ASTR 330: The Solar System Announcements Dr Conor Nixon Fall 2006 Mid-term #2 exams returned today. Class average =148.6 = 74.3% Range: 76-194 pts. Compare.

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• Mid-term #2 exams returned today.

• Class average =148.6 = 74.3%• Range: 76-194 pts.• Compare to Mid-term #1: avg. 167.8 (83.9%)• What were the problem areas?

• Course class average so far: 457/600 = 76%.

• HW#5 due today.


Lecture 24:

Rings and Shepherds

Dr Conor Nixon Fall 2006Picture credit: solarviews.com (Voy 1: 11-12-81)


Saturn’s Rings: Discovery


• Whenever you think of a planet with rings, you probably think of Saturn. Saturn’s rings were first seen in 1610 by the first ever astronomical observer to possess a telescope: Galileo Galilei.

• Galileo was mystified by the phenomenon: it looked like the planet had ‘ears’ ! In fact, many of his contemporaries argued that he was seeing illusions through his new-fangled device.

• This taunt was not helped any when the ‘ears’ disappeared several years later! Why?

• Galileo theorized that he was seeing ‘bumps’ on the planet, or perhaps a triple planet.

Picture credit: St Andrews Univ


An Explanation At Last


• The mystery of the ‘ears’ was eventually cleared up by further observation: The Dutch Astronomer Christian Huygens in 1659 correctly guessed that Saturn was surrounded by “a thin flat ring, nowhere touching” the planet and lying in the equatorial plane.

• The reason for the periodic disappearance also became apparent to observers over time: as the planet orbits the Sun on its inclined axis, over one full orbit we see the ring(s) change from edge on, to more face on, and back again, with a 15 (Earth) year cycle.

Picture credit: (I) St Andrews University (ii) NASA / Hubble Heritage Team (STScI /

AURA)


First Structure in the Rings


• In 1675 another major event took place: the astronomer Giovanni Cassini (right) spotted a dark lane, or division in the ring. The ring was therefore two rings: an outer ‘A’ Ring and an inner ‘B’ Ring.

• In 1837, J.F. Encke (who, like Halley, connected 4 comet sightings as one) spotted a dark lane (minimum) in the A Ring. This was confirmed to be a gap, or division, in 1888 by James Keeler. Keeler’s original drawing is shown (right).

• Meanwhile, in 1850 the third ring, the inner C or Crepe Ring was found by W. Bond, G. Bond and W. Dawes. You can also see it on the sketch.

Picture credit: (I) wikipedia.com (ii) Eric Jamison


1850-1973: Ring Particles


• Until the mid-1800s, it was not conclusively known whether the rings were solid or composed of orbiting pieces, although there were sound mathematical reasons for preferring the latter.

• We now know that the rings are indeed composed of billions of tiny moons, with the inner ones traveling faster than the outer ones, each on a circular Keplerian orbit around Saturn (with a few exceptions).

• At the inner edge, the particles take just 5.6 hrs to orbit once, compared to 14.2 hours at the outer edge.

• In 1970, infrared spectroscopy revealed that the rings were made primarily of water ice.

• Radar signals bounced off the rings were able to give a size estimate in 1973, with the typical size turning out to be around 10 cm.


1980-1981: Voyager Results


• The encounters of Voyagers 1 and 2 were the main sources of knowledge about the rings prior to Cassini.

• The Voyagers found that the ring particles range in size from ‘grain of sand’ size, up to boulders as big as a house. Small particles greatly outnumber large ones: as we saw in the case of the main asteroid belt.

• Most of the particles have a visible albedo of 50-60%, with a spectrum of water ice.

• Some are darker, perhaps composed of organics or silicates.

• Almost all the particles move in circular orbits in the equatorial plane, as they must. Why do we say this? Because any particle moving out of a circular orbit would bump against others, and lose energy. The effect is to circularize the orbits.


The Main Rings of Saturn


• The rings are very broad and thin. They stretch from 7000 km above the cloud tops, to 70,000 km and more. However, they are just 20 meters thick (yes, I did write meters!).

• The analogy (from M&O) is that if the rings were as thick as notepaper, then they would be eight city blocks (1 km) across!

• The main rings we have mentioned already are the A and B Rings; and the inner Crepe (C) Ring. We will discuss others as well.

Name Outer Edge Outer Edge(Rplanet) (km)

D 1.233 74,400C 1.524 91,900B 1.946 117,400Cassini Division 2.212 133,400A 2.265 136,600F 2.324 140,180

Table: Morrison and Owen

ASTR 330: The Solar System Lord of the Rings

Dr Conor Nixon Fall 2006Picture credit: JPL/NASA


Gaps in the Rings


• The spectacular Voyager (false color) image of C-Ring detail below emphasizes one fact: every one of the ‘lettered’ rings is not a single ring at all, but rather many hundreds of individual ‘ringlets’.

• Each ring is separated by an apparent ‘gap’ (or division), but few of these divisions are really empty: they are merely regions of thinner material.

• A few of the gaps are really empty (we will see the reason for that later).

Picture credit: JPL/NASA


Inner rings: C and D


• Voyager discovered several thin rings between the planet and the start of the C-Ring proper at 7000 km: collectively these are called the D-Ring.

• The C-Ring is dense enough to reflect substantial sunlight, and be seen from the Earth, although it is more transparent than the A and B Rings.

• The C-Ring has two major gaps (several hundred km wide).

• In one of these gaps is at least one narrow eccentric ribbon ringlet. The color of this ring is different from the nearby C-Ring material, hinting at a different composition and origin. More have now been found.


The B-ring


• The B-ring is what we would consider the ‘main’ ring of Saturn, being the brightest, and containing most of the mass.

• It begins 32,000 km from Saturn, and continues unbroken out to 57,000 km. The structure is very complex, and although we see thousands of individual separate ringlets; the ‘gaps’ between them are not empty, just less dense.

• The ring particles here range from 10s of cm to meters in diameter.

Picture credit: JPL/NASA/Space Science Institute


Cassini Division

Dr Conor Nixon Fall 2006Picture credit: JPL/NASA/Space Science Institute

• At the outer edge of the B-Ring is the famous Cassini Division. Far from being empty space, as it appears from the Earth, there are in fact:

• several faint sparse rings, with ‘real’ gaps between them.• one known eccentric ringlet (Huygens).

• Scientists considered whether to target the Pioneer 11 spacecraft to pass through the Cassini Division, but the idea was luckily rejected!


The A-ring


• Beyond the Cassini Division, at 61,000 km, the A-Ring begins. The A-Ring is intermediate in brightness and density to the B and C Rings.

• The A-ring contains two main gaps or divisions - Encke and Keeler - and then ends sharply at 96,000 km from Saturn

Picture credit: JPL/NASA/Univ. of Colorado

• The image (left) shows clumping in the A-ring (falsely colored from ultraviolet) compared to a computer simulation (right).


A-ring: Encke Division


• The most interesting feature is the 360 km wide Encke Division (1/10 the width of the Cassini Division).


• In this gap we see:

• 2 discontinuous, kinky ringlets: 20 km wide ribbons of material (above).

• at least one small satellite (Pan) (left).


Voyager 2: Saturn’s Rings (False Color)


• Can you spot the:

• A RING

• B RING

• C RING

• CASSINI DIVISION

• ENCKE DIVISION



The F-Ring


• The width varies along its length: from 30 km to 500 km. It is also eccentric, like the ones inside the C-Ring and the Cassini Division.

• The F-Ring appears to split in places, and divide into multiple strands which appear intertwined or braided!

• Much of the structure in the F-Ring may be caused by small nearby moons..

• 4000 km beyond the edge of the A-Ring is the interesting, faint F-Ring. Unlike the previous A-D Rings we have looked at, which were broad bands, the F-Ring is an isolated bright ribbon.

Image credit: NASA/JPL


The G and E Rings


• Beyond the F-Ring are two extremely faint outer rings: the G and E-Rings.

• The G-Ring is just 8 km wide and composed of small particles of neutral tint.

• The very broad E-Ring (180,000 to 640,000 km from the planet), is composed of very small (1-micron), same-sized particles, which have a distinct blue color.

• The E-Ring stretches from the orbit of Mimas, peaking in density at Enceladus, and then tapering off to Tethys and Dione.

• Enceladus is now known to be the source of E-ring particles, which are the result of its active geysering.


Enceladus and the E-ring


• This Cassini image shows amazing detail of the moon Enceladus encased within the E-ring. Bright streamers of material can be seen leaving the moon.

Picture credit: NASA/JPL/Space Science Institute


Discovery of the Rings of Uranus


• On March 10th 1977, a group of scientists were set to observe an occultation of a star by Uranus. (What do you think they were trying to see?)

• The most important results came not from the planet, but from before and after the planet went in front of the star.

• Scientists observing the occultation around the world all saw the same thing: the starlight flickered on and off several times before the main eclipse, and then on and off again in the same pattern after the eclipse.

• At first the scientists thought that they might be seeing a swarm of small satellites.

• However, the fact that the same pattern was seen from different places on the Earth ultimately led to the conclusion that Uranus was surrounded by several narrow dark rings.


Studying Rings with Occultations


• The occultation technique is now a well-established method for studying the ring structure of Uranus.

• This technique has one particular advantage over direct observation: the resolution (fineness of detail seen) is not limited by the size of the telescope or by the turbulence in the Earth’s atmosphere, as is regular astronomy. The limit on resolution is how fast we can sample the changing brightness of the star.

• In the case of Uranus additional advantages are that:

1. The rings are dark, so do not reflect sunlight to interfere with the starlight.

2. At near-infrared wavelengths, the planet is also dark.3. Uranus has a high tilt, so we can see the rings almost full-face on

(at certain times).

• Would these advantages also apply when observing Saturn’s rings?


Uranian Rings


• How do these compare to Saturn’s rings in width?

Ring Distance WidthName (km) (km) Eccentricity

6 Ring 41,850 1-3 0.00105 Ring 42,240 2-3 0.00194 Ring 42,580 2 0.0011Alpha 44,730 8-11 0.0008Beta 45,670 7-11 0.0004Eta 47,180 55 0.0000Gamma 47,630 1-4 0.0000Delta 48,310 3-9 0.0000Lambda 50,040 1-2 ?Epsilon 51,160 22-93 0.0079


From Moons to Rings


• This image shows the Uranian ring system in false color, from right to left:

• Epsilon (white)• Delta (green)• Gamma (blue)• Eta (blue)• Beta (light blue)• Alpha (light blue)• 4• 5• 6



Uranian rings: description


• The Uranian rings are almost the opposite of the Saturnian ones: rather than having mainly broad rings with a few narrow gaps; instead we have a few narrow rings with large spaces in between.

• Also, the Uranian rings are dark, unlike the bright, reflective rings of Saturn. Hence they cannot be composed of ice; and must instead be made of some dark carbonaceous material.

• Most of the 10 rings are nearly circular and very narrow: typically less than 10 km. Voyager 2 made close-up occultation measurements in 1986, determining fine structure.

• The Epsilon Ring probably contains as much mass as the rest put together: it also has a variable width: 22 km closest to Uranus, and 93 km when furthest away.

• The Eta Ring is also strange: having a broad, diffuse ring 55 km wide, and a narrow denser component at the inner edge.


View From the Shadows


• This image was taken while Voyager 2 was in the shadow of Uranus.

• This is a high phase angle image, meaning that the Sun is lighting up the ring particles from behind, which are scattering the light forward to the camera rather than blocking it.

• The broader dust lanes are un-named. What are the streaks on the image?



Does Jupiter Have Rings?


• When the Voyager 1 spacecraft arrived at Jupiter in 1979, it was deliberately set to make a long exposure of the planet near the equator, seen from behind to look for rings. The image below was the result: a positive one! Voyager 2 was then programmed to take more pictures.

Picture credit: JPL/NASA - Voyager


Jovian Rings


• The main ring of Jupiter is a relatively narrow (5000 km) ring 54,000 km from the cloud tops.

• The Jovian ring is much fainter than those of the other planets: the ring material is 10,000 times less absorbing than window glass - we can only see it when we look across the densest part.

• Inside the main ring, are the orbits of the small moons Adrastea and Metis, which may serve as sources of ring material.

• The main rings merges inwards into the halo: a faint broad torus about 10,000 thick (north-south) which extends halfway from the main ring to the cloud tops.

• Outside the main ring are two very faint gossamer rings: one bounded by the orbit of the moon Amalthea, and the other by the orbit of Thebe.


Dr Conor Nixon Fall 2006Picture credit: JPL/NASA - Galileo


Rings of Neptune?


• Once rings were discovered at Uranus, it was natural to search for rings of Neptune.

• Several occultations were observed, but the evidence was contradictory. Sometimes the starlight dimmed on one side the planet only, not on both as you might expect. This led to the idea that the rings were in fact ‘arcs’ if they were there at all.

• When Voyager 2 arrived at Neptune, it found an even more complex picture. There are in fact 3 major continuous rings, named Galle, Leverrier and Adams in increasing order of distance (who were they?).

• The Adams Ring is the one with the arcs: three enhancements of material at different points along its length. Why has the material not spread out uniformly? We do not know.


Rings and Arcs


• The upper image (right) shows 3 prominent arcs in the Adams Ring: now named Liberty, Equality and Fraternity! From Voyager 2.

• The lower image shows the three main rings, with the glare of Neptune masked out.



Patterns In The Rings


• Over time, as our knowledge of Saturn’s rings has improved, and especially the technology for taking images, we have discovered structure at finer and finer scales.

• From the Earth we were able to see the major Divisions of the rings, but when the Voyagers arrived we saw much more detail: thousands of ringlets, and strange spokes and wave patterns.

• The fine structure of the rings changes in a matter of hours, like the surface of the sea. Two mathematical types of waves were predicted to occur, and both were seen:

• Spiral density waves: ripples of more and less dense material, like sound waves in air, or P-waves in the Earth.

• Spiral bending waves: vertical oscillations of ring material, like EM radiation or S-waves in the Earth.


Waves in the A-Ring


• This Voyager image shows both types of waves in the A-Ring, 400 km apart.

• The outer (left side) ripples are the spiral density wave, propagating outwards.

• The inner (right side) disturbance is the spiral bending wave, propagating inwards.

• These were formed due to resonance perturbations from the satellite Mimas, in this case a 5:3 resonance.

Picture credit: solarviews.com


Ring Spokes


• Spokes are an even stranger effect. These are dark radial markings up to 20,000 km long, which initially perplexed Voyager scientists, as they seemed to defy gravity.

• Spokes are seen on both sides of the ring plane, near the densest part of the B-Ring. They start at about 104,000 km from Saturn center and extend to the Cassini Division, with a characteristic hour-glass shape.

• These are now thought to be made of dust particles suspended above the ring plane. They rotate synchronously with the magnetic field, which indicates that they are charged.

• Possible explanations are:

(i) material which has been ‘punched out’ by the passage of a small moonlet, or,(ii) particles which have acquired charge and been drawn off the ring plane.


Spoke Formation

Sequence


• This 35-minute sequence of images from Voyager 2 shows the rapid formation of a new spoke, indicated by the arrow in the lowermost frame.

Picture credit: solarviews.com


Stability of Rings over time


• Over time, science predicts that the rings should spread out, both away from the planet and towards it, due to interactions between the ring particles and fragments (gravity, friction, collisions etc).

• At the inner edge, the pieces would fall into Saturn as meteors: at the outer edge, the pieces would disperse to greater and greater distances.

• If the rings are to last then, something must be keeping them in place. In fact, we now know that the outer edge of the A-Ring is ‘policed’ by the co-orbital satellites, Janus and Epimetheus, in a 6:7 resonance. Their gravity keeps the ring pieces trapped.

• The F-Ring is also confined by the action of moons: in this case, the shepherd moons Prometheus and Pandora, one on each side. It is probably the influence of these moons which causes the ‘braiding’ effect, but our understanding is incomplete.


F-Ring Shepherds


• The left image from Voyager 2 shows the shepherd moons Prometheus (inner) and Pandora (outer), either side of the F-Ring.

• The right image shows a closer view of Prometheus from Voyager 2. The shape of the moon is clearly seen, but there are no kinks in the Ring evident near the satellite.

Picture credit: (I) solarviews.com (ii) NASA ARC.


U-Ring Shepherds


• The narrow rings of Uranus seemed to be a slam-dunk case for shepherd moons, in an analogue to the F-Ring of Saturn.

• Therefore, Voyager 2 was tasked to search for ring shepherds during the Uranus encounter, a search which was not as successful as anticipated.

• The image (right) shows two Epsilon Ring shepherds discovered, now named Cordelia and Ophelia. Each is less than 50 km in size, and they orbit some 2000 km either side of the Ring.

• No other shepherd satellites were found, leaving a mystery for the other rings: however the cameras could only resolve detail down to 10 km in size.

Picture credit: JPL/NASA ARC


New Moons and Rings

Dr Conor Nixon Fall 2006Picture credit: wikipedia.org


Resonances with large satellites


• It is not only the small satellites close to the rings which have an effect on ring structure: the larger, further moons also play a part.

• Mimas is particularly influential: in fact, the inner edge of the Cassini Division is an exact 2:1 resonance with Mimas (remember the gaps in the asteroid belt?).

• This Voyager 1 image was taken looking across the ‘dark side’ (shadowed) of the rings, showing the A-Ring, thin F-Ring, and the moon Mimas.

Picture credit: JPL/NASA ARC


Origin of Rings and Small Satellites


• There are two main theories of ring formation, and both are closely tied to the presence of the small satellites.

1. The ‘break-up’ theory: in essence, this theory suggests that the rings are the remains of a shattered small satellite.

2. The second theory suggests that the rings are material which was unable to form an actual satellite, due gravitational interference.

• We will examine each of these theories in turn.


Tidal Stability


• The tidal stability limit is the distance from a planet where tidal forces become stronger than the forces keeping a body together.

• This limit, known as the Roche Limit after the 19th century mathematician who proposed it, depends on factors such as tensile strength of the body, as well as its self gravitation.

• For example, two bodies just touching (zero tensile strength) will pull apart at about 2.5 planetary radii from the planet center.

• On the other hand, clearly objects such as the shuttle do not get tidally pulled apart in orbit around the Earth: their intrinsic strength keeps them whole.

• Therefore, we can propose that inside the Roche Limit of the planets, the particles of the rings were unable to stay together long enough to form an actual moon.


Simulation Of Ring Particles


• This image shows a visualization (artists impression) of the ring particles.

• In this image, the largest pieces are house-sized.

• Clumps of boulders are continuously being created and being pulled apart again by tidal forces, as in the foreground (‘S’ shape).

Picture credit: William K Hartmann


Satellite Break-Up


• There are several ways in which we imagine a satellite being destroyed. One is by tidal forces, if a moon or even a comet (e.g. S-L 9) wandered too close to a planet.

• The other possibility is that a satellite suffered a massive impact, while inside the Roche Limit and was unable to re-form again afterwards (unlike Miranda).

• The embedded satellites provide evidence for this type of scenario: if we could find a lot more larger pieces, the theory would be stronger.

• We can calculate the mass of ring systems, and see if the amount is appropriate. For Saturn the total ring mass (that we know of) is 1018 kg: about equivalent to a 250 km diameter satellite, very close to the size of Janus.

• Hence, the theory is plausible, but by no means proven. We cannot rule out that this material never formed a moon.


Long Term Stability


• Even if we know how the rings formed, we have not answered the question of long-term stability. This depends on particle size. Small particles would be eroded more quickly; in just a few 100 million years.

• Therefore, either the rings are a recent occurrence, or they are constantly being renewed by the break-up of km-sized objects which are now been discovered by Cassini (right).

Picture credit: NASA/JPL/Space Science Institute


Compositions


• The chemical compositions of the 4 ring systems must also be explained. Saturn’s rings are mostly icy material: the easiest to explain. Some silicates (rock dust) must also be present.

• The J-Rings are mainly silicate: erosion from the rocky satellites.

• The dark rings of Uranus and Neptune are the hardest to explain, as the large satellites are mostly icy. However, the dark coatings seen on the inner satellites seems to suggest that they were formed from a moon break-up which also created the rings.


Quiz-Summary


1. Who discovered (a) the main rings of Saturn (b) the explanation for the ‘ears’ (c) the two major Divisions?

2. How was the ring particle size and composition first measured? What results were obtained?

3. Why are the ring particles mainly in circular, low-inclination orbits?

4. Are the ring divisions or gaps really empty?

5. What types off outer rings (beyond the A-Ring) do we see?

6. How were the rings of Uranus discovered, and what advantages are there to this method?

7. What are the main similarities and differences between the rings of Saturn, and Uranus?


Quiz-Summary


8. How were the rings of Jupiter discovered, and what are they?

9. The rings off Neptune were once thought to be incomplete ‘arcs’ of material rather than true rings. Was this theory true? Describe them.

10. Why do we see (a) waves and (b) spokes in the rings?

11. What confines (a) the outer edge of the A-Ring (b) the F-Ring?

12. Do all the Uranian rings have shepherds?

13. What causes the Cassini Division?

14. What is the Roche or Tidal Stability Limit?

15.What two theories have been suggested for the origin of rings.

ASTR 330: The Solar System Announcements Dr Conor Nixon Fall 2006 Mid-term #2 exams returned today. Class average =148.6 = 74.3% Range: 76-194 pts. Compare.

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