Lunar and Planetary Science XXXII (2001) 1653.pdf

GEOLOGIC MAPPING OF VENERA 13 LANDING SITE REGION. A. M. Abdrakhimov, Vernadsky Insti- tute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Russia, 117975, Moscow, Kosygina Ulitsa 19, [email protected].

This work is a continuation of the studies of [1] and studied region. It embays tessera and is embayed by [2] where analyzed the Magellan images of the Ven- younger plains (Pwr, Psh). Interpretation: Mafic lava eras' and Vegas' landing sites and concluded that all the locally deformed by compressional deformation. landers sampled Venusian plains of volcanic origin. FB — Fracture Belts material forms linear, radiat- Geological analysis of Venera 13 landing site made by ing and arcuate bands with densely spaced radar-bright [3] showed that Pwr and Pl materials are the most lineaments (fractures?). FB-forming fractures, when in probable units, analyzed by the lander. In this work we contact with Pwr plains, most of the belt fractures are used images with higher resolution (FMAP) than those embayed by this plains, but some of the fractures ex- used in our previous work (C1MIDRP) than led to new tend into Pwr plains deforming them. So FB unit is interpetation in some subareas. evidently made of older (pre-Pwr) and younger (Pwr) The goal of this work is to answer what is the stra- units. The small FB islands, several dozens of km, are tigraphic position of the surface material chemically observed at SE and NE parts of the mapped area. The analyzed by the Venera 13 lander, considering modern morphology of fracture belts suggests their formation knowledge about Venus geology. The area under study due to extensional faulting, so they may be old zones of (298.00-308.01º E, 4.00-12.01º S) is situated at west- rifting. ern part of Navka Planitia to the east of Phoebe Regio. Psh — Shield Plains material forms areas, several The landing circle is centered at 7.55º S, 303.69º E. dozens of km across, of abundant to coalescing small Radius of ellipse of probable error in the lander posi- shields with varying amounts of smooth plains in- tion is 150 km [4]. The geologic mapping of this area between. It embays Pfr and partialy Pwr and is em- was done using the Magellan FMAP mosaics (Fig. 1). bayed by younger plains. Sometimes Psh is deformed It led to identification of nine material stratigraphic by wrinkle ridges. The Psh area are observed at the units, which are correlative to those identified by [5, 6] eastern, central and SW parts of the mapped area. In- in other regions of Venus. The units (from older to terpretation: Fields of shield-like volcanic edifices. younger) are as following: Pwr — Wrinkle-Ridged Plains materials form Tt — Tessera material composes tessera terrain, smooth intermediate-dark to intermediate-bright plains which morphology is determined by two or more inter- complicated by the net of narrow wrinkle ridges. Wrin- secting systems of ridges and grooves of evident tec- kle ridges has primarily NWW trends at the northern tonic origin. Large tessera islands, several hundreds of parts and NNE trends at the southern parts of the km across, are observed at the NW part of the studied mapped area. Pwr is the most widespread unit in the area and some small islands, several dozens of km mapped area. Interpretation: Mafic lavas deformed by across, are observed at the central and southern parts. compressional deformation. Interpretation: Heavily deformed material of unknown Pl — Lobate Plains material forms radar-bright and nature. dark flow-like features often coalescing into a radial or Pdf — Densely Fractured Plains material forms fan-like pattern. It is not deformed by wrinkle ridges. linear, radiating and arcuate bands with dense radar- The fields of Pl in the mapped area associates with the bright lineaments organized into subparallel, radial rift-like zone at 288-301º E and slopes of volcano and/or concentric patterns. These bands are embayed domes at the eastern parts of the mapped area. Inter- by younger plains (Pwr). The cluster of Pdf islands, pretation: Mafic lava flows. several dozens of km across, is observed at the central RT — Rift Terrain material forms linear zones with part of the studied region. Interpretation: Mafic lavas densely spaced radar-bright rift-like fractures, several heavily deformed by extensional faults. hundreds of km longwise. The RT fractures cut Pl and Pfr — Ridged and Fractured Plains material forms Pwr plains, so they are younger than these units. The smooth plains of intermediate radar brightness with area of RT material is observed at the western part of small densely fractured inlets. It is locally warped into the mapped area. The morphology of rift terrains sug- broad linear NW trending ridges (thus forming Ridge gests their formation due to extensional faulting, so Belts). The Pfr islands, several dozens km longwise, they may be recent zones of rifting. are observed in the central and SW parts of the mapped Cu — Crater materials form impact crater walls, area. The small Pfr islands, several dozens of km floor, central peak, ejecta and outflows. Within the across, are observed at the SW and central parts of the mapped area two impact craters were identified. The Lunar and Planetary Science XXXII (2001) 1653.pdf

VENERA 13 GEOLOGY: A. M. Abdrakhimov

crater at 11.6º S, 302.2º E (D~ 30 km) is superposed (~90% of the ellipse area), the Pl lobate plains (~3%, at on the Pwr and Pl plains. The crater at 5.0º S, 303.8º E the SE part of the ellipse) and Psh shield plains mate- (D~ 30 km) is superposed on the Psh and Pwr plains, rial (~3%, at the western part of the ellipse). Pdf and is embayed by Pl material. Interpretation: Material densely fractured plains (~1% of the ellipse area), Pfr of impact craters and their ejecta. fractured and ridged plains (~1%), and Tt tessera ter- About 230 km SE of the estimated landing point at rain (<0.5%) are also present at the western part of the 8.9º S, 305.3º E there is a pancake dome, a volcanic ellipse. steep-sloped dome, Muru Tholus (9.3º S, 335.3º E, So if to assume that probability of landing on some D~25 km).Probably it was made of lava with high vis- unit is proportional to the percentage of its area within cosity. This dome is surrounded by Pl lavas, some of the landing ellipse, the most probable candidate for the them reached the SE edge of the landing site. material sampled by Venera 13 is material of the Pwr Geologic history of the mapped area is practically and less probable ones are the Pl or Psh. Panoramas the same as in the majority of other areas of Venus [5, taken by Venera 13 [7] show that the spacecraft landed 6]. The earliest event seen in morphologic records on rather smooth plains thus probably excluding land- probably was tessera formation due to intensive (here ing on Tt and Pdf. mostly extensional) deformation of material of an un- Acknowledgments: Author appreciates known nature. This was probably followed by em- Dr. A. T. Basilevsky for assistance and constructive placement of the Pdf mafic lavas and their dense exten- reviews of this work. sional fracturing. Next was probably emplacement of the Pfr mafic lavas and their local compressional de- References: [1] Basilevsky A. T. et al. (1992) formation into ridge belts. This was followed locally by JGR, 97, 16315-16335. [2] Weitz C. M. and Basilev- formation of fracture belts and fields of volcanic sky A. T. (1993) JGR, 98, 17069-17097. shields. Next was emplacement of the most widespread [3] Abdrakhimov A. M. And Basilevsky A. T. (1998) now mafic lavas of Pwr plains and their compressional Solar System Res., 32, 171-186. [4] Akim E. L. and deformation with the formation of wrinkle ridges. Then Stepanyanz V. A. (1993) Astron. Vestnik (in Russian). there was rifting resulted in the formation of bands of [5] Basilevsky A. T. and Head J. W. (1995) Earth, fractures and throughs. Probably eruption of smooth Moon, and Planets, 66, 285-336. [6] Basilevsky A. T. lobate lava flows (Pl) was associated with rifting. and Head J. W. (2000) Planet. Space Sci., 48, 75-111. The mapping showed that the Venera 13 landing [7] Basilevsky A. T. et al. (1985) Geol. Soc. Am. Bull., ellipse is dominated by the Pwr wrinkle ridged plains 96, 137-144.

Fig.1. Geologic map of the Venera 13 landing site. The circle represents the error radius in the lander position (7.55º S 303.69º E) Legend: 1. Tessera 2.Densely Fractured Plains 3.Ridged and Fractured Plains 4.Fracture Belts 5.Shield Plains 6.Wrinkle-Ridged Plains. 7.Lobate Plains. 8. Rift Terrains. 9.Crater Units. 10. Ridges. 11. Wide ridges. 12. Fractures. 13. Pancake volcano dome. 13. Gentle- slope volcano dome. 14. Corona-like features. 15. Landing ellipse.