Saturn's E ring

Saturn's E ring

57, 1984 .4dv. Space Roe. Vol.4, No.9, pp.lL+9—l Printed in Creat Britain. All rights 0273—1177/84 $0.00 reserved. Copyright + .50 © COSPAR ...

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57, 1984

.4dv. Space Roe. Vol.4, No.9, pp.lL+9—l Printed

in Creat Britain.

All rights

0273—1177/84 $0.00

reserved.

Copyright

+

.50

© COSPAR

SATURN’S E RING T. W. Hill Space Physics and Astronomy Department, Rice University, Houston, TX 77251, U.S.A.

ABSTRACT The E ring is a loose collection of debris orbiting Saturn well outside the classical ring system. Compared to the classical ring system, the E ring is extraordinarily tenuous but occupies an enormous volume. The optical depth has a pronounced maximum near the orbit of Enceladus, which is therefore widely assumed to be the source of ring material. Optical properties of the ring suggest a collection of micron—sized spheroids whose composition is generally supposed to be water ice in accordance with the assumed Enceladus source. Apart from its unusual physical properties, the very existence of the E ring as a permanent feature of the Saturn system is problematical. The particles of the ring are susceptible to erosion by ion sputtering and outward transport by ion drag. Lifetimes against either of these processes in the presence of observed ion populations are estimated to be no more than a few thousand years, and it is not clear that Enceladus can replenish the ring particles continuously on such a time scale. It has thus been suggested that the E ring is a transient feature of the Saturn system, resulting from an episodic disturbance of Enceladus (e.g., a sizeable meteoroid impact or tectonic event) in the recent past. In any case, the E ring presently provides a convenient natural probe of magnetospheric transport processes because it presents a marginally significant cross section for the absorption of magneto— spherically—trapped particles traversing its volume. INTRODUCTION The E ring is an anomaly. Its volume dwarfs that of the classical ring system, yet its matter is spread so thinly through this volume that it contains only a minute fraction of the total ring mass. The E ring is virtually transparent (its optical depth normal to the ring plane is of the order of 10—6 as compared to optical depths of the order of unity for the classical ring system), and hence difficult to detect optically. We therefore know rather little about its detailed structure. What little is known, however, is sufficient to raise some interesting theoretical questions. For example, the large extent of the E ring, particularly normal to itself problematical : whatever mechanism has operated to reduce all rings to thin equatorial discs (perhaps even monolayers) has failed Could it be, as Baum et al. /1/ have suggested, that the E ring is lapse toward the equator is just beginning?

the ring plane, is in other known planetary utterly in the E ring. so young that its col-

The E ring, being the outermost, is also (and perhaps not coincidentally) unique among Saturn’s rings in being fully exposed to the destructive effects of interaction with magne— tospherically—trapped plasma and energetic particles. The lifetime of E—ring particles against sputtering by magnetospheric ions and/or outward transport due to ion drag is estimated to be no more than a few thousand years /2,3,4/, suggesting again that the E ring may be a very young structure indeed. I will attempt here to review the evidence bearing on the following questions: What is known of the composition and structure of the E ring? What is the life expectancy of E—ring particles against various known destruction and transport mechanisms? What is the likelihood that known source mechanisms can maintain a permanent E—ring structure in the presence of such losses? EMPIRICAL KNOWLEDGE The E ring was first

detected photographically during the 1966—67 ring—plane crossing by 149

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T.W. Hill

Feibelman /5/, following a number of conflicting reports based on visual observations during the previous century. During the next ring—plane crossing in 1980, the E ring was actively observed with techniques that afforded vast improvements both in angular resolution /1,6,7, 8,9/ and in spectral resolution /10,11/.

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DISTANCE FROM THE CENTER OF SATURN IN UNITS OF SATURN RADII

Fig. 1. Brightness distribution of the E ring as observed by Baum et al. /1/. The top panel shows a smoothed radial profile of brightness observed within narrow slices of the sky oriented perpendicular to the ring plane, while the bottom panel shows the spread of the brightness distribution away from the ring plane within each such slice. Figures la and lb (reproduced from Figures 9 and 8, respectively, of Baum et al. /1/) Illustrate the two—dimensional brightness distribution of the E ring as observed from Earth during ring—plane transit. Figure la (top) shows, as a function of distance from Saturn’s center, the integrated brightness within narrow slices of the sky taken perpendicular to the ring plane, while Figure lb (bottom) shows, as a function of distance from Saturn’s center, the effective thickness, perpendicular to the ring plane, of the brightness distribution within each such radial slice. From these brightness distributions, supplemented with further information provided by Baum and Kreidl /12/, one can infer a meridian—plane distribution of E—ring mass density similar to that sketched in Figure 2. The viewing geometry dictates /1/ that the density maximum (with respect to radial distance) lies somewhat out—

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151

side the brightness maximum, and in fact coincides within experimental uncertainty with the orbital distance of the icy satellite Enceladus. Beyond the orbit of Enceladus, the equatorial density decreases rapidly but the scale length of the density gradient normal to the ring plane increases noticeably. (Figure 2 is intended merely as a pictorial representation of these two well—established features; it is not based on a quantitative deduction from the observations, nor should it be interpreted as such.) On the basis of their high—resolution observations, Baum et al. /1/ made two important deductions: (1) that the E—ring material is supplied by Enceladus, and (2) that the E—ring structure represents a very early stage in the natural evolution toward equatorial concentration of ring material.

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I 4

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A qualitative

sketch

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distribution

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meridian plane as inferred from optical observations such as those shown in Fig. 1. Infrared /10/ and optical /11/ reflectance spectra indicate that E—ring particles have dimensions of the order of a few microns or less; this conclusion is supported by the fact /13/ that the E ring was detected by the Voyager 1 imager only in forward—scattered light, not in backscattered light. The tenuous E ring is, in some ways, probed more readily by means of charged—particle observations than remote optical observations. This is because photons traverse the ring but once, with a very low probability of encountering a single ring particle, whereas charged particles bouncing between mirror points in Saturn’s magnetic field traverse the ring many times in the course of their slow radial transport through the ring volume, with a correspondingly enhanced encounter probability. Thomsen and Van Allen /14/ anticipated this opportunity for probing of E—ring properties, and presented model calculations of the effects of E—ring absorption on hypothetical charged—particle populations. These calculations illustrated two fundamental points: (1) that, for particles of a given species and energy, absorption should be most noticeable at equatorial pitch angles near 900 because such particles spend the greatest fraction of each bounce cycle near the equator (and hence within the ring volume); and (2) that, for particles of a given species and equatorial pitch angle, absorption effects should decrease with increasing energy above that energy at which the particle’s stopping range in water ice becomes comparable to the typical dimension of E—ring particles. Thus the radial evolution of charged—particle energy and pitch—angle distributions can, in principle, be used to probe both the spatial structure and the particle size distribution of the E ring. Thomsen and Van Allen also noted that, by requiring consistency among the E—ring properties deduced from examination of various different particle species, one can, in principle, derive the magnitude and radial dependence of the “radial diffusion coefficient” that is used to describe the (common) radial transport rate of the charged particles.

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In situ observations of charged particles by Pioneer 11 and Voyagers 1 and 2 are generally consistent with these expectations /15,16,17,18,19,20,21,22/. Ouantitative inferences of E—ring properties based on charged—particle absorption signatures have, however, been rather few, and have revealed inconsistencies both among themselves and with earlier astronomical results. For example, Bastian et al. /15/ interpret their results as indicating that the typical size of E—ring particles is “probably larger than the range of a 1 MeV proton, but much smaller than the range of a 10 MeV electron, or on the order of a few hundred microns to less than 1 cm,” considerably larger than the typical size of a few microns or less inferred from optical and infrared observations. On the other hand, McDonald et al. /16/ find their results to be consistent with an average grain radius of the order of 2.5 microns, consistent with the optical and IR results, but they note a requirement for an 2, which, for E—ring ice massspheroids density (per unitgiven equatorial the order of 0.1depth microgram/cm water of the radius, area) would ofimply an optical of at least 10’4, In conflict with the optical determination of, at most, a few times 106. The estimate of McDonald et al. is based on an assumed ten—day transit time of energetic particles through the E—ring volume; independent estimates /3,23/ indicate that the actual transit time may he closer to 1000 days, which would remove the discrepancy. Notwithstanding these uncertainties and discrepancies, Hood /23/ was able to model successfully the available energetic—particle data from Pioneer 11 and Voyager 1 within the framework of a radial—diffusion calculation assuming absorption by the icy satellites and an E ring having properties as deduced from optical observations. Hood’s results indicate that the effect of E—ring absorption is subtle but noticeable next to that of satellite absorption. Carbary et al. /20/ have observed an absorption “microsignature” near the orbit of Enceladus which, however, is difficult to attribute to Enceladus itself because of geometric considerations. Carbary et al. suggest that this signature may have arisen from a longitudinally localized clump of E—ring material, analogous to the clumping that is observed optically in the F ring. Sittler et al. /18/ have also reported an energy—dependent extinction of “suprathermal” (several hundred eV) electrons in the E—ring region, which they attribute to energy degradation and/or absorption by E—ring particles. This conclusion has been challenged by Haff et al. /3/ who argue that such energy degradation is insignificant, primarily on the grounds that the electrons in question mirror at high latitudes and thus spend little of their time within the E ring. However, Haff et al. apparently overlooked the fact that the results of Sittler et al. apply specifically to electrons that mirror near the equator. Moreover, Haff et al. parameterize the field—aligned electron distribution by an electron scale—height expression which actually evaluates to at least 1000 R 5 (!). A more appropriate choice would be an oxygen—ion scale height (just a few R5), whereupon the calculation outlined by Haff et al. would predict significant (at least five—fold) energy degradation, thus supporting rather than refuting the interpretation of Sittler et al. Energetic—particle observations are thus broadly consistent with properties of the E ring inferred from optical and infrared observations. The peak optical depth (a few times 10—6) occurs very near the orbit of Enceladus at 4 R5, and It is difficult to avoid the conclusion that Enceladus is the source of ring material. It then follows that the ring particles are composed primarily of water ice, and a typical particle size of a few microns or less seems well established, although some admixture of larger particles cannot be ruled out. The shape of the particles is important in discriminating among candidate source mechanisms; Pang et al. /24/ have interpreted Voyager 1 observations as indicating approximately spherical particles, which would argue /3/ in favor of condensation processes vis—a—vis fragmentation processes. LOSS PROCESSES There are several processes that act to destroy and/or remove small particles orbiting within a magnetosphere. Destruction processes include sublimation, sputtering by solar photons, sputtering by magnetospheric ions, and erosion by meteoroid bombardment. Removal processes include orbit decay due to solar radiation—pressure drag, outward transport due to ion drag, and orbit diffusion due to electric charge fluctuations. Of the particle destruction mechanisms, ion sputtering appears to be the fastest and hence dominant process. For example, Cheng et al. /2/ estimate an erosion rate of about 1O~ micron/yr due to sputtering of water ice by energetic protons (> 50 key) in the E ring, while Haff et al. /3/ and Morfill et al. /4/ have independently estimated erosion rates of 10~_1O2micron/yr due to sputtering by low—energy oxygen ions (< 500 eV). By comparison, the erosion rates due to photosputtering and sublimation are estimated to be about 4 x l0~micron/yr /25/ and < iO’~micron/yr /26/, respectively. The rate of erosion by small meteoroid impact is difficult to estimate reliably (see the following section), but it might plausibly be comparable to the sputtering erosion rate /3/.

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Ion drag appears to be the dominant mechanism for removal (as distinct from destruction) of E—ring particles /4/. The E ring is located well outside the synchronous orbital distance (1.86 Rç). The corotation speed of magnetospheric ions therefore exceeds the Kepler orbit speed of E—ring particles, and their mutual collisional drag produces a small but systematic increase of the orbital energy, hence orbital radius, of the E ring particles. Morfill et al. /4/ have calculated the drag effect of observed ion populations in the vicinity of the E ring and have estimated a time scale of the order of 1O~yr for the resultant expansion of 1—micron particle orbits from 4 R 5 to 9 Rç. (Note an apparent misprint in the relevant equations (17, 18, 19) of Morfill et al. /4/ — cf. their Table 1 and the recapitulation by Morfill /27/.) Time scales for orbit perturbations due to other causes (e.g., radiation pressure drag, charge fluctuations) are estimated to be at least an order of magnitude larger — see Grun et al. /28/ for a discussion of the various physical mechanisms and Morfill et al. /4/ for estimates of the relevant time scales. Thus the time scales for destruction of E—ring particles and for their removal from the E—ring volume are expected to be comparable (both —iO~yrs), and it is not clear which effect, if either, controls their loss. It is, however, clear that the present E—ring particle population cannot be expected to survive unaltered much longer than iO~years; this conclusion follows from the observed properties of the ion population that interacts with the E ring, and from the assumption that the E—ring particle dimensions are not much larger than a few microns, as seems to be required by the optical and infrared observations. SOURCE PROCESSES The rather short life expectancy of E—ring particles seems to imply one of two possibilities: either (1) the ring is constantly regenerated by a source that is continuous on 1000— yr time scales and capable of replacing the total ring mass on such time scales, or (2) the ring was produced in the recent past (within historical times) by an unusual event (unusual with respect to 1000 year time scales), and may be expected to “vanish utterly in a similar span of time” /3/. McKinnon /29/ has advocated the latter possibility; Haff et al. /3/ have addressed the former possibility and have demonstrated the difficulties imposed by our present lack of knowledge of the important physical processes and environmental parameters. Haff et al. /3/ have listed and discussed the following five source mechanisms, all concerned with the removal of material from Enceladus: (1) sublimation, (2) photosputtering, (3) ion sputtering, (4) meteoroid impact ejection, and (5) internal venting. Of these five, the first can certainly be eliminated on quantitative grounds just as it can be eliminated as a loss mechanism for E—ring particles. The same statement probably applies, but with less certainty, to the second candidate. We are then left with ion sputtering, meteoroid impact, and internal venting. In the first two cases, it is required not only that material be liberated from the surface of Enceladus at a sufficient rate, but also that the same process not disrupt the E—ring particles at a comparable rate. Given the lifetime estimate described above, the required source rate is determined by the total cumulative mass of ring particles. This is estimated easily, if crudely, from the optical depth profile under the reasonable assumption that the ring is composed of ice spheres having radius a -~ 1 micron. The optical depth r is related to the column particle density a (number of ring particles per unit equatorial area) by = awa~

(1)

and the total mass of ring particles is simply

2 f 2r(L)LdL (2) M = (8~/3) paR5 1 where p C— 1 gm/cm3) is the material mass density of ring particles, RS (— 6 x 1O’~ km) is Saturn’s radius, L is radial distance in units of R 5, and L1 and L2 represent the inner and outer radial limits of the sensible ring. A radial profile of perpendicular optical depth in the E ring, attributed to H. J. Reitsema (private communication, 1981), has been published by Hood (/23/ — his Figure 1). Numerical integration of this profile gives a value of about 1O~5 for the above integral, and the total ring mass is thus estimated as 8kg (3) M-.3xlO If a lifetime of iO~yr is adopted (see previous section), the required rate of mass supply is dM/dt which corresponds to an average mass flux

JAss 4/9-~

--

10 gm/s

(4)

154

T.W. Hill

F

2—sec



1015 gm/cm

(5)

from the surface of Enceladus. It is also of interest to estimate the total cross—sectional area presented by the aggregate of ring particles, which is given by A = 3M/(4pa) — 2

x

1011 m2

(6)

which happens to be almost exactly equal to the cross—sectional area of Enceladus (i.e., one—fourth its surface area). This coincidence has the following notable consequence. If the ion sputtering yields of Enceladus and of E—ring particles are comparable, then the sputtering process removes mass from the ring at the same rate as it removes mass from Enceladus, and thus cannot represent a net source of E—ring material. If, on the other hand, the ring—particle sputtering yield were significantly less than the Enceladus yield (e.g., by virtue of a fluffy regolith coating on ring particles), then sputtering could In principle provide a net source of mass to the ring. Haff et al. /3/ have investigated this possibility and rejected it on the grounds that the ionization cross—section for neutral gas in the E—ring vicinity is larger (by some two orders of magnitude) than its cross—section for collision with E—ring particles, so that most of the neutral gas released from Enceladus becomes ionized before accreting on E—ring particles. If the net sputtering yield of E—ring particles is less than unity, then ion implantation represents a net mass source to the E ring irrespective of the above considerations. Sputter yields less than unity are not expected for a smooth ice surface, but may be plausible for a rough surface /3/. The flux of oxygen ions in the vicinity of the E ring is of order 107_1O8 ions/cm2—sec /4/, corresponding to a mass flux of 3 x 1016_3 x 1015 gm/cm2—sec, which may be marginally adequate (cf. (4) and (6) above). However, there is no apparent explanation in this scenario for the pronounced concentration of E—ring particles near the orbit of Enceladus, there being no such maximum in the observed ion flux. And one would still, of course, require a primordial source of seed grains (ions in space do not spontaneously cluster to form grains). Estimation of the E—ring mass source arising from meteoroid impact ejection from Enceladus suffers from the rather large uncertainty in our knowledge of the Incident meteoroid mass flux at Saturn’s orbit. On the basis of an earlier estimate by Cook and Franklin /30/, Haff et al. /3/ have adopted a meteoroidal mass influx of 3 x 10—17 gm/cm2—sec at Saturn’s orbit, and have assumed that impact vaporization liberates 5 meteoroid masses from the surface of Enceladus for each impact, thus arriving at an estimated mass flux of 1.5 x 10—16 gm/cm2— sec. This is about an order of magnitude shy of the requirement (5) derived above. (Haff et al. also point out that a much larger mass, of the order of 1O~meteoroid masses, is likely to be ejected in the form of particulates, but that only a fraction < 10’~of the ejecta are likely to attain escape velocity from Enceladus, so that the escaping mass flux from fragment ejection is less than that from impact vaporization.) On the other hand, Morfill et al. /31/ have (in a different context) compiled recent measurements and estimated therefrom a meteoroid mass flux at 10 AU that exceeds the earlier Cook and Franklin result by two orders of magnitude. If this larger value (reduced by a factor —3 to account for the reduced gravitational focusing effect at E—ring distances compared to the distance of the classical rings as considered by Morfill et al.) is substituted into the estimation of Haff et al., the resulting mass flux from Enceladus exceeds the requirement (5) by an order of magnitude. The meteoroid—impact vaporization process then appears as a plausible candidate for continuous regeneration of the E—ring mass. (Meteoroid impact on the E—ring particles themselves is, of course, a potential destruction mechanism whose effect should be subtracted from the above estimates in order to derive the net rate of mass supply to the E ring. This correction is not, however, expected to be important because the bulk of the meteoroid mass influx is expected to occur in the form of particles having radii of 100—1000 microns /31/, which may well liberate several times their own mass from Enceladus, but which can hardly remove several times their own mass from an E—ring particle which is assumed to have an initial radius of only a few microns.) There are, however, three qualitative arguments against this hypothesis, as noted by Haff et al • : Voyager 1 polarimeter observations /24/ indicate that E—ring particles are characterized by (1) a narrow size spectrum peaked at a few microns, and (2) roughly spherical shapes, both of which characteristics argue in favor of a condensation process rather than a fragmentation process, and (3) there is no obvious reason why the meteoroid impact volatilization process should single out Enceladus among the icy satellites for the production of a particulate ring. There is one possible scenario involving meteoroid impact that avoids all three of these difficulties, namely, a single “catastrophic” meteor impact on Enceladus /29/ of sufficient energy to release subsurface liquid water (if any) and/or to melt a noticeable portion of the surface. The size and shape distributions of E—ring particles might then be attribu—

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table to the effects of the phase transitions that would inevitably accompany an impact of such energy, while the unique selection of Enceladus among the several icy satellites could be attributed quite plausibly to happenstance. The unusually smooth topography of portions of the surface of Enceladus provides evidence of recent resurfacing /32/, a fact which is consistent, although not uniquely so, with such a scenario. The final candidate under consideration for supplying the E—ring mass is venting of subsurface water from Enceladus driven by intrinsic tectonic activity. Such a process would be intrinsically episodic, but perhaps less so than the large—meteor—impact scenario described above. It would be equally consistent with the inferred size and shape spectra of E—ring particles. The question of the uniqueness of Enceladus among the icy satellites would then require consideration of the forces that may potentially drive internal heating, hence tectonic activity, within the various satellites. Such considerations are well beyond the scope of this review. There can be no doubt that Enceladus exhibits a unique topography suggestive of recent, if not frequent, resurfacing. The question of whether or not such resurfacing can be explained in terms of internal tectonic activity driven by tidal dissipation has been addressed by Squyres et al. /32/. They conclude that tidal interactions associated with presently—observed satellite orbits are insufficient to melt the interior of Enceladus, but may be marginally sufficient to maintain such a molten state if established previously by, for example, more eccentric satellite orbits in the past /33/. On this basis they admit the possibility “that Saturn’s E ring is the result of ongoing geologic activity on Enceladus.” If this were the case, then Enceladus would be analogous to Jupiter’s satellite lo in the sense that each is uniquely capable of ejecting a sensible distribution of co—orbiting macroscopic particles, not by virtue of any unique plasma or meteoroid flux environment, but by virtue of internal heat derived from their unique positions among their respective families of tidally interacting satellites. I am not aware of any estimate of the escaping mass flux to be expected as the result of such internal venting, nor of any empirical evidence for or against its occurrence. The observed surface topography of Enceladus /32/ certainly suggests “recent” resurfacing with respect to cosmogonic time scales, but whether such resurfacing has resulted from external disturbances (e.g., large meteor impacts) or from intrinsic tectonics (driven, e.g., by tidal dissipation) is apparently an open question. As in the case of the meteor—impact scenario, the internal venting hypothesis has no problem satisfying the mass supply requirement if one is willing to entertain the possibility that the E ring is a transient rather than a permanent phenomenon. CONCLUSION Numerous lines of evidence indicate that the E ring consists of particles measuring a few microns or less, emanating (or having emanated) somehow from Enceladus, and ranging outward nearly to Rhea. A single Voyager 1 measurement /24/ indicates further that the ring particles have nearly spherical shapes and a narrow distribution of sizes. Confirmation of this last observation is an important goal because it places serious constraints on admissible source mechanisms. In particular, it suggests a process (e.g., internal venting and/ or catastrophic meteor impact) that involves a solid/liquid phase transition in association with mass ejection from Enceladus. Such a process would also eliminate the problem of the uniqueness of Enceladus among the many icy satellites in producing such an extensive particulate halo. Moreover, the mass ejection rate required to balance the rapid (—1000 yr) anticipated loss of ring particles may prove difficult to achieve with a continuous source such as sputtering or routine micrometeoroid bombardment. The mechanism of mass removal from Enceladus is surely the key outstanding problem. Given an appropriate mass source, the resulting spatial distribution of E—ring particles, although bizarre, may prove to be a relatively straightforward theoretical problem. There are two possible scenarios /34/ that appear to be consistent with the known radial distribution of E—ring material. One alternative involves mass ejection with a broad spectrum of velocities emanating from the leading face of Enceladus with respect to its orbital motion. The resulting trajectories form a family of ellipses sharing a common periapsis near 4 R 5 and having a distribution of apoapses determined by the distribution of ejection velocities. The other alternative involves low—velocity mass ejection producing a relatively dense circular torus of ejected particles near 4 RS, whose size and density diminish as they drift away from Saturn under the influence of ion drag and sputtering. In this case the near equality of time scales associated with removal by ion drag and erosion by ion sputtering would be no coincidence but would, in fact, be the necessary criterion that determines the radial extent of the ring. ACKNOWLEDGMENTS I have benefitted from discussions with A. F. Cheng and G. L. Siscoe. This work was supported by the National Science Foundation (Division of Atmospheric Sciences) under grant ATM—8311146.

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