Lunar and Planetary Science XXXII (2001) 1093.pdf CLASSIFICATION OF ARACHNOIDS ON CYTHEREAN SURFACE. V-P. Kostama, Astronomy, Univer- sity of Oulu, FIN-90014, Finland (<[email protected]>). Introduction: This group of circular concentric plains, more than half of their total population are as- features with radial lineaments was first seen on the sociated with a deformation zone, usually with a com- Venera 15/16 radar images and later defined in detail pressional one. These Class I arachnoids form about 53 by the Magellan mission [1, 2]. They were given a % of the global Venusian arachnoid population. There morphologically descriptive name after their appear- are somewhat more features in the Subclass Ia, which ance, which does resemble a spider (in Greek, arachne consist of arachnoids which are located on a deforma- = spider, -oid = resemblance) [1]. The origin of these tion zone or as a continuation of a deformation zone. features is still uncertain. We do not know their rela- The Subclass Ib arachnoids are located in the vicinity tion to the other circular or elongate structures, such as of a deformation zone, which has probably affected to coronae or calderas [3]. The arachnoids and coronae do the formation of these surrounding features. One other have many similarities and this is why arachnoids have deviation between the subclasses is the topography of been thought to be corona-like features [2], a sub-class the arachnoids. Arachnoids with dome-like topography of coronae [4] or possibly a part of the corona forma- are frequent within the Subclass Ia, while in the Sub- tion process itself, presenting an earlier phase of class Ib the arachnoids usually have topographic an- coronae evolution [3]. The term is sometimes consid- nulus. The dome-like arachnoids are generally quite ered as an archaic way to present a concentric corona small. The most important characteristic for the Class I with radial lineaments around the annulus [5]. How- arachnoids is their tendency to exist in groups. These ever, the term “corona” is a more broader morphologi- groups are very original and form large scale features. cal term, indicating any circular or elongate structure Class II. The Class II is composed of arachnoids defined primarily by an annulus of concentric fractures that are located on plains and do not have any clear and ridges [1, 6, 7]. The use of the term “arachnoid” association to any surrounding geological formation or does not rule out the possibility that the arachnoids structure. The arachnoids in this class do generally may be subtypes of coronae. The term “arachnoid” is form groups, but the groups do not have such close based on the morphology and it is convenient to use connected structures as in the case of Class I arach- and readily characterizes the specified features [8]. noids. The topography of all the plains arachnoids is Arachnoids, as a separate group of morphological fea- usually similar. The depressed center is surrounded by tures, do have a clear basis for their own classification. elevated rim. In a few cases the topography is dome- like. This class includes 21 % of the arachnoid popula- Arachnoid classification based on the geological tion. environment: Arachnoids do not have any prior speci- Class III. The arachnoids near the tessera strips or fied classifications. All such features were earlier in- islands belong to the Class III. The tessera terrain is cluded to the class of radial-concentric corona-like thought to be remnant of the older Venusian surface, features [7] in the corona classification by Pronin and and the crust may be thicker in those areas. The Class Stofan [6]. In addition to that, we have only several III arachnoids are never located directly on the tessera lists of volcanic formations, which feature the global surface. Their main characteristic is that they are single population of arachnoids on Venus [2, 3, 4, 9]. features, without forming any groups. About half of Because arachnoids form a group of morphologi- the arachnoids in this class have some sort of double- cally distinguishable features, it is rather difficult to ring structure, but the rim is usually irregular in shape. classify a single arachnoid according to its morphol- The topography of the arahnoids is generally dome- ogy. The variations in their appearance are quite small like, only two of the features in this class have a clear and reflect probably differences in the geological envi- topographic rim. Nine percent of arachnoids belong to ronment [8]. Variations in arachnoid locations and in this class. their geological environments can be used to classify Class IV. The arachnoids close to volcanoes and the arachnoid population. Differencies in geological those located in highly volcanic areas belong to this environments give us the main variables which can be class. These features are located in areas with high used in classifying arachnoids or any other morphol- concentration of extrusive volcanism. Typical to the ogy-based features. In arachnoid case, we know that arachnoids in this class is the diversity of their charac- they are generally located below the mean planetary teristics. They also differ clearly from the arachnoids surface (3051,84 km) and their global distribution is in other classes. The diversity of the features in this not random [8, 10, 11]. In this study the arachnoids class is reflected in the topographies of the arachnoids have been classified into four main groups, Classes I – as well. About 50 percent has some sort of elevated IV, with Class I having two subclasses, Ia and Ib. topographic rim, which is usually irregular and some- Class I. It is essential to notice that although the what partly depressed. The arachnoids with depressed majority of arachnoids are located on the Venusian topography are usually found in this class too. In these Lunar and Planetary Science XXXII (2001) 1093.pdf CLASSIFICATION OF ARACHNOIDS: V-P. Kostama cases the lava flows have covered the center of the arachnoid, and the lava load has resulted in the depres- sion of the center.Class IV includes 16 % of Venusian arachnoids. The distribution of arachnoid classes reflects cer- tain noticeable facts. All but three of the features in Class I are on the northern hemisphere. These arach- noids usually exist in larger groups. The arachnoids in Subclass Ia have the same distribution as the other northern arachnoids, in general. In contrast to the Class I arachnoids, the arachnoids of Class IV seem to con- centrate on the southern hemisphere. Three features part from this trend. Two of these are located next to Sif and Gula Mons volcanoes close to the equator. Conclusion: Arachnoids are morphological fea- tures. The characteristics are unique to this group of Cytherean structures. They have a distribution that probably reflects certain qualities of the Venusian geological development. Their distribution can be used to classify the features into four major classes accord- ing to the geological environment. The different geo- logical surroundings give each class several unique characteristics with clear deviations in tectonics, vol- canism and topography. References: [1] Barsukov et al. (1986), JGR 91, pp. 378 - 398. [2] Head, J.W., C.S. Crumpler and J.C. Aubele (1992), JGR 97, pp. 13153 - 13197. [3] Ham- ilton, V.E. and E.R. Stofan (1996), LPSC XXVII, pp. 483 - 484. [4] Price, M. and J. Suppe (1995) Earth Moon and Planets 71, pp. 99 - 145. [5] Chapman, M.G. and J.R. Zimbelman (1998), Icarus 132, pp. 344 - 361. [6] Pronin and Stofan (1990), Icarus Vol. 87, pp. 452 - 474. [7] Stofan, E.R., V.L. Sharpton, G. Schubert, G. Baer, D.L. Bindschadler, D.M. Janes and S.W. Squyres (1992), JGR 97, pp. 13347 - 13378. [8] Aittola, M. and V-P. Kostama (2000), PSS 48, pp. 1479 – 1489. [9] Crumpler, C.S. et al. (1996). Venus Volcanic Feature Catalogue. http://porter.geo. brown.edu/planetary /databases.html. [10] Kostama, V-P. and M. Aittola, this conference. [11] Aittola, M. and V-P. Kostama, this conference..