Encyclopedia of the Solar System 2nd ed

Planetary Rings 505

TABLE 1 Locations of Major Ring Components. The inner limits of Jupiter’s Amalthea and Thebe rings are

poorly constrained. The Saturnian F ring has multiple narrow strands that are part of a continuous

spiral ring in addition to the bright core listed above. The uranianηring has a diffuse component that

extends∼55 km beyond the ring. R1 and R2 were discovered by HST in 2003. New Saturnian rings

discovered by Cassini that lie exterior to the main rings are given below; those within the main rings

have been omitted.

Ring Radial Location Optical

Planet Component or (width) in km Depth

Jupiter Halo 89,400–123,000 10 –^6

(Radius: 71,492 km) Main 123,000–128,940 10 –^6

Amalthea Ring 140,000?–81,000 10 –^7

Thebe Ring 140,000?–221,900 10 –^7

Thebe Extension 221,900–280,000 10 –^8

Saturn D 67,000–74,500 10 –^5

(Radius: 60,330 km) C 74,500–92,000 0.05–0.35

B 92,000–17,580 0.4–> 3

Cassini division 117,580–122,200 0–0.1

A 122,200–136,780 0.4–1.0

R/2004 S1 (Atlas) 137,630 ∼ 10 −^4

R/2004 S2 138,900 ∼ 10 −^5

F 140,200 (∼1) 0.1–1

R/2006 S1 151,500?

(Janus/Epimetheus)

G 166,000–173,000 10 –^6

R/2006 S2 212,000?

(Pallene)

E 181,000–483,000 10 –^6

Uranus 1986 U2R ∼38,000?

(Radius: 26,200 km) 6 41,837 (1.5) 0.3

5 42,234 (∼2) 0.5

4 42,571 (∼2) 0.3

α 44,718 (4–10) 0.4

β 45,661 (5–11) 0.2

η 47,176 (1.6) <0.4

γ 47,627 (1–4) >0.3

δ 48,300 (3–7) 0.5

λ 50,024 (∼2) 0.1

ε 51,149 (20–96) 0.5–2.3

R2 66,100–69,900 10 –^8

R1 86,000–103,000 10 –^8

Neptune Galle 41,000–43,000 10 –^4

(Radius: 25,225 km) Le Verrier 53,200 (<100) 0.01

Lassell 53,200–59,100 10 –^4

Arago 57,200 (∼10)?

Adams 62,933 (15–100) 0.01–0.1

2.2.1 STELLAR OCCULTATIONS

A stellar occultation occurs when, as viewed from Earth,
a bright star passes behind a planetary ring system. These
events occur rarely, typically last for hours, and can yield
data on the location of ring features that rival spacecraft
resolutions. The Uranian ring system was discovered in

1977 by stellar occultation, and the first hint of the Nep-

tunian ring arcs also came during such an event. The value

of these observations is dramatically illustrated by the 1989

occultation of a particularly bright star by Saturn’s ring sys-

tem that revealed numerous ring features to a precision of

2 km, produced an important refinement of Saturn’s pole