CHAOS TERRAIN ON EUROPA: CHARACTERIZATION FROM GALILEO E12 VERYHIGH-RESOLUTION IMAGES OF CONAMARA CHAOS: 2. MATRIX.J. W. Head, R. T. Pappalardo, N. A. Spaun, L. M. Prockter and G. C. Collins. Dept. Geol. Sci., Brown Univ.,Providence, RI 02912 USA (James_Head@brown.edu)

Introduction: Spaun et al. [1], using 180 and 54m/pixel resolution Galileo coverage of Conamara Chaos,subdivided the 60% of the chaos region that consists ofmatrix into several subunits. Angular matrix polygons aresteep-sided angular fragments of linear texture that show asteep tilt towards one side and a high cliff on the oppositeside. Micro-polygonal matrix blocks are smaller plate-likestructures that show very faint evidence of linear texture;they appear highly degraded and are sometimes at a lowertopographic elevation than the linear textured polygons.Peak material consists of individual, often elongated fea-tures usually less than 2 km across; they show very little tono linear texture and tend to be isolated peaks or groups ofpeaks rising above the topographic level of the matrix.Hummocky matrix consists of a rough-textured surface thatis distributed between other sub-matrix units and includesjumbled surface blocks from ~1km in diameter down to theimage resolution. Using the very high resolution Galileocoverage obtained during orbit E12 (9 m/pixel), we havebeen able to map details of the matrix comprising over halfof the study region (polygons are described elsewhere [2]).First, we find that the matrix subdivision of Spaun et al. [1]derived from mapping in the lower resolution coverage isvalid when viewed at even higher resolution. Examples ofpeak material and angular matrix polygons are seen veryclearly in the high resolution coverage, and new details oftheir characteristics are revealed. In addition, very smallpolygonal matrix blocks are abundant and the structurerevealed at very high resolution confirms that they are de-graded polygons. One of the most striking benefits of thevery high resolution data is that it shows abundant detailsof the hummocky matrix, and permits the subdivision ofthis and the micro-polygonal matrix blocks into severaladditional subunits as outlined below (Fig. 1).

Mapped Uni t s : Subdued Lineated P o l y g o nFacies (SLP): (Fig. 1) There are 19 small plate-like struc-tures that show evidence of relatively continuous lineartextures which bear distinct similarities to more continuouslinear features in polygons. Shapes range from rectangularto polygonal (larger blocks) to rounded (smaller blocks).On the basis of their size, shape, location, and linear tex-ture, we interpret these features to be smaller and more de-graded examples of the major polygons. Because of theirsmall size, their linear patterns are not readily matched topatterns in the larger polygons. Discont inuous Ridgeand Scarp Facies (DRS): Most of the 15 occurrencesare less than ~2.5 km in longest dimension and character-ized by linear scarps less than a kilometer and commonlyless than 0.5 km in length, surrounded by distinctive talusdeposits. Shapes are similar to those of the Subdued Line-ated Polygon Facies; individual short ridges appear to bebroken up segments of longer ridges, and the abundance oftalus attests to increased levels of degradation. Knobby

and Bouldery Facies (KB): Knobs up to several hun-dred meters in diameter, boulders in the size range of severaltens of meters, and minor linear texture characterize thisfacies. Sizes of the 4 occurrences range from ~1 km up to4.5 km, and shapes are varied. The Knobby and BoulderyFacies is interpreted to represent disaggregation and disin-tegration of other facies. In this facies, boulders are notclearly associated with scarps and talus slopes, and theircharacter suggests a more distributed and widespread produc-tion of boulders and hummocks through in situ disaggrega-tion and disintegration of other terrain. Hummocky andPi t ted Fac ies (HP): A wide variety of morphologicalfeatures occur here, many of which are shared with othermatrix facies. It is the most abundant of the mapped matrixfacies and surrounds polygons and occurrences of all otherunits. The most abundant components are hummocks andirregular depressions and pits of similar scale, which to-gether make the topography very rough. Also common arelarger rectangular hummocks, short linear ridges, sinuousscarps, and boulders and knobs.

A Sequence o f Degradation in Matrix Fa-c i e s? : Formation of chaos involves destruction of back-ground ridged plains and formation of matrix and enclosedpolygons [1-4]. Our mapping suggests that matrix subunitsmay be related in terms of morphologic features and mayrepresent a sequence of degradation from background plains,to polygons, through a series of progressively more mor-phologically degraded units. Trends toward increasing deg-radation are manifested by: 1) decreases in continuity oflinear segments within mapped features, 2) increases inabundance of equidimensional hummocks and knobs, 3) ageneral decrease in topographic level, 4) a general decreasein distinctiveness and sharpness of the margins, 5) a gen-eral decrease in average size, and 4) an increase in number ofboulders. The pervasiveness of the destruction and degrada-tion in these facies, in contrast to simply producing smallerand smaller fragments of polygons, suggests that the proc-ess involved operates to disaggregate and disintegrate thepolygons in situ.

Summary and addit ional relevant observa-t i o n s : 1) Matrix makes up more than half of the mappedregion. 2) Three facies (Subdued Lineated Polygon Facies,Discontinuous Ridge and Scarp Facies, and Knobby andBouldery Facies) are distributed in polygonal, rounded andirregularly-shaped occurrences from 1-4 km in diameterthroughout the matrix. 3) A fourth facies, Hummocky andPitted Matrix Facies comprises the vast majority of thematrix area and surrounds occurrences of virtually all otherfacies. 4) These four facies appear to represent degradedversions of polygons and show a wide range of trends insize, shape, and morphology that suggest that they repre-sent a trend toward increasing levels of degradation. 5) Thepervasiveness of the destruction and degradation in these

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EUROPA: CONAMARA MATRIX: J. W. Head et al.

facies suggests that the processes involved in matrix forma-tion operate to disaggregate and disintegrate the polygonsin situ. 6) Two additional facies (Angular Matrix PolygonFacies and Peak Material Facies) appear to be primarilyrelated to fragmentation and rotation of large blocks fromthe margins of polygons into the surrounding matrix. 7) Abordering 'miniscus' effect exists around numerous faciesunit occurrences in the matrix. 8) Rounding of facies occur-rences creates arcuate, circular and oval patterns in severalparts of the matrix. 9) No evidence was found for smoothplains or smooth-textured terrain suggestive of solidifiedextrusive flows or low-viscosity smooth plains resurfacingas has been seen in some other places on Europa [3]. 10)Matrix color is different than the background ridged plainsoutside the chaos and similar to that of several polygonswithin Conamara Chaos, as well as to lenticulae outside the

chaos. 11) No evidence for specific occurrences of lenticu-lae were found in the very high resolution coverage ana-lyzed; however, the typical scale of lenticulae, their colorsimilarities, their similarities to textures in the matrix[5,6], their occurrence at the edges of Conamara Chaos andoutside, and their density in the area mean that one or morelenticulae could be present, but unrecognized, in the largerareas of matrix occurrence.

References: [1] N. Spaun et al., Geophys. Res. Lett.,25, 4277-4280, 1998. [2] J. Head et al., Chaos terrain onEuropa: Characterization from Galileo E12 very high-resolution images of Conamara Chaos: 1. Polygons, LPSC30, 1999. [3] R. Greeley et al., Icarus, 135, 4-24, 1998. [4]M. Carr et al., Nature, 391, 363-365, 1998. [5] J. Head etal., Bull. Am. Astron. Soc., 29, 983, 1997. [6] N. Spaun etal., EOS, 79, F540, 1998.

F i g . 1 . Sketch map of units. Numbered blocks are polygons; images are terrain examples (1.8 km across). Patterns referto facies; names for abbreviations and descriptions are in text. Steps from left to right suggest degradational sequence.

Lunar and Planetary Science XXX 1587.pdf