Lena Fletcher (Geology Dept., Smith College, Northampton, MA 01063) and Heather Wright

(Geology Dept., Whitman College, Walla Walla, WA 99362); Eric Grosfils and Linda Reinen

(Geology Dept., Pomona College, Claremont, CA 91711), Martha Gilmore and Samuel Kozak

(Geology Dept., Washington and Lee University, Lexington, VA 24450)

Overview: The majority of volcanic features on Venus are thought to be of basaltic composition [1]. One exception to the uniformity of this composition may be the unusual festoon-type flow. Festoons, as defined by Head et al. [1], are "radar-bright flows that show organized patterns of internal streamlines (looped or curved components)analogous to the ridge and flow bands typical of viscous terrestrial lava flows known as coulees." Two festoon flows have been studied in detail: one in a lowland plains, the Artemis-Imdr Festoon [2], and one in a highland area, the Ovda Regio Festoon [3,4,5]. However, a third possible festoon, located in the plains region Atalanta Planitia in the northern hemisphere (latitude 69.8° to 70.8°N, longitude 200.9° to 203.1°E), has not yet been studied in detail [1]. Analysis of this flow and using Magellan data leads to the conclusion that its properties are similar to the characteristic properties of the festoon flows previously identified.

Methods and Data Collection: In order to identify the characteristic features of festoon flows, we tabulated specific quantitative attributes of the first two festoons (Table 1). Then, to facilitate comparison, we made the same measurements for the Atalanta flow. The tabulated characteristics of the festoons include data acquired during the Magellan mission, such as emissivity, reflectivity and RMS slope. We used Magellan altimetry data to determine the altitude, topography, and thickness of the flow. We also calculated north-south and east-west dimensions, the area, and the ridge spacing of the ogive-looking flow ridge features. Using the thickness and the area values we calculated the volume of the flow. Finally we determined bulk density, yield strength and viscosity using a combination of previous measurements and calculations utilized by the previous studies done on the identified festoon flows.

Discussion: Our analysis indicates that the Atalanta flow has characteristics both similar to and different from the Artemis-Imdr and Ovda festoons. The primary similarities between the Atalanta flow and the two festoons are:

· all three flows exhibit radar brightness

· ogive pressure ridges are present on all three flows and ridge spacing measurements are similar · thickness measurements for the Atalanta flow and Ovda festoon are most similar, but all three are fairly thick flows

· density measurements of the Atalanta flow are near the lower bounds of the other two festoons

· yield strength and viscosity measurements of the Atalanta flow fall within the range of the measurements of the other two festoons.

The primary differences between the Atalanta flow and the two festoons are:

· smaller area and volume of the Atalanta flow by an order of magnitude

· morphological differences in roughness of the Atalanta flow.

However, these differences do not describe properties intrinsic to a festoon. The quantitative and qualitative characteristics which strongly correlate with the other two festoons are more significant than the differences and indicate a more viscous evolved magma. On the basis of our measurements, we therefore conclude that the Atalanta flow is best defined as a festoon, making it the third such lava flow thus far identified on the surface of Venus.

[1] Head J.W. et al. (1992) JGR, 97, 13153. [2] Moore H.J. et al. (1994) JGR, 97, 13479. [3] Head J.W. and Hess P.C. (1996) LPSC, XXVII, 513. [4] Permenter J.L. and Nusbaum R.L. (1994) LPSC, XXV, 1067. [5] Schenk P.M. and Moore H.J. (1992) LPSC, XXIII, 1217. [6] Ford J.P. et al. (1993) Guide to Magellan Image Interpretation, NASA, JPL. [7] Olhoeft G.R. and Strangway D.W. (1975), Earth and Planetary Science Letters, 24, 394. [8] Moore H.J. and Ackerman J.A. (1989), Reports on Planetary Geology Geophysics Program 1988, NASA Tech. Memo., TM 4130, 387 [9] Izenberg N.R. and Arvidson R.E. (1992) LPSC, XXV, 605.

Acknowledgments: This research was funded as part of a grant from the Keck Geology Consortium during the summer of 1997. Thanks also to Robert Newton and John Brady in the Smith College Geology Department for their assistance and support.