v1.2.2 / chapter 12 of 19 / 01 feb 08 / greg goebel / public domain
* The handful of sophisticated probes sent to Jupiter have greatly expanded knowledge of the planet and its moon system. This chapter describes what is now known about Jupiter, and discusses plans for future exploration.
* The US space probes to Jupiter provided many interesting revelations about the planet, but since Jupiter had long been observed in detail from Earth, did not drastically alter our view of the planet itself, as had happened with missions of some of the other planets.
It had been known that the wind directions in the planet's bands alternated in direction. What came as something of a surprise was how fast the winds were, with the Galileo atmospheric probe measuring constant wind speeds of over 530 kilometers per hour. The high winds appear to be driven by the planet's internal heat, causing the generation of "jet streams".
Cyclonic storms come and go, except for one major exception, the Great Red Spot. This huge storm system spans 14,000 kilometers north-south and 26,000 kilometers east-west. It has been observed consistently since 1830 and intermittently seen as far back as 1664. It has, however, gradually shrunk during the 20th century to half its size, and some astronomers wonder if it didn't disappear at times between 1664 and 1830 since there were occasions when its presence wasn't recorded. Although the Great Red Spot has remained at a constant latitude, it tends to drift irregularly in longitude, meaning that it is not closely associated with any disturbance from the planet's solid core.
Other long-lived storm systems have been observed on Jupiter. A number of relatively small "white spot" storms can be observed at any time, and in 1998 three of them, all of which had been observed before World War II, began to merge, completing the merger in 2000 to become a big white oval designated "Oval BA" -- which then turned red in early 2006. "Junior", as the new Red Spot has been nicknamed, is about half the diameter of the original Red Spot, about as big across as the Earth. Junior is moving eastward while the original Red Spot is moving westward at a higher latitude, and the two passed by each other July 2006.
The fact that Oval BA turned red was very interesting. Clearly the red color is associated with the more violent storms, presumably because they transport material from the lower regions of the atmosphere to high levels, with the materials either being red to begin with or turning red after being exposed to solar ultraviolet light. The idea that a hurricane could persist for at least centuries seems bizarre, but given that Jupiter has no solid surface, there is relatively little to break up the storm. However, given that Jupiter's atmosphere grows substantially denser with depth, making its motions much more sluggish, it is unlikely that the storm goes to any major depth.
* Although the space probes did not reveal much that was radically new about Jupiter, they did provide unprecedented views of the Galilean moons. They also revealed some details of the smaller moons, and of course revealed Jupiter's faint ring system.
* Io turned out to be a bizarre and spectacular place, beyond what anyone could have imagined. Instead of having a cratered, barren surface, it was dotted with volcanoes and volcanic calderas, many of them active, with flows of sulfurous material covering the moon's surface and even "seas" of molten sulfur. Io is so active that its surface changed noticeably between the flybys of Voyager 1 and Voyager 2. However, although there are hot spots with temperatures of up to 2,000 degrees Kelvin and the lavas are extremely hot, the average surface temperature is frigid, about 130 degrees Kelvin.
Io appears to be mostly made of rock, unlike most of the other moons of the outer solar system, which have large amounts of ices. This had been hinted before the arrival of the space probes by the moon's high density. The volcanic activity is clearly caused by tidal forces set up by Jupiter and Io's orbital resonances with Europa and Ganymede. The moon has a thin atmosphere of sulfur dioxide and other outflux from the continuous eruptions, and leaves a trail of plasma behind it, forming a faint gaseous "torus" around Jupiter. In addition, streams of electrons connect the moon along magnetic field lines to Jupiter's poles.
Io is deep inside the planet's strong magnetic field and radiation belts. The Galileo orbiter was unable to perform many close observations of the moon until near the end of its mission, since the intense radiation would have degraded the spacecraft's systems and shortened its life.
* Europa also proved to be surprising, though not as much as Io. Europa, like Io, is a rocky world, but it has a surface layer of ices. Its surface is remarkably smooth, with no large craters, leading some to describe it as an "icy cue ball". However, it is not exactly featureless, being crisscrossed with a network of dark streaks.
Some planetary scientists believe Europa's surface is reminiscent of oceanic ice packs, suggesting that there is a layer of water underneath, kept liquid by heat from tidal forces. Galileo's studies of Europa show that it has a peculiar interaction with Jupiter's magnetic field, consistent with a conductive sphere floating in isolation. The best candidate for such a layer is a buried global ocean of salt water. The fact that Europa may very well have a hidden ocean of liquid water makes it a very high priority for closer observation by new space missions. Europa may be the best candidate for another world in the Solar System that can support life forms.
* Ganymede proved to be a little less unusual, with significant cratering of its icy surface, which is patterned in widespread dark and light regions. However, the moon also features odd networks of grooves and ridges in the light regions, which seem to be fault systems. The craters on the surface tend to be relatively shallow, due to the tendency of ices to flow and level themselves over time.
Galileo has shown that Ganymede is very unusual in having a strong magnetic field; no other moon in the Solar System is known to have any appreciable magnetic field. Planetary scientists have suggested the moon has a molten core of iron or iron sulfide covered by silicate rock, surrounded by an icy mantle about 700 kilometers thick. Since Ganymede is a relatively small world, its core should have solidified long ago, but it appears that tidal strains keep it molten, and are probably also responsible for the systems of faults on the surface.
* The Voyager missions showed Callisto to be the least surprising of the Galilean moons, appearing to be a mostly undifferentiated ball of about 40% ice and 60% rock and iron, with a heavily cratered surface. This is the sort of composition that would be expected for an outer planet moon. However, Galileo took a closer look that showed the surface of Callisto to be covered by fine debris, and that there are few small craters, which are common on most other bodies. There must be some surface processes at work, but so far nobody has proposed one that seems very convincing.
Further data provided by Galileo suggested that Callisto may not be entirely mixed, with a rocky core, a mostly icy surface, and a mixed intermediate region. As with Europa, Callisto has an odd interaction with Jupiter's magnetic field that suggests it may have a buried ocean of liquid water.
* Spacecraft observations of the smaller moons of Jupiter have been more minimal, but have had their insights. All these moons are basically irregular lumps of rock and ice in varying proportions, and are generally dark. The four small regular moons inside the orbit of Io received the most attention, since the irregular moons outside the orbit of Callisto were too distant for detailed observation. Of the four inner moons, Metis is the closest to Jupiter, followed by Adrastea, Amalthea, and Thebe.
Metis and Adrastea are so close to Jupiter that their orbits will certainly decay eventually, causing them to fall into the planet if they don't break up first. Amalthea is the biggest of the four, about 189 kilometers along its longest axis. It is very red, apparently having been coated by sulfur compounds leaked by Io.
* Jupiter's faint ring system appears to have three components:
The rings are very tenuous, and composed of small, sooty, reddish dust particles. There is no evidence of ice fragments in the rings. Galileo observations show that the ring material is very similar to the material making up the four inner moons, suggesting that the rings are not due to the breakup of a parent body but were created from dust and debris kicked off the surface of the four moons by micrometeoroid impacts.
* Although Galileo of necessity neglected Jupiter's small irregular outer moons, Earth-based telescopes made a number of remarkable discoveries in the outer region while Galileo was working over the inner moons:
All 45 moonlets are very small, with diameters of a few kilometers, and all have irregular orbits, like those of the eight previously known irregular outer moons. The new moonlets are really just dark chunks of sky junk, not extremely interesting in themselves, and something of a nuisance to keep track of. However, their discovery was an extraordinary feat of observation, demonstrating how far ground-based astronomy, particularly that of electronic detector systems, has technically advanced over the past decades. It will not be very surprising if many more Jovian moonlets are found.
Astronomers had earlier divided the eight irregular moons known before Voyager into two groups according to their orbits; with the discovery of more irregulars, they were organized into four, named the "Himalia", "Ananke", "Carme", and "Pasiphae" groups after the largest member of each group, with the Himalia group being the innermost and the Pasiphae group being the outermost. The moonlets in the Himalia group have prograde orbits, while those in the other three groups have retrograde orbits. There are two moonlets, Themisto and Carpo, that do not belong to any known groups; interestingly, Themisto was discovered in 1975 as "S/1975 J1" but was lost for 25 years, before its rediscovery in 2000.
* In 2005, NASA committed to a new Jupiter probe named "Juno", a solar-electric spacecraft that will be launched in 2011, perform a gravity assist pass by Earth in 2013, and placed into polar orbit around the planet in 2016. It will focus on Jupiter itself, not its moons, using radiometers to map the clouds below, an imager to take pictures of cloud patterns, and a magnetometer to map Jupiter's magnetic field.
Juno is being built by Lockheed-Martin. As planned, the spacecraft will have three solar power arrays, using high-efficiency solar cells. Since Jupiter is five times farther away from the Sun than the Earth, Juno will only receive 1/25th as much power from sunlight as would an Earth satellite. To ensure adequate power, Juno's orbit will keep it facing the Sun at all times, and will never pass into Jupiter's shadow. It was the second mission in the NASA "New Frontiers" program, following the "New Horizons" probe to Pluto, discussed later.
NASA also conducted studies for a more ambitious mission, the "Jupiter Icy Moons Orbiter (JIMO)" probe, which was intended to successively go into orbit around Ganymede, Callisto, and particularly Europa to perform extended detailed observations. JIMO was envisioned as being powered by an advanced nuclear-electric propulsion system powered by a space fission reactor, and not RTGs. JIMO was to be the biggest planetary probe ever built, with a launch weight of 20 tonnes and with a deployed length of roughly 30 meters. It was so big that nobody was sure how it will be launched into Earth orbit. In fact, it was too ambitious, and work on it has been scaled back more or less to a investigation, not a development program. Work on a more modest "Europa Orbiter" probe has been repeatedly delayed and for the time being seems to be in limbo.
* Statistics for Jupiter:
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mean distance from Sun 5.20 AU (778.3 x 10^6 kilometers)
orbital period (sidereal) 11.86 years
orbital eccentricity 0.048
orbital inclination 1.3 degrees
equatorial diameter 142,800 km (11.2 Earth)
mass (relative to Earth) 317.833
mean density (relative to water) 1.33
gravity (relative to Earth) 2.54
escape speed 59.6 kilometers per second
rotation period 0.41 days
oblateness 1/15
inclination of equator 3.1 degrees
albedo 0.52
max surface temperature -160 degrees Celsius (cloud tops)
atmosphere (major constituents ) H, He, Ch4, NH3, H2O
clouds of NH3, NH4SH, H2O
number of known moons 16
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* Major moons of Jupiter, from outermost to innermost, with pronunciations in
parenthesis. The new moonlets are consolidated in following tables.
Radii are measured from the center of Jupiter. The abbreviation "RJ" stands
for the radius of Jupiter, while "RM" stands for the radius of the orbit of
the Earth's Moon, and "M" stands for the diameter or mass of the Earth's
Moon. Densities are given relative to water, which is equivalent to grams
per cubic centimeter. Orbital and rotation periods are given in days and
fractions of days, with days-hours-minutes added for periods under two days.
For irregular moons, the group is specified as well.
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SINOPE ("sah-NOH-pee") / Jupiter IX / irregular (Pasiphae group):
mean distance from planet 23,370,000 km / 327 RJ / 60.78 RM
orbital period (sidereal) 758 days
orbital eccentricity 0.28
orbital inclination 153 degrees (RETROGRADE)
equatorial diameter 35 km
mass, density, albedo UNKNOWN
year of discovery 1914 (Nicholson)
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PASIPHAE ("pah-SIF-ah-ee") / Jupiter VIII / irregular (Pasiphae group):
mean distance from planet 23,330,000 km / 327 RJ / 60.68 RM
orbital period (sidereal) 735 days
orbital eccentricity 0.38
orbital inclination 148 degrees (RETROGRADE)
equatorial diameter 50 km
mass, density, albedo UNKNOWN
year of discovery 1908 (Melotte)
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CARME ("KAR-mee") / Jupiter XI / irregular (Carme group):
mean distance from planet 22,350,000 km / 313 RJ / 58.13 RM
orbital period (sidereal) 692 days
orbital eccentricity 0.21
orbital inclination 164 degrees (RETROGRADE)
equatorial diameter 40 km
mass, density, albedo UNKNOWN
year of discovery 1938 (Nicholson)
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ANANKE ("a-NANG-kee") / Jupiter XII / irregular (Ananke group):
mean distance from planet 21,200,000 km / 297 RJ / 55.14 RM
orbital period (sidereal) 631 days
orbital eccentricity 0.17
orbital inclination 147 degrees (RETROGRADE)
equatorial diameter 30 km
mass, density, albedo UNKNOWN
year of discovery 1951 (Nicholson)
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ELARA ("EE-lar-uh") / Jupiter VII / irregular (Himalia group):
mean distance from planet 11,740,000 km / 164 RJ / 30.53 RM
orbital period (sidereal) 260 days
orbital eccentricity 0.207
orbital inclination 28 degrees
equatorial diameter 75 km
mass & density UNKNOWN
albedo 0.03
year of discovery 1905 (Perrine)
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LYSITHIA ("ly-SITH-ee-uh") / Jupiter X / irregular (Himalia group):
mean distance from planet 11,710,000 km / 164 RJ / 30.46 RM
orbital period (sidereal) 260 days
orbital eccentricity 0.107
orbital inclination 29 degrees
equatorial diameter 35 km
mass, density, albedo UNKNOWN
year of discovery 1938 (Nicholson)
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HIMALIA ("hih-MAL-yuh") / Jupiter VI / irregular (Himalia group):
mean distance from planet 11,470,000 km / 161 RJ / 29.83 RM
orbital period (sidereal) 251 days
orbital eccentricity 0.158
orbital inclination 28 degrees
equatorial diameter 185 km / 0.05 M
mass & density UNKNOWN
albedo 0.03
year of discovery 1904 (Perrine)
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LEDA ("LEE-duh") / Jupiter XIII / irregular (Himalia group):
mean distance from planet 11,110,000 km / 156 RJ / 28.89 RM
orbital period (sidereal) 240 days
orbital eccentricity 0.147
orbital inclination 27 degrees
equatorial diameter 15 km
mass, density, albedo unknown
year of discovery 1974 (Kowal)
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CALLISTO ("ka-LIS-toh") / Jupiter IV:
mean distance from planet 1,885,000 km / 26.4 RJ / 4.9 RM
orbital period (sidereal) 16.689 days
orbital eccentricity 0.007
orbital inclination 0.2 degrees
equatorial diameter 4,800 km / 1.38 M
mass 1.46 M
mean density 1.83
albedo 0.2
year of discovery 1610 (Galileo)
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GANYMEDE ("GAN-uh-meed") / Jupiter III:
mean distance from planet 1,070,000 km / 15.0 RJ / 2.78 RM
orbital period (sidereal) 7.155 days
orbital eccentricity 0.001
orbital inclination 0.2 degrees
equatorial diameter 5,260 km / 1.51 M
mass 2.03 M
mean density 1.93
albedo 0.4
year of discovery 1610 (Galileo)
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EUROPA ("yoo-ROH-puh") / Jupiter II:
mean distance from planet 671,000 km / 9.40 RJ / 1.75 RM
orbital period (sidereal) 3.551 days
orbital eccentricity 0.010
orbital inclination 0.5 degrees
equatorial diameter 3,140 km / 0.90 M
mass 0.66 M
mean density 3.04
albedo 0.6
year of discovery 1610 (Galileo)
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IO ("EYE-oh") / Jupiter I:
mean distance from planet 422,000 km / 5.91 RJ / 1.1 RM
orbital period (sidereal) 1.769 days / 1 day 18 hours 46 minutes
orbital eccentricity 0.004
orbital inclination 0 degrees
equatorial diameter 3,630 km / 1.04 M
mass 1.21 M
mean density 3.55
albedo 0.6
year of discovery 1610 (Galileo)
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THEBE ("THEE-be") / Jupiter XIV:
mean distance from planet 222,000 km / 3.11 RJ / 0.58 RM
orbital period (sidereal) 0.674 days / 16 hours 11 minutes
orbital eccentricity 0.013
orbital inclination 0 degrees
dimensions 100 x 90 km
mass & density UNKNOWN
albedo 0.05
year of discovery 1979 (Synott)
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AMALTHEA ("am-al-THEE-uh") / Jupiter V:
mean distance from planet 180,000 km / 2.52 RJ / 0.47 RM
orbital period (sidereal) 0.498 days / 11 hours 57 minutes
orbital eccentricity 0.003
orbital inclination 0.4 degrees
dimensions 270 x 166 x 150 km / 0.05 M
mass & density UNKNOWN
albedo 0.05
year of discovery 1892 (Barnard)
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ADRASTEA ("a-DRAS-tee-uh") / Jupiter XV:
mean distance from planet 129,000 km / 1.81 RJ / 0.34 RM
orbital period (sidereal) 0.297 days / 7 hours 8 hours
orbital eccentricity 0
orbital inclination 0 degrees
dimensions 23 x 20 x 15 km
mass & density UNKNOWN
albedo 0.05
year of discovery 1979 (Jewitt, Danielson, Synott)
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METIS ("MEE-tis") / Jupiter XVI:
mean distance from planet 128,000 km / 1.79 RJ / 0.33 RM
orbital period (sidereal) 0.294 days / 7 hours 3 minutes
orbital eccentricity 0
orbital inclination 0 degrees
equatorial diameter 40 km
mass & density UNKNOWN
albedo 0.05
year of discovery 1979 (Synott)
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* As mentioned, the irregular moonlets discovered in the 21st century are all
small, a few kilometers in diameter, and dark. Not much more is known about
them. The orbital parameters for these moonlets roughly match those of the
parent of the group to which the moonlet belongs.
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preliminary
moon designation group
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CALLIRRHOE Jupiter XVII S/1999 J1 Pasiphae group
THEMISTO Jupiter XVIII S/2000 J1 -
MEGACLITE Jupiter XIX S/2000 J8 Pasiphae group
TAYGETE Jupiter XX S/2000 J9 Carme group
CHALDENE Jupiter XXI S/2000 J10 Carme group
HARPALYKE Jupiter XXII S/2000 J5 Ananke group
KALYKE Jupiter XXIII S/2000 J2 Carme group
IOCASTE Jupiter XXIV S/2000 J3 Ananke group
ERINOME Jupiter XXV S/2000 J4 Carme group
ISONOE Jupiter XXVI S/2000 J6 Carme group
PRAXIDIKE Jupiter XXVII S/2000 J7 Ananke group
AUTONOE Jupiter XXVIII S/2001 J1 Pasiphae group
THYONE Jupiter XXIX S/2001 J2 Ananke group
HERMIPPE Jupiter XXX S/2001 J3 Ananke group
AITNE Jupiter XXXI S/2001 J11 Carme group
EURYDOME Jupiter XXXII S/2001 J4 Pasiphae group
EUANTHE Jupiter XXXIII S/2001 J7 Ananke group
EUPORIE Jupiter XXXIV S/2001 J10 Ananke group
ORTHOSIE Jupiter XXXV S/2001 J9 Ananke group
SPONDE Jupiter XXXVI S/2001 J5 Pasiphae group
KALE Jupiter XXXVII S/2001 J8 Carme group
PASITHEE Jupiter XXXVIII S/2001 J6 Carme group
ARCHE Jupiter XLIII S/2002 J1 Carme group
HEGEMONE Jupiter XXXIX S/2003 J8 Pasiphae group
MNEME Jupiter XL S/2003 J21 Ananke group
AOEDE Jupiter XLI S/2003 J7 Pasiphae group
THELXINOE Jupiter XLII S/2003 J22 Ananke group
KALLICHORE Jupiter XLIV S/2003 J11 Carme group
HELIKE Jupiter XLV S/2003 J6 Ananke group
CARPO Jupiter XLVI S/2003 J20 -
EUKELADE Jupiter XLVII S/2003 J11 Carme group
CYLLENE Jupiter XLVIII S/2003 J13 Pasiphae group
KORE Jupiter XLIX S/2003 J14 Pasiphae group
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other irregulars discovered in 2003, unconfirmed or unnamed:
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- S/2003 J2 ?
- S/2003 J3 Ananke group
- S/2003 J4 Pasiphae group
- S/2003 J5 Carme group
- S/2003 J9 Carme group
- S/2003 J10 ?
- S/2003 J12 ?
- S/2003 J15 ?
- S/2003 J16 Ananke group
- S/2003 J17 Carme group
- S/2003 J18 Ananke group
- S/2003 J19 Carme group
- S/2003 J23 Pasiphae group
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Also as mentioned, two of the new moonlets do not belong to any known group:
* Jupiter's ring system:
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GOSSAMER RING 222,000 - 128,900 km / 3.11 - 1.81 RJ / 0.58 - 0.34 RM
MAIN RING 128,980 - 122,500 km / 1.81 - 1.72 RJ / 0.34 - 0.32 RM
HALO 122,500 - 92,000 km / 1.79 - 1.29 RJ / 0.32 - 0.24 RM
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