Transport of SO2 by explosive volcanism on Venus

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E8, PAGES 18,899-18,906, AUGUST 25, 1999 Transport of SO2 by explosive volcanism on Venus Lori S. Glaze Proxemy Research, NASA Goddard Space Flight Center, Greenbelt, Maryland Abstract. Observations by the Pioneer Venus orbiter and many different types of analyses have suggested the possibility of contemporary explosive volcanism on Venus. The rise of volcanic eruption plumes on Venus is reexamined using recent improvements in buoyant plume modeling. The first-order model applied to Venus by previous authors features nonphysical discontinuousolutions for all of the model parameters and lacks internal consistency in the formulation of the governing equations. This makes it difficult to assess the validity of the Venus applications and conclusions derived from these models. The model used here contains several improvements including two corrections to the formulation and a change in the criterion for the transition of the plume from the jet region to the buoyancy-driven region. The model used in earlier works assumed a discontinuous transition between these two regions, resulting in an overestimate of the transition height as well as the maximum plume height. The effect of the transition criterion is magnified on Venus, where the continuous solution appears to have very little dependence on initial vent size. In contrast, the discontinuousolution shows a very strong dependence on initial vent size. The continuous solution used here indicates that plumes on Venus become dominated by buoyancy effects almost immediately above the vent. Use of the discontinuousolution, however, suggests that jets up to 10 km above the vent are possible for the boundary conditions considered. The combined effect of using the older model for conditions on Venus is a 5-8% overestimate of the maximum plume height for vent radii ranging from 20 to 250 m. The influence of latitude and elevation are also explored. For large eruptions on Venus, plumes rising in the Northern Highlands would rise much higher than plumes with identical boundary conditions erupted in the equatorial Lowlands. This is due to the greater stability of the upper atmosphere at higher latitudes and the sharp decrease in atmospheric pressure as a function of altitude. To examine the net effect of all the model assumptions and ambient influences, eruptions are simulated for a range of conditions at Maat Mons and compared with results in the literature. These simulations indicate that for small mass fluxes, the new model predicts smaller plumes than the older model. For larger mass fluxes, however, the new model predicts larger plumes than the older model. Because the Maat Mons summit elevation is already more than 9 km above the mean planetary radius, the reduced atmospheric pressure results in a plume with enough buoyancy to more than compensate for all of the model effects. These results continue to support the possibility that explosive eruptions on Venus may be capable of producing plumes that rise buoyantly to heights detected by the Pioneer Venus orbiter. 1. Introduction subject to extremely high temperatures (-470øC) and pressures (up to 9.8 MPa). This means that the relative density The Pioneer Venus orbiter deployed to Venus in 1978 de- difference between the erupted volcanic material and the amtected anomalously high concentrations of SO 2 at the top of bient is much less than on Earth. The high pressure also means the Venusian troposphere (-70 km above the mean planetary that it is much more difficult for volatiles to exsolve from the radius (ampr)). These initial measurements were followed by a magma [Wilson and Head, 1983]. In addition, Magellan data steady decline in measured concentrations over the next 5 have provided very little evidence that points to explosive volyears. The observations by Pioneer Venus indicated that there canism [Head et al., 1992]. Only a handful of fine-grained is an episodic process currently supplying SO2 to the upper deposits associated with volcanoes have been identified Venusian troposphere. One possible mechanism for explaining [Campbell, 1994; Campbell et al., 1998]. There is, however, such a phenomenon is explosive volcanism [Esposito, 1984; some very compelling evidence in support of modern volca- Moore et al., 1992; Robinson et al., 1995]. nism on Venus. First, a study by Fegley and Treiman [1992] Whether or not volcanoes can erupt explosively on Venus investigated the chemical interactions that occur between the has been the topic of debate for many years [Head and Wilson, planet's surface and its atmosphere. The results of this study 1986; Wood and Francis, 1987; Thornhill, 1993; Robinson and indicated that magma would have to be erupted at a rate of -1 Wood, 1993; Robinson et al., 1995]. The surface of Venus is km /yr in order to supply SO2 at a rate fast enough to keep up Copyright 1999 by the American Geophysical Union. Paper number 1998JE /99 / 1998JE with scavenging by the surface/atmosphere chemical processes. Another independent study by Solomon and Head [1982] suggested that even higher rates of volcanism are necessary to 18,899

2 18,900 GLAZE: TRANSPORT OF SO2 BY EXPLOSIVE VOLCANISM ON VENUS explain the Venusian thermal budget. Finally, Robinson et al. [1995] believe that Maat Mons may currently be active, based on anomalous emissivity patterns and morphology. Of course, these arguments do not necessarily require explosive volcanism, but they do indicate that the planet may currently be active and that magmatic volatiles do exsolve. The study conducted here has attempted to answer the question of whether or not explosive volcanic plumes on Venus would be capable of transporting SO2 to an altitude of 70 km ampr. Previous studies by Thornhill [1993], Robinson et al. [1995], and Kieffer [1995] have also investigated the possibility of explosive volcanic plumes on Venus. Kieffer used a numerical simulation approach based on the work of Wohletz et al. [1984] and Valentine and Wohletz [1989]. The numerical approach focuses on the dynamics during the first few (-5) minutes of an eruption, before it reaches steady state, and is helpful in illustrating the initial formation (or lack of formation) of a sustained, buoyant eruption column. Both the Thomhill [1993] and Robinson et al. [1995] studies were based on the Woods [1988] model for eruption column dynamics, with the exception of a correction for the specific heat of the solid particles (G. D. Thornhill, personal communication, 1993). The Woods model, however, lacks internal consistency in the formulation of the governing equations and contains nonphysical, discontinuou solutions for all of the model parameters [Glaze and Baloga, 1996; Glaze et al., 1997]. This makes it difficult to assess the validity of the Venus applications and conclusions derived from these models. Glaze and Baloga [1996] and Glaze et al. [1997] have recently developed a new model for eruption column dynamics that provides numerous improvements to the Woods approach. In this paper, the Glaze et al. [1997] model is modified to include a gas thrust region and to account for ambient conditions on Venus. To illustrate the differences between the Woods and 2. The Model Over the past 20 years there have been essentially two basic types of models applied to volcanic plumes in order to describe their physical motion: those based on systems of first-order ordinary differential equations, and complex numerical models that attempt to solve the Navier-Stokes system of partial differential equations for two-phase flow within the plume. All of the first-order models of buoyant volcanic plume dynamics [Wilson et al., 1978; Sparks and Wilson, 1982; Sparks, 1986; Woods, 1988, 1993; Glaze and Baloga, 1996; Glaze et al., 1997] are based on the original work of Morton et al. [1956], who investigated the buoyancy and ambient entrainment processes that control convective plume rise. These first-order systems represent integrals over the important terms of the Navier- Stokes formulation and have been validated by laboratory experiments [Morton et al., 1956; Turner, 1979] and numerous terrestrial studies [e.g., Briggs, 1969]. In addition to mass and momentum conservation, these models address, in various ways, thermal dynamic effects due to entrainment of ambient gases, adiabatic expansion, and condensation. Solutions of the system of ordinary differential equations are a function of the boundary conditions for initial plume radius, velocity, density, volatile content, and temperature. Because buoyancy is ultimately driven by the density difference between the plume and the surrounding fluid, ambient atmospheric conditions also play a very important role in plume rise [Glaze and Baloga, 1996]. In order to eliminate some of the limitations inherent in the first-order models, Wohletz et al. [1984], Valentine and Wohletz [1989], and Dobran et al. [1993] have presented models that consider the full two-dimensional system of two-phase Navier- Stokes equations for compressible fluids. Over the last 10 years, this numerical simulation approach has appeared in a variety of volcanic plume applications [Wohletz and Valentine, 1990; Valentine et al., 1991; Neri and Dobran, 1994]. While these complex models may be able to overcome some of the limitations of the first-order models, they are computationally Glaze approaches, they are each applied to a set of common boundary conditions on Earth as well as potential eruption intensive and inappropriate for the sustained, steady state sitconditions on Venus. The net effect of using the Woods apuation of interest here, where plumes are transported through proach is an overestimate of the height at which buoyancy the entire troposphere of Venus. In addition, the numerical begins to dominate the plume dynamics as well as the maxisimulation conducted by Kieffer [1995] for conditions on Venus mum height attained by the plume. Kieffer [1995] has considconsiders only one set of eruption parameters. Under these ered only the case of a small CO2-driven eruption on Venus, specific conditions, the eruption does not generate a buoyant and her results are consistent with those of the model precolumn. Because the primary objective of this study is to desented here. Therefore this study concentrates on the differtermine what eruption conditions are required to transport ences between the Woods and Glaze approaches and the vol- SO2 to the top of the Venus troposphere by a steady state canic conditions that could explain the Pioneer-Venus eruption column, the rest of this discussion will focus on the observations. first-order approach. Glaze and Baloga [1996] concluded that the atmospheric The model used in this study is a first-order model based on lapse rate can have a significant influence on the rise height of the approach described by Glaze et al. [1997](notation is given buoyant plumes. The second objective of this study is to exam- in Table 1). The interested reader is referred to Glaze et al. for ine the effects on predicted plume height of different ambient the complete derivation of the system of first-order ordinary conditions resulting from variations in latitude and elevation. differential equations and a detailed discussion of the model It will be shown that plumes erupted at the higher latitudes are significantly affected by the more stable atmosphere above 40 km ampr, rising at least 8 km higher than identical plumes erupted in the equatorial region. Finally, the net effect of all the modeling changes is explored for conditions on Venus. An eruption at Maat Mons is simulated and compared to analogous results of Robinson et al. [1995]. From this comparison, it can be seen that Robinson et al. systematically overestimate sensitivities. The self-consistent system consists of conservation equations for the mass of each plume component (ambient air, magmatic volatile, solids, and condensed liquid), momentum of the bulk plume, and thermal energy of the bulk plume. The only difference between the system of equations presented in (1)-(7) of Glaze et al. [1997],and those used here for the buoyant portion of the plume is that plumes on Venus would most likely not have a significant condensed, "liquid" compoplume heights for small mass fluxes and underestimate plume nent. Therefore the Glaze et al. condensation rate is set to heights for larger mass fluxes. zero. For this work, it has been assumed that the erupted

3 GLAZE: TRANSPORT OF SO2 BY EXPLOSIVE VOLCANISM ON VENUS 18,901 material consists of solid particulates and some magmatic volatile (either H20 or CO2). The mass flux of the solids and magmatic volatile within the control volume do not change with height. The ambient atmosphere on Venus below 100 km altitude is assumed to be composed of a variety of gaseous species based on spectroscopic observations and in situ measurements (see section 4 below). This ambient gas mixture, which includes some fraction of the species assumed to be the magmatic volatile, is entrained at each step along the rise height. It should be noted that the model formulation assumes that all of the plume components move confluently (no drag) and that no mass is lost as a result of sedimentation or rainout. The final modification made here to the Glaze et al. [1997] model is the inclusion of a gas thrust, or jet, region at the base of the plume. In contrast to the convecting plumes that are driven primarily by buoyancy, plume rise in the jet region is dominated by momentum. Wilson [1976] was the first to model a volcanic plume as a jet, and Woods [1988] later combined the jet and buoyancy models. Following the Woods approach, the mass flux in the jet region can be defined as d ur dz (pbur2) = - (PaPB) /2. (1) When solving the system of equations, this definition for the mass conservation is used in place of (5) by Glaze et al. [1997] until the plume transitions to a convecting column. The point at which this transition takes place is discussed in more detail in the following section. 3. Comparison and Discussion Two recent efforts to look at explosive volcanism on Venus have come from Thomhill [1993] and Robinson et al. [1995]. Both of these studies used the Woods [1988] model for volcanic plume rise. The model used in this study differs from Woods [1988] in three distinct ways, two involving the thermal energy conservation definition and one involving the transition between the momentum-driven and buoyancy-driven regions. These differences and their consequences will now be dis- cussed in turn. First, as noted by Glaze and Baloga [1996], the term in the Woods [1988] model that describes the cooling of the control volume due to entrainment of ambient air is inconsistent with the momentum conservation defined for the system. The second difference between the model presented here and the Woods model is also in the thermal energy conservation def- Table 1. Notation Variable no a R R u o Definition bulk atmosphere specific heat on Venus mass fraction of constituent i maximum plume height gas mass fraction of magmatic volatile plume radius bulk atmosphere gas constant on Venus universal gas constant (= J K - mol - ) bulk plume velocity vertical distance bulk plume temperature molecular weight bulk atmospheric density bulk plume density Table 2. Effect of Variations in the Thermal Energy Conservation Definition H, km ro, m Glaze et al. [1997] Entrainment Adiabatic Both inition. The adiabatic cooling term in both the Woods [1988] and subsequent Woods [1993] models has been applied to the partial density of the gas phase (the density that the gas would have if it occupied the entire control volume, e.g., no particles) within the plume as opposed to the actual density. Of course, as the plume expands adiabatically and entrains additional gas, the relative volume occupied by the solids becomes very small. Table 2 shows predicted plume heights for some typical boundary conditions on Earth and several variations on the thermal energy conservation term. The boundary conditions used in each of the model runs assumed u o = 300 m s - 0o = 1000 K, n o = 0.03 (where the volatile was water vapor), Zo = 0 km, and r o is defined in the first column. For illustration purposes the plumes were erupted into a summertime tropical atmosphere, and no condensation was allowed. The maximum plume heights predicted by the Glaze et al. [1997] model are presented in the second column. The third column shows how the predicted plume heights are affected by changing only the entrainment term in the thermal energy equation to agree with Woods' [1988, 1993] expression. It can be seen that using the Woods entrainment term results in an overestimate of the maximum plume height by -4-7% in these cases. As stated above, the effect of using the partial density should be very small, and Table 2 verifies that this is indeed the case. The maximum plume heights presented in the fourth column for plumes where only the Woods adiabatic cooling term has been used are, in fact, indistinguishable from those predicted by the Glaze et al. [1997] model. The final column in Table 2 shows the cumulative results of using Woods' entire thermal energy expression. As expected, only the entrainment error can be detected by looking at the maximum predicted plume heights. The third major difference between the Woods approach and the model used here is the definition of the transition between the momentum- and buoyancy-driven regions of the plume. While the expression for the conservation of mass flux given in (1) is identical to that used by Woods [1988], the transition between the jet and buoyancy regions is defined very differently here. The Woods [1988, 1993] models both assume that the plume becomes buoyant at the point where the bulk plume density drops below the ambient density. While, at first sight, this seems to be a logical assumption, it is not allowed by a self-consistent formulation of the governing equations. Under this assumption, the solutions for all of the variables (e.g., velocity, radius, and temperature) are discontinuous across the boundary, which is not acceptable in the physical world. The method used here for defining the jet/buoyancy boundary ensures that all of the variables have continuous solutions, and is defined in the following way. It is assumed that the plume is initially described by the jet model for entrainment. At each

4 18,902 GLAZE: TRANSPORT OF SO2 BY EXPLOSIVE VOLCANISM ON VENUS Table 3. Comparison of Definitions for the Transition Between the Jet and Buoyancy Regions Discontinuous Boundary Continuous Boundary ro, m Transition, km H, km Transition, km H, km time step the resultant values from the jet model for each plume variable are input into both the jet model and the buoyancy model. The transition point is defined as the height where all of the variables converge for both models, thus ensuring a continuous progression across the boundary. To illustrate the difference between these two approaches, they have each been applied to the set of Earth boundary conditions used for Table 2. Table 3 presents results for plumes erupted into the U.S. Standard Atmosphere with initial radii as shown in the first column. The second and third columns show = = 0.5 o.o Figure 1. Comparison of the two methods for defining the transition between the jet- and buoyancy-driven regions. The Earth boundary conditions used in both cases were u o = 300 m s -, 0o = 1000 K, no , and ro shown along the x axis. Note that the use of the discontinuous transition criterion results in an overestimate of the maximum plume height (Table 3) due to an overestimate of the transition height. buoyancy under most conditions [e.g., Thomhill, 1993]. The continuou solution lowers the transition region, thus making it easier for plumes to move into the buoyancy controlled region. As a note, it should also be pointed out that according to the CRC Handbook of Chemistry and Physics [Wenst, 1985], the specific heat of steam is J kg -1 K -1, not 1400 J kg -1 K -1 as defined by Woods [1993] or 1617 J kg -1 K -1 defined by Woods [1988]. Also, it should be noted that Woods [1988] has taken the specific heat of the solid particles in a volcanic plume to be 1617 J kg -1 K -1 as opposed to the value of 1100 J kg - K -1 used by Sparks [1986] or the 920 J kg -1 K -1 used in this study [Riehle, 1973]. Because buoyant plume rise is strongly controlled by the thermal energy within the plume, the choice of specific heat can have a significant effect on the predicted plume heights. The choice of a specific heat that is much too large results in the plume retaining its thermal energy better and thus rising unrealistically high. 4. Buoyant Plumes on Venus Table 4 summarizes the most abundant chemical species present in the Venus troposphere [yon Zahn et al., 1983]. In addition to the percentage of the atmosphere (by mass) occupied by each constituent, its molecular weight and specific heat the model results for the discontinuousolution (Woods' approach) and the last two columns for the continuou solution, are also given. The molecular weight has been used to deterwhere everything else is the same. In each case, both the height mine both the bulk specific heat and gas constant for the at which the transition between the jet region and the buoyancy ambient atmosphere by using the definitions region occurs and the maximum height attained by the plume are given. Figure 1 illustrates the transition height results graphically. It can be seen that the transition height for the discontinuousolution is considerably larger than that for the continuous solution. This indicates that, although the bulk and plume density is greater than the ambient, the plume dynamics are described by the buoyancy model. C a : flc1 -n t- f2c2 -n t-...nt- fici, (3) For the particular cases shown here, the effect of the differ- where f/is the mass fraction, i is the molecular weight, and Ci ent transition heights on the final predicted plume heights is is the specific heat for a particular constituent. The universal negligible. However, it becomes significant for predicting what gas constant is given by R u and has a value of J K - conditions result in collapsed columns. The criterion often mol -. Using these expressions, the gas constant and specific used to determine when column collapse occurs is whether or heat for the bulk ambient atmosphere below 100 km are R a = not the plume achieves buoyancy (rises beyond the transition 191 J K- kg- and Ca = 835 J K- 1 kg- 1, respectively. height). This becomes particularly important on Venus, where For comparison with the Woods approach used by both claims have been made that explosive plumes cannot achieve Thomhill [1993] and Robinson et al. [1995], an eruption with a vent elevation of ---5 km ampr has been simulated. Figures 2a and 2b show the temperature and pressure profiles for the globally averaged model of the Venusian atmosphere [Seiff, ] through which the plumes are erupted. The boundary conditions used were an initial gas mass fraction of magmatic Ra = ( f ILl +f2+ + f,)r (2) water vapor of 5%, an initial velocity of 270 m s- an initial temperature of 1400 K, and initial vent radii ranging from 20 m to 250 m. For investigative purposes, water vapor was chosen as the magmatic volatile despite evidence that the atmosphere Table 4. Molecular Weights and Specific Heats for the Most Abundant Chemical Species in the Venus Atmosphere Atmospheric Molecular Weight Specific Constituent Weight, g mol- * Percent Heat, J K- * kg- * CO N SO H CO H2S

5 GLAZE: TRANSPORT OF SO 2 BY EXPLOSIVE VOLCANISM ON VENUS 18,903 on Venus is extremely dry [Fegley and Treiman, 1992] and that CO2 is more likely the primary magmatic volatile species [Head and Wilson, 1986]. This choice is based on the conclusions of Thomhill [1993] and Kieffer [1995] that magmas with CO2 as the primary volatile would not form convecting eruption columns. These conclusions are in agreement with results of the model presented here. As part of a complete discussion of boundary conditions for explosive plumes on Venus, Thornhill [1993] also concluded that plumes with initial temperatures of 1200 K would collapse and form pyroclastic flows unless the,-, = initial heat flux was greater than 2.05 x 10 ls J s - (equivalent to an initial radius of 175 m for the boundary conditions listed above) and that plumes with initial temperatures below 1000 K Figure 3. Comparison of the net effects of the Woods apwould not convect at all on Venus. proach with the Glaze approach used here. The Venus bound- Figure 3 compares maximum plume height results for both ary conditions used for both models were u o = 270 m s -, approaches. Because both approaches are based on the origi- 0 o = 1400 K, no = 0.05 (water vapor), and r o shown along the nal Morton et al. [1956] buoyant plume formulation, the two models are in reasonable agreement, despite significant inconsistencies in the Woods approach. The results from the model x axis. Note that the Woods approach has overestimated the maximum plume height due to both of the issues discussed in the text. used here continue to support the earlier findings of Head and Wilson [1986] that volatile contents in excess of 4 wt % are required to generate explosive volcanic plumes capable of ris- heights seen on Venus is primarily due to the use of the ing to the top of the troposphere. All of these studies, however, discontinuousolution across the boundary (note when comcontradict the contention of Sugita and Matsui [1992] that paring Figure 1 for Earth and Figure 4 for Venus that the much lower mass and heat fluxes are necessary to drive a vertical scales are not the same). It can be seen that, for the plume in excess of 45 km ampr. continuous solution, the altitude of the transition point is fairly The results shown in Figure 3 indicate that the Woods ap- insensitive to the initial radius (or mass flux). The discontinuproach predicts plume heights that are consistently too high by ous transition height, however, is systematically higher than the about 5-8%. The extra altitude gained by those plumes is due continuous solution and exhibits a significant dependence on to the combined use of the Woods [1988] ambiguous thermal initial radius. Use of the discontinuous solution implies that energy definition and the criterion for the discontinuous tran- explosive jets of ash can extend up to 10 km above the vent on Venus. This means that for the discontinuous solution the sition. Figure 4 shows that the larger difference in plume plume is much higher when the buoyancy model begins describing the plume and that, consequently, the final plume is somewhat higher. 50 The fact that the Woods approach overestimates plume heights by 5-8% is an important result of this study in the 40 sense that the difference is relatively small. The nonphysical discontinuou solutions and the lack of internal consistency in the Woods model make it difficult to assess the validity of the. 20 previous Venus applications and conclusions derived from the model. The analyses presented here clearly illustrate the rela- < 10 tive magnitudes for each of the issues discussed above as well as the extent to which their relative magnitudes are magnified 0 or minimized for conditions on Venus. Based on these analy Temperature (øc) 0,.. i... i... i.., i.., 40, 3o Woods...'".. 2O < i0 ß..-'"' Glaze Pressure (MPa) Figure 2. Globally averaged (a) temperature and (b) pressure profiles for the Venus atmosphere as taken from Seiff [1983] Figure 4. Comparison of the two methods for defining the jet/buoyancy transition for conditions on Venus. The boundary conditions used in both cases were u o = 270 m s -, 0o = 1400 K, no = 0.05 (water vapor), and ro shown along the x axis.

6 ... 18,904 GLAZE: TRANSPORT OF SO2 BY EXPLOSIVE VOLCANISM ON VENUS '= 40 < 20 high 1ow latitude ' Temperature (øc) Figure 5. Plot of the latitudinal dependence of atmospheric temperature on Venus. Temperature profiles for both low and high latitudes, as given by Seiff[1983], are shown. The pressure gradients for both cases are very similar to the globally averaged values shown in Figure 2b. ses, the conclusions of Thornhill regarding boundary conditions for explosive plumes on Venus should still be valid. Of course, the primary objective of this study was to investigate the conditions under which a volcano could transport SO2 to the top of the Venus troposphere. In light of the conclusions of Glaze and Baloga [1996] regarding the sensitivity of buoyant plumes to the ambient surroundings, consideration has been given to the effects of varying the atmospheric lapse rate on the maximum heights of explosive volcanic plumes on Venus. Previous theoretical studies of explosive volcanism on Venus (such as the one used by Thomhill [1993] and Robinson et al. [1995] have used the very simple globally averaged temperature and pressure model shown in Figure 2, derived from four Pioneer Venus probes sent into the Venusian atmosphere. While it is generally thought that the Venusian atmosphere does not have the kinds of extreme local variations that occur on Earth, there does appear to be some latitudinal dependence. Seiff [1983] reported that atmospheric temperatures at higher latitudes (above 60 ø ) between 40 and 60 km ampr may decrease much more quickly than at middle and equatorial latitudes at the same altitudes. This steeper lapse rate is indicated by both the one Pioneer Venus probe that made its descent at 59øN and radio occultation data from the Pioneer Venus orbiter. Figure 5 shows the temperature profile for both low and high latitudes taken from Seiff [1983]. The pressure profiles for both cases are very similar to the globally averaged values shown in Figure 2b. The atmospheric lapse rate is very important for volcanic plume rise and hence SO2 transport. Essentially, this means that a plume released at higher latitudes that is able to rise to the lower boundary of this layer (---40 km ampr) will then be able to rise much more easily than if it were released at middle or equatorial latitudes. This indicates that volcanoes erupting explosively with identical initial conditions would produce the highest plumes at latitudes greater than 60 ø. While some of the highest elevation terrain on Venus is at latitudes greater than 60øN [Ford and Pettengill, 1992], it should be noted that there is very little evidence of explosive volcanism at these northern latitudes. The elevation of a volcanic vent can also influence the final height of a buoyant plume. There are two consequences of eruption at higher elevation, the most obvious being that if a plume starts out higher, it should end up higher. This is the effect accounted for by Robinson et al. [1995] when they add the initial vent elevation to the final plume height. However, in addition to this effect, the atmospheric pressure on Venus drops off very quickly with elevation so that a plume released at 8 or 9 km ampr is subjected to only about 5 MPa pressure as opposed to 9 MPa at mean radius. This again adds to the rise potential of an explosive volcanic plume. The ability of a plume to rise buoyantly is driven solely by the density difference between the plume and its surroundings. A lower ambient density can play a significant role in a plume's ability to become buoyant. As an example, if a volcano in the Maxwell Montes region (a region in the northern hemisphere, at about 60øN, with some of the highest topography on Venus) were capable of producing an explosive eruption, it would have a strong potential for producing a comparatively high plume. Likewise, Maat Mons (considered by Robinson et al. [1995]), with a summit elevation in excess of 9 km ampr, is an excellent candidate for producing an explosive eruption plume that can rise buoyantly. The higher a plume is able to rise in the atmosphere, the easier it is to explain the accumulation of SO2 at the top of the troposphere. Using the atmospheric temperature and pressure profiles for the low and high latitudes, the combined jet/buoyancy model has been run for u o = 270 m s -, no = 0.05 (water vapor), 0 o = 1400 K, and z o = 10 km. Figure 6 shows the results for plumes that rise to heights greater than 40 km ampr where the difference between the two atmospheric models is evident. It can be seen that the plume erupted at the higher latitude is significantly affected by the more stable atmosphere, rising more than 8 km higher than an identical plume erupted in the equatorial region. It can also be seen that this effect is somewhat delayed and is not apparent in those plumes that rise only a few kilometers over 40 km. However, there would be very little difficulty in explaining a volcanic injection at 70 km for those plumes that rise through the more stable region. This result again supports the conclusions of Glaze and Baloga [1996] that the atmospheric stability has a strong effect on the final plume height. Figure 7 shows the cumulative effect of all the factors discussed here. The Robinson et al. [1995] data shown in Figure 7 are for an eruption at Maat Mons. Also shown are the results of the model discussed here. The boundary conditions used in m z ',.L w, lalitudc Figure 6. Comparison of maximum predicted plume heights using the low- and high-latitude atmospheric profiles. The boundary conditions used were u o = 270 m s- 1, 0o = 1400 K, no = 0.05 (water vapor), and r o shown along the x axis. Note that the increased atmospheric stability in the km altitude region at high latitudes is capable of boosting the plume an additional 8 km for the 250 m vent.

7 ! GLAZE: TRANSPORT OF SO2 BY EXPLOSIVE VOLCANISM ON VENUS 18,905 both studies are r 0 ranging from 20 to 300 m, K, u0-270 m s -1, z 0 = 9.17 km ampr, and no = 0.05 water vapor. For small vent radii resulting in plumes that are barely buoyant, the Robinson et al. results overestimate the maximum plume height. This is the expected result based on the entrainment and transition issues described in section 3. However, for larger vent radii, the greater buoyancy of the plumes initiated at 9.17 km (as opposed to adding 9.17 to the height attained by a plume released at mpr) clearly outweighs the model effects, resulting in maximum plume heights even higher than those predicted by Robinson et al. 5. Conclusions In this work, a new model based on the Glaze et al. [1997] approach has been used to reexamine the buoyant rise of explosive volcanic eruption plumes on Venus. This new model contains several improvements over the Woods [1988] model used in previous studies of explosive volcanism on Venus. The model used here has corrected the formulation incon- mum plume height for vent radii ranging from 20 to 250 m. The effects of latitude and elevation have also been ex- plored. For large eruptions on Venus, plumes rising in the Northern Highlands would rise much higher than identical plumes erupted in the equatorial Lowlands. This is due to two effects. The first is that the upper atmosphere is more stable near the poles, resulting in a greater buoyancy effect for plumes that rise through that region. Second, vent elevation plays an important role in a plume's ability to become buoyant because of the sharp decrease in atmospheric pressure as a function of altitude.,. 60 = Thi Figure 7. Comparison of maximum plume heights predicted by Robinson et al. [1995] with those predicted by the model presented here. Both models have been used to simulate an explosiv eruption at Maat Mons with the boundary conditions u 0 = 270 m s -1, 0 = 1400 K, n o = 0.05 (water vapor), z 0 = 9.17 km ampr, and r 0 shown along the x axis. sistencies in the Woods [1988] model that result in a 4-7% overestimate of plume heights on Earth. This new model also These results continue to support the possibility that exploredefines the criterion for the transition between the jet and sive eruptions on Venus may be capable of producing plumes buoyancy regions. The Woods discontinuous transition critethat rise buoyantly to heights detected by the Pioneer Venus rion results in an overestimate of the transition height as well orbiter. The boundary conditions used to simulate such a as the maximum plume height and is magnified on Venus. The plume are an initial vent radius of 300 m, an initial bulk plume continuou solution appears to have very little dependence on temperature of 1400 K, an initial bulk plume velocity of 270 m initial vent size, whereas the discontinuous solution shows a s -1, a vent elevation of 9.17 km ampr, and an initial magmatic very strong dependence on initial vent size. The continuous water content of 5 wt %. It may be, however, that large buoyant solution used here indicates that plumes on Venus become plumes are not the only way to transport SO2 and that other dominated by buoyancy effects almost immediately above the possibilitieshould be considered. The Venusian troposphere vent. The Woods approach, however, suggests that jets up to is well mixed, and it may be possible that circulation such as the 10 km above the vent are possible for the boundary conditions Hadley Cell circulation on Earth could be capable of transconsidered. The combined effect of using the Woods approach porting volcanic material from smaller eruptions or from pasfor conditions on Venus is a 5-8% overestimate of the maxi- sively degassing vents through the troposphere [Crisp et al., 1991]. The key to such transport would be the timescale of the circulation. It would be imperative that the transport be completed within the lifetime of the combined SO2/H2SO 4 cycle. On Earth, the complete conversion of volcanic SO2 to H2SO 4 can take several months (SO was observed by satellite instruments more than 170 days after the Pinatubo eruption in 1991 [Read et al., 1993]). Another possible mechanism for transporting volcanic SO might be by co-ignimbrite eruption plumes that result from collapsing eruption columns. Woods and Wohletz [1991] have shown that co-ignimbrite plumes are capable of rising to great heights because most of the larger To examine the net effect of all these influences, an eruption particles are sedimented out during pyroclastic flow, leaving was simulated for conditions at Maat Mons and compared to only very fine particles and hot gas. results presented by Robinson et al. [1995]. These simulations indicate that for smaller initial mass fluxes (smaller vent radii), the plumes produced by the model presented here do not rise Acknowledgments. L. Glaze would like to thank the Geodynamics as high as the Robinson et al. plumes. Robinson et al. have Branch at Goddard Space Flight Center for office space and continued support. Thanks also to S. Baloga for constructive comments throughoverestimated these plume heights due to the cumulative efout this work. This effort was funded by the NASA Venus Data fects of the inconsistencies the Woods [1988] model. 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