V Session: RADAR OBSERVATIONS OF THE PLANETS A Review of Radar Studies of Planetary Surfaces G. H. Pettengill

Arecibo Ionospheric Observatory,! Arecibo, Puerto Rico In recent years, radar has been used to study the surfaces of the planets Mercury, Venus, Mars, and Jupiter. In the case of Venus, attenuation in the planetary atmosphere at short wavelengths has also been reported. For Mercury and Venus, where the diurnal rotation is difficult to establish by other means, radar has provided a clear-cut determination of the sidereal periods as 59 and 247 days, respectively. Mercury is found to possess surface conditions not unlike those on the Moon. Venus appears to have a surface considerably denser and smoother than the Moon, but displaying several localized reo gions of scattering enhancement. Mars appears smoother than the other planets, with a marked degree of surface differentiation. Except for one brief period of observation in 1963, Jupiter appears exceedingly ineffi cient as a refl ector of decimetric radio energy.

1. Introduction Institute of Technology to justify an attempt to observe Venus. In their report of this attempt [Price et al. , The extension to the planets of radar techniques in 1959], success was clai med on the basis of two obser­ use against the Moon presents a severe technical vations made on 10 and 12 February 1958. The signals challenge. The closest and most easily detected were quite weak, however, barely exceeding 3 standard object beyond the Moon is Venus; but even under the deviations of the accom panyin g noise, and were most ideal conditions this planet returns an echo obtained only after exte nsive analysis on a di gital some 5 million times weaker than does the Moon. co mputer. On the basis of the agreement in Doppler Nevertheless, there now exist a number of facilities shift with the expected valu e and the self- consistency with sufficient sensitivity to observe not only Venus, of the values obtained on the two days, it was felt but Mars, Mercury, and perhaps Jupiter. that th e results were genuine. A radar cross secti on Far less is known about the planets than about the for the planet of approximately 7m2 (w ith a being th e Moon, and, ther efore, there is potentially more value planetary radius) was obtained. Measure ments in applying radar methods to the study of planetary made at subsequent close approaches of the planet, surfaces. The techniques are generally the same, however, have found a different valu e for the orbital but because of the weaker echo signal strengths more dimension, and a very much lower valu e for the eros compromises with data resolution mu st be made. It section than the earli er work. Therefore, it must is only in the pas t several years that radar information be assumed that for so me reason a spurious response on planetary surfaces is becoming available. was present which gave ri se to the early results. Of particular importance is the extension to the planets of the range-Doppler mapping techniques At the following conjunction in 1959, using muc h the developed in connection with the Moon. Since the same equipment, Price and Pettengill [1961] failed to resolution inhere nt in these methods depends pri­ observe echoes from Venus. Evans and Taylor [1959] marily on the signal-to- noise availa ble, and not on working at the 10drell Bank Experimental Station of the angular resolution, they may yield far more information University of Manchester, England, reported a success­ on surface details of di stant planets than visual obser­ ful result in agreement with the 1958 measurement. vation. In the case of Venus and Jupiter, the greater Their signal was also weak, however, and they did not penetrating power of radio wavelengths also permits exclude the possibility that it might have been caused investigation of s urface laye rs inaccessible to optical by noise alone. observers. An immediate and obvious application By the inferior conjunction of spring, 1961 , sufficient of this capability is to the problem of determining rota­ capability had become available to permit attempts to tion rates where th ese are poorl y known because of observe Venus by groups not only at the Lincoln Labo­ distance or obscurati on. ratory [Pettengill et al., 1962] and 10drell Bank [Thomson et al., 1961], but also at the Jet Propulsion 2. History Laboratory of the California Institute of Technology [Victor and Stevens, 1961], the Radio Corporation of The first serious discussion of the possibilities and America [Maron et al., 1961], and in the Soviet Union technical difficulties of using radar to explore the [Kotelnikov et al., 1962a]. The 1961 measurements planets was given by Kerr [1952]. It was not until were in reasonably good agreement concerning the 1958, however , that s ufficient radar capability became values of the astronomical unit which were obtained available at the Lincoln Laboratory of Massachusetts and in the cross section -approximately O.17m2 • But the IPL and MIT results required that the planet I Operated by Co rn ell Un iversit y with the support of the Advanced Research Projects Agency under a research contract with tlfe Air Force Offi ce of Scientific Research. OAR. rotate very slowly on its axis, perhaps as slowly as its 1617 orbital rate around the Sun, while the Russians re­ Although largely unpublished, the results of these ported observations which led to a rotational period measurements appear to confirm the earlier conclu­ of 10 or 11 days_ sions that Venus is smoother than the Moon and Thp. inferior conjunction of fall, 1962, was again rotating with a retrograde period of about 250 days. observed in the USSR [Kotelnikov et aI., 1963a], and Among the most interesting of those that have been by JPL [Goldstein, 1964a; Carpenter, 1964). By published, however, is a report of observations made measuring the way in which the bandwidth of the at a wavelength of 3,6 cm [Karp et aI., 1964]. The returned signal varied with the date of observation, low reported cross section of 0.009 and the wide JPL was able to establish that the planet possessed a spectral dispersion has been interpreted by the authors retrograde rotation of about 266 days_ The Soviet as caused, at least in part, by significant absorption workers were able to reproduce their results of 1961 in traversing the atmosphere of the planet. from which they had deduced a rotation of about 10 days, but could not satisfactorily explain them_ They 3. Techniques and Relative Detectabilities also reported results in agreement with a retrograde rotation of about 250 days' period_ As brought out in the introduction, the planet most In addition, in 1962, two new groups reported obser­ easily detected by radar is Venus, which at inferior vations of radio echoes from Venus. Working at conjunction (closest approach) still returns an echo 50 Mc/s, Klemperer et aI., [1964] reported a cross 5 million times (or 67 dB) weaker than the Moon. The section of approximately 0.27Ta 2 • In common with enormous gap in level was spanned in less than 15 the groups working at higher frequencies he noted that years as radar systems and techniques were improved. the Venusian surface slopes were significantly smoother While it seems unlikely that the next 15 years will see than the Moon. A series of measurements at 38 Mc/s a corresponding further improvement, it is interesting was also reported by James a~d Ingalls [1964]. They to ask what targets would be uncovered with an addi­ found a mean cross section of about 0.15 times the tional improvement in performance. Figure 1 plots geometrical cross section but with substantial variation the relative signal strength to be expected from the from day to day. various planets and their moons, under the assumption In June of 1962, radar detection of Mercury was that they scatter like Venus. Although this assump­ reported in the USSR [Kotelnikov et aI., 1962b], using tion is now known not to be strictly true for the inner much the same equipment and techniques as were planets, and is likely to be considerably in error used shortly afterward to observe Venus. The signal for the giant planets, it still serves as an approxi­ was not strong; however, detection seems reasonably mate guide. Along the abscissa is shown the echo certain. A fractional cross section of about 0.06 was delay for each target. The dates shown for Mars and obtained, which is substantially less than for Venus Mercury mark the positions for the opposition and but comparable with the value obtained for the moon. inferior conjunction, respectively, when these occur in In 1963 JPL [Carpenter and Goldstein, 1963] also the months shown, Perhaps the most immediate and reported a detection of Mercury in no severe disagree­ obvious conclusion is how large a fraction of the solar ment with the earlier Russian result. system can be covered with a further improvement Radar measurements of Mars have been reported equivalent to that necessary to achieve the first Venus during the opposition of early 1963 by both JPL [Gold­ echoes. stein 'and Gillmore, 1963], and the Soviet group [Kotelnikov et aI. , 1963b). Both groups of workers found a fractional cross section which varied according to the region of Mars under observation from 0.02 to VENUS over 0,10, Both are in agreement that Mars possesses -10 MARS a surface which at radar wavelengths is significantly flatter than that of Venus. -20 - In the fall of 1963, Jupiter was claimed to have been ~ w > ~PITER observed by both the Soviet Group [Kotelnikov et aI., ·30 0 I 1964] and JPL [Goldstein, 1964b], Since Jupiter z u I possesses a very deep and presumably absorbing w I > -40 I ~ATURN atmosphere, it might be expected that signals would I I MARS I not be returned at frequencies high enough to penetrate ~ I I I I the terrestrial ionosphere, The JPL results appear -50 I to be consistent with reflection from material sus­ I I '\. URANUS pended in the upper atmosphere of Jupiter. In view of -60 I I recent results from the Arecibo Ionospheric Obser­ rPHOBOS NEPTUNE TITAN vatory to be reported subsequently, the USSR results I DEIMOS - 70 10 30 100 300 1000 are in some doubt. DELAY (min) Considerable efforts were undertaken to observe Venus during the 1964 inferior conjunction by groups at FIGURE 1. Relative detectability and echo delay for the 1,Ianets JPL, the Arecibo Ionospheric Observatory of Cornell and their Moons by radar, assuming equal slllface reflectivity. Months listed by ti ck marks for Mars and Mercury show delectability when closest ap­ University, MIT, Jodrell Bank, and in the USSR. proach occurs on that date.

1618 Most of the increase in performance which has made between 0.1 and 0.2, with the higher values generally radar astronomy possible has stemmed from the use of associated with the longer wavelengths. All the meas· very large antennas. Since the gain of the antenna urements indicate a significantly higher reflectivity enters twice in the radar equati on and is proportional than for the Moon. If one applies the same analysis to the area, performance depends on the fourth power used in the case of the Moon, one finds a surface dielec­ of antenna diameter. Currently th e largest antenna tric constant of between 4 and 5. Such a value is in regular use for the radar study of the planets is the quite consistent with a surface composed of dry rocks 1000·ft diam reAector of the Arecibo Ionospheric not unlike many rocks of the Earth's surface. The Observatory in Puerto Rico. Average transmitted presence of significant amounts of liquid water at the power is important; most of the facilities now in opera· surface seems to be ruled out. The low absolute tion use between 100 and 150 kw. Finally, the excess value of the reflectivity and its relative independence noise temperature of the receiving system is significant of frequency also make a dense Venusian ionosphere since it determines the weakest signal which can be ob· appear unlikely as a source of the reflections. A served. Perhaps the most striking example of how low variation in intensity reported by some workers [J ames this quantity may be reduced is given by JPL [Gold· and Ingalls, 1964] raises the possibility that differenti· stein , 1964a] which has achi eved a value as low as ation of the surface may exist which effects the signal 37 oK. By combining all these factors, MIT has intensity as various portions of the planet rotate into achieved a level of sensitivity of 20 dB , JPL and the the central region of the planetary disk. A measure· Soviet group approximately 25 dB , and the AIO ment which falls outside this group and which has inter· 35 dB beyond th e bare ability to detect Venus at closest esting implications has been reported by Karp et aI., approach. [1964]. Taken at a wavelength of 3.6 cm, their results In their attack on the pro blem of pl anetary detec· yield a fractional cross section of only 0.009 ± 0.003, tion , the vari ous groups appear to divide into two which is some 10 times less than the results at larger camps. The older, represented by MIT [Pettengill wavelengths. The most likely explanation seems to lie et aI. , 1962], the earl y JodrelJ work [Thomson et aI. , in a significant amount of absorption as the radiation 1961] , Jicamarca [Klemperer et aI. , ]964], and the traverses twice the planetary atmosphere. In this AIO, send out pulses of energy having peak powers in connection it is interesting to note that at wavelengths the ne ighborhood of several megawatts. Pulse widths just below 3 cm, the planetary radiometric temperature range from 100 ILsec up to many milli seconds, at falls abruptly, indicating absorption sufficient to bring repetition rates s uch that the average power is of the the measured emission into equilibrium with the cooler order of 100 kw. More recently, JPL. [Victor and upper layers of the atmosphere (see the review by Stevens, 1961], the Soviet group [Kotelnikov et aI. , Mayer, [1961]. 1962a] , El Campo [James and Ingalls, 1964] , and Several workers, [Smith, 1963; Kotelnikov et aI., Jodrell Bank [Ponso nby et aI. , 1964] have employed a 1963a; and Muhleman, 1964], have reported angular CW trans mi ssion , with various form s of phase and scattering measurements of Venus. The Soviet frequency coding. In the past several years both results are shown in figure 2. All are in agreement camps have employed in creasin gly sophisti cated modu· that the echo possesses the same qualitative features lation and detection schemes whi ch have had the effect as do lunar echoes, namely, that the bulk of the return of leveling the relative advantages and disadvantages appears to be related to coherent reflection from of the two techniques, making them both de pend smooth areas of the surface near the center of the largely on the average power employed. Suffice it disk. In the case of Venus, however, the surface to say that th e major groups in th e fi eld now have a appears significantly flatter than for the Moon. Muhle· signal delay measurement accuracy of better than 50 man [1964] has applied an analysis similar to that ILsec wh ere signal·to·noise permits, and a Doppler developed for the Moon and derives a mean surface frequ ency shift measurement accuracy approaching slope parameter some 3 times smaller than for the 0.1 cis. Systems now in exi stence are adequate for detecting Venus and Mercury at almost all points in their orbits, and Mars and Jupiter at close approach. A 0.8 0.9 4. Planetary Surfaces

0.6 0.6 In connection with radar studies of the lunar surface t from a: earth O. 37 e ~ p [.0.42(t-fo)] w (see, for example, Evans and P ettengill, [1963]) it 3: 0 ,4 has been shown how the intensity of the target echo Ii'

could be related, at least loosely, to the type of surface 0.2 material comprising the reAecting surface. Similarly for the planets, one can gain some idea of the surface 0.0 'T' ..q::1.=>;.LLj'-'-";~F-<-±;."30""'7~ -10 -7.5 -5 -2.5 0 2.5 5 , 10 500 100 0 1500 7.5 conditions from the strength of the return. (f-folc/ s - 6R (kml - Venus. For Venus, Victor et ai. [1961], Pettengill et ai. [1962], Carpenter [1964], Kotelnikov et ai. [1963a], FI GURE 2. Venus echo frequency dispersion (A) and delay dis· persion (B) as observed during the fall of 1962 in the USSR by James and Ingalls [1964], and Klemperer et aI., [1964] Kotelnikov et al. [1963aJ at a frequency near- 700 Mc/s. have found values for the fractional cross section lying The dotted lines show the e mpirical fit given by the functions shown. 1619

786-228 0-66-8

------Moon. More recently, unpublished results from the MIT and AIO groups indicate that the Venus slopes VENUS ECHO POWER may have more nearly one·half the average inclination VS DELAY found on the Moon. Figure 3 displays the scattering - 5 70 em law obtained at 70 cm at the AIO in 1964, while figure o 00 A.I.O. 22 MAY 1964 4 shows the results obtained at 23 cm at MIT (Evans, o o J. V., private communication). For comparison the -10 lunar scattering laws obtained under similar conditions are also plotted. Again it will be noted that the initial decay of power with range is less than for the Moon, indicating a smoother planetary surface. It is inter­ -15 esting that, at least on the basis of these two sets of P measurements, there appears to be little dependence (dB) on frequency of the angular scattering laws of Venus

-20 as compared to those of the Moon. Also of interest , , is the presence of a rough scattering component for " Venus at greater delays, not departing too greatly ", ,, from the lunar law. -25 , Mercury. For Mercury the available information " , I 0 0 f1- s ec PULSE 0 0 is much more primitive. The USSR [Kotelnikov, " , 1962b], JPL [Carpenter and Goldstein, 1963] and the I msec PULSE = + , -30 \ AIO [Dyce et aI., 1964] are in good agreement that the

2 msec PULSE 0 , '\ surface reflectivity is close to 0.06, similar to the value ," obtained for the Moon. The scattering law for Mer­ cury is currently poorly determined. The recent -35 finding at the AIO [Pettengill and Dyce, 1965] that the PL ANETARY RADIUS rotation rate is about 50 percent faster reduces the 0 .001 0 .002 0 .005 0 .01 0 .02 0 .05 0 .1 0 .2 0 .5 1.0 roughness required to explain their measured fre­ quency dispersion. Preliminary pulse measurements 0 . 05 0 .1 0.2 0 .5 2 5 10 20 40 made at the AIO are compatible with a scattering law VENUS DELAY (milliseconds) similar to that observed for the Moon. In view of the

FIGU RE 3. Venus echo power versus delay observed ill 1964 rtf 430 common environment faced by both these objects, a Mc/s at the AID. conclusion of similarity seems reasonable. The sol id c urve represent s the ('(IfJ'c spol1ding be ha vior of th e' Muon. scaled to the Ve nus radius. Note the simil ari ty bel wee n the two la ws . holding exct-' pl for th e regio n ne ar For Mars, the best measurements until very th e cent e r of the di s k. whe re Ve nu s appears smoothe r and more reflecti ve than the Moon.- Mars. recently were those of JPL [Goldstein and Gillmore, 1963], who have measured the radar cross section as 0 a function of the target longitude facing the radar. Their results are shown in figure 5, where an enhance­ -4 VE NUS 19 64 ment of reflectivity in the region of Syrtis Major 1295 Mel s -... may be seen. The USSR results [Kotelnikov et aI., -8 '- '­ '- .... 1963b] appear to be sufficiently weak to have signi­ -12 " ficance only when all longitudes are added together.

1620 -50.------~ planetary longitude and was well correlated only with the System I coordinates, indicating a source moving with the outermost (visible) atmospheric circulation. The s pectral width of the echo was narrow, indicating a high degree of smoothness of the scattering medium. The source, therefore, may have been (but is not re­ quired to be) narrow in latitudinal extent. The high intrinsic refl ection effi ciency, however, requires that the scattering surface extend through nearly the entire longitude region (about 45°) corresponding to the inte­ +20 gration time of the successful observations. +30 In the fall of 1964, radar observations of Jupiter were carried out at the AIO at a wavelength of 70 cm [pette n­ +40 gill and Dyce, 1965]. Despite a greatly increased system sensitivity as compared to the earlier measure­ +50~L_ __~ ~L---L---L-~L---~--~--~ 180 200 240 280 320 0 40 80 120 140 180 ments, no significant detection was obtained. By Longilude summing over all longitudes of the planet in a band­ width of approximately 100 cIs, to simulate the results FIGU RE 5. Th e radar reflectivity oj Mars at a freqll ency oj 2388 reported in the USSR, the AIO group was able to place Mc/s plotted verSlis planetary longitl/de [Goldstein and Gil/m.ore, 1963 1. an upper limit on the specific refl ectivity of 0.00045, Nute til{' appart'llt correiatiull bl'l wcell hi J! h rd leclivil), and the n!giull or Sy rlis Major. or 200 times less than the earlier result reported at 43 cm. By s umming over regions of longitude in a bandwidth of 700 cIs to simulate the measurements reported by JPL, an upper limit of 0.0036 for the specific re fl ectivity was obtained, a result 160 times A di stribution of surface slopes not unlike that found less than that reported earlier at 12.5 cm. on the Moon would suffi ce. Suppose now, however, It is interesting to note that if the refl ection mecha­ th at unlike the Moon th ere existed certain regions of ni sm behind the JPL observati on is associated with unus ual smoo thness (low areas with sediment a ti on?) suspended particulate matter, and if this has dimen­ which would occasionall y move under the radar beam. sions on the order of 10 cm or less, then by the law of When properl y ali ned normal to the incid ent energy, Rayleigh scattering there need not have been any these fa cets would be very effective in returning detectable energy (i. e., whi ch exceeded the level set energy and would give ri se to an enhancement in th e by the 70-cm observations). Meteorological differ­ total power even if the intrinsic specifi c re fl ectivit y e·nces.between the years 1963 and 1964 are also suffi­ remained low. The re fl ectivit y need not be low, however, and one may manufacture many models from cient to explain the discordant results. mars hes to salt fl ats which would all be compatible with the observed enhancement and relatively narrow spectral characteristics. 5. Planetary Rotation Jupiter. As a radar target, Jupiter is extre mely puzzling. All models proposed for this planet call for an extremely thick and dense atmos phere. Even The appli cation of pulse-coherent analysis to detail ed if a true surface exi sts, it is impossible to vi sualize radar mappin g of the surface of t he Moon has been a penetration by radi owa ves to this surface which described [Pettengill and Henry, 1962]. In reduc in g; would survive extincti on by absorption. Thus only the measurements to actual positions on th e surface refl ection by solid or liquid matter of sufficient size of the Moon, it was necessary to use the know n rates s uspended in the upper laye rs of the atmosphere, or of libration of the lunar surface with respect to the refl ection by an ionospheric plasma, would seem observer. However, by turning the process around, able to return measurable power. it would have been possible to obtain the apparent rotation of the Moon from the radar measure ments to Despite the unlikelihood of an efficient reflection a hi gh degree of accuracy. For Venus, wh ere the mechanism at wavelengths in the decimeter to meter rotation is essentially unknown from vi sual measure· range, groups at JPL [Goldstein, 1964b] and in the ments because of the extensiv e cloud cover, and for USSR [Kotelnikov et aI. , 1964] have reported successful Mercury, wh ere vi sual observation is diffi cult because measurements in the fall of 1963 which require s pecific of the proximity of the Sun , ex tre me interest att aches reflection efficiencies of 0.6 and 0.1 respectively. The to radar measure ment s. statistical significance of the latter measurement is If only frequency s pectrum measure ments are avail­ not high, however, since the reported output signal­ able, it is possible to deduce the apparent instan· to-noise ratio is only 1.3. taneous angul ar velocity from the maximum extent of The JPL measurements cannot be dis missed on the the spectrum, provided suffi cient signal-to- noise same gounds, however, since the best case exhibited ratio is avail ab le to ensure that one is actuall y observ­ an output signal-to-noise ratio of about 8. Positive in g echo power from the limbs of th e planet. Of detection was obtained over only a small region 111 course, one must assume that the radius of the planet 1621 is "known. Working within these limitations Carpenter [1964] has calculated that the rotation of Venus has MERCURY ROTATION 1965 a sidereal period of approximately 266 days in the retrograde sense with an orientation approximately 1.5 ~ perpendicular to the plane of its orbit. The earlier 46 DAYS RETROGRADE 20 ~ Russian result [Kotelnikov, 1962a], that the rotational 0: u « ~ 5~ period was as rapid as 11 days, appears to be dis· 59 DAYS DIRECT '"z -0 credited. '" ..f- to An even more powerful attack is possible if one can z z w ::> a: - obtain frequency spectra at a selected and known .. >­ 1 a. f­ a. - .. u range. As shown in Pettengill and Henry [1962], 15 o I the spectra in such a case are sharply defined and avoid w--' 1.0 > the necessity of assuming that echo power is being returned from the actual planetary limb. Using this 88 DAYS DIRECT technique, Goldstein [1964] has obtained from the 1962 (SYNCHRONOUS) observations a retrograde rotation of 248 days, again with an axis orientation very close to perpendicular to 30 10 15 20 25 30 the orbital plane of Venus. Unpublished data from MARCH APRIL the 1964 conjunction taken at the Arecibo Ionospheric Observatory confirm this latter measurement and yield FIGURE 7. Plot of the apparent rotational angular velocity of the a value for the sidereal rotation of 247 ± 5 days retro­ !)lanet Mercury versus data for several values of rotation during grade with an axis orientation approximately 6° off the inferior co njullction of April, 1965. perpendicularity to the orbital plane. Twenty-one The values inferred from the measure me nt s arc shown with their estimate d errors. measurements, reduced to the equivalent limb·to·limb Doppler spread are shown in figure 6, together with the curve which is the least·mean·squares fit to the data. of the echoes. Nevertheless, Pettengill and Dyce The three parameters which yield the best fit curve [1965] have recently reported radar measurements are also shown. From the extreme accuracy with which indicate that Mercury is rotating in a direct which the 21 observed points may be made to fit sense with a sidereal period of 59 ± 5 days. The data a theoretical curve of 3 deg of freedom, it must be are summarized in figure 7. The direction of the pole concluded with no reservation that we are observing, is not well· determined from these limited data, but is at least at 70 em,: the actual surface of a solid, rotating approximately normal to the planetary orbit. body. We must not rule out the possibility, however, The finding of a value for the rotational period of that there are also weaker reflections from a variable Mercury which differs from the orbital period is and turbulent atmosphere which can give rise to a unexpected and has interesting theoretical implica­ much broader spectral component, and which may tions. It indicates that either the planet has not been become important at short wavelengths. in its present orbit for the full period of geologic For Mercury, the corresponding measurements time or that the tidal forces acting to slow the initial are more difficult because of the relative weakness rota_tion have not been correctly treated previously. 6. References Carpenter, R. L. (1964), Computation of planetary theories, Astron. VENUS ROTATION J. 69, No.2, 136. A. I. O. 1964 Carpenter, R. 1., and R. M. Goldstein (1963), Radar observations 15.0 of Mercury, Science 142, 381. o BEST FIT CURVE Dyce, R. B., G. H. Pettengill, and A. D. Sanchez (1964), Paper W "0: presented at Spring URSI meeting, Wash., D.C. a. ? P ER IOD 247 d , 5 d Vl eEL LAT - 860 ! 2 0 Evans, J. V., and G. N. Taylor (1959), Radio echo observations of 0: W CEL LONG 260·' 30· Venus, Nature 184, 1358-1359. a.--' Evans, 1. V., and G. H. Pettengill (1963), The scattering behavior a. o of the moon at wavelengths of 3.6, 68, and 784 centimeters. o 10.0

L Kotelnikov , V. A. , et al. (1962a), Resultat radiolokatsii vener, Radio· Ponsonby, R. E. B., J. H. Thomson, and K. S. Imrie (1964), Radar techno i Electron. 7, No. 11 , 1860- 1872. observati ons of Venus and a determination of the astronomical Kotelni kov , V. A., et a1. (1962b), Radar observati ons of the pl anet unit, M.N.R.A .S. 128, 1- 17. Mercury , Dok 1. Akad . Nauk. 148, No . 6, 1320- 1323. Price, R., et al. (1959), Radar echoes from Venus, Science 129, Ko telnikov , V. A., et a1. (1963a), Radar observ ations of Venus in 75 f -753. the Soviet Union in 1962, Dokl. Akad. Nauk. 151, No. 3, 532- 535. P rice, R., and C. H. Pettengill (1961), Radar echoes from Venus and Koteln ikov, V. A., et al . (1963b), Radar studies of the planet Mars a new determinati on of the Solar Parall ax, Planetary Space Sci. in th e Sovi et Uni on, Dokl. Akad. Nauk . 151, No.4, 811- 81 4. 5 , 71- 74. Kotelnikov , V. A. , et a1. (1964), Radar observations of the planet Smith, W. B. (1963), Radar Observ ati ons of Ve nu s, Astron. 1. 68, I Venus, Dokl. Akad. Nau k. 155) No.5, 1035- 1038. No. 1, 15-21. Maron, 1. , C. Luchak, and 'W. Blitzstein: (1961), Radar observations Thomso n, J. H., et a1. (1961), A new determinati on of the solar r of Venu s, Science 134, 1419-1420. parallax by means of radar echoes from Venus, Natu re 190, No. Mayer, C. H. (1961) , The Solar System, Vo l. 3 , Planets and Satellites, 4775, 519-520. ch. 12, pp. 442- 472, ed. C. P. Ku iper; and B. M. Middlehurst Victor, W. K. , and R. Stevens (1961), Exploration of Venus by radar, (U niv . of Chi cago Press, Chi cago). Science 134, 46-48. Muhleman, D. O. (1964), Radar scattering from Venus and the Moon, Astro n. J. 69, No. 1, 34- 41. Pett engill , C. H., et a1. (1962) , A radar investi gation of Venus, As tron. J. 67, 181-190. Pett engill , C. H., and R. B. Dyce (1965), Paper presented at Spring URSI meeting, Wash., D. C. Pettengill, C. H., and J. C. Henry (1962), Enhancement of radar re fl ecti vity associated with the lunar crater Tycho, J. Ceophys. Res. 67, No . 12, 4881-4885. (Paper 69D12-609)

Preliminary Venus Radar Results 1

Richard M. Goldstein

Communications Systems Research Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif.

1. Introduction ROTATION AXIS This note presents some of the preliminary results obtained by the Jet Propulsion Laboratory during its radar observations of Venus. These experiments were performed during the months surrounding the inferior conjunction of June, 1964. The radar parame ters were as fo llows: T O EART H

P owe r = 100 k W Freque ncy = 2388 Mc/s Antenna Gain = 108.5 dB (two-way) Syste m Te mperature = 33 oK. FIGU RE 1. Countors of constant time delay and of constant f re­ Several different types of experiments were per­ quency shift. formed, but here we shall re port on only spectrum and range-spectrum analysis of the echo. Each planetary re fl ecting ele me nt has four attributes which may be isolated by the radar: velocity, time delay, intensity The time-delay increments are 125 J.Lsec (l i miles) for normal, and intensity for crossed polarization. and are constant. The frequency increments are not The methods used to isolate these attributes are constant, but are proportional to that rotation com­ described by Gold ste in [1964).2 pone nt which is perpendic ular to the line of sight. As Ve nus passes the Earth in its orbit, the frequency 2. The Rotation of Venus incre me nts first decrease and the n increase. This phenomonon enables the three components of Venus's Figure 1 presents the contours of constant time delay rotation vector to be determined. Goldste in [1964] and of constant velocity (or Doppler frequency shift). gi ves the details of this computation. A sample of the basic data obtained is give n in fi g­ 1 This papt: r presents the result s of one phase of research carri ed o ul at the Je t Propulsio n Laboratory, California In stitut e of Technology. under Contract No. NAS 7- 100. sponsored ure 2. The echo arising from e ac h ring of constant by the National Aeronautics and S pace Admi ni stratio n. time delay has first been separated by the radar and 'l Goldstein. R. M. (1964). Venus charac teri stic s by eart h-based radar. Astron_ J. 69, No. I. 12- 18. the n spread into its frequency components in order

1623