The moon’s frozen surface, made from a mix of ice and rock, has a mottled appearance in which dark, old and heavily cratered regions are separated by lighter, less cratered (and therefore more recently formed) areas. It seems that heat escaping from Ganymede’s interior once drove an icy equivalent of Earth’s plate tectonics, pulling older areas of the crust apart and allowing a fresh mix of rock and ice to well up and fill the gaps. This suggests that, while Ganymede is not subject to tidal heating today, things were different in the distant past.
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222 JUPITER AND ITS MOONS
Tiamat Sulcus
In many places, the brighter segments of Ganymede’s crust take the form of long, broad strips of parallel grooves and ridges, known as ‘sulci’. The longest, such as Tiamat Sulcus, stretch for hundreds of kilometres across the giant satellite’s landscape, forming ranges of rolling hills that may be up to 500 m (1600 ft) high, separated by parallel valleys a few kilometres wide.
Sulci are rather similar to the parallel ridges associated with the generation of new crust on Earth, and it’s likely they have a similar cause. They seem to mark areas where new crust was created in the distant past. Convection in the moon’s interior caused the original crust to crack into icy plates that slowly drifted in different directions, allowing fresh ice to well up through the cracks. The process evidently continued for a considerable part of Ganymede’s history, with older sulci being subjected to the same process; Tiamat, for example, is divided in two by the narrower Kishar Sulcus.
224 JUPITER AND ITS MOONS
Callisto
The outermost of Jupiter’s Galiean moons and the second largest (with a diameter of 4,821 km or 2,996 miles), Callisto has never suffered the tidal heating that helped to shape its inner neighbours. As a result, its surface has been stable since its formation, and it is now saturated with the impacts from more than four billion years’ worth of objects pulled inward by Jupiter’s gravity. Researchers believe it is probably the most cratered world in the solar system. Although the outermost crust is generally dark (a result of ‘space weathering’: bombardment by particles from the solar wind), impacts force out fresh ice from just below the surface, spraying bright rays of ejecta across the landscape.
Callisto’s lack of geological activity suggests that its cold and icy interior never separated to form a distinct core. However, the moon’s effect on Jupiter’s magnetic field suggests there is at least one internal layer – a global salty ocean around 150 km (93 miles) deep, hidden beneath about the same depth of solid, icy crust.
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226 JUPITER AND ITS MOONS
Asgard and Valhalla
Callisto’s dominant features are two enormous impact basins, known as Valhalla and Asgard. Valhalla is the larger of the two, with an overall diameter of about 1,900 km (1,180 miles). Asgard (opposite) is only slightly smaller at 1,600 km (1,000 miles) wide. Each basin consists of a relatively bright, flat central plain known as a ‘palimpsest’, surrounded by rings of concentric hills rather than a single defined crater wall. In places, the Sun’s feeble, but still potent, heat causes ice to sublimate out of the rock-ice mix, weathering these hills into sharply defined, conical peaks. The palimpsests (a term used
by historians to describe manuscripts that have been wiped clean and reused) are believed to have formed where each incoming meteorite smashed through the moon’s dark outer crust and allowed brighter, relatively slushy ice to well up from beneath, eventually healing the central scars. Patterns of radial fractures that cross the surrounding mountain ranges, meanwhile, may have been caused by the crust flexing on top of the moon’s deep underlying ocean as the basin formed.
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228 JUPITER AND ITS MOONS
Amalthea
In addition to its four giant Galilean moons, Jupiter has a host of other satellites. Of these, 61 outer moons are most likely objects captured into orbit around the planet, while four inner moons, orbiting close to Jupiter’s rings, appear to be natural satellites, formed alongside the planet. Amalthea is the largest of these, and third in distance from Jupiter. Some 262 km (163 miles) long and 150 km (93 miles) wide, it is peppered with craters, the largest
of which is the 90-km-diameter (56 mile) Pan. Amalthea also has one of the reddest surfaces in the solar system (one theory is that it sweeps up reddish sulfurous material that escapes from Io). Data from the Galileo space probe showed that Amalthea
has a surprisingly low mass (suggesting that it is little more than an orbiting ‘rubble pile’), while infrared studies have revealed the presence of hydrated minerals that are hard to explain if it formed at its current distance from Jupiter. It’s thought, therefore, that Amalthea is all that survives of a large progenitor body that once orbited further out; when this was destroyed in an impact, some of the pieces reassembled themselves into this moon.
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230 JUPITER AND ITS MOONS
Galileo’s most detailed views of Jupiter’s four small inner moons
Metis
Adrastea
Amalthea
Thebe
Moons of Jupiter
Name
Diameter
Orbital period (days)*
Eccentricity (circular = 0)
Metis
60x40x34 km (37x25x21 miles)
0.29
0.00
Adrastea
20x16x14 km (12x10x9 miles)
0.30
0.00
Amalthea
250x146x128 km (155x91x80 miles)
0.50
0.00
Thebe
116x98x84 km (72x61x52 miles)
0.67
0.02
Io
3643 km (2263.7 miles)
1.77
0.00
Europa
3121.6 km (1939.7 miles)
3.55
0.01
Ganymede
5262.4 km (3269.9 miles)
7.15
0.00
Callisto
4820.6 km (2995.4 miles)
16.69
0.01
Themilessto
8 km (5 miles)
130
0.24
Leda
10 km (6.2 miles)
241
0.16
Himalia
170 km (105.6 miles)
251
0.16
Lysithea
24 km (14.9 miles)
259
0.11
Elara
80 km (49.7 miles)
260
0.22
Dia
4 km (2.5 miles)
287
0.25
Carpo
6 km (3.7 miles)
456
0.43
S/2003 J3
4 km (2.5 miles)
504 (R)
0.24
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S/2003 J12
2 km (1.2 miles)
533 (R)
0.38
Euporie
2 km (1.2 miles)
553 (R)
0.16
S/2011 J1
4 km (2.5 miles)
581 (R)
0.30
* R = Retrograde orbit
Name
Diameter
Period*
Ecc.
Name
Diameter
Period*
Ecc.
S/2010 J2
4 km (2.5 mi)
588 (R)
0.31
Isonoe
3.8 km (2.4 mi)
726 (R)
0.26
S/2003 J16
4 km (2.5 mi)
595 (R)
0.27
S/2011 J2
4 km (2.5 mi)
727 (R)
0.39
S/2016 J1
3 km (1.9 mi)
604 (R)
0.14
Erinome
3.2 km (2 mi)
728 (R)
0.27
S/2003 J18
4 km (2.5 mi)
606 (R)
0.12
Kale
2 km (1.2 mi)
730 (R)
0.26
Mneme
4 km (2.5 mi)
620 (R)
0.23
Aitne
3 km (1.9 mi)
730 (R)
0.26
Euanthe
3 km (1.9 mi)
621 (R)
0.23
Taygete
5 km (3.1 mi)
732 (R)
0.25
Orthosie
2 km (1.2 mi)
623 (R)
0.28
S/2017 J1
2 km (1.2 mi)
734 (R)
0.40
Harpalyke
4.4 km (2.7 mi)
623 (R)
0.23
Carme
30 km (18.6 mi)
734 (R)
0.25
Praxidike
6.8 km (4.2 mi)
625 (R)
0.22
Cyllene
4 km (2.5 mi)
738 (R)
0.32
Thyone
4 km (2.5 mi)
627 (R)
0.23
Hegemone
6 km (3.7 mi)
740 (R)
0.33
Thelxinoe
4 km (2.5 mi)
628 (R)
0.22
Kalyke
5.2 km (3.2 mi)
743 (R)
0.24
Ananke
20 km (12.4 mi)
630 (R)
0.24
Pasiphae
36 km (22.4 mi)
744 (R)
0.41
Iocaste
5.2 km (3.2 mi)
632 (R)
0.22
Eukelade
8 km (5 mi)
746 (R)
0.27
Hermippe
4 km (2.5 mi)
634 (R)
0.21
Sponde
2 km (1.2 mi)
748 (R)
0.31
Helike
8 km (5 mi)
635 (R)
0.16
Megaclite
5.4 km (3.4 mi)
753 (R)
0.42
S/2003 J15
4 km (2.5 mi)
668 (R)
0.11
Callirrhoe
8 km (5 mi)
759 (R)
0.28
S/2003 J9
2 km (1.2 mi)
683 (R)
0.27
Sinope
28 km (17.4 mi)
759 (R)
0.25
S/2003 J19
4 km (2.5 mi)
701 (R)
0.33
S/2003 J5
8 km (5 mi)
760 (R)
0.21
Autonoe
4 km (2.5 mi)
715 (R)
0.20
S/2003 J23
4 km (2.5 mi)
760 (R)
0.31
Pasithee
2 km (1.2 mi)
716 (R)
0.29
Aoede
8 km (5 mi)
762 (R)
0.43
Herse
4 km (2.5 mi)
717 (R)
0.28
Arche
3 km (1.9 mi)
763 (R)
0.33
S/2003 J4
4 km (2.5 mi)
723 (R)
0.20
Kallichore
4 km (2.5 mi)
765 (R)
0.26
S/2010 J1
4 km (2.5 mi)
723 (R)
0.32
S/2003 J10
4 km (2.5 mi)
767 (R)
0.21
Chaldene
3.8 km (2.4 mi)
724 (R)
0.24
Kore
4 km (2.5 mi)
779 (R)
0.33
Eurydome
3 km (1.9 mi)
724 (R)
0.26
S/2003 J2
4 km (2.5 mi)
983 (R)
0.38
Saturn
Like Jupiter, Saturn is a gas giant with a huge atmosphere wrapped around a small solid core. The sixth planet from the Sun and the most distant visible to the naked eye, it orbits at an average distance of 9.6 AU. A tilted axis similar to Earth’s gives rise to a comparable cycle of seasons – albeit stretched over the 29.5 Earth years it takes to complete a single orbit.
Aside from its spectacular rings (see pages 242-51), Saturn appears at first to be placid in comparison to Jupiter, with an orderly pattern of cream-toned cloud bands. However, the two worlds are far more alike than such appearances suggest – much of Saturn’s activity is simply ‘muted’ to distant observers by creamy ammonia clouds that condense at high altitudes in its atmosphere. The planet’s average diameter is 116,464 km (72,367 miles) – some 83 per cent of Jupiter’s, but it has less than one-third of that planet’s mass and, consequently, low density and gravity. Coupled with the planet’s rapid 10.5-hour rotation, this creates a noticeable bulge around Saturn’s equator.
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234 SATURN AND ITS MOONS
pages 242-51
Inside Saturn
As a gas giant, Saturn has a fairly similar internal structure to Jupiter. It consists primarily of hydrogen and helium, with a gaseous atmosphere transforming into a liquid mantle at high pressures about 1,000 km (620 miles) beneath the visible surface. Further in, liquid molecular hydrogen breaks down into a sea of ‘liquid metallic’ hydrogen that stretches down to a central core perhaps twice the size of Earth. Overall, however, the planet’s average density is less than that of water – Saturn would float if immersed in a big enough ocean.
Currents running through the electrically conductive sea generate a magnetic field somewhat weaker than Jupiter’s.
Just as the Jovian field is boosted by interaction with material escaping from Io (see page 206), so Saturn’s field is shaped and intensified by the influence of water vapour ejected from the small moon Enceladus into the doughnut-shaped E ring (see page 250). The magnetic field also channels trapped particles from the solar wind onto the poles, to create spectacular aurorae.
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236 SATURN AND ITS MOONS
page 206
page
250
Gaseous outer atmosphere
Liquid molecular hydrogen/ helium mantle
Metallic hydrogen ocean
Probable solid rocky core
Saturnian storms
Saturn’s most obvious weather features are bands of darker and lighter cloud that wrap their way around the planet, moving in opposite directions under the influence of prevailing winds. Lighter bands are defined as zones and somewhat darker ones as belts, but both are generally wider and less well-defined than the similar features on Jupiter (see page 202).
Although there is no semi-permanent storm to match Jupiter’s Great Red Spot, certain regions of Saturn’s atmosphere are prone to generating storms on a more or less predictable basis. At mid-southern latitudes, a region known as Storm Alley produces electrical storms deep within the atmosphere. Radio bursts from one particularly large example, called the Dragon Storm, were detected by the Cassini probe as it neared Saturn in 2004. A visually more impressive storm called the Great White Spot (opposite) recurs in the northern hemisphere roughly every 30 years, usually bursting into visibility around the height o
f the northern summer when the atmosphere is warmest.
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238 SATURN AND ITS MOONS
page 202
The polar hexagon
Saturn’s north pole is marked by a striking hexagonal structure of dark clouds, about 13,800 km (8,600 miles)
along each side, with a whirlpool-like vortex embedded at its centre. One possible explanation for this strangely geometric feature is that its shape is created by the sharp boundary between faster- and slower-moving regions of Saturn’s atmosphere. Careful measurement from NASA’s Cassini probe suggests that, unusually, the hexagon is rotating at exactly the same rate as the planet’s deep interior – viewed from that frame of reference, it has no significant directional winds (in striking contrast to the rest of the atmosphere). Wind speeds around the central vortex, however, are some of the strongest in the solar system, reaching around 1,800 km/h (1,120 mph). A prominent ‘eye’ at its centre, some 2,000 km (1,240 miles) across, is surrounded by a wall of clouds up to 75 km (47 miles) high. These strange features may be connected to Saturn’s unusually warm poles – temperatures here can be up to 60°C (140°F) warmer than in regions close to the equator.
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240 SATURN AND ITS MOONS
Saturn’s rings
The ring system around Saturn is the largest and most complex in the solar system. Its prominent inner section extends across more than twice Saturn’s own diameter, but is just 1 km
(0.6 miles) thick at most. Seen from orbiting spacecraft, these razor-thin platters dissolve into countless narrow ringlets of different brightness and transparency, separated by distinct gaps.
Each ringlet is itself a stream of closely packed individual particles following near-perfect circular orbits above Saturn’s equator. Small-scale disturbances in the rings are common, but the dynamics of the system naturally restore an orderly rotation. Any particular ring particle that is pushed into an elliptical or inclined orbit is much more likely to collide with
its fellows, these collisions tending to cancel out its rogue movement. Astronomers have understood the basic nature
of the rings since 1859, when Scottish mathematician James Clerk Maxwell showed that if the rings were solid structures, they would inevitably be torn apart by tidal forces.
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Solar System in Minutes Page 9