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Venus: Concentrations of radar-reflective minerals by wind
ICARUS90, 123--128 (1991)
Venus: Concentrations of Radar-Reflective Minerals by Wind RONALD GREELEY,* JOHN R. MARSHALL,*'t DREW CLEMENS,:t: ANTHONY R. DOBROVOLSKIS,*'t AND JAMES B. POLLACKJ"
*Department of Geology, Arizona State University, Tempe, Arizona 85287-1404; and ~NASA-Planetary Geology Program and tSpace Sciences Division, National Aeronautics and Space Administration, Ames Research Center, Moffett Field, California 94035-1000 Received February 28, 1990; revised October 22, 1990
(1982) indicate that pyrite may be stable at the lower Areas of high radar reflectivityand low emissivityon Venus may atmospheric pressures and temperatures found at high be covered with windblown material rich in Fe and Ti, such as elevations on Venus, but not at lower altitudes. Head et ilmenite, a mineral common in some basaltic lava flows. Experi- al. (1985), Garvin et al. (1985a,b), Garvin and Head (1985), ments under Venus-like conditions show that dense mineral grains and Garvin (1985) have argued from geological considerabecome concentrated on the surfacesof windblownsedimentswhen tions that large amounts of pyrite are not likely on the subjected to the low winds typical for Venus. ©1991AcademicPress, Inc. surface of Venus, but suggested instead that other minerals enriched in Fe, Ti, or Mn could account for the observations. Pieters et al. (1986) have reported spectral eviINTRODUCTION dence for hematite (Fe203) in the soil of Venus, while Pioneer Venus radar data at a wavelength of 17 cm Head et al. (1985) pointed out that some lunar basalts are reveal localized regions of unusually high reflectivity (Pet- rich in ilmenite (FeTiO3), and that analogous lava flows tengill et al. 1982, 1983, 1988) associated with high eleva- may cover the radar-reflective regions on Venus. Head et tions, such as Maxwell, Rhea Mons, and Theia Mons, al. (1985) also suggested that "secondary occurrences most of which are believed to be volcanic terrains. Petten- of high-dielectric materials might arise from erosion and gill et al. (1982) suggested that "the highly reflecting re- concentration in lag deposits" and, in particular, t h a t " eogions contain a significant amount of conducting material lian a c t i v i t y . . , might concentrate materials on the basis as inclusions in the rock," while Pettengill et al. (1988) of their density." In the absence of flowing water, wind may be an imestimate the conducting fraction at about 10% by volume. portant agent in transporting and sorting materials on Ford and Pettengill (1983) analyzed Pioneer Venus obserVenus (Greeley et al. 1984a). Despite relatively sluggish vations of 17-cm thermal emissions from Venus and found winds near the surface (Counselman et al. 1979, Ksanfothat areas of unusually low radio emissivity also corremality et al. 1983, Von Zahn et al. 1983), the high density lated with the highlands. They concluded that the presence of conductive inclusions could account for the low of the Venus atmosphere permits even gentle winds to emissivities as well. Venera 13 and 14 measurements of initiate movement of loose particles. Theoretical models of the circulation also indicate that enhanced winds over the DC resistivity of the Venus soil also suggest a high slopes produce the greatest rates of aeolian erosion (and, concentration of conductive minerals on the flanks of thus, particle sorting) at the higher elevations (DobroPhoebe Regio (Kemurdzhian et al. 1983). Jurgens et al. volskis and Saunders 1986, Saunders et al. 1990), which (1988) found evidence for high-reflectivity spots at lower have high reflectivities as well. Our experiments demonelevations on Venus from Earth-based radar data at a strate that aeolian processes can produce lag deposits of wavelength of 12.5 cm. By interpreting these rings as dense conductive material on the surface of Venus, and bright crater ejecta and comparing them with the fraction that such deposits need be only a few centimeters thick of radar-bright haloes in Venera 15/16 crater counts, Jurto explain the observed local enhancements of radar regens et al. estimate that the reflective material has been flectivity. chemically stable for the past 250 million years. What minerals could affect the conductivity of the EXPERIMENTS Venus surface this way? Pettengill et al. (1982, 1983) proposed pyrite (FeS2) as a candidate material on chemical Laboratory experiments were conducted to assess the grounds. Thermodynamic models of Nozette and Lewis effectiveness of wind in concentrating certain minerals 123
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GREELEY
TABLE I E x p e r i m e n t Conditions a
Run
Material
Grain size (,~m)
16 17 19 21 22
Basalt Quartz Pyrite Chromite 10% Pyrite 90% Basalt 10% Chromite 90% Quartz I% Pyrite 99% Basalt 0.25% Pyrite 99.75% Basalt 10% Pyrite 90% Basalt
125-180 125-180 125- 180 125-180 125-180 125-180 125-180 125-180 90-125 180-250 90-125 180-250 90-125 180-250
24 26 28 29
Bulk density (g cm -3)
Grain density (g cm -3)
1.5 1.6 2.5 2.7 2.5 1.5 2.7 1.6 2.3 1.5 2.3 1.5 2.3 1.5
3.3 2.7 5.0 4.5 5.0 3.3 4.5 2.7 5.0 3.3 5.0 3.3 5.0 3.3
u, (m s Ij
Run time (s)
0.040 0.040 0.050 0.063 0.063
700 700 700 700 700
0.063
700
0.074
364
0.080
450
0.080
260
a u, is wind friction velocity.
on the surface of Venus (Table I). Tests consisted of subjecting beds of particles to wind in the Venus Wind Tunnel [VWT (Greeley et al. 1984a)] under CO2 densities appropriate to Venus. Surface processes were viewed directly through ports in the wind tunnel during the experiments. Afterward, the bedforms that developed (such as microdunes) were analyzed for particle sorting. Experiments were limited to sand (diameter = 90-250/zm) of four mineral compositions to assess the effects of particle density on sorting by the wind. The sands used were basalt (presumed common on Venus), quartz (common on Earth, used here for comparison), pyrite, and chromite. The other candidate minerals suggested to explain the radar data were not tested, but it is assumed that their wind-sorting characteristics will be similar to t h o s e of pyrite and chromite because their densities are comparable. All experiments were performed at wind speeds close to the minimum needed to initiate particle motion for the low-density grains. Four tests using monomineralic grains (runs 16, 17, 19, 21) served as a basis for comparing tests involving grains of different densities. Microdunes developed in all cases, although they formed best with quartz and basalt grains. Typical microdunes grew to 10-20 cm long by 1-3 cm high in - 1 0 min (Fig. 1), and have slip faces, internal foreset laminae, and a high ratio of saltation pathlength to dune length, all characteristics of full-sized dunes on Earth. Microdunes have been reported from previous VWT experiments (Greeley et al. 1984b), and have also been inferred to exist on Venus (Florensky et al. 1977, 1983). Two tests were run to assess the separation of dense mineral grains from less dense particles of the same size. Run 22 started with basalt sand uniformly mixed with 10% pyrite grains, while run 24 involved quartz sand with 10%
ET AL.
chromite grains. In both cases, the denser grains (pyrite, chromite) concentrated on the upwind slopes and crests of the dunes. As Fig. 2 shows, the percentage of heavy grains decreased by - 5 0 % in the basal layers of the dunes, while in the crest zones, the pyrite concentration increased nearly fivefold and the chromite concentration multiplied sevenfold. Such sorting also occurs in mixtures containing a lower initial proportion of dense grains. Three tests (runs 26, 28, 29) were run to determine the sorting of pyrite from basalt as a function of initial concentration. Again, microdunes developed and the pyrite was concentrated in the upper layers. For run 29, initially containing 10% pyrite, the upwind slopes of the dunes developed a shiny layer of pyrite grains that covered the entire dune crest. Pyrite grains also became concentrated on the upwind slopes of dunes in runs 26 and 28 (containing 1% and 0.25% pyrite), but coverage did not go to completion within the limited duration of the tests. Had each experiment continued for a longer time, we infer that the area covered by pyrite would have increased. We now propose an explanation of the sorting process by the wind. Beds of particles are readily organized into microdunes under Venusian conditions at wind speeds slightly above threshold. Once the dunes form, there is flow separation from the dune crest, and flow reattachment in the lee of the dunes. This is a highly erosive part of the bed, and the turbulence of the reattaching flow is sufficient to inject both materials into the wind flow. Although this wind-scoured zone has very little surface concentration of heavy minerals, entrainment of both dense and light materials occurs in the lee of microdunes. However, the reattachment zone injects more material than the flow can accommodate and deposition occurs on the upwind slope of the next microdune. Presumably the denser minerals will be deposited first, while the lighter material is carrier farther and ultimately onto the slip face, where it becomes buried during migration of the dune. As a result, the basal layers concentrate the lighter particles. Meanwhile, dense grains are concentrated on the surface of the bed by the removal of less dense (more easily moved) grains by the wind, a winnowing process common on Earth which forms a " l a g " surface. Thus, the combination of winnowing of less dense grains at the surface and of their burial during dune migration results in the concentration of dense grains on the upwind slope and crest zone of the microdunes. REFLECTIVITY How much conductive material must be present to account for the radar observations? The radar reflectivity of the Venus lowlands averages about 14%, while that of the highlands clusters around 40% (Pettengill et al. 1988). If
125
VENUS: WIND CONCENTRATION OF MINERALS
Fig. 1. "Microdune" developed in the Venus Wind Tunnel showing steeply dipping cross beds that are parallel to the slip face (right side). Area shown is about 10 cm wide; wind is from the left, illumination is from the right.
the surface is modeled as a homogeneous half-space, the corresponding dielectric constant lel is - 5 . 0 in the lowlands, within the range of ordinary rocks. In the highlands, however, lel is on the order of 20, well beyond that of dry silicates. By comparison with data on "loaded dielectrics," Pettengill et al. (1988) estimate that the reflective regions contain about 10% conductive inclusions by volume. Such concentrations are high by terrestrial standards, exceeding ore-grade levels. However, the highdielectric material need not extend very deep below the surface.
We model the soil of Venus as a horizontal layer of depth d and complex dielectric constant e2 atop a homogeneous half-space with real dielectric constant e3. The complex quantity e 2 may be written a s [ez[ e -i8, in which tan 8 is the loss tangent of the soil. The values of [e2[ and 8 depend on the concentration of conductive grains in the soil, their conductivity, and the porosity of the soiK The dielectric constant for solid samples of ilmenite, magnetite, and pyrite typically ranges from - 3 0 to 80, while [el can exceed l0 s in magnesium oxides (Garvin and Head 1985). Nozette (1982) has measured sulfide ores so con-
DEPTH WIND ~
(mm)
15I( UNDISTURBED I
0
|
I
I
I
I
5cm
BED
30~ '
CHROMITE '
' 4'0 '
'
' 80
VOLUME %
Fig. 2. Sorting of particles by density within microdunes; left side shows dune geometry (vertical scale is exaggerated; "slip face" is the steep, downwind side of the microdune on the right) and zones that were sampled; right side shows volume percentage of the dense grains for each zone. Note that in both cases, dense minerals become concentrated in the crest zone and are depleted in the basal layer of the microdune; not shown is the concentration on the upwind surface of the dunes. O, Run 22 initially 10% pyrite grains and 90% basalt grains 125-180 txm in diameter; O, run 24 initially 10% chromite grains and 90% quartz grains 125-180 ~m in diameter. In most experiments a layer of pyrite (or chromite) formed an almost continuous veneer on the windward face of the microdunes.
126
GREELEY
ET AL.
ductive that their equivalent 17-cm dielectric constants range f r o m 1.2 × l03 to 3.0 × l 0 6. Although these measurements were m a d e at r o o m t e m p e r a t u r e ( - 3 0 0 K), m o s t minerals increase in conductivity at Venus surface t e m p e r a t u r e s ( - 7 0 0 K). H o w e v e r , the admixture of pores and n o n c o n d u c t i v e grains reduces the effective dielectric constant. The theory of w a v e propagation in heterogeneous m e d i a is not yet mature, but it is c o m m o n to take the bulk dielectric constant of a mixture of grains as the geometric m e a n of its individual constituents (Ulaby et al. 1990). Thus, soil c o m p o s e d of a pure mineral with dielectric constant e and a volume fraction V of void space would h a v e an effective dielectric constant - e I v. C o m p a r i s o n of the bulk densities of crushed material to those o f single grains (Table I) indicates that the porosity of our e x p e r i m e n t a l samples was on the order of 50%. Such high porosities are c o m p a r a b l e to those inferred at the V e n e r a 13 and 14 landing sites ( K e m u r d z h i a n et al. 1983, A v d u e v s k i i et al. 1983, Basilevsky et al. 1985). Accordingly, we consider three possible values of the effective dielectric constant: l ezl = 20, 100, and 1000. The value of the loss tangent is also uncertain, but does not affect the results strongly; for definiteness, we take 8 = 0.40 so that tan 8 = 0.42. The dielectric constant of the substrate is taken as e3 ~ 5.0, characteristic of the Venus lowlands. The dielectric constant el of the a t m o s p h e r e varies slightly with height, but will be taken as 1.034, characteristic of the t e m p e r a t u r e and pressure at the mean surface of Venus (Pettengill et al. 1988). The corresponding refractive index of each layer is n = X/~e., in which /~ is its magnetic permeability. Although magnetite, hematite, and maghemite are still below their Curie temperatures on Venus, we may s e t / ~ ~ i within a few tens of percent. Then n I ~ 1.017, //2 = x/le21 e i6/2 and n 3 2.236. We consider only vertically incident radar waves, as appropriate for the Pioneer Venus geometry, so that the polarization state of the radiation is irrelevant. Then the c o m p l e x amplitude reflectivities at the u p p e r and lower interfaces are
1.0
//2 - - //I
P12 - - -
(1)
//2 + //1
and /'/3 - - //2 P23 -- - //3 + n 2
(2)
respectively. The net reflection coefficient is then P 1 2 + P23 e-iA P = 1 + plzP23 e-i~
(3)
(Klein 1970), in which A ~- nzd/X and X is the radar wavelength in free space. F o r d = 0 or n 2 = n3, expression (3) gives the amplitude reflectivity of the bare substrate:
'
'
''""1
'
'
' ' " " 1
'
'
''""1
'
'
' ' " "
0.8
0.6 0 LLI _J
~.1 rr
0.4
0.2
0
I 0.01
I
I
I
lllll 0.I
I
I
i i ,i,il
i
,
I
i i ,~,iI 10
i
~
i i ~lJJ I00
THICKNESS (cm) F i g . 3. P o w e r r e f l e c t i v i t y a t a w a v e l e n g t h o f 17 c m a s a f u n c t i o n o f t h e t h i c k n e s s o f a l a y e r o f c o n d u c t i n g g r a i n s . L o w e r c u r v e , e 2 = 2 0 e - 0.40i; m i d d l e c u r v e , e 2 = 1 0 0 e - ° 4 ° i ; u p p e r c u r v e , e~2 = 1 0 0 0 e o.4o(
//3 - - //I
P13 - - -
(4)
//3 + //I
Using n I = 1.017 and n 3 = 2.236 then gives PJ3 ~ 0.375, while the corresponding p o w e r reflectivity is IP~3[ ~0.140, equal to the m e a n reflectivity of the Venus lowlands. In the limit of a deep soil layer (d ~ ~), P reduces to P12. For c o m p a r i s o n with Pettengill et al. (1988), consider the nominal case [e21 = 20. T h e n Pl2 ~ 0.636e3'°5i, while 10221 ~ 0.405, characteristic of the Venus highlands. Between these two e x t r e m e s , the reflectivity depends on the depth d of the soil layer. The b o t t o m curve in Fig. 3 graphs the p o w e r reflectivity It)z ] as a function of d for the nominal case e2 = 20e -°'4°i. It is clear f r o m this plot that high reflectivities (Ipzl ~ 40%) are achieved with highdielectric layers only - 5 cm thick. This greatly reduces the total quantity of conductive material required by the radar observations. This a m o u n t can be reduced even further if the dielectric constant of the soil is greater; the middle and u p p e r m o s t curves in Fig. 3 display Io2l versus d for e2 = 100e 0.40iand e z = 1000e 0.40i, respectively. The corresponding depths of a highly reflective layer are only 1 cm and - 1 mm. As Fig. 2 shows, high-dielectric layers 1 cm thick are easily f o r m e d in the Venus Wind Tunnel within - 10 min. Finally, suppose that the surface of Venus consists of patches of conductive material interspersed with bare ground. The a b o v e theory should still apply as long as each patch is continuous o v e r scales longer than the radar wavelength. This a s s u m p t i o n is justified for the Pioneer Venus radar (X = 17 cm), since m i c r o d u n e s on Venus
VENUS: WIND CONCENTRATION OF MINERALS
should have wavelengths of at least 10-20 cm and may reach tens of meters in wavelength. Then the net reflectivity R in a given radar footprint should be
g
= f [ p 2 [ + (1 -- f ) P l 23 ,
(5)
in which f is the fraction of its surface area covered by conductive patches. For example, if [p2[ __ 0.67 and Pl3 0.14, R -- 0.14 + 0.53f. Thus, reflectivities of 40% can be obtained for values of f around one-half. ----
CONCLUSIONS
The foregoing experiments and analysis lead to the following model: 1. Many of the areas ofhigh radar reflectivity and corresponding low emissivity are interpreted to be surfaced with lava flows, possibly of basaltic composition (Basilevsky and Head 1988). Fe- and/or Ti-rich flows containing ilmenite are common on the Moon and are reasonable candidate materials for Venusian flows (Head et al. 1985, Garvin et al. 1985). 2. Chemical and physical weathering and large-scale fragmentation (e.g., impact cratering) of ilmenite-bearing lava flows would generate a wide range of particle sizes, including sands capable of being moved by the wind (Greeley et al. 1984a) and minerals of different densities. 3. When subjected to winds appropriate for Venus (Counselman et al. 1979), mixtures of grains with different densities form microdunes, and the denser grains become concentrated on the upwind slopes and in the upper layers of the microdunes. 4. Thus, the areas identified as having high radar reflectivities and low emissivities might be explained by the presence of grains of ilmenite (or of minerals with similar electrical properties) which weather free from igneous rocks and are concentrated by the wind. At the very least, this aeolian mechanism could contribute substantially to enhanced reflectivities, even if it were not the sole factor involved. 5. Additional experiments to verify these conclusions are planned, using other dense minerals such as hematite and magnetite (Fe304). ACKNOWLEDGMENTS The authors thank J. B. Garvin and an anonymous referee for their helpful comments. This work was supported by the NASA Planetary Geology Program under Grant NCC 2-346 through the NASA Ames Research Center.
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127
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Venus: Concentrations of Radar-Reflective Minerals by Wind RONALD GREELEY,* JOHN R. MARSHALL,*'t DREW CLEMENS,:t: ANTHONY R. DOBROVOLSKIS,*'t AND JAMES B. POLLACKJ"
*Department of Geology, Arizona State University, Tempe, Arizona 85287-1404; and ~NASA-Planetary Geology Program and tSpace Sciences Division, National Aeronautics and Space Administration, Ames Research Center, Moffett Field, California 94035-1000 Received February 28, 1990; revised October 22, 1990
(1982) indicate that pyrite may be stable at the lower Areas of high radar reflectivityand low emissivityon Venus may atmospheric pressures and temperatures found at high be covered with windblown material rich in Fe and Ti, such as elevations on Venus, but not at lower altitudes. Head et ilmenite, a mineral common in some basaltic lava flows. Experi- al. (1985), Garvin et al. (1985a,b), Garvin and Head (1985), ments under Venus-like conditions show that dense mineral grains and Garvin (1985) have argued from geological considerabecome concentrated on the surfacesof windblownsedimentswhen tions that large amounts of pyrite are not likely on the subjected to the low winds typical for Venus. ©1991AcademicPress, Inc. surface of Venus, but suggested instead that other minerals enriched in Fe, Ti, or Mn could account for the observations. Pieters et al. (1986) have reported spectral eviINTRODUCTION dence for hematite (Fe203) in the soil of Venus, while Pioneer Venus radar data at a wavelength of 17 cm Head et al. (1985) pointed out that some lunar basalts are reveal localized regions of unusually high reflectivity (Pet- rich in ilmenite (FeTiO3), and that analogous lava flows tengill et al. 1982, 1983, 1988) associated with high eleva- may cover the radar-reflective regions on Venus. Head et tions, such as Maxwell, Rhea Mons, and Theia Mons, al. (1985) also suggested that "secondary occurrences most of which are believed to be volcanic terrains. Petten- of high-dielectric materials might arise from erosion and gill et al. (1982) suggested that "the highly reflecting re- concentration in lag deposits" and, in particular, t h a t " eogions contain a significant amount of conducting material lian a c t i v i t y . . , might concentrate materials on the basis as inclusions in the rock," while Pettengill et al. (1988) of their density." In the absence of flowing water, wind may be an imestimate the conducting fraction at about 10% by volume. portant agent in transporting and sorting materials on Ford and Pettengill (1983) analyzed Pioneer Venus obserVenus (Greeley et al. 1984a). Despite relatively sluggish vations of 17-cm thermal emissions from Venus and found winds near the surface (Counselman et al. 1979, Ksanfothat areas of unusually low radio emissivity also corremality et al. 1983, Von Zahn et al. 1983), the high density lated with the highlands. They concluded that the presence of conductive inclusions could account for the low of the Venus atmosphere permits even gentle winds to emissivities as well. Venera 13 and 14 measurements of initiate movement of loose particles. Theoretical models of the circulation also indicate that enhanced winds over the DC resistivity of the Venus soil also suggest a high slopes produce the greatest rates of aeolian erosion (and, concentration of conductive minerals on the flanks of thus, particle sorting) at the higher elevations (DobroPhoebe Regio (Kemurdzhian et al. 1983). Jurgens et al. volskis and Saunders 1986, Saunders et al. 1990), which (1988) found evidence for high-reflectivity spots at lower have high reflectivities as well. Our experiments demonelevations on Venus from Earth-based radar data at a strate that aeolian processes can produce lag deposits of wavelength of 12.5 cm. By interpreting these rings as dense conductive material on the surface of Venus, and bright crater ejecta and comparing them with the fraction that such deposits need be only a few centimeters thick of radar-bright haloes in Venera 15/16 crater counts, Jurto explain the observed local enhancements of radar regens et al. estimate that the reflective material has been flectivity. chemically stable for the past 250 million years. What minerals could affect the conductivity of the EXPERIMENTS Venus surface this way? Pettengill et al. (1982, 1983) proposed pyrite (FeS2) as a candidate material on chemical Laboratory experiments were conducted to assess the grounds. Thermodynamic models of Nozette and Lewis effectiveness of wind in concentrating certain minerals 123
0019-1035/91 $3.00 Copyright © 1991 by AcademicPress, Inc. All rights of reproduction in any form reserved.
124
GREELEY
TABLE I E x p e r i m e n t Conditions a
Run
Material
Grain size (,~m)
16 17 19 21 22
Basalt Quartz Pyrite Chromite 10% Pyrite 90% Basalt 10% Chromite 90% Quartz I% Pyrite 99% Basalt 0.25% Pyrite 99.75% Basalt 10% Pyrite 90% Basalt
125-180 125-180 125- 180 125-180 125-180 125-180 125-180 125-180 90-125 180-250 90-125 180-250 90-125 180-250
24 26 28 29
Bulk density (g cm -3)
Grain density (g cm -3)
1.5 1.6 2.5 2.7 2.5 1.5 2.7 1.6 2.3 1.5 2.3 1.5 2.3 1.5
3.3 2.7 5.0 4.5 5.0 3.3 4.5 2.7 5.0 3.3 5.0 3.3 5.0 3.3
u, (m s Ij
Run time (s)
0.040 0.040 0.050 0.063 0.063
700 700 700 700 700
0.063
700
0.074
364
0.080
450
0.080
260
a u, is wind friction velocity.
on the surface of Venus (Table I). Tests consisted of subjecting beds of particles to wind in the Venus Wind Tunnel [VWT (Greeley et al. 1984a)] under CO2 densities appropriate to Venus. Surface processes were viewed directly through ports in the wind tunnel during the experiments. Afterward, the bedforms that developed (such as microdunes) were analyzed for particle sorting. Experiments were limited to sand (diameter = 90-250/zm) of four mineral compositions to assess the effects of particle density on sorting by the wind. The sands used were basalt (presumed common on Venus), quartz (common on Earth, used here for comparison), pyrite, and chromite. The other candidate minerals suggested to explain the radar data were not tested, but it is assumed that their wind-sorting characteristics will be similar to t h o s e of pyrite and chromite because their densities are comparable. All experiments were performed at wind speeds close to the minimum needed to initiate particle motion for the low-density grains. Four tests using monomineralic grains (runs 16, 17, 19, 21) served as a basis for comparing tests involving grains of different densities. Microdunes developed in all cases, although they formed best with quartz and basalt grains. Typical microdunes grew to 10-20 cm long by 1-3 cm high in - 1 0 min (Fig. 1), and have slip faces, internal foreset laminae, and a high ratio of saltation pathlength to dune length, all characteristics of full-sized dunes on Earth. Microdunes have been reported from previous VWT experiments (Greeley et al. 1984b), and have also been inferred to exist on Venus (Florensky et al. 1977, 1983). Two tests were run to assess the separation of dense mineral grains from less dense particles of the same size. Run 22 started with basalt sand uniformly mixed with 10% pyrite grains, while run 24 involved quartz sand with 10%
ET AL.
chromite grains. In both cases, the denser grains (pyrite, chromite) concentrated on the upwind slopes and crests of the dunes. As Fig. 2 shows, the percentage of heavy grains decreased by - 5 0 % in the basal layers of the dunes, while in the crest zones, the pyrite concentration increased nearly fivefold and the chromite concentration multiplied sevenfold. Such sorting also occurs in mixtures containing a lower initial proportion of dense grains. Three tests (runs 26, 28, 29) were run to determine the sorting of pyrite from basalt as a function of initial concentration. Again, microdunes developed and the pyrite was concentrated in the upper layers. For run 29, initially containing 10% pyrite, the upwind slopes of the dunes developed a shiny layer of pyrite grains that covered the entire dune crest. Pyrite grains also became concentrated on the upwind slopes of dunes in runs 26 and 28 (containing 1% and 0.25% pyrite), but coverage did not go to completion within the limited duration of the tests. Had each experiment continued for a longer time, we infer that the area covered by pyrite would have increased. We now propose an explanation of the sorting process by the wind. Beds of particles are readily organized into microdunes under Venusian conditions at wind speeds slightly above threshold. Once the dunes form, there is flow separation from the dune crest, and flow reattachment in the lee of the dunes. This is a highly erosive part of the bed, and the turbulence of the reattaching flow is sufficient to inject both materials into the wind flow. Although this wind-scoured zone has very little surface concentration of heavy minerals, entrainment of both dense and light materials occurs in the lee of microdunes. However, the reattachment zone injects more material than the flow can accommodate and deposition occurs on the upwind slope of the next microdune. Presumably the denser minerals will be deposited first, while the lighter material is carrier farther and ultimately onto the slip face, where it becomes buried during migration of the dune. As a result, the basal layers concentrate the lighter particles. Meanwhile, dense grains are concentrated on the surface of the bed by the removal of less dense (more easily moved) grains by the wind, a winnowing process common on Earth which forms a " l a g " surface. Thus, the combination of winnowing of less dense grains at the surface and of their burial during dune migration results in the concentration of dense grains on the upwind slope and crest zone of the microdunes. REFLECTIVITY How much conductive material must be present to account for the radar observations? The radar reflectivity of the Venus lowlands averages about 14%, while that of the highlands clusters around 40% (Pettengill et al. 1988). If
125
VENUS: WIND CONCENTRATION OF MINERALS
Fig. 1. "Microdune" developed in the Venus Wind Tunnel showing steeply dipping cross beds that are parallel to the slip face (right side). Area shown is about 10 cm wide; wind is from the left, illumination is from the right.
the surface is modeled as a homogeneous half-space, the corresponding dielectric constant lel is - 5 . 0 in the lowlands, within the range of ordinary rocks. In the highlands, however, lel is on the order of 20, well beyond that of dry silicates. By comparison with data on "loaded dielectrics," Pettengill et al. (1988) estimate that the reflective regions contain about 10% conductive inclusions by volume. Such concentrations are high by terrestrial standards, exceeding ore-grade levels. However, the highdielectric material need not extend very deep below the surface.
We model the soil of Venus as a horizontal layer of depth d and complex dielectric constant e2 atop a homogeneous half-space with real dielectric constant e3. The complex quantity e 2 may be written a s [ez[ e -i8, in which tan 8 is the loss tangent of the soil. The values of [e2[ and 8 depend on the concentration of conductive grains in the soil, their conductivity, and the porosity of the soiK The dielectric constant for solid samples of ilmenite, magnetite, and pyrite typically ranges from - 3 0 to 80, while [el can exceed l0 s in magnesium oxides (Garvin and Head 1985). Nozette (1982) has measured sulfide ores so con-
DEPTH WIND ~
(mm)
15I( UNDISTURBED I
0
|
I
I
I
I
5cm
BED
30~ '
CHROMITE '
' 4'0 '
'
' 80
VOLUME %
Fig. 2. Sorting of particles by density within microdunes; left side shows dune geometry (vertical scale is exaggerated; "slip face" is the steep, downwind side of the microdune on the right) and zones that were sampled; right side shows volume percentage of the dense grains for each zone. Note that in both cases, dense minerals become concentrated in the crest zone and are depleted in the basal layer of the microdune; not shown is the concentration on the upwind surface of the dunes. O, Run 22 initially 10% pyrite grains and 90% basalt grains 125-180 txm in diameter; O, run 24 initially 10% chromite grains and 90% quartz grains 125-180 ~m in diameter. In most experiments a layer of pyrite (or chromite) formed an almost continuous veneer on the windward face of the microdunes.
126
GREELEY
ET AL.
ductive that their equivalent 17-cm dielectric constants range f r o m 1.2 × l03 to 3.0 × l 0 6. Although these measurements were m a d e at r o o m t e m p e r a t u r e ( - 3 0 0 K), m o s t minerals increase in conductivity at Venus surface t e m p e r a t u r e s ( - 7 0 0 K). H o w e v e r , the admixture of pores and n o n c o n d u c t i v e grains reduces the effective dielectric constant. The theory of w a v e propagation in heterogeneous m e d i a is not yet mature, but it is c o m m o n to take the bulk dielectric constant of a mixture of grains as the geometric m e a n of its individual constituents (Ulaby et al. 1990). Thus, soil c o m p o s e d of a pure mineral with dielectric constant e and a volume fraction V of void space would h a v e an effective dielectric constant - e I v. C o m p a r i s o n of the bulk densities of crushed material to those o f single grains (Table I) indicates that the porosity of our e x p e r i m e n t a l samples was on the order of 50%. Such high porosities are c o m p a r a b l e to those inferred at the V e n e r a 13 and 14 landing sites ( K e m u r d z h i a n et al. 1983, A v d u e v s k i i et al. 1983, Basilevsky et al. 1985). Accordingly, we consider three possible values of the effective dielectric constant: l ezl = 20, 100, and 1000. The value of the loss tangent is also uncertain, but does not affect the results strongly; for definiteness, we take 8 = 0.40 so that tan 8 = 0.42. The dielectric constant of the substrate is taken as e3 ~ 5.0, characteristic of the Venus lowlands. The dielectric constant el of the a t m o s p h e r e varies slightly with height, but will be taken as 1.034, characteristic of the t e m p e r a t u r e and pressure at the mean surface of Venus (Pettengill et al. 1988). The corresponding refractive index of each layer is n = X/~e., in which /~ is its magnetic permeability. Although magnetite, hematite, and maghemite are still below their Curie temperatures on Venus, we may s e t / ~ ~ i within a few tens of percent. Then n I ~ 1.017, //2 = x/le21 e i6/2 and n 3 2.236. We consider only vertically incident radar waves, as appropriate for the Pioneer Venus geometry, so that the polarization state of the radiation is irrelevant. Then the c o m p l e x amplitude reflectivities at the u p p e r and lower interfaces are
1.0
//2 - - //I
P12 - - -
(1)
//2 + //1
and /'/3 - - //2 P23 -- - //3 + n 2
(2)
respectively. The net reflection coefficient is then P 1 2 + P23 e-iA P = 1 + plzP23 e-i~
(3)
(Klein 1970), in which A ~- nzd/X and X is the radar wavelength in free space. F o r d = 0 or n 2 = n3, expression (3) gives the amplitude reflectivity of the bare substrate:
'
'
''""1
'
'
' ' " " 1
'
'
''""1
'
'
' ' " "
0.8
0.6 0 LLI _J
~.1 rr
0.4
0.2
0
I 0.01
I
I
I
lllll 0.I
I
I
i i ,i,il
i
,
I
i i ,~,iI 10
i
~
i i ~lJJ I00
THICKNESS (cm) F i g . 3. P o w e r r e f l e c t i v i t y a t a w a v e l e n g t h o f 17 c m a s a f u n c t i o n o f t h e t h i c k n e s s o f a l a y e r o f c o n d u c t i n g g r a i n s . L o w e r c u r v e , e 2 = 2 0 e - 0.40i; m i d d l e c u r v e , e 2 = 1 0 0 e - ° 4 ° i ; u p p e r c u r v e , e~2 = 1 0 0 0 e o.4o(
//3 - - //I
P13 - - -
(4)
//3 + //I
Using n I = 1.017 and n 3 = 2.236 then gives PJ3 ~ 0.375, while the corresponding p o w e r reflectivity is IP~3[ ~0.140, equal to the m e a n reflectivity of the Venus lowlands. In the limit of a deep soil layer (d ~ ~), P reduces to P12. For c o m p a r i s o n with Pettengill et al. (1988), consider the nominal case [e21 = 20. T h e n Pl2 ~ 0.636e3'°5i, while 10221 ~ 0.405, characteristic of the Venus highlands. Between these two e x t r e m e s , the reflectivity depends on the depth d of the soil layer. The b o t t o m curve in Fig. 3 graphs the p o w e r reflectivity It)z ] as a function of d for the nominal case e2 = 20e -°'4°i. It is clear f r o m this plot that high reflectivities (Ipzl ~ 40%) are achieved with highdielectric layers only - 5 cm thick. This greatly reduces the total quantity of conductive material required by the radar observations. This a m o u n t can be reduced even further if the dielectric constant of the soil is greater; the middle and u p p e r m o s t curves in Fig. 3 display Io2l versus d for e2 = 100e 0.40iand e z = 1000e 0.40i, respectively. The corresponding depths of a highly reflective layer are only 1 cm and - 1 mm. As Fig. 2 shows, high-dielectric layers 1 cm thick are easily f o r m e d in the Venus Wind Tunnel within - 10 min. Finally, suppose that the surface of Venus consists of patches of conductive material interspersed with bare ground. The a b o v e theory should still apply as long as each patch is continuous o v e r scales longer than the radar wavelength. This a s s u m p t i o n is justified for the Pioneer Venus radar (X = 17 cm), since m i c r o d u n e s on Venus
VENUS: WIND CONCENTRATION OF MINERALS
should have wavelengths of at least 10-20 cm and may reach tens of meters in wavelength. Then the net reflectivity R in a given radar footprint should be
g
= f [ p 2 [ + (1 -- f ) P l 23 ,
(5)
in which f is the fraction of its surface area covered by conductive patches. For example, if [p2[ __ 0.67 and Pl3 0.14, R -- 0.14 + 0.53f. Thus, reflectivities of 40% can be obtained for values of f around one-half. ----
CONCLUSIONS
The foregoing experiments and analysis lead to the following model: 1. Many of the areas ofhigh radar reflectivity and corresponding low emissivity are interpreted to be surfaced with lava flows, possibly of basaltic composition (Basilevsky and Head 1988). Fe- and/or Ti-rich flows containing ilmenite are common on the Moon and are reasonable candidate materials for Venusian flows (Head et al. 1985, Garvin et al. 1985). 2. Chemical and physical weathering and large-scale fragmentation (e.g., impact cratering) of ilmenite-bearing lava flows would generate a wide range of particle sizes, including sands capable of being moved by the wind (Greeley et al. 1984a) and minerals of different densities. 3. When subjected to winds appropriate for Venus (Counselman et al. 1979), mixtures of grains with different densities form microdunes, and the denser grains become concentrated on the upwind slopes and in the upper layers of the microdunes. 4. Thus, the areas identified as having high radar reflectivities and low emissivities might be explained by the presence of grains of ilmenite (or of minerals with similar electrical properties) which weather free from igneous rocks and are concentrated by the wind. At the very least, this aeolian mechanism could contribute substantially to enhanced reflectivities, even if it were not the sole factor involved. 5. Additional experiments to verify these conclusions are planned, using other dense minerals such as hematite and magnetite (Fe304). ACKNOWLEDGMENTS The authors thank J. B. Garvin and an anonymous referee for their helpful comments. This work was supported by the NASA Planetary Geology Program under Grant NCC 2-346 through the NASA Ames Research Center.
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127
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