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O Lunar and Planetary Institute Provided by the NASA Astrophysics VENUS: A FIRST GEOLOGIC MAP BASED ON RADAR ALTIMETRIC AND IMAGE DATA; Harold Masursky, A.L. Dial, Jr., G.G. Schaber, and M.E. Strobe1 1, U.S. Geol ogical Survey, Fl agstaff , AZ 86001 Continued acquisition of image and a1 timetric data by the Pioneer-Venus radar mapper during the extended mission has resulted in the interleaving of data lines and the filling in of gaps so that data acquired on individual orbits are now spaced 1ess than 75 km apart. Resol ution, therefore, has Seen increased by a factor of two, and new versions of the Venus maps have been drawn from the combined data set. The new spacecraft data show the geomorphic forms in much crisper detail, and confirm the earl ier division of the planet into three geologic provinces (accompanying figure) . The highl and province lnakes up about 13 percent of the imaged surface, the rolling uplands province includes about 60 percent, and the lowlands province includes about 27 percent. Earlier estimates of the composition and possible geologic age of the surficial rocks in each province also seem to be confirmed by the new data. Basaltic rocks associated with kta Regio (centered at 1at 28" V., 1ong 283") may be younger than the rocks of possible intermediate composition that compose the rugged highland area Maxwell ilontes (centered at 1at 65" N., 1ong 5") , and the more degraded and probably still 01 der highl and region at Aphrodite Terra (situated on the equator between 1ong 60" and 205") ; the 1atter may be intermediate to sil icic in composition. Lowland areas appear to he almost uncratered; they may be areas where the early crust of Venus was thinned by mantle convection or impact and then, at a much 1ater time, partly fill ed by mare1 ike basal ts. The venusian lowlands would then be analogous to lunar mare areas, and to the extensive 1owl and plains in the northern hemisphere of Plars. Radiating 1ava fl ow compl exes from Beta Regio probably over1 ie the adjacent 1owl and basal ts. Abundance of radioactive elements at the Venera 8, 9, and 10 sites (accompanying figure) has been deduced by gamrna-ray spectrometers on board the Soviet spacecraft. Analysis at the Venera 8 site in the roll ing plains province indicates a "granitic" composition, whereas analyses of the Venera 9 and 10 sites, on the flank of Beta, are similar to those of terrestrial basal ts. Future 1andings by Venera 13 and 14 in the area southeast of Beta will shed additional light on the composition of the rocks at their sites. These spacecraft will use X-ray fluorescence devices to measure major-element chemistry. Estimates of the colnposition of highland reqions and other areas removed from the Venera landing sites are based on radar return and gravity data. The newly acquired data, both from Pioneer-Venus and from computer mosaics of Earth-based images, have allowed us to discriminate with more certainty between volcanic and impact features. New profiles across Hathor Mons (centered at 1at 21" N., 1ong 0") and Sappho (centered at 1at 15" N., long 15"), and combined Earth-based and spacecraft image data, show thern to he circular features with positive topographic expression; near the top of each feature, circular depressions that may be calderas suggest that they are volcanic constructs. Theia and Rhea Montes in Beta Regio display simil ar topographic forms, though on a much larger scale. They probably make up a 1arge irregul ar volcanic shield complex of basal tic composition. The features Lise Meitner (centered at 1at 55" S., 1ong 321") , Colette (centered at 1at 64" N., 1ong 324") , and Eve (centered at 1at 0.5" S., 1ong 3") are shown from the new data to be circular depressions with raised rims; although this configuration might result from radar imaging of maar volcanoes, the features are more likely to be impact craters. Large circular dark features with bright central spots may be shallow impact craters that have been partly flooded by mare1 ike material. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System DESCRIPTION OF MAP UNITS VOLCANIC CONSTRUCTS-Inegul~r shcld-shaped mountains with summit calderas or troughs; "bdtic" from V-9, 10 LOWLAND PLAINS-Smooth dark tenain with no craters. May be covered by badtic lava flows MODIFIED VOLCANIC CONSTRUCTS-Bnght rough mountams Wth possible caldera. May be fehc volcanic rocks HIGHLANDS-Bright rough tcrram, many rift valleys and ndgcs, some crates and mountain ridges (mr), may be felsic and mafic lavas. very de- graded in places ROLLING PLAINS-Heavily cntcrcd rolling temm w~thisolated, pogdbly volcanic constructs and somcanc~enthuvily fractured tenain (ft). "granitic" from V-8 Probable impact craters Contact Possible fault Location of Venera site I I 1 I I I I 1 1 I I I 140° 270" 300' 330' no 30" 60- 90' 170D 150. 1800 210- 240' 0 5000 10 000 bm I I I *pp,e.ilo.. r.1. .t .q..*oc PRELIMINARY GEOLOGIC MAP OF .VENUS VENUS GEOLOGY Masursky, H. --et al. Evidence of possible tectonism is also enhanced by the new altimetric and i~naginqdata [I ,2,3]. The most striking tectonic features dre valleys with r~arqinalramparts; these valleys are more than 100 km wide, more than 2 km Icep, and Inore than 1000 km 1ong. Several of the marqinal ramparts may he vol canic constructs. These va? 1eys strong1 y resembl e the African rift val 1eys on Earth. Other features thouqht to be caused by tectonic disruption of the crust appear as radar-bright l~neamentsin the Pioneer-Venus and Earth-based images. A1 timetric data show iome of these 1ineaments to he ridqes that stand about 500 m above the adjacent country; however, unlike analagous terrestrial features, no central trough is recognized. Their apparent 1ack of a central trough may he a resolution effect. Two larqe arcuate structures a1 so are composed of double ritlves that bound central troughs; these features may be anal oqous ti, terrestria1 isl and arcs or, more probably, to resprunq irnpact structures. Other discontinuo~~s,bright linear features do not have topograpl~icexpression hut are probably disrupted crustal zones of increased rouqhness [4]. 4t this resolution, features such as terrestrial ridqes that transect mid-ocean basins, and marqinal troughs, are not apparent; an integrated system of qlobal plate tectonics may rlot exist. However, the disrupted zones, rift valleys, and plateaus bounded by scarps strongly indicate that the crust has heen disrupted by faulting at iany places. 9 positive qravity anomaly of 70 milligals over Beta Regio has been noted by Id. Yjoqren (personal communication) . On the basi s of photogeol ogic analysis, Reta may he the youngest of the 1arge terrain features on Venus. It nay be similar to younq volcanic constructs on the Earth, such as the Hawaiian Is1ands. Tlie 01 der, more disri~pted volcanic construct, ?laxwe11 Montes, has no associated qravity anomaly. This lack of anomaly may be a result of its qr-eater aqe and its composition being intermediate to silicic rather than basal tic. References 111 Hasursky, H., El iason, E., Ford, P. C;., McGill, C. FI., Pettinqill, G. H., Schaber, G. C., Schubert, G., (1980) Jour. Geophys. Qes., v. 85, no. Al?. n. P232-Q260. [?I ~ettin~ill,G. ti., Eliasor?, E., Fori-l, P. G,, I-oriot, G. R., F.lasursky, H., McGill, C. F., (1980) Jour. Geophys. iles., v. 85, no. A13, p. 8251- 8270. 131 Schaber, G. G., and Plasursky, H., (19QO) VPSA Tech. Memo. 82385, Reports of Dl anetary Ceol ogy Jro ran - lQRn, p. 32-84. 141 Schabcr, G. C., and Masursky, 4hs. Lunar and Dlanet. Science Conf. XII, this volume. O Lunar and Planetary Institute a Provided by the NASA Astrophysics Data System .
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