Faint planetary rings, their dynamical behavior and physical properties, were the main focus of the research efforts. The motion of weakly-charged dust through the gravitational and magnetic fields of Jupiter were examined. Several topics concerning features of Saturn's rings were addressed. The origin and fate of the Uranian ring dust is presently being studied

Burns, Joseph A.

English

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INVESTIGATIONS OF PLANETARY RING PHENOMENA 
Joseph A .  Burns, Cornell Universitlly 
Faint planetary rings--their dynamical behavior and 
physical properties--have been the main focus of research 
efforts during the past year. A reanalysis of the structure 
and physical properties of the Jovian ring system was 
completed and will be published shortly (2). Jupiter's ring 
is composed of i) a flattened main ring (t<300 km) that 
terminates abruptly at its outer edge, and thaF has features 
associated with the two known embedded moonlets, ii) a 
toroidal halo that rises quickly at the main ring's inner 
edge to a half thickness of ~ 1 0 4  km and fades into the 
background at -1.4 RJ, and iii) the exterior "gossamer" ring 
(see last year's progress report) that is only 1/20 as bright 
as the main ring and that is enhanced at synchronous orbit. 
The ring is reddish like nearby Amalthea and its particles 
have a power law size distribution of slope -2.5(+0.5) in the 
range m0. 1-1004m, 
We have examined the motion of weakly-charged dust 
through Jupiter's gravitational and magnetic fields 
(1). Resonances occur where the frequency of the perturbing 
Lorentz force matches a grain's natural orbital frequency; 
near such resonances, large radial and out of plane motions 
appear. Ongoing research shows that during passage through 
such resonances large vertical amplitudes develop and 
persist; the boundaries of the jovian halo are near such 
locations. Current studies include the character of 
stochastic variations in a particle's electric charge and the 
orbital consequences of this, an analytical investigation of 
resonance passage, the details of the dust-plasma 
interaction, and application to other ring systems. 
Several topics concerning features of Saturn9s rings 
have been addressed. Cuzzi and Burns (4) have shown that 
depletions in MeV charged particles measured by Pioneer 11 
near the F ring are only partly caused by the ring and thus 
that unobserved localized clouds of debris must be 
present. We hypothesize that these faint clouds are ejecta 
thrown off during collisions amongst moonlets (radius 0.1-10 
km) which populate the entire annulus between the orbits of 
the shepherds. We have developed a self-consistent scenario 
in which mutual collisions of unseen parents generate debris 
clouds, which then shear out and become part of an overall 
patchy background of faint material; the particles comprising 
the background are continually re-accreting onto the parents, 
only to be thrown off in some later collision. According to 
this model, about 102 clouds (with ??lo-3-10-4 and '"102kmx1O4 
km size) should be present at any typical time. With a steep 
size distribution of parents, even the F ring itself could be 
the outcome of a collision between two of the largest 
moonlets, in which case the F ring is a temporary feature of 
the ring system. We suggest that arcs like those in 
https://ntrs.nasa.gov/search.jsp?R=19870013927 2020-03-20T10:36:58+00:00Z
Neptune's retinue may be produced by this scheme. Kolvoord 
(7) has developed a perturbation routine to study the orbital 
consequences of an embedded moonlet on a narrow ring and is 
presently incorporating an exterior satellite perturber into 
his program as well as considering the consequences of 
particle collisions within the narrow rings. 
Showalter and co-workers (3) have interpreted quasi- 
periodic optical depth variations that were found on either 
side of the Encke gap as the gravitational "wakest' of 
a moonlet orbiting pear the gap's center and at a longitude 
of 320. A single moonlet of -10 km radius is able to produce 
the observed features. Studies of the signal scatter of the 
PPS occultation data by Showalter and Nicholson (8), 
initiated under this grant, give promise of providing an 
independent measure of the size distribution of ring 
particles. 
The origin and fate of Uranian ring dust is presently 
being studied (6). Such ring particles have brief lifetimes 
primarily due to sputtering and orbital collapse under 
atmospheric drag. An unresolved quandary is that abrupt 
transitions in ring brightness observed at interior ringlet 
edges seem to imply that the expected orbital evolution does 
not proceed. The dust discovered on ring plane crossing at 
4.57 R v  must have nearby sources; its broad and asymmetric 
distribution about the planet's equatorial plane suggests 
that resonant electromagnetic forces are at work (5). 
Our studies of faint rings are pertinent to the possible 
presence of fine material near Neptune; such grains could 
prove hazardous to Voyager 2 as it swings through the ring 
plane on its way toward a flyby of Triton; we have been 
helping to advise the Voyager project office on the potential 
danger to this precious spacecraft. 
Several review articles were also prepared during the 
funding period. They concerned dust motion ( 9 ) ,  planetary 
rings (lo), satellites (ll), and satellite orbital evolution 
(12) 
1, L. Schaffer and J,A. Burns (1986). The dynamics of 
weakly-charged dust: Motion through Jupiter's gravita- 
tional and magnetic fields. Jnl.Geophys.Res. 92, in 
press, Abstracts in BAAS 17, 921; BAAS 18, 777-778 and 
838, Nat'l Congress Applied Mech. Paper F3a (Austin, 
June 1986). 
2. M.R. Showalter, J.A. Burns, J.N. Cuzzi and 
J.B. Pollack (1986). Jupiter's ring system: New 
results on structure and particle properties. Icarus 
69, in press. 
3. M.R. Showalter, J.N. Cuzzi, E.A. Marouf and 
L.W. Esposito (1986). Satellite "wakes" and the orbit 
of the Encke gap moonlet. Icarus 66, 297-323. 
4. J.N. Cuzzi and J.A. Burns (1986). Charged particle 
depletion surrounding Saturn's F Ring: Evidence for a 
moonlet belt. Icarus, submitted; Abstracts in BAAS 17, 
922; 18, 768; EOS 67, 1077. 
5 .  D.A. Gurnett, W.S. Kurth, F.L. Scarf, J.A. Burns, 
J.N. Cuzzi and E. Grun (1986). An analysis of 
micron-sized particle impacts detected near Uranus by 
Voyager 2. Jnl.Geophys.Res., in preparation; Abstract 
in EOS 69, 340. 
6. J.A. Burns, L.E. Schaffer, J.N. Cuzzi, and 
D.A. Gurnett (1986). Dust in the Uranian system: Its 
origin and fate. BAAS 18, 770-771. 
7. R.A. Kolvoord (1986). The effect of an embedded 
satellite on narrow rings using a perturbation 
approach. BAAS 18, 771. 
8. M.R. Showalter and P.D. Nicholson (1986). Saturn's 
rings through a microscope: Constraints on particle 
size from the Voyager PPS scan. BAAS 18, 767. 
9. J.A. Burns (1986). The motion of interplanetary 
dust. In Evolution of the Small Bodies in the Solar 
System (M. Fulchignoni and L.' Kresak, Eds.), in press. 
10. J.A. Burns (1986). Rings around planets. In 
Evolution of the Small Bodies in the Solar System 
(M. Fulchignoni and L .  Kresak, Eds.), in press. 
11. J.A. Burns (1986). Some background about 
satellites. In Satellites (J.A. Burns and 
M.S. Matthews, Eds.), University of Arizona Press, 
1-38. 


Investigations of planetary ring phenomena

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