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Aphrodite Terra, Venus: Characteristics Ofgeologi'3 Provinces

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Aphrodite Terra, Venus: Characteristics Ofgeologi'3 Provinces APHRODITE TERRA, VENUS: CHARACTERISTICS OFGEOLOGI'3 PROVINCES. W.3. Ehmann and J .W. Head, Dept, of Geol. Sci ., Brown Univ,, Providence, RI 02912. Aphrodite Terra is the largest of the three major upland regions on Venus. We have defined the base of Aphrodite to be the 6051.5 km elevation level, which is the mean planetary radius (mpr) (I), This choice delineates a contin- uous upland, twice the size of Africa, that covers 6.85 X lo7 km2 (15%) of the venusian surface, Aphrodi te trends domi nantly east-west for 22 ,I 80 km be- tween 45" and 255" longitude and ranges between 60" N and S latitudes. We have refined our earlier analysis of the topography of Aphrodite Terra (2) by incorporating data from hypsometric plots. The plots are derived from raw Pioneer-Venus altimetry data that has been filled in using a five degree boxcar filter. The filter interpolates between points nearest a data gap, taking into account the average elevation within the surrounding five degree area. In this study, maximum filling (37%) occurs in the Eastern Pinnacles province, while minimum filling (9%) takes place within Artemis Chasma province. Aphrodite Terra contains a variety of geologic features, including broad mountains , isolated peaks, individual and a1 igned pits , 1 i near and arcuate troughs, and circular structures. We establish six geomorphic provinces within Aphrodite (see Figure 1, ref. 2) using altimetry, surface roughness, and reflectivity data. The Western Highlands province covers 13.6 X 106 km2 as a continuous, elliptical mountain region bounded by steep slopes and marked by high surface roughness and reflectivity values. As shown in Figure 1, the province has an even distribution of topography with nearly equal percentages between the first three 1.5 km intervals above mpr. The Interior Highlands (15.9 X lo6 km2) province is twice the size of Australia and is characterized by a less continuous mountain area (Thetis Regio) set on a gently sloping NE/SW trending upland. The broad topography results in a shift of the hypsometric curve towards lower elevations (Fig. lb). RMS slopes are generally lower, and high reflectivity values are less common than in the Western Highlands. The Eastern Highlands (13.0 X lo6 km2) province contains two mountain districts set on a broad upland. A hypsometric curve (Fig. Ic) shows an even greater skewness toward rnpr than the Interior Highlands because of less extensive mountain regions. High reflectivity values are associated with Atla Regio, the most distinctive topographic prominence, and are moderate elsewhere. RMS slopes are variable in the province. Artemis Chasma (4.4 X lo6 km2) province has a circular outline (2600 km in diameter) defined by two arcuate troughs. Artemis Chasma proper spans 280" of arc, averaging 200 km wide and 1 km deep, and is flanked by ridges of varying height (3,4). The other trough lies to the north, is wider and deeper, and is not bounded by ridges. The province has a unimodal hypsometric curve (fig. Id) centered at 1 km above mpr. Unlike the highland provinces, Artemis has high rms slopes associated with low reflec- tivity values. A band of high rms slope trends west for 4000 km from the center of the province and may reflect the presence of a subdued chasma. The Central Chasma (11.6 X 106 km2) province contains a broad east-west trending arch cut by linear troughs and scattered pits. Dali Chasma extends 3600 km along the crest of the arch flanked by elongate ridges of variable height (3,4). Lowlands are symmetrical about the trough axis. Hypsometrically, the Central Chasma province (Fig. le) is nearly identical to Artemis Chasma province. Four quasi-circular outlines of high rms slope occur south of Dali , while re- flectivity is consistently low throughout the province. The Eastern Pinnacles (10.1 x lo6 km2) province is characterized by two intersecting trends of 300 km diameter peaks set on a patchwork of arcuate lows. The 8200 km N-S trend crosses the 5400 km E-W trend at 20°, 255'. Hypsometry shows that the O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Aphrodi te Terra: Geological Characteristics W. J. Ehmann and J. W, Head Pinnacles province resembles the A. WESTERN D. ARTEMIS provinces, except for being HIGHLANDS 11°1 CWSMA centered closer to mpr (at . +0.6 km) . Moderate rms slopes 20.0' 20.0- \ and low reflectivity values are aLT aLL 0- = 4 1a.0- = common. W aW a Several Aphrodi te provinces 4 4 w 12.0- W 11.0- are similar in morphology and U 0 elevation to Beta Regio, but are distinct from terrain in zg a.0- B ..- Ishtar Terra, Each province b b is roughly the size of the # 4.0- # 4.0- Tharsis region on Mars. We / are presently compi 1 i ng com- 0.0 , I 1 I I 00 I I 1 -2.0 0.0 rt.O 4.0 ta.0 -2.0 0.0 t2.O r4.0 *O parable plots of roughness KM FROM MPR KM FROM MPfl and reflectivity data and com- 0. "RIOR E. CENTRAL ""] CHASMA paring the provinces and fea- WGHLANDS tures within Aphrodite to 20.0 to~ I geologic structures on the other terrestrial planets in an effort to test hypotheses for the origin and evolution of the venusian surface. References: 1) H. Masursky et al. (1980) JGR 85, 8232.2). J. EhmK and J. W. Head (1982) Planet. Geol. Principal Investigators Mtg., Reston, VA. 3) G. KM FROM MPR KM FROM MPR Schaber (1981) LPI Cont, #457, C EASTERN F. EASTERN 31, 4) G, Schaber (1982) HIOILANDO *O] PINNACLES --GRL 9, 499, KM FROM MPR KM FROM MPR Figure 1. Hypsometric curves for pro- vinces in Aphrodite. Plots are derived from raw PV altimetry data that has been filled using a five degree boxcar filter. Maximum fill ing is 37% for the Eastern Pinnacles, while the minimum filling of 9% occurs for Artemis Chasma Province. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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