From: mmhedman@astro.cornell.edu Subject: Review comments on the Encke Gap Paper Date: October 10, 2012 9:49:54 AM EDT To: , , Well, it looks like we got some nice reviews on the paper. mmh -------- Original Message -------- Subject: Your Submission MS No. ICARUS-12583 Date: 10 Oct 2012 14:44:28 +0100 From: "ICARUS - Editorial Office" To: mmhedman@astro.cornell.edu Cc: Alessandro.MORBIDELLI@obs-nice.fr, morby@oca.eu Ms. Ref. No.:  ICARUS-12583 Title: Of Horseshoes and Heliotropes: Dynamics of Dust in the Encke Gap Icarus Decision:  Minor Revise Dear Dr. Hedman, The reviewers have commented on your above manuscript.  Both reviewers are generally favorable and have offered some minor suggestions to improve the paper. Would you please consider revising your manuscript along the lines of these reviews? If you feel that you can suitably address the reviewers' comments (included below), I invite you to revise and resubmit your manuscript. Your revision is due by Nov 09, 2012. Please carefully address the issues raised in the comments. If you are submitting a revised manuscript, please also: a) outline each change made (point by point) as raised in the reviewer comments  AND/OR b) provide a suitable rebuttal to each reviewer comment not addressed To submit your revision, please do the following: 1. Go to: http://ees.elsevier.com/icarus/ 2. Enter your login details 3. Click [Author Login] This takes you to the Author Main Menu. 4. Click [Submissions Needing Revision]. I look forward to receiving your revised manuscript. PLEASE NOTE: The journal would like to enrich online articles by visualising and providing geographical details described in Icarus articles. For this purpose, corresponding KML (GoogleMaps) files can be uploaded in our online submission system. Submitted KML files will be published with your online article on ScienceDirect. Elsevier will generate maps from the KML files and include them in the online article. Best regards, Alessandro Morbidelli Editor Icarus Editor's Comments:  When revising your manuscript, please address the style issues listed below. When revising your manuscript, be sure that the text is in single-column format and double-spaced, as well as page and/or line numbered. Please do not submit a manuscript that is marked-up to show changes, unless specifically asked for by the editor, as they are no longer required. When re-submitting your paper, please remember that you will need to upload the individual source files (.doc or .tex, for example, as well as any supporting files to build a TeX or LaTeX document) for the text and tables, and .tif or .eps for the figures/images. PDF files will not be accepted at this stage. Reviewers' comments: Reviewer #1: Using a variety of observational datasets from Cassini, the authors thoroughly scrutinize the motion of clumpy dusty ringlets in the Encke gap of Saturn's ring system. Their analysis of spatial and temporal variations in the ringlets reveal some remarkable and inexplicable dynamical phenomena. The radial confinement of the ringlets is a long-standing mystery, but the results of the authors analysis here reveal motions of clumps in the Pan ringlet that are inconsistent with horse-shoe orbits, including longitudinal confinement, and the slow merging and splitting of clumps. The authors go on to briefly propose ideas that may explain the confinement of the Pan and inner ringlet, as well as the motions of clumps and their apparent longitudinal confinement. Their thorough compilation and reduction of data on the Encke gap ringlet from many different Cassini datasets makes this paper an important contribution to ring science and certainly worthy of publication, particularly because of the puzzling dynamics that emerges from these observations. This paper is of great interest to ring dynamicists. It is reasonably concise given the scope of its investigation and cites prior work appropriately. Their ideas to explain the dynamics involve gravitational effects combined with plasma drag, an intriguing and as far as I know novel combination of forces invoked in Saturn's rings, and although their idea is not developed in detail and can't explain all the observations, at first glance they seem plausible and a reasonable attempt to make sense of a real mystery. Minor issues: The authors should clearly define the scope of their theoretical work to explain the dynamics in the Encke gap. Their idea attempts to explain the radial confinement of the ringlet, clump formation and the longitudinal motion of the clumps. They do not attempt to explain how free eccentricities are excited, or how the outer ringlet is confined, or whether we can get away with ignoring these other mysteries in evaluating their Pan plus plasma drag model. In section 6 the authors seek evidence of heliotropic dynamics in the ringlets of the Encke gap. The authors appear motivated to seek heliotropic (forced) eccentricities because free eccentricities will dissipate (and yet they are clearly excited constantly, somehow). The forced eccentricity component is due to the size dependent effect of solar radiation pressure and therefore constrains particle size. I was unsure just how heliotropic they have found the inner ringlet to be. For the Pan ringlet, the authors were satisfied from just two observations that the ringlet is displaced towards to sub-solar longitude. A similar search for the inner ringlet in Section 6.3 finds that the ringlet is closer to the planet on the sunward side, and the authors argue this cannot disprove the hypothesis of heliotropicity. Section 6.4 provides detailed models of the orbital parameters, and the authors find evidence of significant forced eccentricities in the clumpy regions for the Pan ringlet, but for the inner ringlet, the free eccentricity is close to or even higher than the the forced eccentricity (thus explaining the lack of heliotropicity found in Section 6.3). This should be explained in more detail. Does the heliotropicity rely on clump formation? If the clumpy regions were ignored, what would the conclusion be regarding heliotropicity? Is the model in Section 7 contingent on forced eccentricities? Are there independent estimates of grain size in the ringlets? Section 7 introduces ideas to explain the observations, but the ideas are not developed into a detailed hypothesis. It is not clear if the authors are planning to follow up on this paper with more details, or if their order-of-magnitude checks are where they intend to leave the theory. This is by no means a critique of their ideas, or a request to put more modeling into the paper before publishing it. Rather this is a request for them to state their aims for this paper more clearly in the introduction. Although the authors are consistent in their thorough treatment of just the Pan and inner ringlets, a small subsection explaining the limitations of analyzing the outer ringlet might be worthwhile. Is it possible to search for heliotropic behavior in the outer ringlet? The model of ring confinement proposed for the Pan and inner ringlet clearly struggles for the outer ringlet, since the plasma drag and perturbations from Pan will combine instead of balancing each other.  A more detailed discussion of the outer ringlet would certainly be of interest. Typos: p6: last paragraph "equtions". p11: last sentence of second paragraph missing a word "data are then fit (with?).." p12: last line "reative" p21: horizonatal axes "deegres". p21: vertical axis labelled "NEW", only explained later at page 24. p24: horizonatal axes "deegres". p28: 4th paragraph, "minima is radius" p33: 5th paragraph, "consistent what" p39: horizonatal axes "deegres". p45: 1st paragraph "mangitude" Reviewer #2: Dear Editor, I have read with pleasure the manuscript "Of Horseshoes and Heliotropes: Dynamics of Dust in the Encke Gap " by Hedman, Burns, hamilton and Showalter. This paper deals with Cassini observations of ringlets in the Encke gap and close to the satellite Pan. These ringlets are known since Voyager to have a clumpy structures. The dynamics and origin and the origin of these clumps is inanunderstood and is a matter of debate. The present paper first presents 360° observations of these structures as well as and then detailed dynamical study. The authors report the Cassini observations in terms of radial structure and longitudinal structure relative to pan, in which they show that in a couple of years the distribution of clumps embedded in the rings has largely evolved. The authors present a dynamical study of the ringlets and they show in particular that the dynamics of the ringlets is largely influenced by both Pan's gravity as well as the Sun's radiation pressure. In addition they show that apparent radial migration of the ringlets are observed and attribute them to plasma drag forces and/or collisions. I think this is a very good paper, clearly presented and clearly written. It mixes both observation and a detailed dynamical interpretation and shows in a convincing way the action of the sun and, possibly of pan's gravity.  This paper is really a step forward in our understanding of dynamics of Saturn's ringlets. I do not have much comments in the content of the paper, as the data processing is carefully presented and the discussion is sound. I have only one main comments, (see after) where I think the authors should clarify their implicit assumptions when discussing the clumps motions (section 4.1 and 4.2) and where, more or less, they conclude that they do not follow keplerian orbits. The authors might be much more careful here and say that these bright regions may *not* be real condensation of materials.. and so the motion of the bright structures could just be an observational artifact due to the low resolution and the radial averaging of signal.  See my comments below. Apart from this I have only very minor comments below, This is a nice paper that should be published after minor revision. Best, Sebastien Charnoz ********************************************* Page 12, end of section 3 : "we do not plot error bars" :  At least give an estimate of how reliable are these plots, or of the estimated amplitued of the systematic bias. My sole and only main comment is the following : Whereas the analysis if data is well presented and the dynamical study of the ringlets is very convincing,   I find that the discussion in some sections of the paper (Section 4.1 , 4.2) dealing with the location of bright clumps in radially avegared profiles,  much less convincing. Indeed, it seems to me that, there are some implicit assumptions behind these discussions that should be clarified, and I would appreciate that the authors do so, in the way they prefer. Sections 4.1 and 4.2 assumes implicitly that a bright clumps is made of some concentration of material, and in consequence, that their motion tracks the physical motion of material in the ringlets. However somewhere the author admit that the clump may also appear and disappear. This results in a discussion which may not very convincing where clumps are said to gather, scatter each other etc.. (sections 4.1 and 4.2).   At the end section concludes, more or less, that the bright clumps follow non-keplerian motion.. giving the feeling that the material may do so... and that this complex motion illustrates the complexity of the inter-clumps interactions. Such an interpretation of motion is highly degenerated (since clumps may appear or disappear) : any modification in the clumps distribution may be interpreted in a variety of ways... so the conclusions of sections 4.1 and 4.2 are confusing. In fact we have no proof that these bright structures in the averaged profiles are indeed accumulations of material. It is indeed possible that these bright clumps are photometrical effects produced by the poor radial resolution, and are just the result of the 3D complex wavy structure of the ringlets, but averaged in the images. For example, for the F ring : Voyager images of the F ring at low resolution, and during the 1995 ring plane crossing showed the present of longitudinal overdensities, that were also called "clumps" that exhibithed  a complex orbital motion (see, for example, papers by myself, Charnoz et al., 2001) . However, Cassini images of the F ring at high resolution did not show such clumps and rather a complex 3D structure... these clumps where the results of the averaging of the variety of structures in the F ring... that are known to have keplerian motions... It is possible that the same effects are at play in the ringlets of the Encke gap and that should be mentioned. 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