NASA Technical Reports Server (NTRS) 19930005160: Flexural models of trench/outer rise topography of coronae on Venus with axisymmetric spherical shell elastic plates

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by  NASA Technical Reports Server (NTRS)

72 International Colloquium on Venus
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Surface Processes: Surface processes include eolian, mass 72, 285. [16] Arvidson R. E. et al. (1991) Science, 252, 270-275.
wasting, and other processes [16]. Examples where measurements [17] Guest J.E. et at. (1992) JGR, special Magellan issue, in press.
of relief will be useful include (1) analyses of erosion-deposition [18] Moore H.J.et at. (1992) JGR, special Magellan issue, in press.
patterns behind obstacles [16], (2) slopes of erosion-deposition [19] Hsu K. J. (1975) GSA Bull., 86,129-140. [20] Plaut J.J.(1992)
environments, (3) slope stability analyses, and (4) estimates of LPSCXXIII, 1085-1086. [21] Leberl F. etat. (1991)Photog. Engr.
landslide volumes. Rein. Sens., 57, 51-59. [22] Tyler G. L. (1991) Science, 252,
Rheological Analyses: There is ahost of applications of relief 265_-270." I -
measurements to the analyses of the theological properties of ,N93-14348 '"
venusian flow associated with volcanism [8], impact cratering [7],
FLEXURAL MODELS OF TRENCH/OUTEi_ RISE TOPOG-
and debris flows [17]. These applications include flow thicknesses
and relations between the flows and the adjacent topography. Lava RAPHY OF CORONAE ON VENUS WITH AKISYMMETRIC
flow thickness aslarge as 1{30to700 m have already been measured SPHERICAL SHELL ELASTIC PLATES. W. Moore t, (3.
using parallax [18]. According to Magellan altimetry, bright out- SchuberO, and D. T. Sandwell 2, IUniversity of California, Los
flows from impact craters flow up slopes, and flow margins may be Angeles CA, USA, 2Scripps Institution of Oceanography, Univer-
100 m orso above the centers of the outflow s. Iftrue, these relations sity of California-San Diego, La Jolla CA. USA.
have important implications about the kinematics and rheology of
the outflows. Relations between the relief and runout may reveal the Magellan altimetry has revealed that many coronae on Venus
rhcological properties of venusian landslides [17,19]. have trenches or moats around their peripheries and rises outboard
Backscatter Functions: A better understanding of the rela- of the t_enches [1,2]. This trench/outer rise topographic signature is
tions between back.scatter cross sections and incidence angles can be generallyassociated with thetectonic annulus ofthe corona. Sandwell
gained by analyscs of given classes of landforms and terrains with and Schubert [3,4] have interpreted the n'ench/outer rise topography
variable slopes and sufficient relief for stcreome_ic analyses. and the associated tectonic annulus around eoronae to bc the result
Muhiplc viewing conditions arc essential in undcrstanding (1) the of elastic bending of the Venus lithosphere (though the tectonic
forms of the scattering laws, (2) the dielectric properties, (3) the structures are consequences of inelastic deformation of the lihhos-
contributions of conducting materials to scancring, behaviors, (4) phere).They usedtwo-dimensional elastic plate flexuretheorytofit
thefree-scale roughnesscs, and (5)the contributions ofquasispecular topographic profiles across anumber of large coronae and inferred
and diffuse cchocs toaverage backscatter cross section sof tesserae, elastic lithosphere thicknesscs between about 15and 40 krn,similar
impact craters, and volcanic edifices, craters, and flows [20]. An to inferredvaluesofelasticthickness[i_he Earth'slithosphereat
understanding of the above will assist in geologic interpretations of subductionzonesaround thePacificOcean. Here,wc reportthe
tesserae, impact cratering, and volcanism. resultosfusingaxisymmctric_:lastifclexurcthcoryforthedcforma-
Radarclinometry and Shape from Shading: Once the back- tionofthinsphericalshellplates[5]tointerpretthetrcncWoutcrrise
scatter functions of thevarious classes of landforms are established, topographyofthelargecoronacmodeled bySandwcll andSchubert
shapo from shading can be used to refine the topography of [3,4]andofcoronaeassmallas250 km indiameter.Inthecaseof
landforms with stereo-relief data [21], and radarclinometry can be acorona only a few hundred kilometersindiameter,themodel
used to estimate the relief and shapes oflandforms of the same class accountsforthesmallplanformradiusofthemoat andthenonradial
where there isno stereoscopic covcrage and where thelandforms are orientationof altimctrictracesacrossthecorona.By fittingthe
too small for stereo-parallax mcasuremants. flcxuraltopographyofcoronacwe determinetheelasticthickness
Topographic Analyses: Radargramme_c reduction of ste- and loadingnecessarytoaccount fortheobserved flexure.Wc
reoscopic models and radarclinomctry (shape from shading) [21] calculatetheassociatedbendingmoment anddeterminewhetherthe
may provide information on the topography of venusian surfaccs at coronainteriotropographicloadcanprovidetherequiredmoment.
slope lengths smallcr than those achier able with Magellan ahimetrj We alsocalculatesurfacestressesand compare thestressdistribu-
and largcr than those obtained by analyses of quasispecular echoes tionwiththelocationofannulartectonicfeatures.
from level surfaces with surface tilts smaller than the image Thc model lithosphere_sa sphcricalelasticshcllbuoyantly
resolution [22]. Derived topographic information includcs slope supportedby adenseinternalfluid.Although themodel includes
probabilities, power spectral densities, and fractal dimcn.sions. membrane st_sses,foraplanetthesizeof Venus thebuoyant
Altimetry: Radargrammetric reduction of stcreoscopic mod- supportprovidesthedominant reactionto_c load.The load is
els can confirm, refutc, or supplement Magellan ahimctry wherc modeled aseitheranaxisymmctricdisk(uniformloading)oraring
problcms with the altimetry exist. The current problem of the stccp (peripheralloading).Other loadgeomctriesmay bc achievedby
slopes of Maxwell Monies is an example, but there are others. superpositiotTth.e wavelengthoftheflcxuralfeaturedcpcndsonly
References: [1] Leberl F. ctat. (1992)JGR, special Magellan on thethicknessoftheplateandnoton thedetailsof theloading,
issue, in press. [2] LeberlF. etal. (1991) Photog. Engr. Rein. Seas., allowingauniquedeterminationoftheelasticthicknessfrom the
57, 1561-1570. [3] McKenzie D. etat. (1992)JGR, special Magellan best-fittinmgodel. Verticalstrainsarcnot includedso thatthe
issue, in press. [4] Pike R. L (1980) USGS Prof. Paper 1046-C, verticaldisplacementatthetopofthelithosphereisthesame asthat
77 pp. [5] Pike R. J. and Davis P. A. (2984) LPSC XV, 645-646. atthebottom where thebuoyancy forcesareacting.This model
[6] Schenk (1991)JGR, 96, 15635-15664. [7] Schabcr(1992)JGR, includesthecffcctsofadistributedloadand acontinuouslithos-
special Magellan issue, inpress. [8]Head J.W, ctat. (1991)Science, pherethatarcabsentintwo-dimensionalmodels and thatbecome
252, 276-288. [9] Pike R. J. (1978) Proc. LPSC 9th, 3239-3273. importantwhen theradiusoftheloadisreducedtoafew flexural
[10] Pike R. J. and Clow G. D. (1981) USGS Open-file Rept. 81- wavelengths.
1038, 40 pp, [11]Wood C. A. (1979) Proc. LPSC 10th, 2815-2840. The models arcfittothetopographyusingleastsquaresfitting
[12] Blakc S. (1990) IAVCEI Proc. Volcanol., 2, 88-126. and therelevantparametersaredetermined from thebest-fitting
[13] Solomon S. et at. (1990) Science, 252,297-312. [14] Connors model.For thecoronaLatona(diameter= 800 kin)we reproduce
C. and Suppc J. (1991) Eos, 72,285. [15] McGill G.E. (1991) Eos, Sandwcll and Schubert's[3]valueofapproximately30km forthe
LPI Contribution No. 789 73
elastic thickness, demonstrating the agreement of the axisymmetric uncertain and arguments are made that it is actually varying with
and two-dimensional models in the case of a large corona. For time on ascale of I0 yr.
smaller coronae, we find that elastic lithosphere thicknesses be- Although the geochemical arguments arerather compelling, itis
tween l0 k:m and 15 km provide the best fits to the flexural vitally important to rationally look at other explanations for the
topography (Table 1). radar and radio emission measurements such as that presented by
Tryka andMuhleman [3]. The radar reflectivity values are retrieved
from the raw MageLlan backscatter measurements by fitting the
TABLE i.
Hagfors' radar scattering model in which a surface roughness
Corona Loc_don Diameter Elastic parameter and a normal incidence electrical reflectivity are esti-
Name 0un) Thickness (kin) mated. The assumptions of the theory behind the model must be
considered carefully before the results can be believed. These
lama 17oS,17°E 310 15
include that the surface roughness exists only at horizontal scales
Selu 43°S, 6OE 300 10
Aramaiti 26°S, 820E 350 10 large comp_'ed to the wavelength, the vertical deviations are
Boann 270N. 136°1_ 300 5 gaussianly distributed, there isno shadowing, and thatthe reflection
Latona 20aS, 17lee 800 30 occurs at the interface of two homogeneous dielectric half-spaces.
Probably all these conditions are violated at the anomalous fea-
tures under discussion! The most important of these is thehomoge-
The disk loading model can be used to deduce the gravity neity of the near surface of Venus, particularly in highlands. Under
signature of acorona. We will report calculations of gravity using the assumptions of the theory, all of the radio energy is reflected by
the disk loads inferred for the larger coronae and compare with the impedance jump at the very boundary. However, in heteroge-
recent gravity data, e.g., over Artemis [6]. neous soil some fraction of the illuminating energy is propagated
References: [1] Squyres S. W. et al. (1992) JGR, in press. intothe soil and then mattered back out by impedance discontinuities
[2] Stofan E. R. et al. (1992) JGR, in press. [3] Sandwell D. and such as rocks, voids, and cracks. In light soils, the latter effect can
Schubert G. (1992a)JGR, submitted. [4] Sandwell D. and Schubert overwhelm the scattering effects of the true surface and greatly
G. (1992b) Science, submitted. [5] Brotchie I. and Sylvester R. enhance the backscatter power, suggesting amuch higher value of
(1969) JGR, 74, 5240-5252. [6] Sjogren W. L. (1992) Eos, 73, 83. an effective dielectric constant that would be estimated from
Hagfors' model.
N93-14349; The phenomenon of emission is similar but has several impor-
tant different characteristics. In the case of thermal emission from
RADAR-ANOMALOUS, HIGH-ALTITUDE FEATURES ON asmooth, homogeneous dielectric into vacuum, some of the radia-
VENUS. Duane O. Muhleman and Bryan J.Buffer, Division of tion generated in the effective black body passes through the
Geological and Planetary Science, California Institute of Technol- interface to the observer and afraction is reflected back downward
ogy, Pasadena CA 9l 125, USA. into the material where it is reabsorbed. In the simple case of an
isothermal layer (such asthe near surface of Venus), radiating from
Over nearly all of the surface of Venus the reflectivity and a homogeneous layer, the emissivity is determined by the Fresnel
emissivity at centimeter wavelengths are about 0.15 and 0.85 reflection coefficients atthe observing angle to the normal. How-
respectively. These values are consistent with moderately dcnsc ever, ifthe layer contains multiple scatterers in alight soil, radiation
soils and rock populations, but the mean reflectivity is about afactor generated even at small depths cannot reach the surface since the
of 2 greater than that for the Moon and other terrestrial planets (in tendency is to scatter the energy backward, similar to the strong
the case of the Earth, regions free of moisture). Pettingill and Ford backscattering reflection from above such a surface. Thus, the
[1], using Pioneer Venus rcflectivities and emissivities, found a emissivity can be greatly depressed and the observed brightness
number of anomalous features on Venus that showed much higher temperature will be low. This phenomenon for Venus was discussed
reflcctivities and much lower emissivities with both values ap- in 1979 [4] as an explanation for the decrease in the average disk
proaching 0.5. These include Maxwell Montes, anumber of high temperature of Venus at wavelengths longward of 10-20 cm.
regions in Aphrodite Terra and Beta Regio, and several isolated The most outstanding and relevant example of the importance of
mountain peaks. Most of the features are at altitudes above the the multiple scattering or volume scattering in radar and microwave
mean radius by 2 to3km ormore. However, such features have been emission are the icy satellites of Jupiter [5]. The radar reflectivity
found inthe Magellan data atlow altitudes andthe anomalies do not of the full disk of Europa at 13-era wavelength is 0.65 and the
exist on all high structures, Maat Mons being the most outstanding emissivity is about 0.42! Certainly, the surface of Europa is almost
example. A number of papers have been written that attempt to pure water ice that, if itexisted inthe form of dense ice, would have
explain the phenomena in terms of the geochemistry balance of areflectivity of 0.07 and an emissivity of 0.93. If the Europa icewas
weathering effects on likely surface minerals; see reference [2] and inthe form of ahomogeneous layer, under dense frost the reflectivity
papers cited therein. The geochemists have shown that the funda- would be even lower. It is obvious that the reflection and emission
mentally basaltic surface would be stable at the temperatures and phenomena on Europa are independent of the Fresnel surface
pressures of the mean radius in the form of magnetite,but would reflection coefficients and dependent entirely on the physical struc-
evolve to pyrite (FeS2) and/or pyrrhodte (F%,-r.tS) in the presence ture of the near surface, i.e, the existence of lumps, voids, cracks,
of sulfur-bearing compounds such as SO 2.Pyrite will be stable at etc. It is also very important that ice as cold as 130 K is highly
altitudes above 4 or 5km on Venus. The details of the stability of transparent at centimeter wavelengths and very little of the energy
these rather good electrical conductors depends on the availability is ohmically absorbed in the near surface. If that were not the case,
of O in excess over that tied up in equilibrium with the parent the surface would be agood emitter and arather poor reflector.
constituent of the atmosphere, CO 2.This isclearly explained in [2]. The radiative transfer calculations for the emission and reflec-
However, the abundance of the sulfur compound SO2 is very tion from alayer with volume scattering are very complex, with the