So? and Ch4 Levels in the Venusian Atmosphere, Measured by Pioneer Venus: Caused by Plinian-Style Volcanic Activity .At Maat Mons? C.A

LPSC XYY 1139

SO? AND CH4 LEVELS IN THE VENUSIAN ATMOSPHERE, MEASURED BY PIONEER VENUS: CAUSED BY PLINIAN-STYLE VOLCANIC ACTIVITY .AT MAAT MONS? C.A. Robinson (Harvard-Smithsonian Center for Astrophysics, Cam- bridge, MA 02138) and G. D. Thornhill (Brown University, Providence, RI 02912).

Magellan (and PVO) found high mountain terrains on Venus ( -2.5 km above 6051 km planetary radius) exhibit anomalously low radiothermal emissivity [1,2,3]. This is thought to result from weathering of primary Venus rock, which at high altitudes produces a high- dielectric-constant, low-emissivity mineral assemblage [1,3]. Deviations from the nominal altitude-emissivity pattern have been used as a crude chronometric tool with which to date Venusian landforms [4,5]. This technique indicates Maat Mons (an unusually large, basaltic shield volcano. standing 9.17 km above 6051 km planetary radius. at 2.1°N. 194.3OE) has undergone a "recent" episode of large-scale volcanic activity [3,4]. The timescale for weathering defines "recent" to be anywhere in the past 2-3 My [5]. Recent volcanic activity is also implied morphologically with impact crater-free flanks, and young, medium-bright flow fields that emanate from the summit region and overlie all other landforms in their paths [4]. The presence of possible, stratigraphically-young ash flows on the northern flanks [3] and at the summit of Maat Mons [6] indicate the recent volcanic activity was explosive and, therefore, gas rich. We investigated whether a plinian eruption at Maat Mons could explain the enhanced concentrations of SO2 and CH, gas in the middle (-26 km above 6051 km planetary radius) and upper atmosphere (-50 km above 6051 km planetary radius) of Venus, as detected by the Pioneer Venus atmospheric probes [7,8,9, 101. Modelling of the eruption plume is based on the mathematics of (111 and adapted for the Venusian environment [12]. A plinian-style eruption at Maat Mons is considered appropriate since plinian eruptions are characterised by explosive activity at the vent. Our results show for a minimum vent radius of 156 m and minimum eruption temperature of 1200 K (Fig. I), a plinian eruption at Maat Mons can explain the anomalous SO2 and CH4 concentrations recorded by PVO. This would mean Maat Mons is an active volcano.

References: 1. Pettengdl, G.H. et al. (1988) J. Geophys. Res., 93, 14,881. 2. Garvin, J.B. et al. (1985) J. Geophys. Res., 90, 6859. 3. Klose, K.B. et al. (1992) J. Geophys. Res. 97, 16353. 4. Robinson, C.A., and J.A. Wood (1993) Icarus, 102, 26. 5. Robinson, C.A., and J.A. Wood (1993) JGR-Planets, submitted. 8. Robinson, C.A. et al. (1994)

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in prepamtion. 7. Donahue, T.M., and R.R. Hodges (1992) LPI, 789, 29. 8. Donahue, T.M., and R.R. Hodges (1993) Geophys. Res. Lett., 20, No. 7, 591. 9. Esposito, L.W. (1984) Science, 233, 1072. 10. Esposito, L.W. et al. (1988) J. Geophys. Res., 93, 5267. 11. Woods, A.W. (1988) Bull. Volc., 50, 169. 12. Thornhill, G.D. (1993) J. Geophys. Res., 98, No. E5, 9107.

Fig. 1. Graph of plume heights as a function of vent diameter and eruption temperature (1200 K and 1400 K). H20 is the magmatic volatile. 0 height datum represents the vent height - this is 9.17 km for Maat Mons.

TheonUal plum helghts tram Maat Mons summtt

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