Rings in the Solar System

RINGS IN THE SOLAR SYSTEM

A SHORT REVIEW

By Kaarin Goodburn

1. Introduction

2. Ringed Planets

2.1 Background

2.2 Jupiter

2.3 Saturn

2.4 Uranus

2.5 Neptune

3. Other solar system ring structures

4. Asteroid Belt

5. Edgeworth-Kuiper Belt

6. Oort Cloud

4. Further reading/references

1. Introduction

Rings dominate the solar system. Circular/ellipsoid motion is the rule owing to the interaction of angular momentum and gravitational forces between, e.g. the sun, planets, moons, asteroids and comets, as described by Kepler’s laws of planetary motion.

The planets were formed by accretion, growing from coalesced ‘planetesimals’ (1m-1km) in a disk-like nebula of dust and gas rotating around a hot young star. As larger objects grew larger they agitated and accelerated smaller bodies in the nebula, producing millions of micrometer-sized particles from high speed collisions, reflecting light from the central star, which is seen as a disk, as recently discovered around epsilon Eridani. Inside a ring, new planets circle the central star, outside the formation process continues.

A 1998 submillimetre image of epsilon Eridani’s dust disk.

JAC/JLA, Astronomy Now, September 2000

2. Ringed Planets

2.1 Background

From 1610 to 1977, Saturn was thought to be the only planet with rings. However, it is now known that Jupiter, Uranus and Neptune also have ring systems, although Saturn's is by far the largest.

If rings were formed at the same time as a planet, material must feed them otherwise they would have long vanished. Roche proposed (1850) a region close to a planet where gravitational forces are so powerful that they can destroy a body, resulting debris colliding and producing ever-smaller particles settling into flat rings around the planet’s equator. The Roche limit is the closest distance an object can come to another without being destroyed by gravitational forces. The gas giants’ outer planets’ ring systems are inside their respective Roche limits:-

Planet	Roche Limit (km)
Jupiter	175,000
Saturn	147,000
Uranus	62,000
Neptune	59,000

JPL/NASA

(www.jpl.nasa.gov.saturn.faq.html)

2.2 Jupiter

Jupiter has a ring system 92,000-250,000 km from its centre, discovered by Voyager I. Galileo has gathered much more data, finding it has three parts:

i. flat main ring

ii. halo above and below the main ring caused by electromagnetic forces pushing the smallest, electrically-charged particles out of the ring plane; and

iii. gossamer ring: two rings embedded within each other

Jupiter’s Rings and Inner Satellites (NASA)

Jupiter’s rings are without ice, comprising dark debris particles from micrometeroid strikes on its inner moons, which escapes their gravitational fields and is captured by Jupiter’s, forming an equatorial ring.

Jovian Ring Formation (NASA)

Jupiter’s Inner Moons. From left: Metis, Adrastea, Amalthea, Thebe (NASA)

Table 1: Jupiter’s Inner Moons and their Relationship to its Rings

Moon	Discovered	Radius (km)	Mass (kg)	Distance from Jupiter (km)	Orbital Period (days)	Relationship to rings
Metis	1980 (Voyager 2)	30	9x10¹⁶	128,000	0.294780 inside Jupiter’s synchronous orbit radius - will decay and fall into Jupiter	Embedded in main ring: source of main ring material
Adrastea	1979 (Voyager 2)	13x10x8	2x10¹⁶	129,000	0.29826 inside Jupiter’s synchronous orbit	Along main ring’s outer edge: source of main ring material
Amalthea	1892 (Barnard)	131x73x67	7x10¹⁸	181,300	0.49817905	Outer periphery of gossamer ring: source of gossamer ring material
Thebe	1980 (Voyager 1)	55x45	8x10¹⁷	222,000	0.6745	Near outer edge of gossamer ring: source of gossamer ring material

Main ring (NASA)

False colour image: halo (NASA - edited)

3.3 Saturn

Galileo was the first person to observe Saturn’s water ice and dust particle rings, in 1610. Although the particles range from microns to metres in size the rings stretch 380,000 km – about the distance from Earth to our Moon – but are only 10-200m thick.

The rings, which Galileo first thought were ‘handles’ or two moons either side of Saturn, have complex structure, some related to gravitational perturbations by Saturn's moons, but much unexplained. They comprise hundreds of thousands of ringlets, possibly caused by gravitational waves from nearby moons, and the gaps/divisions between them, once thought to be empty, are full of fine-grained particles. Natural boundaries appear within the rings where particle densities change.

Every particle in the rings is in balance between Saturn’s gravity and its own orbital velocity. If something caused them to slow down (lose momentum velocity), they would plummet into Saturn.

Galileo’s 1610 sketch of Saturn

Cassini’s 1676 sketch of Saturn

Enhanced false colour: possible chemical composition variations (NASA).

Saturnian Ring Formation Theories

1. They are remnants of early solar system material (about 4.6 billion years old) never forming into satellites because they were within the Roche limit

2. A moon strayed inside the Roche limit and was destroyed by gravitational forces

3. They are the remains of a meteor-shattered moon

4. They include material from captured comets and asteroids

5. Saturn’s icy moons, e.g. Enceladus, could be a source of material via water volcanoes or geysers.

The rings were given letter names in the order of their discovery. (Table 2).

Table 2: Characteristics of Saturn’s Rings

Ring	Distance from Saturn’s centre (km)	Particle size	Structure	Rotation period (hrs)
D	60,000-74,400	Fine grains	Thin complex ringlets without well-defined edges	4.91-5.61
C	74,400-91,900	10cm-2m	Regular alternative light/dark bands. 2 major gaps	5.61-7.93
B	91,900-117,400	cm to few metres, upper limit not known	Approx 25,000 km wide, contains most of rings’ mass. Thousands of ringlets. Spokes	7.93-11.41
Cassini Division	117,400-121,900	10cm-8m	Approx 4,500 km wide. Contains several ringlets and regions of fine dust, appearing as gaps. May contain small moons. Maintained by 2:1 resonance with Mimas	11.75 (middle)
A	121,900-136,600	Fine dust to metres	Inner ¾ has ringlets. Remainder contains ringlets and several sharp-edged gaps, possibly caused by shepherding moons.	11.93-14.24
Encke Division	133,370-133,640	10cm-2 m	270 km gap. Contains two discontinuous ringlets and ‘kinky ringlets’, (clumps/mini-arcs). Contains 10km moon Pan.	13.82
F	140,600	10cm–1m (upper limit unknown)	Non-circular ring about 500 km wide. Confined gravitationally between two shepherding moons, Prometheus and Pandora. Includes several discontinuous ringlets with sub structures such as clumps, kinks and inter-twining	14.94
G	170,000	Fine dust to boulders	Tenuous region lying far beyond the main rings. Apparently lacks ringlet structure	19.9
E	180,000-480,000	Fine dust to boulders	Thought to be associated with the moon Enceladus	31.3 (middle)

‘Shepherding moons’ travel around the inner and outer edges of the F-ring, keeping it in place by accelerating slowing inner ring particles, and decelerating outer ring particles that have increased their orbits. This concept, proposed in 1979 to explain Uranus’ narrow rings, was followed by the 1981 discovery of Prometheus and Pandora.

Prometheus, Pandora and the F-ring (NASA)

Braided F Ring (NASA: Voyager 1)

B-ring Spokes up to 20,000 km long are visible only from Saturn’s dark side and are thought to be charged microscopic grains levitating away from the ring plane, possibly after a meteor strike. Since the spokes rotate at the same rate as Saturn’s magnetic field, electromagnetic forces are involved.

Spokes in the B ring (NASA)

Cassini, arriving at Saturn on 1/7/04 seeks to discover more about Saturn and its rings.

3.4 Uranus

The Uranian ring system was discovered accidentally in 1977 during a stellar occultation by Uranus. A symmetric pattern of five dips in the stellar signal was seen either side of Uranus and later confirmed to be rings, Voyager 2 detecting several more and imaging the entire system.

Hubble Space Telescope Image, 14/8/94

Uranus has 9 major, very dark rings, surrounded by fine dust belts.

False colour: Uranus’ 9 major rings – Epsilon, delta, gamma, eta, beta, alpha 4, 5, and 6 (NASA)

The ring system, which is made of millions of particles, wobbles, possibly due to Uranus’ slightly flattened shape, and its many moons’ gravitational effects.

Cordelia and Ophelia shepherd the epsilon ring.

Two shepherding moons associated with the rings of Uranus (NASA, Voyager)

3.5 Neptune

Neptunian Rings (NASA: Voyager 2)

Neptune’s rings were detected using stellar occultations, but were not always symmetric, indicating gaps (‘ring arcs’), believed to be clumps in the original ring material, streaked as it orbits Neptune.

Table 3: Neptunian Ring Features

Ring	Distance from centre of Neptune (km)	Notes
Adams	63,000	Continuous. 3 prominent arcs: Liberté, Egalité and Fraternité. May be confined by the moon Galatea.
Leverrier	53,000	Continuous
Lassell		Relatively faint extension to the Leverrier ring
Arago	57,000	Relatively faint

‘Ring arc’ outside the orbit of moon 1989N4. (NASA, Voyager 2)

‘Twisted Rope’ in Fraternity Arc (Voyager 2, NASA)

4. Other Solar System Ring Structures

4.1 Asteroid Belt

Asteroids are rocky objects orbiting the sun between Mars and Jupiter, moving in the same direction as the planets and believed to be pieces of a planet that never formed since gravitational forces between Jupiter and Mars would have prevented them from coalescing.

Ceres is the largest, the smallest being pebble-sized. ‘Apollo Asteroids’, of which there are believed to be around 1,000 of >1 km diameter, cross Earth’s orbit.

Source: NASA

4.2 Edgeworth-Kuiper Belt

Orbiting bodies beyond Neptune (30-100 AU from the Sun) were first hypothesized by Edgeworth (1949), and Kuiper (1951) and may be remnants from the accretional phases of the solar system. The Edgeworth-Kuiper Belt is believed to be the source of short-period comets (orbital periods <200 years, e.g. Halley), in the same way as the Oort Cloud is for long-period comets.

Edgeworth-Kuiper Belt

Computer simulations of the solar system's formation predicted that a disk of icy debris that never coalesced to form planets would form beyond Neptune’s orbit. This remained theoretical until 1992 when a 200 km-wide body (1992QB1) was detected at the suspected belt’s distance. More than 300 objects have been detected in the region and it is estimated there are ≥100,000 bodies and >1,000,000,000 comets in this region (SwRI, 2000). Pluto and some gas giants’ moons in unusual orbits may be captured Belt objects, possibly affected by the giants’ gravity sending them into an orbit crossing them or even into the inner solar system.

4.3 Oort Cloud

Jan Oort (1950) noted that

1. No observed comet’s orbit indicates it came from interstellar space

2. Aphelia of long-period comet orbits tend to be at 50,000 AU, and

3. There is no preferential direction of comet origin.

Oort proposed that long-period comets came from a shell of icy bodies at about 50,000 AU - the Oort Cloud.  When the planets were condensing, possibly trillions of comets were ejected from the Sun to follow near circular orbits of hundreds of millions of years. Occasionally, the gravity of a passing star nudges comets inwards but 20-30% may disintegrate on their first trip inwards.  Since each comet is so small and at such large distances, there is no direct evidence for the Cloud.

4.            Further Reading/References

General

‘Formation of the solar system’, NASA: http://www.solarviews.com/cap/misc/ssanim.htm

‘Comparing the rings of Jupiter, Saturn, Uranus and Neptune’, NASA/JPL: http://learn.jpl.nasa.gov/projectsapce/bkg470b.html

‘The solar system’, http://www.solarviews.com/eng/solarsys.htm

‘Encyclopedia of the solar system’, Eds Weissman, McFadden, Johnson, Academic Press

‘A Dusty Ring may be the Tell-tale mark of an Emerging Planetary System’, Harvard-Smithsonian center for Astrophysics, 20/10/99, NASA/JPL: http://stardust.jpl.nasa.gov/news/news76.html

SwRI News Release, 11/8/00, Spaceflight Now

Asteroid Belt

‘The environment of space’, JPL reference doc JPL D-9774A

Jupiter

‘Galileo finds Jupiter’s rings formed by dust blasted off small moons’, NASA: http://galileo.ivv.nasa.gov/status980915.html

‘The formation of Jupiter’s ring system’, NASA: http://galileo.ivv.nasa.gov/images/R08-satorb.html

‘Moons and rings of Jupiter’, NASA: http://galileo.ivv.nasa.gov/moons/rings.html

Saturn

‘Saturn planet profile’, NASA/JPL: http://pds.jpl.nasa.gov/planets/welcome/saturn.htm

‘The A and B rings’, NASA: http://ringmaster.arc.nasa.gov/saturn/voyager/saturn55.html

‘Saturn’s Ring System’, NASA: http://ringmaster.arc.nasa.gov/saturn/saturn.html

‘Historical background of Saturn’s rings’, NASA/JPL: http://www.jpl.nasa.gov/saturn/back.html

‘What is the Structure of Saturn’s rings?’, NASA/JPL: http://learn.jpl.nasa.gov/projectspacef/bkg410b.html

‘What forces created Saturn’s rings?’, NASA/JPL: http://learn.jpl.nasa.gov/projectspacef/bkg430b.html

‘Do Saturn’s rings change?’, NASA/JPL: http://learn.jpl.nasa.gov/projectspacef/bkg450b.html

’How long will Saturn’s rings last?’, NASA/JPL: http://learn.jpl.nasa.gov/projectspacef/bkg420b.html

‘Frequently asked questions about Saturn’s rings’, NASA/JPL: http://www.jpl.nasa.gov/saturn/faq.html

‘Could a spacecraft pass through Saturn’s rings and survive?’, NASA/JPL: http://learn.jpl.nasa.gov/projectspacef/bkg460b.html

‘Rings and dust in the Saturn System’, NASA/JPL: http://www-b.jpl.nasa.gov/cassini/Science/MAPS/IDSCuzzi.html

‘Voyager 2 images of Saturn’s ring system’, NASA: NASA: http://ringmaster.arc.nasa.gov/saturn/voyager/voyager2.html

‘What keeps planetary rings in place?’, NASA: http://imagine.nasa.gov/docs/ask_astro/answers/981027a.html

Uranus

‘Voyager gallery of Uranus’ ring system’, NASA: http://ringmaster.arc.nasa.gov/uranus/voyager

‘Uranus’ rings’, NASA: http://ringmaster.arc.nasa.gov/uranus/voyager/PIA01984.html

‘The Uranian ring system’, NASA: http://ringmaster.arc.nasa.gov/www/uranus/uranus.html

‘Hubble Shows Rings-Moons of Uranus’, (2/11/94), NASA Spacelink http://www.nasa.gov/

‘Earth-based gallery of Uranus’ ring system’, NASA: http://ringmaster.arc.nasa.gov/www/uranus/earthbased