IMPACT-RELATED LOW-EMISSIVITY ANOMALIES ON VENUS S. L. Lawson1 and J. J. Plaut*; 'University of Colorado, Department of Astrophysical, Planetary, and Atmospheric Sciences, Boulder, CO 80309; 2Jet Propulsion Laboratory, MS 230-225,4800 Oak Grove Dr., Pasadena, CA 9 1109. Magellan radiothermal emissivity data were analyzed to find low-emissivity impact- related areas on the surface of Venus. Although low-emissivity parabolic crater-related features have been previously reported [I, 21, it was the goal of this study to conduct an independent detailed examination of lowemissivity areas and investigate the relationship between these areas and impact craters. In addition, while the previous investigations incorporated only Magellan Cycle 1 data, this work analyzed data from the first three Magellan data acquisition cycles. The reduced emissivity data used here had an absolute error of 0.02, with 0.005 variations detectable [3]. The Venus surface was divided into 34 subsections which were individually analyzed. Large low-emissivity regions were defined as totaling approximately half of each subsection. In addition, small lower-emissivity regions were subjectively defined as being the relative lowest- emissivity areas on each subsection. Up to four large and thirteen small areas were defined on each subsection. Plotted over these low-emissivity areas were 919 craters from Schaber et al. [4] and their additional unpublished data, and a few craters independently found from Magellan Cycle 3 SAR images. The low-emissivity areas were arranged into four categories: 1) clearly associated with impact craters; 2) likely associated with impact craters; 3) possibly associated with impact craters; and, 4) not associated with impact craters. The categorization of the areas was subjective, but required the agreement of both authors. Areas were generally considered to be clearly associated with an impact crater if the crater was in or to the east of the low-emissivity area; often the low-emissivity areas were related to more than one impact crater. Although the crater thought to be the primary cause of the low-emissivity area could often be easily identified, sometimes the relationship between the craters and the areas was ambiguous, with perhaps more than one crater being the cause of the anomalous emissivities. High elevation areas and crater floors, which are also known to have anomalously low emissivities, were not categorized. The average difference between the clearly crater-related emissivity lows and the emissivity of the surrounding terrain was 0.04. The largest value of this difference was 0.09, while the smallest was 0.01. The diameters of the craters clearly associated with the low- emissivity areas ranged in size from 2.5 km to 150 km; see Table 1 for a detailed listing of craters. The total number of craters clearly associated with anomalous low-emissivity areas is 88, with an additional 34 craters likely related to emissivity lows. These numbers include craters that lie within the defined low-emissivity areas, but perhaps were not the cause of them. It was found that not all the anomalous low-emissivity areas are parabolic shaped and neither do they all have corresponding SAR parabolas. Campbell et al. [2] reported finding eight craters associated with emissivity parabolas. The identification of over 60 low-emissivity features clearly or likely associated with impact craters indicates that these surfaces are more common and perhaps longer-lived than previously thought. When combined with extended ejecta deposits observable in SAR data [5], the fraction of craters that retain such signatures comprises a majority of the crater population. Impact events are clearly the dominant source of low-elevation low-emissivity anomalies; in many plains regions >50% of the surface has emissivities lowered by impact-related features. Enhanced dielectric constants appear to be the most likely cause of the low ernissivity values [6]. The precise physical mechanisms producing higher dielectric constants (compositional andlor density differences), and their implications for impact processes and target properties remain to be determined.

REFERENCES [I] Arvidson, R. E. et al. (1991). Science 252, 270-275. [2] Campbell, D. B. et al. (1992). JGR 97, 16,249-16,277. [3] Pettengill, G. H. et al. (1992). JGR 97, 13,091-13,102. [4] Schaber, G. G. et al. (1992). JGR 97, 13,257-13,301. [5] Izenberg, N. R. et al. (1992). Roc. Lunar Planet. Sci. Conf. XXIV, 703-704. [61 Plaut, J. J. and R. E. Arvidson (1992). JGR 97, 16,279-16,291.

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TABLE 1. Low-emissivitv areas clearlv associated with im~actcraters 1 Crates Latitude Longitude- Diameter I Crater Latitude Longitude- Diameter (ON) ('E) ) ("N) ('E) MAGNADI 58.60 337.00 26.6 DEWIlT -650 275.60 21.0 Kaaini 57.80 333.00 24.8 unnamed 25.11 285.28 12.0 RUTH 43.29 19.90 18.0 ROSE -35.15 248.20 15.0 Prichard 44.00 11.50 22.1 Moore -30.30 24830 21.0 FEDORETS 59.65 65.15 54.0 unnamed -30.30 249.40 10.0 ZHIL€)VA 66.53 125.50 56.0 Kanik -32.60 249.80 16.0 TSVETAYEVA 64.65 146.90 40.0 UNNAMED -13.00 328.00 35.0 YABLOCHKINA 48.27 195.15 63.0 CARSON -24.20 344.10 41.0 unnamed 48.30 191.40 5.0 Danilova -26.40 337.25 49.0 ANYA 39.50 297.80 20.0 Aglaonice -26.50 339.95 65.0 Lenore 38.70 29230 15.5 Saskia -28.60 337.15 37.0 UNNAMED 6.00 331.85 12.6 FRANK -13.10 12.90 23.0 UNNAMED 0.90 338.75 10.8 unnamed -6.00 6.00 75.0 UNNAMED 10.60 346.30 12.8 Xantippe -10.80 11.75 40.0 Rhoda 11.50 347.70 13.0 unnamed -11.20 13.50 7.2 NADINE 7.80 359.10 19.0 BATHSHEBA -15.10 49.35 36.0 Hellman 4.75 356.30 35.0 Gillian -15.20 49.95 15.5 unnamed 9.30 358.00 10.0 BASSI -18.95 64.70 33.0 UNNAMED 2.90 4.95 6.0 BOULANGER -26.55 99.30 62.0

UNNAMED 36.70 1.70 12.5 1 Trollo~e -54.80 246.40 26.0 unnamed 36.75 3.60 2.5 ~odi6 -56.10 251.60 32.0 EDGEWORTH 32.20 22.75 30.5 Delilah -57.85 250.50 18.0 Noreen 33.45 22.70 19.5 unnamed -58.70 252.25 22.0 ~nnamed 34.40 18.90 10.0 ABINGTON -47.75 277.80 22.5 unnamed 28.40 14.50 75 unnamed -49.00 275.80 11.0 UNNAMED 21.95 37.20 75 MEITNER -55.60 321.60 150.0 MIRIAM 36.50 48.20 15.0 SARTMA -63.40 67.10 28.0 RILEY 14.05 72.30 24.0 unnamed -61.90 70.90 12.0 ADIVAR 8.95 76.10 31.0 Lucia -62.10 67.80 17.0 unnamed 7.95 74.00 35 unnamed -61.00 53.50 6.0 Naorni 6.00 70.10 18.0 Berggolts -63.40 53.00 31.0 MERIT PTAH 11.30 115.65 18.5 Danute -63.50 56.50 14.0 unnamed 13.25 112.80 4.0 Rand -63.75 59.50 27.0 HIMIKO 19.00 124.17 36.5 GUAN -61.50 181.80 46.0 DAOSHENG BAN ZHAO 17.20 146.90 42.0 unnamed -62.20 17830 9.0 GREEN AWAY 22.95 145.00 92.0 Bryce -62.55 197.10 24.5 1 Marie Celeste 23.45 140.20 95.0 Vacarescu -63.00 199.60 29.5 ' DU CHA'IELET 21.50 164.95 18.5 EUDOCIA -59.05 201.80 27.5 unnamed 23.55 165.35 85 Banymore -52.30 19555 50.0 CACCINI 17.40 170.40 38.0 unnamed -53.20 19850 6.0 COWMAN 0.35 151.85 52.0 STOWE -43.20 233.00 82.0 BOLEYN 24.40 219.90 69.0 RUSLANOVA 84.00 16.50 45.0 Craters are separated into groups corresponding to a single low-emissivity area. Craters in upper case are believc to be the primary cause of the associated low-emissivity area, while the craters in lower case may be additional responsible. In some areas, there is believed to be more than one primary crater, while in other areas the prima crater is not easily identified.

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