* 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. It continued to grow, being observed as half the size of the Great Red Spot in 2008.
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.BACK_TO_TOP
* 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 the core 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.
There has been some curiosity about why Ganymede is differentiated and Callisto is not, despite the fact that the two bodies are of comparable size and general overall composition. An interesting explanation has been advanced that proposes the difference traces back to the early days of the Solar System, about 3.9 billion years ago, when planetary bodies were being pounded heavily by comets and asteroids. Jupiter was a prime target for such bombardments, with comets and asteroids falling into the giant planet's gravity well; since Ganymede is deeper in Jupiter's gravity well than Callisto, it was more in the line of fire -- with modeling suggested that it was hit twice as often by objects that had been accelerated by Jupiter's gravity to higher impact velocities.
The end result was that the amount of energy dumped into Ganymede by impacts was about 3.5 times greater than impacts dumped into Callisto, with the greater number of impacts helping reduce Ganymede to a molten state that then sorted itself into differentiated layers. The idea is more or less informed speculation at this time, but it has been seen as very plausible and attractive.
* 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 51 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 orbiter named "Juno", named after the Roman god Jupiter's wife. Juno is, unlike all previous Outer Planets probes, solar powered, featuring three solar power arrays arranged around its central body, giving it a diameter of 20 meters. The central body, which mounts communications antennas, instruments, and system electronics, is 3.5 meters across and 3.5 meters tall. Since Jupiter has savage radiation belts, core electronics are stowed in a box with titanium walls over a centimeter thick.
Although Galileo was placed into an orbit around Jupiter's equator, Juno was placed in a highly elliptical polar orbit, rising high over the planet to drop down to an altitude of about 4,000 kilometers for close observations. Each orbit takes 11 days, with the probe taking six hours of observations on its closest approach, then returning data and being set up for the next pass before diving down again. Although the orbit avoids the worst of Jupiter's radiation belts, the probe is still only designed to tolerate 30 orbits, less than a year's observations, before it fries. Juno is focused on observations of Jupiter itself and will pay little attention to Jupiter's elaborate Moon system.
Solar power was chosen for Juno to reduce cost. No probe has ever operated with solar power that far from the Sun, five times as far away as the Earth. The solar panels were built with ultrahigh efficiency solar cell and have thick cover glass to protect them from radiation, though they will still degrade over time and lose efficiency. At the distance of Earth, the arrays can produce 18 kilowatts, but at Jupiter they are only able to generate 400 watts -- and half that power is required for heaters to keep the probe's payload systems from freezing up. However, thanks to careful power budgeting, the probe is still able to operate on 200 watts.
During its orbits around Jupiter, Juno spins at 2 RPM while it performs observations with a suite of nine instruments and 25 sensors, including:
The spacecraft had a launch mass of about 3.5 tonnes, 2.5 tonnes of that being fuel. It was launched from Cape Canaveral by an Atlas 5 551 booster on 5 August 2011, performed an Earth flyby for gravity assist on 9 October 2013, and arrived in Jupiter orbit on 4 July 2016.
To ensure adequate power, Juno's orbit keeps it facing the Sun at all times, the probe never passing 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.
NASA and the ESA conducted studies for a "Europa Jupiter System Mission (EJSM)" that would involve multiple orbiters, with one orbiting Europa and another orbiting Ganymede. EJSM was selected in 2009 for further study, with an alternative, the "Titan / Saturn System Mission", dropped for the time being. NASA funding cutbacks left EJSM in doubt, so the ESA focused on mission named the "Jupiter Icy Moon Explorer (JUICE)", based on the EJSM Ganymede orbiter.
JUICE was selected for development in 2012, with launch scheduled for 2022 and arrival in the Jovian system in 2033. It will spend three and a half years orbiting Jupiter, then settle into an orbit around Ganymede. JUICE will be solar powered; it will have a launch mass of 5,500 kilograms, and will have a payload suite of ten instruments, including optical, radar, magnetic-electric, plasma and particle sensors. NASA will contribute an ultraviolet spectrometer, and is collaborating on two European instruments. JAXA is also collaborating on three JUICE instruments.
NASA has not given up on a Europa mission, with planning now in progress for a "Europa Multi-Flyby Mission" that could be launched early in the next decade. The mission will include an orbiter and a lander, with the orbiter dropping the lander after scouting out an interesting landing zone.BACK_TO_TOP
* Statistics for Jupiter:
__________________________________________________________________________ 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 moons > 10 km in size 13, plus dozens of known moonlets __________________________________________________________________________
* 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.
__________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________ 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) __________________________________________________________________________
* 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.
_______________________________________________________________ preliminary moon designation group _______________________________________________________________ CALLIRRHOE Jupiter XVII S/1999 J1 Pasiphae group THEMISTO Jupiter XVIII S/2000 J1 - KALYKE Jupiter XXIII S/2000 J2 Carme group IOCASTE Jupiter XXIV S/2000 J3 Ananke group ERINOME Jupiter XXV S/2000 J4 Carme group HARPALYKE Jupiter XXII S/2000 J5 Ananke group ISONOE Jupiter XXVI S/2000 J6 Carme group PRAXIDIKE Jupiter XXVII S/2000 J7 Ananke group MEGACLITE Jupiter XIX S/2000 J8 Pasiphae group TAYGETE Jupiter XX S/2000 J9 Carme group CHALDENE Jupiter XXI S/2000 J10 Carme group DIA Jupiter LIII S/2000 J11 Himalia 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 EURYDOME Jupiter XXXII S/2001 J4 Pasiphae group SPONDE Jupiter XXXVI S/2001 J5 Pasiphae group PASITHEE Jupiter XXXVIII S/2001 J6 Carme group EUANTHE Jupiter XXXIII S/2001 J7 Ananke group KALE Jupiter XXXVII S/2001 J8 Carme group ORTHOSIE Jupiter XXXV S/2001 J9 Ananke group EUPORIE Jupiter XXXIV S/2001 J10 Ananke group AITNE Jupiter XXXI S/2001 J11 Carme group _______________________________________________________________ ARCHE Jupiter XLIII S/2002 J1 Carme group EUKELADE Jupiter XLVII S/2003 J1 Carme group - S/2003 J2 ? - S/2003 J3 Ananke group - S/2003 J4 Pasiphae group - S/2003 J5 Carme group AOEDE Jupiter XLI S/2003 J7 Pasiphae group HEGEMONE Jupiter XXXIX S/2003 J8 Pasiphae group - S/2003 J9 Carme group HELIKE Jupiter XLV S/2003 J6 Ananke group - S/2003 J10 Carme group KALLICHORE Jupiter XLIV S/2003 J11 Carme group - S/2003 J12 ? CYLLENE Jupiter XLVIII S/2003 J13 Pasiphae group KORE Jupiter XLIX S/2003 J14 Pasiphae group - S/2003 J15 Ananke group - S/2003 J16 Ananke group HERSE Jupiter L S/2003 J17 Carme group - S/2003 J18 Ananke group - S/2003 J19 Carme group CARPO Jupiter XLVI S/2003 J20 - MNEME Jupiter XL S/2003 J21 Ananke group THELXINOE Jupiter XLII S/2003 J22 Ananke group - S/2003 J23 Pasiphae group _______________________________________________________________ - S/2010 J1 ? - S/2010 J2 ? _______________________________________________________________ - S/2011 J1 ? - S/2011 J2 ? _______________________________________________________________
Also as mentioned, two of the new moonlets do not belong to any known group:
* Jupiter's ring system:
__________________________________________________________________________ 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 __________________________________________________________________________BACK_TO_TOP