1966AJ 71. . 902E energy atfrequenciesoutside thelimitsexpectedfor wavelengths. Inaddition,there wassomeevidencein these earlymeasurements for theexistenceofecho The half-powerwidthofthesignalswas75cps,though roughness androtationrate asdeterminedatlonger 20 cpsorlessduetothecombinedeffectsofsurface one wouldhaveexpectedahalf-powerwidthofonly indicated aconsiderableamountofDopplerbroadening. spectrum ofthesignalsobtainedat3.6cmin1964 Walker andSagan1966).However,thefrequency THE ASTRONOMICALJOURNAL temperature measurements(BarrettandStaelin1964; number ofpossiblemodelsfortheatmosphereVenus of attenuationandsatisfyexistingradioinfrared absorption, wouldplacedefiniteconstraintsuponthe low value,ifattributabletotheeffectofatmospheric that couldbeconstructedtoprovidetherightamount radar crosssectionofonly1%(Karpetal.1964).This Ford radarsystemat3.6-cmwavelengthindicateda same conjunctionusingtheLincolnLaboratoryWest for themoon,Mercury,orMarsandimpliesarather during aseven-monthperiodcenteredontheinferior Shapiro 1965;Muhlemanetat.1965). compacted typeofsurfacematerial(Pettengilland conjunction whichtookplacein1964.Theseresults, cm usingtheLincolnLaboratoryMillstoneHillradar range 12.5cm-6m,indicatearadarcrosssectionfor IN apreviouspaper(Evansetat.1965)wedescribed Venus ofabout15%theprojectedareadisk together withthoseofotherobserversinthewavelength (Evans etal.1965).Thisishigherthanencountered * Operatedwithsupportfromthe U.S.AirForce. Separate observationsofVenusmadeduringthe © American Astronomical Society • Provided by theNASA Astrophysics Data System radar observationsofVenusatawavelength23 yielded crosssectionssimilartothosepreviouslyobtainedatlongerwavelengthsandlentconfidencethat observe theplanetVenus.Theradarcrosssectionobservedatthiswavelengthwasonlyaboutone-tenthas permits theVenusrotationperiodtobeestablishedas244±afewdays. in theseobservationswiththepositionsreportedbyCarpenter(1966)during1964inferiorconjunction for thelowcrosssectionobserved.Acomparisonofpositionsanomalouslyreflectingregionsobserved ments attwofrequencies,whichshowedthattheechodelaytimesagreedto5/¿sec,i.e.,wellwithin firmation thatthesurfaceisscatteringagenthasbeenprovidedbynearsimultaneousrangingexperi- much ashasbeenreportedatwavelengthsof12cmorlonger.SeparateobservationstheplanetMercury vertical absorptionoccursintheatmosphereofVenus.Variationsobservedcrosssectionwithtime experimental accuracy.Thedepolarizedcomponentoftheechoeswasobservedandfoundtobeorder radar. Ifanyenergyisreflectedbytheatmosphereitappearstobeonlyasmallportionoftotal.Con- on thebasisofknownrotationrateandcenterDopplershiftmeasuredusingaseparateX=23cm section reportedbyKarpetal.(1964).Theechospectrumisfoundtobecomparativelynarrow,suggesting made, andwhencomparedwiththelawobservedatlongerwavelengthsindicatesthatabout4-6dBtwo-way the solidsurfaceasreflectingagent.Noechopowercanbeobservedoutsidefrequencylimitsexpected the radarequipmentwasfunctioningproperly.Thusourmeasurementssupportlowvalueforcross of 14dBweakerthantheexpectedcomponent.Ananalysisscatteringlawforplanethasbeen (by asmuchafactorof2)supporttheviewthatatmosphericattenuationisresponsibleatleastinpart During theearlypartof1966LincolnLaboratoryHaystackRadar(X=3.8cm)wasemployedto I. INTRODUCTION Lincoln Laboratory*MassachusettsInstituteofTechnology,Lexington, Radar ObservationsofVenusat3.8-cmWavelength J. V.Evans,R.P.Ingalls,L.Rainville,andSilva (Received 12July1966) VOLUME 71,NUMBER9 902 4 proportional to(f/X),whereristheradiusofparti- length measurementswithbetter precisionarosearound difficulty. surface andfromrainispossibly onewayoutofthis situation inwhichthereis both scatteringfromthe as 1%oftheprojectedarea of theplanet.Ofcourse,a would havecrosssectionsintheRayleighregion,i.e., enough particlestoobtainatotalcrosssectionaslarge cle. Inthiscaseitbecomesratherdifficulttointroduce This explanationrequiredrelativelylargescatterers.If small icecrystalsorraindropsareinvokedthescatterers Doppler widthofthesignalswouldthenbeattributable energy wouldbereturnedfromtheplanetarysurface. R. E.Newell,privatecommunication).Thelarge from scatterers(e.g.,rainoricecrystals)intheVenusian atmospheric absorptionwouldberequiredtosohigh to therandommotionofthesescatterersand atmosphere (I.I.Shapiro,privatecommunication; reflections mightarisenotfromthesurfaceitselfbut were considered.Oneofthesewasthatthe3.6cm being duetoatmosphericattenuationofasignal limbs. Thusthefrequencyspectrumobservedcast reflected fromthesurface. doubt uponanexplanationofthelowcrosssectionas it shouldbedifficulttorecognizeanyenergyatfre- quencies correspondingtoreflectionfromnearthe sponsible forloweringthecrosssectionbyafactorof10 narrowed. Infact,ifatmosphericabsorptionisre- limbs (duetothegreaterpathlengthinVenusian atmosphere) onewouldexpectthesignalspectrumtobe absorption alongeachraywouldincreasetowardthe reflections fromthelimbsofplanet.Since (e.g., 10-20dBoneway)thateffectivelylittleorno The firstopportunitytorepeat theseshort-wave- Alternative explanationsforthisanomalousbehavior NOVEMBER 1966 1966AJ 71. . 902E Location Wavelength Frequency Antenna gain Antenna diameter System temperature Maximum peak Total waveguide Maximum average Beamwidth ments weremadeatawavelengthof3.8cmusingthe This paperreportstheresultsof3.8-cm-wavelength was madeusingtheMillstoneHill23-cmradar,sothat the timeof1966inferiorconjunctionVenus depolarization ofthesignals(VII).Adiscussion briefly describetheradarsystememployed(Sec.II) system. Atthesametimeafreshsetofmeasurements points andtheerrorinpositioningbeamisof cross section(V),thescatteringlaw(VI),and and theoperatingdatareductionprocedures companion paper.Inthesectionswhichfollowwe observations. The23-cmobservationsarereportedina recently completedLincolnLaboratoryHaystackradar order ofone-sixteenththisamount.Theangular The antennabeamwidthis4'arcbetweenhalf-power results follows. a directcomparisonbetweenthetwowouldbeavailable. power andalargerantenna. system isapproximately16dBmoresensitivethan ters oftheWestFordsystemasemployedbyKarpetal. turn, viz.,theidentityofscatteringagent(IV), offset thepointingfrom apparentpositionduring approach sothatnoattempt wasmadeatthattimeto apparent and“true”positions oftheplanetnearclose of digitalcommandsfromaUnivac490computer. radar. Forcompletenesswehaveincludedtheparame- the transmissionperiod.Subsequently, itwasnecessary the apparentpositionofVenuscontinuouslybymeans the WestFordsystemduetoincreasedtransmitter to directtheantenna“ahead” oftheapparentposition extent ofthebeamisadequate toincludeboththe (which occurredon25January).Thenewmeasure- (Sec. HI).Theprincipalresultsarethenpresentedin (1964) forobservationsat3.6cm.TheHaystackradar transmitter power transmitter power on Venus and otherlosses Table Iliststheparametersof3.8-cmHaystack At Haystacktheantennawasdirectedtopoint Table I.ParametersoftwoLincolnLaboratoryradars. © American Astronomical Society • Provided by theNASA Astrophysics Data System Tyngsboro, 66.1 dB 0.07° 7750 Mc/sec 0.5 dB 120 ft 105 kW 105 kW ~3.8 cm ~120°K II. EQUIPMENT Massachusetts Haystack OBSERVATIONS OFVENUSAT3.8cm Westford, Massachusetts Pleasanton, California 0.14° 59.8 dB 59.8 dB 8350 Mc/sec 40 kW 0.14° Receiving 60ft Transmitting 60ft 40 kW 74°K ~3.6 cm bistatic radar West Ford measurements thetransmitterwasenergizedfora during transmissiontoplacethegreatestamountof ization wereavailableforreception.Foralltypesof sense weretransmittedandbothsensesofcircularpolar- the transmittedpoweronplanetarysurface. length oftimeequaltothatexpectedforapulse modulated carrierwave.Inothers,pulsesof4,Land for anequalintervaloftimeduringwhichthereceived mean-square sense)theexpectedchangeinDoppler 0.5 msecwereemployedatrepetitionfrequenciesof15, signals weresampledandrecorded.Inthesimplest travel totheplanetandback,wasthenturnedoff serious consequence.Attimesremovedfrominferior less than110cps(limb-to-limb)this1-cpserrorisofno with respecttotheobserveratthiswavelengthisnever broadening duetotheapparentrotationofplanet experiments thetransmissionconsistedofanun- spread infrequency. owing tothelowertotalechopoweranditsgreater larger, buttheresolutionwithwhichitisusefulto introduced bythislinearapproximationissomewhat conjunction theerrorinDopplercompensation near inferiorconjunction.Sincetheover-allDoppler sufficiently high(10°)topermitusefulobservations frequency duringthatparticularreceivingperiod.The was accomplishedbychangingthefrequencyin0.1-cps Doppler shiftofthesignalsarisingfrommotion was adjustedcontinuouslytocompensateforthe were transmittedandamatchedfilterdelay-linewas determinations. Inthisschemephase-codedpulses 50, and60cps(approx),respectively.Also,apulse explore thesignalspectrumiscorrespondinglypoorer error introducedbyalinearfitislessthan±1cpsat steps atalinearratewhichbestmatched(inleast- employed atthereceivertoeffectcompression compression schemewasemployedforaccuraterange times whentheplanetisatanevaluationthat the planet’scenterwithrespecttoobserver.This were firsttranslatedinfrequencyto200000cpssuch an unmodulatedcarrierwaveandperformingaspec- the Haystackradarconsistedoftransmittingan was compensated.Thesignalsat200000cpswere trum analysisofthereturns.Thereceivedsignals 500 cps/sec.Apartialanalysis oftheechoeswas samples inphasequadrature weretakenatarateof a waythatthecontinuouslychangingDopplershift sized abankofdigitalfilters intheU490computer.A then filteredbymeansofa300-cps-widefilterand wide. Thedatasampleswere alsorecordedonmagnetic afforded byaFourieranalysis programwhichsynthe- total of13filterswasconstructed eachbeing12.5cps (Evans etal.1965). Radio waveshavingacircularpolarizationofone Throughout eachreceivingperiodthereceivertuning The simplesttypeofexperimentcarriedoutwith Ill OBSERVINGPROCEDUREANDDATAREDUCTION 903 CMH O'!O
904 EVANS, INGALLS, RAINVILLE, AND SILVA h) Table II. Continuous wave observations of Venus. UD OïUD Correction for atmospheric Cross attenuation section Date (1966) /max (cps) Trecho ( °K) b (cps) (dB) (%7rfl2) (a) Polarized component 18 Jan. 65.7 400 0 ±0.4 130.85 0 1.44 1 Feb. 61.0 325 63 ±0.5 121.08 0 1.10 9 Feb. 76.9 213 55 ±0.3 152.33 0.2 1.34 15 Feb. 91.6 105 25 ±0.51 181.6 1.2 1.11 21 March 140.8 8 89 ±0.54 289.08 0 1.22 30 March 146.5 9 28 ±0.16 289.08 0 2.08 15 April 154.3 4 51 ±0.25 304.69 0 2.92 5 May 161.4 1 28 ±0.24 304.69 0 1.33 17 May 164.5 1 39 ±0.21 304.69 0 2.29 28 May 166.0 1 02 ±0.33 304.69 1.3 3.28 15 June 165.3 1 23± .09 117.19 0 1.56 27 June 163.2 0 .83 ± .09 117.19 0 1.29 (b) Depolarized component 19 Jan. 64.0 15.94 ±0.41 132.82 0.057 4 Feb. 65.9 12.63 ±0.40 132.82 0.052 Fig. 1. Venus power spectrum observed at 3.8-cm wavelength on 1 February 1966. The expected position for reflections from the limbs have been indicated by arrows. Note the bumps in the spectrum which are thought to be reflections from anomalously divided by the mean of the values in these two windows rough regions on the surface of Venus. and then multiplied by the value of the system tempera- ture Ts which was determined by a separate calibration tape for later non-real time processing using a CDC procedure using a wide-band radiometer. The samples 3200 computer. Either the polarized (i.e., expected) or in the “noise only” spectrum were similarly normalized depolarized component of the signals could be analyzed by dividing by the mean value of the points in the in real-time, but not both. Thus the other polarization same two windows and multiplied by Ts. In this way was recorded in analogue form for later sampling and one ensured that effects due to differences in receiver processing. Here we discuss only the real-time results. gain between the signal and the noise calibration runs were removed. The noise temperature values were then Table II lists the days on which these observations subtracted from the signal-plus-noise ones to leave the were made. The signals obtained on 18 January and equivalent echo temperature in °K. These individual 1, 9, and 15 February were processed with an equivalent spectra were averaged over each day after weighting bank of 256 filters each 1.9 cps wide. For the measure- each in proportion to the duration of the receiving ments of the depolarized component of the signals period. (Table II) as well as the component displaying the expected polarization sense on 21 March and subse- Pulsed operation of the radar system was also quently, 64 filters (each 7.8 cps wide) were employed. possible, but imposed a loss in the average transmitted power. That is, it was not possible to raise the peak The contribution of receiver and sky background power in the pulses above the 100 kW level obtained noise (^120°K, Table I) has been removed from these when transmitting cw. Despite this a number of experi- spectra in the following way. After each period of ments were carried out with pulses of 4-, 1-, and 0.5- reception data samples were taken of noise alone, msec duration. In these, the reflected signals were while the antenna continued to point at Venus for a converted to the same 200 kc/sec intermediate fre- time equal to the length of the receiving interval. These quency in such a way that the Doppler shift was noise samples were processed in precisely the same way compensated. They were then transmitted to the Mill- as the signal-plus-noise to yield a noise spectrum. A stone Hill radar by cable, where an apparatus was mean was then taken of all the noise _spectra obtained available that could take data samples in such a way in any one day to obtain a function Pv(/) which was that they were made to keep in step with the motion taken to represent the power passband characteristics of the echo along the time base. These data samples of the receiver, since the noise can safely be assumed were recorded on magnetic tape and subsequently “white” over such a narrow interval. That is, the processed using the Millstone Hill radar computers weighting imposed by the 300-cps-wide receiver filter in the manner outlined by Evans et al. (1965). on the frequency spectrum was determined and used to obtain corrected spectra for both the signal run and IV. IDENTIFICATION OF THE SCATTERING AGENT the noise run which followed it. It was next assumed that the power in two narrow A spectrum obtained for the expected, i.e., “polar- spectral windows farther removed from the center of ized” component of the echoes on 1 February is shown the spectrum consisted of noise alone. All the values for in Fig. 1, which is representative of all others obtained. the power in the “signal-plus-noise” spectrum were If the signals are reflected only from the planetary
© American Astronomical Society • Provided by the NASA Astrophysics Data System 1966AJ 71. . 902E Table III.Short-pulserangedeterminationsat3.8and23cm. measured relativetothevalueforcenterof disk, i.e., lie inafrequencyband2/wide,where/isthe surface thenweshouldexpectalltheechoenergyto motion pasttheearthwasincluded,butsmallcom- Doppler shiftoftheechoesreflectedfromlimbs period forVenusandpolepositionofô=—70°,a=212° where aistheplanetaryradius(m)coapparent ponent (diurnallibration)introducedbytherotationof /max werecomputedusingEq.(1)inordertodetermine measurements at23-cmwavelength(Evansetal.1966). group (Carpenter1966).Theeffectoftheplanet’s as determinedbyShapiro(1964).Thesevaluesarein reasonable agreementwithonesobtainedbytheJPL are listedinTableII.Theangularrotationwowas the expectedDopplerwidthofsignals,andthese angular spinoftheplanetwithrespecttoearth planetary surfaceforthefollowingreasons: are indicatedinFig.1byarrows. computed assuminga247-dayretrograderotation sponds preciselytothatestimatedfromsimultaneous be statedthatthe3.8-cmenergyisreflectedfrom the earthwasneglected.Thevaluesof/soobtained ponent wouldberenderedundetectable bythenormali- with thesamemeanspeedassubradarpointon et al.1965),thisrequiresthatthereflectingagentmove zation procedureadoptedto produce thesespectra. echoes arereflectedonlyfromthesurface.Thatis, in allcasestothatpredictedontheassumption noted, however,thataweak, verybroad-bandcom- quencies beyondthepositionsmarkedbyarrowsin there isnoreliableevidenceforechoenergyatfre- the surfaceonbasisofanumbertests(Evans Since theechoat23cmhasbeenestablishedasfrom max surface. Indeed, itispossibletoidentify thesamefeaturesin Fig. 1norinanyoftheotherspectra.Itshouldbe (rad/sec) andXtheradarwavelength(m).Valuesof 0 max (e.g., Fig.1)canbeattributed tolocalroughregions. 20 Jan. 24 Jan. 4 Feb. Date lengthvalions23-3.8cm (1966) (¿¿sec)(cm)(UT)delay On thebasisofthesespectrumresultsaloneitcan (i) TheDopplershiftofthepeakechocorre- (ii) Thespectralwidthofthesignalscorresponds (iii) Someofthefinestructure inthespectrashown © American Astronomical Society • Provided by theNASA Astrophysics Data System Equivalent CentertimeTimedelay pulse Wave-ofobser-difference, 360 40 40 C /max= =t(2acOoA)PS,(1) 23 23 23 3.8 3.8 3.8 OBSERVATIONS OFVENUSAT3.8cm 2017 1519 1913 1944 1716 1708 - 5±14 - 5.5±14 -14 ±70 nals processedinanidenticalfashion.Theabsolutedelaysatthe lated usingthesamecodedpulsewaveform,andreflectedsig- the surfaceisresponsibleforscatteringatbothwavelengths. error, andtheechoshapesarestrikinglyalike.Thisconfirmsthat two frequencieswerefoundtoagreewithintheexperimental wavelength on24January1966whenthetworadarsweremodu- features areapparentlythesameasthoseobservedin rotation oftheplanet.Inaddition,somethese successive spectrashiftedinpositionduetotheapparent in Fig.1thebumpatabout+43cpsisthoughtto mitted and“compressed”onreceptionusingamatched- feature ß).Thisidentificationisdiscussedfurtherin bumps atapproximately—31and—40cpsare correspond toCarpenter’sfeatureG,thatat+26cps Haystack. Inbothcasesphase-codedpulsesweretrans- were madenearlysimultaneouslyatMillstoneand and featureF(Goldstein’sa),thetwo using cablestolinkthetwostations.Thedelayintro- filter delayline.Themodulationanddemodulationof Sec. VII. 1964 byGoldstein(1965)andCarpenter(1966).Thus the planetarysurface,short-pulse-rangedeterminations Carpenter’s featuresBandC,respectively(Goldstein’s accurately inseparateclosed-circuitmeasurements. duced bythesecables(^10/xsec)couldbedetermined the 3.8-cmsignalswereaccomplishedatMillstone in allcaseswithintheexperimentalerror.There observed atthetwofrequencies.Itcanbeseenthat range differences(23cmminus3.8cm)thatwere type weremade,thepulselengthsemployed,and the agreementinresidualsisremarkablygoodand This iscontraryinsenseto what wouldbecausedby which hasnotbeenaccounted for;i.e.,thedelay delays atthetwofrequencies oftheorder5/¿sec appears tobeasmallsystematic differencebetweenthe the presenceofinterveningionization, andinanyevent appears somewhatlargerat theshorterwavelength. Fig. 2.Plotsofechopowervsdelayobservedat23and3.8cm As aconclusivetestthattheechoesarereflectedfrom Table IIIliststhedayswhenobservationsofthis DELAYfyusec) 905 1966AJ 71. . 902E 2 906 ¿isec onthedelayscalehasbeenmadetoimprove either frequencybytheionizationlyingbetween earth andVenusonthebasisofpresentknowledge a delayaslargethiscouldnotbeintroducedat shape ofthetwoplotsprovidesseparateevidencethat distribution observedintheVenusechoindicates correspondence. Theshadedregionshowstheecho at thetwofrequencieson24Januarywhenashiftof4.5 subradar point.Theremarkableagreementbetweenthe spread inthedepthsofreflectingcentersnear expected froma“point”(i.e.,small)target.Thelarger the 3.8-cmechoisfromplanetarysurface. (Evans etal.1965). it laterbecamenecessarytowidenthewindowson is approximately2/inalltheearlyruns.However, The bandhoverwhichtheechoenergyiscomputed of thefiltersplottedinFig.1)hasbeenaveragedto obtain ameanvalueTechowhichislistedinTableII. reliably) andthisrequiredavalue¿><2/.The either sideoftheecho(toestablishbaselinemore section (i.e.,percentofira)hasbeencomputedandis corresponding valueofthepercentageradarcross measured, andthetotalechopowergivenby measurements thepeakechotemperatureTchois from thepulseexperimentswhichwerecarriedout, largely thatimposedbyimpreciseknowledgeofsome also listedinTableII.Theuncertaintythesevaluesis must beappliedtothecalculatedvaluesofcross of theparametersequipment(e.g.,antenna product ofthisandthereceiverbandwidth.Acorrection gain) andisprobablyoftheorder±1.5dB. probably oftheorder±1dBorbetter.Forcw fully illuminatethenearhemisphereofplanet.For section toallowforthefactthatashortpulsedoesnot and thesearelistedinTableIV.Inthecaseofpulse listed inTableIVisoftheorder±2dB,but significantly differentfromthatat23cm,andhence max measurements listedinTableIItherelativeaccuracy does notintroduceanyseriouserror(Sec.V).The corrections obtainedfromFig.11ofEvansetat.(1965) is probablyoftheorder±0.5dB. relative accuracyofthemeasurementsishigher— absolute uncertaintyinthecrosssectionmeasurements could beemployed.Asshownbelow,thisassumption the scatteringlawat3.8-cmwavelengthwasnot the purposesofmakingthiscorrectionweassumedthat max has notbeenoverestimated by10dB.Theantenna parameters oftheradarequipment (TableI)andthus Tables IIandIVdependsupon theknowledgeof gain hasbeenestablishedby observingradiosources, one mayquestionwhetherthe performanceoftheradar e Figure 2comparestheechopowervsdelayobserved The echoenergyineachofthespectra(i.e., A fewadditionalvaluesofcrosssectionareavailable The determinationofthe cross sectionsgivenin © American Astronomical Society • Provided by theNASA Astrophysics Data System V. RADARCROSSSECTION EVANS, INGALLS,RAINVILLE,ANDSILVA directional couplersandthereceiversystemtempera- the transmitterpowerismeasuredusingcalibrated which areinturncalibratedagainsthotandcoldloads. These testsofthecomponentsradarlargely preclude thepossibilitythatsystemasawholemay be malfunctioning.Nevertheless,anadditionaltestof ture canbedeterminedusinggas-dischargenoisetubes previously withthisradar(Ingalls1965),andvaluesof measurements istheattenuationintroducedin days, viz.,22and31March.Thecrosssectionof respectively. Thisisthesameasvaluereported Mercury observedonthesedayswas4.6and4.1%, cw radarobservationsofMercuryweremadeontwo the entiresystemwasconsideredessential.Accordingly, nikov etal.(1962)at40cm. 5 and6%reported,respectively,byCarpenter Goldstein (1963)at12.5-cmwavelengthandbyKotel- earth’s atmosphere.Resonantabsorptionbyneutral Handbook ofGeophysics)andindicatethatthetwo-way sphere beingnegligibleatthisfrequency.Estimatesof causes ofattenuation—thatintroducedbytheionos- oxygen anduncondensedwatervaporaretheprincipal loss inthezenithislessthan0.2dBatourwavelength. from anumberofsources(e.g.,theU.S.AirForce these constituentsintheatmosphereareavailable the totalverticalattenuationduetopresenceof due tothissourcewouldrise0.4dB.Thissmall correction hasnotbeenappliedtotheresultsinTable out atazenithdistanceofabout70°theattenuation Since thelargestpartofourobservationswerecarried II orIV. in Fig.3.Theerrorbarsshownarethesumsof indicated inthetable. introduced ontheseoccasionscouldbeestimatedfrom observations duringrainordensecloudweremadeon determinations). Asfarascan bedeterminedthecross computed standarddeviation inTcho(TableII)and only threeoccasionslistedinTableII.Theabsorption attenuation intheearth’satmospherecanbeappreci- junction andthemeanvalue atthattime(^1.2%)is section remainedroughlyconstant nearinferiorcon- the estimatedrelativeuncertainty inthemeasurements correction hasbeenapplied,themagnitudeofwhichis the observedriseinsystemtemperature,anda ably largerthantheaboveestimate.Fortunately, (i.e., 0.5dBforcwmeasurements and1dBforpulse e The onlyremainingsourceofsystematicerrorinthe 26 Jan.4.0msec500cps92.0°K1.32% 21 Jan.4.0msec250cps138.9°K1.03% 11 Feb.1.0msec1000cps17.6°K1.09% In thepresenceofrainorverydensecloud The variationofcrosssectionwithdateisplotted 2 Feb.4.0msec500cps79.6°K1.32% Date PulselengthBandwidthTechocrosssection Table IV.Long-pulseobservationsoíVenus. Percent 1966AJ 71. . 902E close tothatreportedbyKarpetal.(1964).From30 in whatseemsarandomfashion.Partofthisvariation March onwardsthecrosssectionappearstohavevaried can beattributedtothepoorersignal-to-noiseratio variations inreflectioncoeflicient,caused,forexample, point, butcouldconceivablyresultfromlarge-scale seen herearetoolargetoresultfromvariationsinthe by avariationinthedepthofhypotheticallayer which seemassociatedwiththerelativeroughness large aswereobserved.Variationsincrosssectionwith ever, thisseemsinadequatetoexplainvariationsas and henceincreaseduncertaintyinthevalues.How- if thesamesequenceofvariationsisobservedata lossy material.Atestofsuchamodelwouldbetosee scattering behaviorofthesurfacenearsubradar later inferiorconjunction. of theterrainatsubradarpoint.Thevariations these havebeensmall.Forexample,Evansetat. time havebeenobservedatotherwavelengthsbut introduced intheplanetaryatmosphere.Itispossibly appear tobechangesintheamountofattenuation fortunate thatpressureofotheractivitiesprevented is discussedfurtherinSec.VIII.Itsomewhatun- observed untilthesubradarpoint(whichcontributes significant thatnovariationincrosssectionwas light. basis ofthepresentdata. fluctuations wasoftheorderadayorweekon terminator. Itisconceivablethattemperaturechanges the bulkofechopower)laywithin10° using short-pulsemeasurements.Theseresultspermit because mostoftheechopowerisreflectedfrom as wecannotsaywhetherthetimescaleofthese occur intheatmosphereandtheseresultachange where aistheplanetaryradius andcthevelocityof between therelativedelaytand theangleofincidence0 be determined,becauseoftheuniquecorrespondence observed at12.5cm(Carpenter1966)wouldrequire the radarfrombeingusedtoviewVenusalmostdaily, the totalplanetaryatmosphericattenuation.Thispoint (1965) report±30%variationsat23-cmwavelength mean echopowervsdelaytmeasuredwithrespectto center ofthevisiblediskwherewavestraverse between theradiowavesand meansurfacenormal the angularpowerspectrumforsurfaceP(
908 EVANS, INGALLS, RAINVILLE, AND SILVA h) Table V. Puise observations employed to determine UD the echo power vs delay curve. OïUD Useful Date Pulse length Sample spacing range of delays 24 Jan. 40 ¿¿sec 20 Msec 10 - 200 Msec 20 Jan. 360 ¿usee 100 Msec 350 1000 Msec 28 Jan. 500 Msec 100 Msec 0.5- 3.0 msec 11 Feb. 1 msec 0.2 msec 1 5.0 msec 21 Jan.] 26 Jan. 4.0 msec 0.3 msec 24 msec 1 Feb.J
the same as at 23 cm, i.e., P()23. The difference between the two curves in Fig. 4 in the limb region (/> 15 msec) corresponds to a value 2d =5-6 dB [Eq. (3)]. The difference is not, however, as large as would be expected if 10 dB two-way attenuation occurred in the atmos- phere of Venus. In the second approach the spectra obtained on 18 January and 1, 9, and 15 February (Table II) were examined. In order to reduce the uncertainty in the echo intensity in the vicinity of the limb a mean spectrum was first constructed. To do this it was necessary to remove the variation of the spectral width /max as a function of date (Table II). Thus the power Fig. 4. The echo power vs delay law observed at 3.8-cm wave- length compared with that reported for 23 cm by Evans et al. (1965). The two curves have been normalized at 10 Msec, and the difference for large values of t is thought to indicate the effects of limb darkening caused by atmospheric attenuation at the shorter wavelength.
nature of the surface of Venus, and in part to the fact that few of the experiments conducted so far against Venus have achieved a resolution in delay comparable ^ to that which can be employed in studying the moon. ^ Two separate attempts were made to obtain the 2 distribution of echo power with delay. In the first, a g series of short-pulse experiments was carried out 2 using different pulse lengths in order to obtain the best > resolution near the leading edge, and yet measure over the largest possible delay interval. Table V summarizes d these measurements. An echo power vs delay curve was constructed from the results in the manner described by Evans et at. (1965). Some uncertainty in the curve was introduced by the fact that some of the sections of the curve did not overlap sufficiently (Table V). Despite this, the law obtained (Fig. 4) agrees well with that obtained previously at 23 cm for small values of delay. For delays greater than 1 msec the two curves diverge in a manner consistent with what would be expected as a result of atmospheric attenuation. That is, when the curves are renormalized at the origin we Fig. 5. The spectra obtained on 18 January through 15 February expect a difference of the form are here plotted against a normalized frequency scale (/max is the value of the computed Doppler shift of the limb with respect to that from the center of the disk). The two halves of each 10 logioP(0)23—10 logP()3.8= 2A (sec—1) dB (4) spectrum (e.g., Fig. 1) were first averaged and then adjusted vertically to obtain the best agreement in the vicinity of when we assume that in the absence of absorption the ///max = 0.1 to 0.2. The full line is a mean curve drawn through relative scattering law at 3.8-cm wavelength would be the points by eye.
© American Astronomical Society • Provided by the NASA Astrophysics Data System CMH O'!O
OBSERVATIONS OF VENUS AT 3.8 cm 909
in the two halves of each spectrum was averaged to UD obtain a single set of points for each day, and the echo power was then replotted on a logarithmic scale against a normalized frequency scale ///maX. These curves were then adjusted vertically for best agreement near the origin. It was found that this could be accomplished better at ///max=0.1 than at ///max = 0 because ap- parently the intensity of the reflection at the subradar point varies slightly from day to day. The results for the polarized component are shown in Fig. 5, together with a mean curve that has been drawn through points obtained for the four days. Figure 6 compares this mean curve with the A-band (12.5-cm) results reported by Muhleman (1965) when these two are adjusted for best correspondence at ///max=0.1. The angular power spectrum P($) and the normal- ized power spectrum P (/) are related through 2 2 1 2 (/max -/ ) / P(.4>) -dy, (5) J o (/, where Fig. 7. The echo power vs delay function for 5 band and X band <£= sin-'C^+y2) V/max] obtained by transforming the power spectra presented in Fig. 6 as described in the text. The dotted line indicates the effect of the smoothing that was introduced during the transformation (see, for example, Carpenter 1964). Equation (5) is process on the behavior near the origin. usually inverted to obtain P($) through /max (¿/¿/)[P(/)] undesirable operation of differentiating the spectrum P (0) °c COS0 dj. (6) 2 P(J) required in (6). In this method one obtains P(0) /maxSÍii(/> (/ —/max Sin)i by a simple Bessel transform of the autocorrelation An alternative method of inverting (5) to give the function of the signal p(r) where angular power spectrum has been developed by T. P M = (y (i)y (¿+ r) )/ (y2 {t) ), (7 ) Hagfors (private communication) which avoids the where y{t) is the amplitude of the signal envelope at time t and y(/+r) at a time r later. The autocorrelation function p(r) could be obtained from the data samples directly, but in practice it is faster to employ high-speed methods of digital frequency analysis (Cooley and Tukey 1965) and then take the Fourier transform of the resultant power spectrum to obtain p(r). 1 /+00 p(t) = — / P(j)eiwtdu. (8) 2tt 7-00 This procedure was followed in the present instance. The angular power spectrum P(0) was then obtained from cos0 r / k \ P(4>)d © American Astronomical Society • Provided by the NASA Astrophysics Data System CMH O'!O 910 EVANS, INGALLS, RAINVILLE, AND SILVA and X-band results and hence treats them equally. It has, however, tended to smooth out the variation of h) echo power vs delay in the region t<\ msec and the UD broken line in Fig. 7 indicates the variation obtained in this region when no smoothing is applied. As a test of the accuracy of the procedure carried out here we have plotted in Fig. 8 the echo power vs delay observed at 70 cm (Pettengill 1965), at 23 cm (Evans et al. 1965), and the 12.5-cm power spectrum results (Muhleman 1965) when transformed in this way. In the region t<\ msec there are differences in the behavior at the three wavelengths which in part reflect differences in the equivalent range resolutions achieved, and possibly day-to-day variations in the behavior at the subradar point. At larger delays the agreement is regarded as satisfactory. If we assume that in the absence of an atmosphere the X-band power spectrum would be the same as that observed at S band, the difference in decibels between the two curves should depend upon the secant of the angle of incidence 0 as in Eq. (4). The observed differ- ence (Fig. 7) appears to be consistent with the behavior Fig. 8. A comparison of the echo power vs delay laws obtained at three wavelengths. The curves have been adjusted vertically expected for total atmospheric attenuation of 2^4 =3.5 to demonstrate the agreement for delays ¿>1 msec. There are dB approximately. Thus, again we find evidence for differences for delays t less than 1 msec which are thought to represent the different delay resolution obtained in the three absorption, but of an insufficient amount to fully measurements, and possibly day-to-day differences in the scatter account for the reduced cross section. ing behavior near the subradar point. For the region msec the agreement seems reasonably good, thereby lending confidence to the transformation procedure employed to obtain VI. DEPOLARIZED COMPONENT the results shown in Fig. 7. Only a limited amount of time was devoted to examining the depolarized component of the signals. carried out large oscillations of the function P( VENUS X = 3.8 cm DEPOLARIZED COMPONENT 4 FEB 1966 ^ /N -o—o V-v/ ^ -80 -60 - 40 - 20 0 20 40 60 8< FREQUENCY RELATIVE TO PREDICTED DOPPLER (Hz) Fig. 10. The mean curve of Fig. 5 is here compared with the Fig. 9. Venus power spectrum observed at 3.8-cm wavelength mean “depolarized” spectrum obtained from the results obtained on 4 February 1966 for the depolarized component of the echoes. on 19 January and 4 February 1966. Error bars have been plotted Note the absence of any central peak and the greatly reduced to indicate the rms scatter of the points. It can be seen that intensity compared with Fig. 1. unfortunately the scatter is quite large for ///max^0.9. © American Astronomical Society • Provided by the NASA Astrophysics Data System CMH O'!O OBSERVATIONS OF VENUS AT 3.8 cm 911 pated, the echo power is considerably weaker than that UD in the specular component. In addition, the spectrum, unlike that shown in Fig. 1, does not exhibit a central peak corresponding to the strong reflections from the region near the subradar point. Figure 10 compares the mean spectrum (Fig. 5) obtained for the polarized component with that for the depolarized signals. The displacement of the curves reflects the difference in the total echo power in the two components, i.e., approximately 14 dB. This ratio may be compared with that obtained by Levy and Schuster (1964) and Carpenter (1966), at 12.5 cm (12 dB) and at 23 cm by Evans et al. (1966), namely 15 dB. It seems possible that the value obtained will depend to some extent upon how many rough regions Fig. 12. The percentage polarization of the 3.8-cm results are visible on the surface at a given time, because rela- plotted in Fig. 11 has been obtained using Eq. (10). Also shown tive to their environs these reflect more strongly in the (shaded) are the results reported by Carpenter (1966) for 12.5-cm depolarized sense. However, to the accuracy of the wavelength. present measurements there seems no marked variation of this ratio with wavelength, again suggesting that the same Bessel transform routine [Eqs. (7)-(9)]. Because surface of Venus does not exhibit a wide distribution of of the large uncertainty in the original spectrum (Fig. small-scale structure sizes as is encountered for the 10) the curve shown in Fig. 11 for the depolarized moon (Evans 1965). component D(t) should be treated with caution. We Figure 11 shows the polarized P(t) and depolarized do not believe, for example, that the echo power D(t) components of the echo power when plotted with increases with delay up to /= 10 msec as shown—merely respect to delay. The polarized component shown is the that there is little variation in D(t) over this interval. solid curve of Fig. 7 and the depolarized plot was Also, it is probably unlikely that the depolarized obtained from the mean spectrum (Fig. 10) using the component D(t) is as strong as the polarized component P(t) at the limb. Since any atmospheric attenuation would reduce D(t) and P(t) equally, we may ignore its presence, and compute the percentage polarization p as P{t)-D(t) p = X100. (10) P{t)+D{t) The curve obtained for p from the data shown in Fig. 11 is presented in Fig. 12, where it is compared with the results reported by Carpenter (1966) at 12.5-cm Fig. 11. The plot of the echo power vs delay P(t) for X = 3.8 cm shown in Fig. 7 is here replotted with that obtained for the depolarized component [/}(/)] from the mean spectrum shown Fig. 13. Spectrum of Venus echoes observed by Carpenter in Fig. 10. The uncertainty in the original spectrum is sufficiently (1966), shown together with one obtained at Haystack (this large that the behavior shown for />20 msec has not been deter- paper). Note the correspondence in the positions of the features mined reliably. in the two spectra. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1966AJ 71. . 902E 912 scales havebeenadjustedforthebestcorrespondencebetween frequency difference)superimposed(closedcircles).Thevertical the positionsobservedinpresentobservations(scaledfor cannot beexpectedsincetheapparentspinofVenuswithrespect Carpenter (1966)inFig.13ofthatpaperisherereproducedwith VII. SURFACEFEATURESANDTHEVENUSROTATIONRATE the twoDopplerhistories.Perfectagreementbetween of theearth’sorbit. the sameatsuccessiveinferiorconjunctions,owingtotiltof to aterrestrialobserverasfunctionofdatewillnotbeprecisely the poleofVenuswithrespecttoeclipticandeccentricity parable amountofuncertaintyexistsinthe3.8-cm decreasing wavelength(EvansandHagfors1966). moon echoesithasbeenshownthatpdecreaseswith wavelength. TheshadedregioninFig.12denotesthe dependence inthisquantityforVenus.Inthecaseof draw attentiontothepresenceofanumberregions curve, sothatitisnotpossibletoinferanywavelength range ofuncertaintyinCarpenter'sresults.Acom- on thesurfaceofVenus,whichareanomalously regions andgiventheirpositions(albeitthelocationsof in Fig.8.Carpenter(1966)hasidentifiedsevensuch reflecting, i.e.,theydonotconformtothelawshown first beseenontheapproachinglimb(positiveDoppler position oftheplanetand inclinationoftheplanet's by Goldstein(1965).ThemethodwhichCarpenter drawn attentiontothe nearly earth-synchronous shift) andlastontherecedinglimb(negativeDoppler some areambiguous)inacoordinatesystemproposed relative tothecenterofdisk.Mostfeatureswill an examinationofthetimehistoryitsDopplershift inferior conjunctions.Thus, because thepoleofVenus depends uponthelocationofregion,pole shift), buttheprecisepathfromoneextremetoother present nearlythesameface towards earthatsuccessive orbit withrespecttotheecliptic. Carpenterhasalso rotation ofVenus,i.e.,thefact thatVenusappearsto (1966) hasidentifiedthepositionofaregioninvolves Fig. 14.TheDopplerhistoryofseveralfeaturesidentifiedby Carpenter andGoldstein(1963)werethefirstto © American Astronomical Society • Provided by theNASA Astrophysics Data System EVANS, INGALLS,RAINVILLE,ANDSILVA is nearlyperpendiculartotheplaneofitsorbit(Shapiro coplanar, featuresseeninVenusspectratendtore- published byCarpenter(1966)obtainedapproximately mately thesamepositions.Sincewedonothavean appear inthesameplaceatsuccessiveinferiorconjunc- obtained atHaystackaboutanequalamountof 6 daysbeforeinferiorconjunctionin1964withone four featurescanbeseenineachspectrumapproxi- tions. ToillustratethisweshowinFig.13aspectrum 1964), andtheorbitsofearthVenusarenearly time priortoinferiorconjunctionin1966.Thesame formation publishedbyCarpenter(1966)incomparison with ourowntoestimatetherotationperiod.Thatis, adequate numberofspectrawithwhichtoredetermine fication ofthefeaturesiscorrect.Itpossiblethat the positionsoftheseregions,wehaveusedin- Venus rotationisearth-synchronousassuggestedby features obtainedafteranintervalthatcorrespondstoa rotation of—244days.Itshouldbeemphasizedthat attempted toidentify(B,C,D,F,andGinCarpenter's then comparedwiththetimehistoryreportedby the relativeDopplershiftsoffeaturesthatwehave reich andPeale1966;Bolomo,Colombo,Shapiro earth-synchronous attitudeareextremelyweak(Gold- couples whichwouldtendtomaintainVenusinan close, butnotpreciselyequaltothis.Itseemsthat this resultdependsupontheassumptionthatidenti- Carpenter. Figure14showsthebestalignmentof of howthislockedpositioncouldbebroughtabout. the basisofpresentevidenceonecannotruleoutaperiod rotation) havebeenplottedasafunctionofdate,and by theobserverslistedEvansetal.(1965).Insome Carpenter (1964)withaperiodof243.16days,yeton wavelength isplottedinFig.15usingvaluesreported instances, whereobservationshavebeenmadebythe 1966), andhencethereisatpresentnounderstanding value, buttheerrorbarsaredrawntoinclude mean (1.2%)observedatinferiorconjunction.The Thus, Fig.16indicatesthemostrecentlyreported consistencies inthereportedvaluesaretobefound. same groupatsuccessiveconjunctions,somein- largest andsmallestvaluesthatwouldbepermittedby earlier observations.The3.8-cmvalueplottedisthe fall inthecrosssection. difficult toexplainunlessabsorptionintheatmosphere rapid fallincrosssectionbetween12and4cmseems surface everywhere,andis partly responsibleforthe of Venusisinvoked.Itis,however,conceivablethata at 3.8cmbyatmospheric absorption,onewould thin layerofextremelylossymaterialoverliesthe require approximately5-dB one-way attenuationofa ray whichpenetratestheatmosphere ofVenusverti- The radarcrosssectionofVenusasafunction In ordertoaccountforthelow crosssectionobserved A. CauseoftheLowCrossSection VIII. DISCUSSIONOFTHERESULTS 1966AJ 71. . 902E 2 complete computationyieldstheresultthata5dB energy isreflectedfromthecenterofdisk.A cally. Thisfollowsbecausethelargestpartofecho presence ofatmosphericattenuation.Inbothcases section of10.6dB. one-way attenuationwouldcauseareductionincross definite evidenceofsuchlimbdarkeninghasbeen observed atalongerwavelengthdetermineifindeed scattering lawat3.8cm,andbycomparingitwiththat largely uponthebehaviorobservedforreflections not regardedasserious,sinceatestofthistypedepends found, butthetwo-wayattenuationdeducedisinone there islimbdarkeningaswouldbeexpectedinthe from thelimbswheresignalsareextremelyweak case 5.5dBandtheother3.5dB.Thisdiscrepancyis and subjecttothelargestuncertainty.Despitethis, crepancy canberesolvedatleastinprinciple.Thefirst absorption aslarge10dB. is tosupposethattheangularscatteringlawof dependence hasbeendetectedinthescatteringbe- discount entirelythisexplanation,butnowavelength comparison ofthescatteringlawsreportedinSec.V This typeofchangewithdecreasingwavelengthis surface at3.8cm,unmodifiedbyatmosphericabsorp- the resultsdonotpermitavalueoftwo-way must causethevaluefortwo-wayabsorption depolarization behavior.Itistrue,however,thatthe would provideanunderestimateand,inreality,the observed forthemoon(Evans1965).Inthiscase havior observedatlongerwavelengthsorinthe deduced inSec.Vtobealowerlimit.Thismeansthat that relativelymorepowerisreturnedfromthelimbs. tion, doesdifferfromthatatlongerwavelengths,and true valueofAmightbe5dB.Itisnotpossibleto effects ofachangeinthescatteringlawwithwavelength Hence, byargumentssimilartothoseofEvansand absence ofabsorptioncannotbelessthan3.5%ira. taking avalue2^4=5dB,thecrosssectionin measurements (Pettengillet at. 1962;Carpenter1966). constant ofthesurfacemateriale>2. Pettengill (1963)inthecaseofmoondielectric model isavailablefromtheradiopolarizationresults However, ifClarkandKuzmin’s valueforthedielectric values e~4obtainedfromlonger-wavelengthradar homogeneous (e.g.,thedensityincreasingwithdepth) The lattermightariseifthesurfacewerebothin- intrinsic reflectioncoefficientat3.8-cmwavelength. be toinvokebothatmosphericattenuationandalower constant istakentoapplyfor theuppermostmaterial reflection coefficientof5.3% andwouldthusrequire responsible forthereflections at3.8cm,weobtaina of ClarkandKuzmin(1965)whichyieldavaluefor and somewhatlossy.Supportingevidenceforsucha atmospheric attenuationwith atwo-waytotal2^4—6 the dielectricconstantofe=2.5ztl,incontrastto Two attemptshavebeenmadetoderivetheangular There arethreewaysinwhichthisapparentdis- The secondmeansofresolvingthediscrepancywould © American Astronomical Society • Provided by theNASA Astrophysics Data System OBSERVATIONS OFVENUSAT3.8cm point obtainedat3.8-cmwavelengthformmostofthediscussion according tovariousobservers.Reasonsforlowvaluethe with theamountofattenuationinferredinSec.V. dB. This,aswehaveseen,canperhapsjustbereconciled Thus a25%temperaturedifferentialbetweenthe with thetemperatureatbaseofatmosphereas limbs. Thedifficultywiththismodelisthatonewould agreement wouldbetointroducevariationsinthe center ofthediskandlimbswouldallowavalue absorption acrossthefaceofdisk.If expect thespectratoappearlopsidedasVenusmoves 2^4 =10dBatthediskcenterand4near and N2thentheone-wayattenuationAwouldvary arises inadenseatmosphereconsistingonlyofCO2 basis ofthismodel,ifitbesupposedthatthevariation spectra, althoughtheechopowersintwohalvesof such systematicasymmetryhasbeendetectedinthe distribution wouldcertainlybecomeasymmetrical.No away fromcloseapproach,sincethetemperature (Sec. VIII). mean surfacevariesbyafew km,sincetheabsorption significant amount(seebelow).Asystematicchangeof one spectrumsometimesdifferbyastatistically lengths (BarrettandStaelin1964)appliesalsotothe cross sectionwithdatewouldalsobeexpectedonthe of temperaturewithphaseobservedatradiowave- a denseCO2-N2atmosphere with thepresentresultsif systematic fashion.Itwould still bepossibletoreconcile atmosphere. Whilethecross section hasbeenobserved the heightofsubradar point withrespecttothe to risesinceinferiorconjunction ithasnotdonesoina (Barrett andStaelin1964) Fig. 15.ThecrosssectionofVenusasafunctionwavelength The thirdmeansofbringingalltheresultsinto 14 16 A cr o ui in in 10 UJ o Q 12 o cr O 8 in 4 6 RADAR CROSSSECTION OF VENUS A cc(l/r4.6)( dB)i (11) 10cm Im WAVELENGTH 913 1966AJ 71. . 902E models wereconsidered.The firstofthesemodels particles withlargerdiameters. Finally,threecloud length thereislittlescattering, andhencethepre- dominant effectisoneofpure absorption.Inthefifth introduced. Forparticlesmuch smallerthanthewave- and sixthmodelsscatteringwas introducedbyassuming particles witharatioofimaginaryandrealparts models wereattemptstomatchtheaeolosphericmodel the complexdielectricconstante,/e=0.01were of Öpik(1961)totheresults.Inthirdmodel,dust Plummer andStrong1966).Thethird,fourth,fifth is somereluctancetoacceptamodelofthistype represent theatmosphereofVenus(Plummer1965; become importantonlyatveryhighpressuresthere process—the transientcollisioninduceddipolemoments in aCO2-N2atmosphere(Barrett1961).Sincethese models theabsorptionisintroducedbyanonresonant Table VIsummarizeseightsuchmodelsconstructed varies withthesquareofatmosphericpressure for variousmodels(afterBarrettandStaelin1964). by BarrettandStaelin(1964).Inthefirsttwoofthese value of3.8-cmradarobservationsasameans and 12cmfortransmissionthroughtheatmosphereofVenus for thevariationofradiotemperaturewithwavelength. H2O intheatmosphere,variationscrosssection 914 distinguishing betweenanumberofmodelsthe Pollack andSagan1966)havedrawnattentiontothe For example,ifwatervaporweretheresponsibleagent, and largeweatherpatternsgovernthedistributionof absorption mightoccuracrossthediskofplanet. atmosphere ofVenusthatcanbeinvokedtoaccount that weobservecouldbeexplained. r (D. H.Staelin,privatecommunication). Model 8 (a) 3 (a) 7 (a) 6 (a) 5 (a) 4 (a) 2 (a) 1 (a) No. Table VI.Valuesofone-wayattenuationatwavelengths3 A numberofauthors(BarrettandStaelin1964; Other wayscanbeinvokedinwhichvariationsof (b) (b) (b) (b) (b) (b) (b) (b) © American Astronomical Society • Provided by theNASA Astrophysics Data System Dust, D<0.6mm DuSt e/€r=0.01 Cloud Cloud Cloud Dust, Z)<0.3mm CO2-N2 C0-N (H2O) (t^v) (scattering) (scattering) (pure abs.) (Lapse rate (Lapse rate i 2 = 7°K/km) = 4.86°K/km) B. VenusAtmosphere Model EVANS, INGALLS,RAINVILLE,ANDSILVA /i: =5.55X10- ir=l.94X10- 3 3 3 1000 atm 3 300 atm 3 300 atm 3 100 g/m 100 g/m 100 g/m 100 atm 3 20 atm 3 10 g/m 10 g/m 10 g/m 2 atm 0.1 g/m 1.0 g/m attenu- 330 ation 3 cm12 33 16 (dB) Wavelength 0.97 0.75 0.50 5.1 0.34 0.61 0.07 0.05 6.1 0.58 5.2 3.0 1.6 20.6 0.32 0.06 0.15 0.02 4.05 0.40 0.32 0.04 2.1 0.19 1.55 section withwavelengthshown inFig.15. in partisresponsibleforsome ofthevariationcross variation ofreflectioncoefficient withwavelengththat exclude thepossibilitythat thereisasystematic of model6(TableVI),but unfortunatelywecannot discriminate betweenthetwo remainingmodelsinfavor and notthesquareoffrequency.Thiswould models (1and2)wemaysaythatanatmospheric pressure ofatleast200atmseemstoberequired,and tion coefficientthatdependsdirectlyuponfrequency way attenuationintheatmosphereat3.8cmisnotless that themicrowaveabsorberresponsiblehasanabsorp- at thiswavelength.Ifisthecase,thenitclear and 7)canbeeliminatedimmediately.OftheCO2-N2 moments, andhenceareeffectiveabsorbers,whereas observed at12.5-cmwavelength(11.5%)islowerthan cloud models(6and8)mightreadilypermitsuchvaria- attenuation couldresultfromsuchanatmosphere.The a meansmustbefoundinwhichlargevariations dependent uponmolecularcollisionsfortheinduction tions. Itisinterestingtospeculatethatthecrosssection than 2dB,sothatanumberofthesemodels(e.g.,4,5, of thedipolemoment. stems fromthefactthatthesemoleculespossessdipole model 8)cangivegoodagreementwiththeobservations prevailing onVenus,orthatmorethanoneofthese the meanofalllongerwavelengthmeasurements without requiringexcessivesurfacepressures.This how molecularresonantabsorbers(e.g.,watervapor— the CO2typeabsorptionisanonresonantprocess do, however,bringuponeimportantpoint.Theyshow VI) isachievedbyintroducingforthecloudsubstance second cloudmodelafrequencysquareddependenceof with complexdielectricconstant5(1-0.05J).Inthe absorbing agentsisinvolved.ThemodelsinTableVI considered absorptionbywatervapor.Itispossible tions areresistedbyaviscousforce.Thelastmodel that noneofthesemodelsresemblesthesituation a liquidcomposedofpolarmoleculeswhosefreerota- assumed acloudlayerconsistingoforganicparticles the absorptioncoefficient(oropticaldepthr—Table (say 16%)becausethereis1.5-dBtwo-wayattenuation The apparentlimbdarkeningsuggeststhattheone- 27 June•1.29 28 May2.943.28 30 March0.402.08 21 March0.661.22 15 June•••1.56 17 May0.712.29 15 April1.052.92 15 Feb.1.081.11 18 Jan.1.061.44 Table VII.Ratioofpowersinthetwohalves 5 May0.721.33 1 Feb.1.141.10 9 Feb.0.981.34 Date topos.DopplerCrosssection of theVenusechospectra. Ratio neg.Doppler 1966AJ 71. . 902E spectrum (recedinglimb)tothatinthepositivehalf for eachofthecwmeasurementslistedinTableII is giveninTableVII,togetherwiththecrosssection when measurementshavefollowedatapproximately by valuesfortherationearunity.Thesignificanceof no obviousrelationtotheratiopertainingonsame for thatday.Ascanbeseenthecrosssectionbears always precededbyalowvaluefortheratio,and day. Itiscurious,however,thatfrom21Marchonwards, in theatmosphereandthattheseareevidentfirston cases. However,itispossiblethat“clearings”appear two lowvaluesforthecrosssectionwerebothpreceded this behaviorishardtoassesssincethereareinsufficient 15-day intervals,alargevalueforthecrosssectionis cross sectioninagreementwithourobservations.The at positiveDopplershifts.Some10-20dayslaterthese the approachingsidewheretheygiverisetomorepower microwave absorber,e,g.,watervaporwhoseabundance ratios wouldthengiverisetosuccessiveincreasesin the crosssectiontorisemarkedly.Asuccessionofsmall little fasterthantherotationofsurface. time intervalinvolvedisconsistentwitharotationof “clearings” maycrossthesubradarpointandcause the limbdarkening)andnotmorethan6dB(from of 3.8-cmwavelengthwavesnotlessthan2dB(from the atmospherewhichiscomparablewithorperhapsa some ofthese,andwouldfavormodelsinvolvinga atmosphere, thevariabilityseemsinconsistentwith could bereproducedbyanumberofmodelsthe dielectric constantof2.Whilethetotalabsorption reduced crosssection).Thisplacesalowerlimitonthe atmosphere ofVenusintroducesaone-wayattenuation might varyovertheplanetarydisk. operation oftheradarequipmentrequiredtomake numerous tomentionbynamewhocontributedthe The ratioofthepowerinnegativehalf C. ModelAccountingfortheVariableAttenuation The radarresultspresentedhereinindicatethatthe We aregratefultoanumberofourcolleaguestoo © American Astronomical Society • Provided by theNASA Astrophysics Data System ACKNOWLEDGMENTS IX. CONCLUSION OBSERVATIONS OFVENUSAT3.8cm measurements describedherein.Specialthanksaredue Barrett, A.H.,1961,Astrophys.J.133,281. Beckmann, P.,andKlemperer,W.K.1965,/.Res.Natl.Bur.Std. Barrett, A.H.,andStaelin,D.H.1964,SpaceSei.Revs.3,109. N. M.Brenner,C.A.Clark,andJ.E.Morriellofor writing thecomputerprogramsinvolvedinthiswork. Bolomo, E.,Colombo,G.,andShapiro,I.1966,Presentedto and supportofJ.S.Arthur,P.B.SebringM.L. R. E.Newell,G.H.Pettengill,I.Shapiro,andW.B. Smith. Wearealsopleasedtoacknowledgetheinterest .1966,ibid.71,142. Carpenter, R.L.1964,Astron.J.69,2. Stimulating discussionshavebeenheldwithT.Hagfors, Cooley, J.W.,andTukey,W.1965,Math.Computation19, Clark, B.G.,andKuzmin,A.D.1965,Astrophys.J.142,23. Stone. Carpenter, R.L.,andGoldstein,M.1963,Science142,381. Evans, J.V.1965,Res.Natl.Bur.Std.69D,1637. Ingalls, R.P.1965,PresentedtothefallURSIMeetingin Evans, J.V.,andPettengill,G.H.1963,Geophys.Res.68,423. Evans, J.V.,Brockelman,R.A.,Henry,C.,Hyde,G.M., Karp, D.,Morrow,W.E.,andSmith,B.1964,Icarus3,473. Muhleman, D.O.,Goldstein,R.,andCarpenter,R.1965,IEEE Muhleman, D.O.1965,J.Res.Natl.Bur.Std.69D,1630. Kotelnikov, V.A.,etal.1962,Dokl.Akad.Nauk.SSSR145,1035. Evans, J.V.,Brockelman,R.A.,Dupont,E.N.,Hanson,L.B., Öpik, E.J.1961,/.Geophys.Res.66,2807. Goldreich, P.,andPeale,S.J.1966,Nature209,1117. Pettengill, G.H.1965,/.Res.Natl.Bur.Std.69D,1617. Goldstein, R.M.1965,RadioSei.,J.Res.Natl.Bur.Std.69D, Walker, R.G.,andSagan,C.1966,Icarus5,105. Pettengill, G.H.,andShapiro,I.1965,AnnualReviewof Pettengill, G.H.,andHenry,J.C.1962,/.Geophys.Res.67,4881. U. S.AirForce.1960,HandbookofGeophysics(TheMacmillan Pettengill, etal.1962,Astron.J.67,181. Shapiro, I.1964,Trans.IAU12F.SeealsoJ.Res.Natl.Bur. 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