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Saturnian Ring System Han Zhang Oct-­‐25, 2013 Outline 1.  What’s the structure and composiAon of Ring system? 2.  What’s the Cassini Division? How does it form? 3. Possible theories about formaAon of ring system. Components From outside to inside G, F (thin and difficult to see) A, B and C (broad and easily visible) Total mass of the ring: 5×10^-­‐8 M_sat Sources: Burns et al., 2001, Nicholson and Dones, 1991. Colwell et al. (1) Tiscareno et al.(2006) and Sremčević et al.(2007) The radius of Saturn: 60268km Roche Limit Edouard Roche (1847) , α_r is the radius limit, ρ_p is the average density of central Planet, R is the radius of central planet and ρ is the parAcle’s density. Gradient in a planet’s gravitaAonal force vs the gravitaAonal a^racAon of the satellite’s own material. Larger ρ -­‐-­‐-­‐ smaller Roche limit A purely fluid satellite would be pulled apart by Adal forces inside the Roche limit. Roche Zone Region surrounding the Roche limit AccreAonary growth vs Adal disrupAon Main ring system ŸContaining ring A, Cassini Division, B, and C Ÿbelow Saturn’s Roche limit for Ice (about 140,000km from Saturn’s center) ŸParAcles larger than ~1cm across ŸStrong Adal force Diffuse ring system ŸInclude E and G rings Ÿmainly made of micrometer-­‐sized dust Ÿfaint, hardly visible F ring Ÿlocated near Roche limit Ÿmixture: big bodies (larger than 1m) & micrometer-­‐ sized dust Composi;on: Icy parAcles (also have li^le rocky material) Cassini Division The largest of these gaps, named ager the discover Jean D. Cassini. Between A and B ring; about 4,800 km wide, although this varies quit a bit around the planet. A complex region containing an array of gaps and ringlets. Origin and EvoluKon of Saturn's. Ring System. Chapter 17 of the book. “Saturn ASer Cassini-­‐Huygens” Resonance Ÿ P_i/P_j=n/m (n, m integer) Ÿ Orbital resonances greatly enhance the mutual gravitaAonal influence of the bodies (i.e., their ability to alter or constrain each other’s orbits). In most cases, this results in an unstable interacAon in which the bodies exchange momentum and shig orbits unAl the resonance no longer exists. Ÿ Example: Mimas 2:1 resonance likely plays an important role in creaAng the Huygens Gap Hedman et al 2009 AJ There is still no definitive explanation for most of the numerous gaps
present throughout the inner part of Cassini Division.
Possible theory: each gap in Cassini contains a tiny moon, but that have
not yet been reported.
Main Ring Forma;on vRemnants from Saturn’s sub-­‐nebula disk? Ÿ Type II migraAon Ÿ  IsolaAon mass Ÿ  The same environment in which the satellites formed, which implies at least some parts of the ring are as old as the Solar System. But the observaAon shows that the rings are geologically young. Main Ring Forma;on vDebris from a destroyed satellite? Ÿ  the total ring mass is comparable to a Saturnian mid-­‐sized satellite (like Mimas; 200-­‐300 km radius ) Ÿ  Saturn’s Ades alone is not enough Ÿ  “Late heavy bombardment” (LHB). A short global phase of intense bombardment that may have happened throughout the solar system about 700-­‐800Myr ager its origin. The reality of LHB is sAll a ma^er of debate. Main Ring Forma;on vDebris from Kdally disrupted comets? Ÿ Saturn’s rings could have arisen through the Adal disrupAon of a large “comet” or outer Solar System planetesimal. Ÿ Similar model of the origin of the Moon. But the objects larger than 300km are not that many. F Ring The F ring’s origin seems to be linked to km-­‐sized moonlets. F ring dusty component is a cloud parAcles released by the collisional erosion of an underlying populaAon of 0.1-­‐1km sized moonlets. Origins of Diffuse rings E Ring spans the region between 3 and 8 R_saturn, has its peak brightness near the orbit of the satellite Enceladus (3.95 R_s). Enceladus is the primary source of E ring material. Cassini showed the s o u t h p o l e o f E n c e l a d u s i s geologically acAve, generaAng a plume of small parAcles that extends thousands of kilometers above its surface. Significance Giant planets all have the ring systems, but Saturn’s rings are the largest and the brightest of the four ring systems surrounding each of the giant planets. T h e s e r i n g s y s t e m s s e r v e a s prototypes for more massive disk systems such as accreAon disks and spiral galaxies occurring else where in astronomy. Summary 1.  Saturn’s ring system is consist of different rings and divisions. Each part has its own paAcle size, density and opAcal depth. 2. Cassini division may be explained by the resonance of satellites. 3. The formaAon of Saturn’s ring system is sAll unclear. Many different theories have been put forward to explain that. Reference Chapter 17 of the book ''Saturn ASer Cassini-­‐Huygens'' Saturn from Cassini-­‐Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009) 537-­‐575 Origin od Saturn’s E ring: Self-­‐Sustained, naturally, Douglas P. Hamilton&Joseph A. Burns, Science, Vol 264, 22 April 1994 The architecture of the Cassini Division, Hedman et al, 2009, arxiv:0911.2438v2 The dynamics of planetary rings, Goldreich. P. & Tremaine. S. 1982, ARAA, 20:249-­‐83