Though the Magellan mission was not designed to collect geochemical or petrological information, it has done so nonetheless. Since the time of the Pioneer Venus mission it has been known that high-altitude (greater than 2.5-5 km) mountainous areas on Venus exhibit anomalously low radiothermal emissivity (e less than 0.6). Magellan has greatly refined and extended these observations. The low emissivity requires surface material in the uplands to have a mineralogical composition that gives it a high bulk dielectric constant, greater than 20. The dielectric constant of dry terrestrial volcanic rocks seldom exceeds 7. The high-dielectric character of high-altitude surface material cannot be a primary property of the local volcanic rock, because there is no reason why rock having the required special mineralogy would erupt only at high altitudes. Therefore it is a secondary property; the primary Venus rock has reacted with the atmosphere to form a mineralogically different surface layer, and the secondary minerals formed are controlled by the ambient temperature, which decreases with altitude on Venus. A further investigation of venusian mineralogy is presented

Wood, J. A.

English

NASA Technical Reports Server

132 International Colloquium on Venus
Fig.2, Mona LisaCrater,centeredat25.6°N,25.2"Enearthe edge ofEisila
Regio.Note partialfloodingof outermoat structureand brightfracture
patterninthecentralfloorplate.Scalebaris-17 km (enlargedsecdonof
CI-30N027;I).
recognized. Although transcctcd by a number of later wrinkle
ridges, both the outer moat region and the central floor appear to be
relatively smooth and may reflect a sequence of crater-centered
volcanism after the uplift event, also observed in the lunar crater
Posidonius. The second crater (Fig. 2) is larger (-75 km diameter)
and somewhat less modified. In this case, a bright ridge nearly
surrounds the central floor plate, and volcanism has only flooded the
central floor and the northern half of the outer moat structure,
allowing identification of the moat fractures in the south. Inside the
central floor plate, concentric fractures bound the central floor with
a set of radial/polygonal fractures in the center of the floor plate.
Both craters occur in ridged lowland plains with elevations close to
the mean planetary radius. Since the ejecta pattern and the scalloped
southern crater rim indicate an oblique impact from the north
[16,17], the northward offset of these central fracture patterns
relative to the crater center is consistent with the uprange offset of
both central peaks and basin rings in other oblique impact structures
[5,16,17], whereas the distribution of moat-filling volcanism is
consistent with the enhanced failure uprange proposed by [5.18,19]
for cavity collapse in an oblique impact event.
The intrusion parameters derived from the relations of [13] can
be related to the local crustal structure. For floor uplift of -1.5 km
(inferred for the craters described above), both craters indicate a
magmatic driving pressure of -375 bar for a basaltic melt. If this
pressure then reflects the magmastatic head resulting from the
density contrast between a basaltic magma (-2800 kg]m 3) and a
basaltic crust (3000 kg]m3), a magma column length of 22 km is
indicated for both regions. Since the effective thickness of the floor
plate is estimated at 2-6 km for crater 1 and 4-8 km for crater2, this
simple model requires a crustal ('basaltic) thickness exceeding
-2530 kin. Alternatively, if the basaltic crust on Venus is less than
10-20 km, as proposed in [20], the base of the magma column occurs
within a denser mantle unit at depths of less than 20--25 kin. If the
base of the magma column corresponds to a deep, regional magma
chamber, these magma column models should indicate either the
base of the basaltic crust or theological boundaries with the crust or
lithosphere [21 ]. Consequently, the implications of floor-fractured
craters on Venus for subsurface density provide an additional test
for models of regional crustal structure.
References: [1] Sehultz P. H. (1976) Moon, 15, 241-273.
[2] Schultz P. H. (1978) GRL, 5, 457-460. [3] Schultz P. H. (1976)
Moon Morphology. [4] Wichman R. W. and Schultz P. H. (1991)
LPSCXXll, 1501-1502. [5] Wichman R. W. (1992) Ph.D. thesis,
Brown University. [6] Brennan W. J. (1975) Moon, 12, 449-461.
[7] Whifford-Stark J. L. (1974) Nature, 248, 573-575. [8] Young
R. A. (1972)Apollo 16 Prelim. Sci. Rep., NASA SP-315, 29---89 to
29--90. [9] Bryan W. B. et al. (1975) Proc. LSC 6th, 2563--2570.
[10] Danes Z. F. (1965) Astrogeol. Stud. Arm. Prog. Rep. A
(2964-1965), 81-100. [11] Scott R. F. (1967) Icarus, 7, 139-148.
[12] Hall I. L. et al. (1981)JGR, 86, 9537-9552. [13] Pollard D. P.
and Johnson A. M. (1973) Tectonophysics, 18, 261-309. [14] Head
J. W. et al. (1992) Magellan 6 Month Rep., in press. [15] Solomon
S. C. (1975) Proc. LSC 6th, 1021-1042. [16] Schultz P. H. and Gault
D. E. (1991) LPSC XXIII, 1231-1232. [17] Schultz P. H. (1992)
Magellan 6 Month Rep., in press. [18] Wichrnart R. W. and Schultz
P. H. (1992) LPSC XXilI, 1521-1523. [19] Schultz P. H., this
volume. [20] Grimm R. E. and Solomon S. C. (1988) JGR, 93,
11911-11929. [21] Head J. W. and Wilson I,.. (1992)JGR, 97,
3877-3903.
N93-143941 :-, i: .,-i
vENus:C .OC.EMICALCbNC'USIONS  R0 
MAGELLAN DATA. ]. A. Wood, Harvard-Smithsonian Center
for Astrophysics, Cambridge MA 02138, USA.
Though the Magellan mission was not designed to collect
geochemical or petrological information, it has done so nonetheless.
Since the time of the Pioneer Venus mission it has been known that
high-altitude (>2.5-5 kin) mountainous areas on Venus exhibit
anomalously low radiothermal emissivity (e < 0.6) [1 ]. Magellan
has greatly refined and extended these observations. The low
emissivity requires surface material in the uplands to have a
mineralogical composition that gives it a high bulk dielectric
constant, >-20. The dielectric constant of dry terrestrial volcanic
rocks seldom exceeds 7. The high-dielectric character of high-
altitude surface material cannot be a primary property of the local
volcanic rock, because there is no reason why rock having the
required special mineralogy would erupt only at high altitudes.
Therefore it is a secondary property; the primary Venus rock has
reacted with the atmosphere to form a mineralogically different
surface layer, and the secondary minerals formed are controlled by
the ambient temperature, which decreases with altitude on Venus.
Klose et al. [2] showed that, for a plausible assumption o foxygen
fugacity in the Venus atmosphere (-10 -21 bar), variation of the
equilibrium weathered mineral assemblage with altitude and tem-
perature would create such a situation: above a few kilometers
altitude the stable assemblage includes pyrite (FeS2) in sufficient
abundance to create a "loaded dielectric" with a bulk dielectric
constant >-20; at lower altitudes the stable Fe mineral is magnetite
(Fe304), and this is present in insufficient abundance to give rise to
such a high bulk dielectric constant. Pettengill et al. [3] first
concluded that pyrite was the mineral responsible for the high-
dielectric materials observed at high altitudes on Venus.
Fegley et al. [4] reject this interpretation. They note that the gas
species COS is much less abundant in the Venus atmosphere than the
equilibrium concentration, and argue that the reaction
3FeS 2 + 2CO + 4CO 2 = 6COS + Fe304
https://ntrs.nasa.gov/search.jsp?R=19930005206 2020-03-17T09:05:02+00:00Z
LPI Contribution No. 789 | 33
would tend to oxidize pyrite to magnetite at all altitudes, using its
S to build up the deficit of atmospheric COS. This argument is
specious. Fegley et al, argue from only one reaction and three
species abundances in the Venus atmosphere, while [2] used an
energy-minimization procedure that effectively considers all pos-
sible reactions simultaneously, loose et al. [2] used as input the
bulk elemental composition of the atmosphere, not the abundances
of selected gas species in it. They determined the total equilibrium
system, including concentrations of gas species in the atmosphere
as well as surface minerals, lOose et al. found, as Fegley et al. say,
that the equilibrium abundance of COS is greater than the concen-
tration the Venus aunosphcrc appears to contain (- 15 ppm vs. -0.25
ppm). Apparently kinetic barriers prevent COS from attaining its
equilibrium concentration at ground level. Thus the Venus system
is not completely at equilibrium. One can debate how badly the
system is out of equilibrium, but the nonequilibrium abundance of
the minor species COS in the atmosphere by no means requires that
the surface mineralogy is completely out of equilibrium; and to be
in equilibrium with the atmosphere's concentration of SO 2, -185
ppm, at the fo2 cited above, the mountaintop mineral assemblage
must contain pyrite,
Fegley et al. [4] suggest that primary perovskite (CaTiO3) in the
rock might be responsible for low-emissivity mountaintops on
Venus, since this mineral has a particulkrly high dielectric constant.
They note the occurrence of perovskite in (relatively rare) SiO z-
undersaturated igneous rocks on Earth. but do not examine its
thermodynamic stability in more typical basalts, or in Venus basalts
having the compositions reported by Soviet landers. In fact,
perovskite is not a stable primary mineral at crystallization tempera-
turcs in these expected Venus rock types: instead Ti appears as
rutile, and most Ca as diopside and anorthite.
With time, the weathering reactions discussed by [2] work
deeper and deeper into surface rock on Venus. The timescale of
weathering--the time needed to weather rock to a depth sufficient
to control the radiothermal emissivity measured by PVO and
Magellan---is not known, hut is expected to be long. Water is an
essential ingredient of terrestrial weathering, and its near absence on
Venus must greatly retard the process. To some extent this effect is
offset by the much higher temperatures on Venus, but in spite of this
it may take tens or even hundreds of millions of years to weather a
high-altitude surface to a high-dielectrlc assemblage.
This is comparable to the timescales of other important pro-
cesses on Venus, and an interplay between weathering and (e.g.)
volcanism and tectonism is to be expected. In other words, it may
be possible to use the presence or absence of weathering effects to
distinguish between relatively young features on Venus and older
land forms.
Volcanism: Klose et al. [2] and Robinson and Wood [5] have
drawn attention to large and small flow units high on the volcanos
Maat, Sapas, Ozza, and Theia Mons that display relatively high
emissivities. They conclude these units are too young to have had
time to weather to the high-dielectric pyritic state. This criterion,
if confirmed, provides a crude basis for establishing a chronology
of volcanism.
Tectonlsm: loose et al. [2] and Pathare [6] have explored the
complex relationship between altitude (a) and emissivity exhibited
by Maxwell Montes. An a/e scaaer plot of measurements made over
the broad Maxwell landmass shows a well-defined band at e - 0.4,
which presumably reflects the presence locally of completely
weathered surface material; but there is also abroad scatter of points
to Mgher values ofe (<--0.6) in the diagram. Pathare [6] found these
latter data points are provided by a belt of mountainous terrain
0atitude -67 °, longitude 3500-05 °) that defines the northern edge
of the Maxwell Montes low-emissivity zone. Pathare [6] attributed
the less-than-minimal emissivities in this belt to incomplete weath-
ering, and concluded that this belt was uplifted more recently than
other parts of Maxwell. The emissivity incr*ases, and presumably
the time of uplift decreases, westward along the belt.
The Magellan emissivity records contain other information of
interest. For example, [5] found that some small volcanic domes
and clusters of domes near plains level display anomalously low
emissivity. At these altitudes, according to the model of [2],
magnetite should be the stable Fe mineral, and weathered surface
material should not have high-dielectric properties. Robinson and
Wood [5] attributed this phenomenon to continued seepage of
volcanic gas through the soil covering these domes, and showed that
an admixture ofordy--0.02% of sulfurous gas of the type emitted by
the K.ilauea volcano on Earth, with normal Venus atmospheric gas
in the pores of the soil, would stabilize pyrite in the soft at plains
altitude. They noted a correlation between the presence of appar-
ently unweathered volcanic flows at the summits of volcanos, and
nearby low-emissivity domes; both should be manifestations of
recent volcanic activity.
Another interesting aspect of weathering and emissivity on
Venus is the fact that the mineral reaction boundary separating
pyrite 0ow-emissivity) from magnetite (high-emissivity), which
presumably follows an isothermal surface in the Venus atmosphere,
does not intersect aU highlands at the same altitude. The observed
"snowline" varies in altitude from -2.5 km (Sapas Mons) to.--4.7 km
(Maxwell). Curiously, the "snowline" altitude correlates well with
the total height of the mountain; the "snowline" occurs at roughly
half the peak height. This suggests that the thermal structure of the
atmosphere is somehow modulated by topography on Venus, a
concept that has not found favor with atmospheric scientists. An
alternative explanation has not been forthcoming, and this phenom-
enon remains to be understood.
ReFerences: [1] Pettengill G. H. et al. (1988)JGR, 93,
14881-14892. [2] Klose K. B. et al. (1992) JGR, submitted.
[3] Pettengill G. H. et al. (1982) Science, 217, 640---642. [4] Fegley
B. Jr. et al. (1992) Proc. LPSC, Vol. 22, 3-19. [5] Robinson C. A.
and Wood I. A. (1992) in preparation.-[6] Patham A. (1992) B.S.
thesis,Harvard Univers_. N93 14395 ' "
FINITE AMPLITUDE GRAVITY WAVES IN THE VENUS
ATMOSPHERE GENERATED BY SURFACE TOPOG-
RAPHY. R.E. Young I. H. Houben 2. R. L. Wa]terscheid 3, and
G. Schubert 4, lNASA Ames Research Center, Moffett Field CA,
USA, 2Space Physics Rcscarch Institute, Sunnyvale CA. USA.
3The Aerospace Corporation, Los Angeles CA, USA. aOniversity of
California. Los Angeles CA, USA.
A two-dimensional, fully nonlinear, nonhydrostatic, gravity
wave model is used to study the evolution of gravity waves
generated near the surface of Venus. The model extends from near
the surface to well above the cloud layers. Waves are forced by
applying a verticalwind atthe bottom boundary. The boundary
verticalwind isdctcrmincd by the product of the horizontalwind
and the gradient of the surface height. When wave amplitudes are
small, the near-surface horizontal wind is the zonally averaged
basic-state zonal wind. and the length scales of the forcing that
results are characteristic of the surface height variation. When the
forcing becomes larger and wave amplitudes affect the near-surface


Venus: Geochemical conclusions from the Magellan data

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