Cassini had a relatively close flyby at Iapetus on New Year's Eve 2005. The 288 ISS images set various constraints on the origin theories of the dark/bright dichotomy, as revealed multiple surface structures at up to 740 m/pxl size

Denk, T.

Giese, B.

Helfenstein, P.

Johnson, T.V.

McEwen, A.S.

Neukum, G.

Perry, J.E.

Porco, C.C.

Roatsch, T.

Thomas, P.C.

Turtle, E.P.

Veverka, J.

Wagner, R.J.

Institute of Transport Research:Publications

FIRST IMAGING RESULTS FROM THE IAPETUS B/C FLYBY OF THE CASSINI SPACECRAFT.T. Denk1, G. Neukum1, T. Roatsch2, A.S. McEwen3, E.P. Turtle3, P.C. Thomas4, P. Helfenstein4, R.J. Wagner2, C.C.Porco5, J.E. Perry3, B. Giese2, T.V. Johnson6, J. Veverka4, and the Cassini ISS Team. 1Freie Universität Berlin, Mal-teserstr. 74-100 Haus D, 12249 Berlin, Germany (denk@zedat.fu-berlin.de), 2DLR (German Aerospace Center)Berlin, 3University of Arizona, Tucson AZ, 4Cornell University, Ithaca NY, 5Space Science Institute, Boulder CO,6JPL, Pasadena CA.Abstract/ Introduction.  The first of two relatively closeIapetus flybys in Cassini's primary mission occured on Dec31, 2004 18:49 UTC near apoapsis from orbit "B" to "C" atan altitude of ~123,400 km over the northern leading hemi-sphere, resulting in a minimum pixel scale of 740 m for theISS narrow angle camera (NAC) [1]. Data revealed details ofa >1300-km-long ridge that had been discovered just oneweek earlier in optical navigation images [2, 3]. Individualmountains within the western part of the ridge reach heightsof ~20 km over surrounding terrain. The data set providesconstraints on the origin of the albedo dichotomy. It appearsvery likely that the dark material is overlying an ice crust, butno evidence for emplacement of dark material via surfaceflows is apparent. Instead, signs for dark-material emplace-ment through processes that included ballistic transportationare visible. No bright-floor ("punch-through") craters havebeen found on the dark hemisphere. The ridge discovery mayrevive the idea of an endogenic origin of the dark side [4].Data acquisition. All 288 planned images were success-fully transmitted to Earth. A total of 8 NAC mosaics wereobtained, ranging in spatial resolution from 1030 to 740 andthen back to 1100 m/pxl. The phase angle increased from 51°to 140°. Three sequences included multi-color observations(from 0.25 to 1 µm). 55 images were taken of parts of thesurface that were primarily illuminated by Saturn shine. Dur-ing the observation period from 31 Dec 2004 01:53 UTC to01 Jan 2005 14:30 UTC, the sub-spacecraft point movedfrom 29°N/ 70°W north-eastwards to 66°N/ 0°W and thenback in a southeast direction to 47°N/ 300°W. The sub-solarpoint moved from 106°W to 113°W at 7.5°S latitude. There-fore, almost no part of the trailing side or poles was illumi-nated by the sun and visible from Cassini's perspective.Ridge system. The ridge system shows a large variety oflandforms [3]. It includes isolated peaks as well as singleridges >200 km long, and even parts where two or threeridges run in parallel [3]. Its average height decreases to-wards eastern parts of Cassini Regio, and the whole systemmight end near 50°W, close to the large 550 km impact ba-sin. It is covered by numerous craters, and some large craterscompletely destroyed it locally [3]. This indicates that itsformation occured relatively early in Iapetus' history. Onecrater near 100°W shows a white rim, suggesting that theridge is composed primarily of icy material. Its origin is apuzzle so far, and no similar geologic feature has been ob-served elsewhere in the solar system. Possibly, further evi-dence about formation may come from topographic data.Craters in the dark hemisphere. Craters of all sizes canclearly be seen at 1 km/pxl resolution. In general, craterflanks appear very steep with an abrupt transition to a flatfloor. This gives them a "pan-shaped" appearance, similar towhat was observed by Cassini on Dione. Central peaks arecommon.Deep within Cassini Regio, rims and central peaks of cra-ters appear somewhat less covered by dark material thantheir floors and walls, reminiscent of Callisto [5]. However,bright spots on Callisto can be recognised at pixel scales of10 km or worse, while the dark hemisphere of Iapetus ap-pears homogeneously dark at the 10 km scale  (Fig. 2 in [6]).Further away from the equator and closer to the northerntransition zone, several large craters exhibit steep, brightrims, especially on their north-facing flanks. No dark layerscan be seen on top of these bright crater walls. Since the bestimage resolution is 740 m/pxl, it is probable that the darklayer is less than ~1/2 km thick in regions about 30° north ofthe equator.Dark crater rims and streaks in the transition zone.The color image of the northern transition zone removes alldoubts about asymmetric deposits of dark material on craterwalls. Almost every crater at high latitudes shows a dark,south-facing wall and central peak (when present) [3]. Addi-tionally, numerous dark streaks cover wide parts of the tran-sition terrain. Whitish patches are in most cases correlated tonorth-facing slopes. These observations strongly suggest thatthe dark material did not flow over the surface, like lava fromthe interior, but was distributed over this part of the surfacefrom a distant source [3]. The relative brightness of localareas appears to depend on local topography and on latitude.This is also consistent with the bright, polewards facing cra-ter walls, which might have never been hit by dark material.A possible dependence on longitude is less obvious and un-der investigation.Landslide crater. A 120 km-sized crater is located at thesouthwestern edge within the 550-km basin [1] at~5°N/35°W. Almost half of its floor is covered by a giantlandslide, which appears to have originated from the wall ofthe giant basin which is ~15 km high and has a very steepslope of at least 45°. Unfortunately, large parts of the wall liein shadow so that no detailed structure can be seen. However,an area immediately adjacent to the crater on its northern sideis also covered by the landslide. As in the crater, this area israther sparsely cratered.Lunar and Planetary Science XXXVI (2005) 2268.pdfA second curiosity of the landslide crater is that the cra-ter floor shows no impact craters, but is very flat. Since thelandslide crater itself is quite large, and because the landslideitself is covered by a few craters, it appears more likely thatit has been resurfaced rather than being a very recent impact.Possibly, a relaxation of a locally weak ground removed alltopography. Alternatively, deposits of fine materials mightbe considered, although the reason for this very localizedappearance is not clear in either cases. Interestingly, the dark material coverage on the landslideand on the crater floor do not look different from each otheror from other locations. This suggests that the process caus-ing the dark blanket ended later than the flattening of thecrater -- perhaps even ongoing today.Trailing side. Parts of the trailing side including the"moat structure" (an ~250 km wide crater, see [1] for details)have been imaged in Saturn shine. Various mid-sized andsmall craters are visible, as is a complex albedo pattern of the"moat area" in the vicinity of the equator. One relativelylarge dark-floor crater with a diameter of ~50 km has beendetected near ~10°N/ 300°W, more are possible but uncertaindue to their closer position near the 270°W meridian whichforms the boundary for Saturn-shine illumination. Severalvery small (mainly <10 km sized) craters closer to CassiniRegio also exhibit dark floors or partial coverage by darkmaterial. The overall appearance of dark/ bright materialdistribution on the trailing side still appears very puzzling.No equatorial ridge is obvious, but illumination condi-tions might have prevented an immediate detection. Note thateven the crater morphology of the "moat" is not obvious inthe Saturn-shine images.Origin debate. The new Iapetus data set several con-straints. No bright-floored crater was found on the darkhemisphere, which means that either the dark material isyounger than resolved craters, too deep to be excavated bycraters, or most likely a combination of both. The dark mate-rial looks like a blanket on top of an icy crust with a maxi-mum thickness of 1 km, probably less than 1/2 km (at thelocations where bright north-facing crater walls are visible; itcould be thicker near the center of Cassini Regio). Thestreaks and equatorwards-facing dark crater walls indicatethat the dark material was emplaced as "spacefall" from adi rection close to the apex of motion. This is in principleconsistent with accretion of exogenic material or with ballis-tic emplacement from an equatorial source [3].A new endogenic origin hypothesis was developed in theImaging Team after seeing the newly detected ridge. Follow-ing this idea, the ridge, its flanks or the area nearby might actas a source of the dark material [3]. Possibly, venting-likeplumes regularly erupt intimately mixed dark and brightmaterial onto the surface, with the lighter icy componentdiffusing into space and the heavier dark component deposit-ing on the surface. The position of the ridge system on Iape-tus is well suited for this: It is located along the equator, bi-secting Cassini Regio. The poles on the leading side wouldhave remained bright because the plume deposits would notextend this far. Eruption along the ridge might explain whythe dark area extends to the trailing side on the equator.However, a major problem is the energy source for suchvolcanism. Within the ISS Team, it was proposed that energyfrom Saturn is dissipated through the relatively rapid preces-sion motion of Iapetus' orbit plane. This would imply that theprocess might be ongoing even today, weakening the prob-lematic finding that no kilometer-scale bright-floor craterwas detected inside Cassini Regio. However, no detailedtheoretical modelling on this problem has been done so far,including a determination of the power of such a hypotheticalenergy source.Alternative options of an exogenic origin remain valid.The data are consistent with models that predict that the darkmaterial should have been deposited in a direction from theapex of motion or nearby. However, the existence of thedark-floor craters on the trailing side or the complex patternof the transition zones at the equator remain difficult to ex-plain by either model. The latter is in particular a problem forthe "Phoebe-dust theories" [7], while catastrophic collisionhypotheses [8] suffer the problem of the very low probabilityof such an event at more recent times.This leaves the origin debate open; more work, maybenew ideas (and/or data) are needed. Possibly, several proc-esses were acting independently, and different explanationsare needed for the genesis of the ridge, the dark blanket, andthe dark-floor craters? Despite our lack of understanding, acertain fact is that Cassini Regio exists.References. [1] Porco, C.C. et al. (2004) Space Sci. Rev. 115, 363-497.[2] Denk T. et al. (2005) LPSC XXXVI , abstract. [3] Porco C.C. et al. (2005)Science, under review. [4] Smith B.A. et al. (1982) Science 215 , 504-537. [5]Greeley R. et al. (2000) Plan. Space Sci. 48, 829-853. [6] Denk T. et al. (1999)LPSC XXX, abstract #1841. [7] e.g., Burns J.A. et al. (1996) Phys. Chem.Dyn. of Interplan. Dust, ASP Conf. Series 104, 179-182; Buratti B.J. and J.A.Mosher (1995) Icarus 115, 219-227. [8] e.g., Tabak R.G. and W.M. Young(1989) Earth, Moon, and Planets  44, 251-264; Matthews R.A.J. (1992) Q. J.Roy. Astron. Soc.  33, 253-258; Denk T. and G. Neukum (2000) LPSC XXXI,abstract #1660; Owen T.B. et al. (2001) Icarus 149, 160.172; Ma rchi S. et al.(2002) Astron. Astrophys. 381, 1059-1065.Acknowledgements. The authors are very thankful to all individuals whomade the acquisition of the data possible, esp. of the data from New-Year'sEve flyby. Many thanks also to their patient families. Particular thanks areowed to the person(s) in the Huygens project who oversaw the receiver Dop-pler problem and those who redesigned the Cassini tour, enabling the Iapetusflyby to have an altitude of 123,400 km instead of ~670,000 km.Images. There's so much new information in the data that the availablespace did not allow us to include figures to the abstract. However, images areavailable at various places in the WWW, such as:Cassini Imaging Team:     http://ciclops.org    NASA:     http://photojournal.jpl.nasa.gov/target/Iapetus   JPL:     http://saturn.jpl.nasa.gov/multimedia/images/images.cfm?subCategoryID=14   FU Berlin:     http://userpage.fu-berlin.de/~planeten/de/projekt/cassini/N_ia.htm     Lunar and Planetary Science XXXVI (2005) 2268.pdf

First imaging results from the Iapetus B/C flyby of the Cassini spacecraft

First imaging results from the Iapetus B/C flyby of the Cassini spacecraft

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