confirmed before.
\n\t\tVoyager 2 observations during its Jupiter-to-Saturn
\ncruise showed the magnetotail extends at least to the orbit of
\nSaturn, 650 million kilometers (400 million miles) away.
\n\t\tScientists interpreted whistler emissions as lightning
\nwhistlers in the atmosphere. Lightning was suspected, and it has
\nbeen proven, from the emissions and detection of bolts; lightning
\nis a major energy source for many activities on Jupiter.
\n\t\tVoyager also measured radio spectral arcs (from about 1
\nmegahertz to more than 30 megahertz) in patterns that correlate
\nwith Jovian longitude.
\n\t\tBoth Voyagers continued on to encounters with Saturn.
\nVoyager 1 is bound out of the solar system. Voyager 2 completed
\nencounters with Uranus (in January 1986) and Neptune (in August
\n1989). It is now also leaving the solar system.
\n\t\tThe next mission to Jupiter will be Galileo, launched
\nin 1989. Galileo, an orbiter and an atmospheric probe, will
\ncontinue the exploration of Jupiter begun by the Pioneers and
\ncontinued by the Voyagers. Both the missions are managed for
\nNASA by the Jet Propulsion Laboratory.
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\n5\/7\/90DB<\/p>\n
