ill Session: PASSIVE RADIO OBSERVATIONS OF VENUS, SATURN, MERCURY, MARS, AND URANUS Passive Radio Observations of Mercury, Venus,
Mars, Saturn, and Uranus I Alan H. Barrett
Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Mass. The radio observa tions of Mercury, Venus, Mars, Saturn, and Uranus are reviewed and discussed in relation to knowledge of these planets acquired by other means. In the case of Mercury, it is show n tha t the radio observations imply a temperature of - 300 OK for the unilluminated hemisphere, a result which appears to be in sha rp di sagreement with infrared measurements of Mercury. Two de tail ed measure ments of the Venu s spectrum near I·cm wa velength are presented and compared. 1. Introduction the radio domain, hv/kT ~ 1, so (1) may be expanded to give the fa miliar Rayleigh·J eans approxima li on, Astronomi cal observati ons at radio wavelengths, say, from 100 m to 1 mm, have given rise to unexpected 2 l ev) dv = 2kTv dv = 2kT dv' (2) results in many branches of astronomy, and the plane· c2 '1.. 2 tary studies branch is no exception. The first radio emission from an y planet was detected from Jupiter The power received by a radio telescope having an at 13.5·m wavelength by Burke and Franklin [1 955], effective area Ae from a radio source of solid angle a nd it was obvious immediately from the inte nsity a nd fls is give n by time depe ndence of the emission that the radiation process could not be ascribed to thermal mechanisms. (3) Long.wavelength studies of Jupiter emission have proceeded at a rapid rate since 1955 a nd fo rm the major part of the observing programs of several radio where TA is the "ante nna temperature," and a factor astronomy observatori es. In 1956, a new phase of 1/2 has been included because only one polarization pla netary radio as tronomy was introduced by the detec· can be received at a time. The wavele ngth de· tion of thermal radiation from Venus, Mars, and Jupiter pendence of (3) shows clearly the advantage of short by Mayer, McCullough, and Sloanaker at the U.S. wavelengths when trying to detect thermal radiation. Naval Research Laboratory [1958 a, b, c]. Unlike For all pe ncil-b eam antennas now in use the effective the observations that led to the Jupiter detection, area of the aperature is essentially constant over the Venus was first observed at 3.15-cm and 9.4·c m wave· solid angle sub tended by the source, so (3) may be length, and all subsequent planetary detections approximated as have been made initially at centime ter wavelengths. At the present time, radio emission has been detected from Mercury, Venus, Mars, Jupiter , Saturn, and (perhaps) Uranus . (4) The fact that planetary radiation has been primarily studied at centimeter and millimeter wavelengths where Ta represents the radio brightness te mperature can be readily understood from the basic law of thermal averaged over the he mi sphere of the pla ne t vi sible radiation, Planck's law, and the usual radio.frequency from Earth. approximation, the Rayleigh·Jeans law. The thermal Equation (4) has been used to compute th e values energy emitted per unit time into unit solid angle by shown in table 1. For the computation of these values a unit area of a perfect emitter at a temperature T we have assumed a parabolic antenn a of 30.5·m (100·ft) in the frequency range between v and v dv is given + di a meter operating at a wavelength of 3 c m with a n by Planck's law, aperature e ffi ciency of 0.50. The values of ta ble 1 2hv3 dv are only intended to serve as a guide to the typi cal J(v) dv =7 (e'w/kT -1)' (1) orders of magnilude in volved in pla netar y radi o astron omy. Note that the detecti on of radi o e mi ssion fro m where h, c, and k are Planck's cons tant, the velocity Neptune and Pluto is a formidable task by present of light, and Boltzmann's constant, respectively. In standards. Equation (4) was derived under the assumption that I This work was supported in part by the Joint S ervices Electroni cs Program under the effective area of the antenna does not vary appre Contract DA36--o39- AMC-03200(E); and in pa rt by the National Aeronautics and S pace Adminis tratio n (G rant NsG-419). ciably over the solid angle sub tended by the planet.
1565 TABLE 1. Typical values 0/ received power and antenna tempera Measurement of Mercury's radio emi ssion is diffi tures Jar planetary radiation at 3-cm wavelength a cult for two well·known reasons: (a) The planet is of small angular size, subte nding approximately 11 sec Pl ant:! Solid an1-!le h !\lean bri ~ htn e..§. s Ant e nna Recei ved power temperature. Tn te mperature of arc at. mean conjunction ; and (b) the planet's angular dIstance from the Sun is never more than S(a adian oK oK W 28°. The first implies that the radio emi ssion, as .\'Ie rc ur), 2.2 X 10 -" 40 0 0.36 5.0 X 10- " V Cll US 7. 0 X 10 -" 600 17. 1 2.4 x 10- 15 measured on the Earth, is very weak, as can be seen ~1 ar s 5.9 X 10 -" 200 0.48 6.6 X 10- 17 from table 1. Actuall y, table 1 is misleading because Jupite r 4.1 X 10 -" ISO 2.5 3.4 X 10- 16 Sat urn 7.0 X 10 -" 100 0. 28 3. 9 X lO - 17 of the second difficulty. The values in table 1 were Uranus 2.4 X 10 - 111 , lOO .0098 lA x 1O - 11l Neptune 8.3 X 10 - 11 '· lOO .0034 4. 7 X lO- ll1 computed fo~ Mercury at conjunction, when the radio Plut o 5.0 X 10- " '· 100 .0002 2.8 X 10- 2u energy receIved on Ear th is maximum and wh en
11 A paraiJoll(. a nt e ~n a. 30.5 m. 000 [I) III diame ter. with an a pe rat ure efT'l<..: u: n<:y of 0.50 at Mercury is at its minimum angular separation from A = 3 C Ill. and a re<.: c m ..: r bandwidth of 10 !\'1c'!s is ass umed. 10 At mean cunjunction ur opposition. the Sun. Rece ption of solar radio emi ssion in the l' Es timated te mpe rature fur purpuses uf c omparison. wide·angle lobes of the a ntenna pattern can exceed the radio emission from Mercury, particularly when This is simply another way of stating that with the the planet is separated from the Sun by a small an o-le antenna beam widths that are now in common use one thereby making detection of Mercury a difficult ob;er: does not have sufficient angular resolution to resolve vational problem. the planet and to directly measure the distribution of Radio emission was firs t detected from Mercury, in the radio brightness temperature over the planetary 1960, at 3.45-cm and 3.75-cm wavelengths by Howard, disk. In an observational sense, this limitation can Barrett, and .Haddock [1962], as shown in fi gure 1. be circumvented by three radically different methods: The observatIOns were not of sufficient precision to (a) Use of variable-baseline interferometers to deter reveal a de pendence of the mean radio brightness mine the Fourier angular spatial-frequency components temperature on the phase of Mercury, if such a de of the brightness distribution; (b) observation of the pendence exists, but merely gave an av erage bright diffraction pattern resulting from a lunar occultation ness temperature of 400 ± 80 OK for phase angles near of the planet (also to determine the Fourier components 90°. To compare this value with planetary tempera of the brightness distribution); and (c) measurement tures determined from the infrared measurements of planetary emission from spacecraft passing close it is necessary to assume a temperature di s tributio~ to, or orbiting, the planet. All three methods have over the planetary surface. If Merc ury has no atmos been employed in recent years; it is certain that use phere and is in synchronous rotation about the Sun of these techniques will increase in the future_ Vari then a reasonable assumption is that the s urfa c~ able-baseline interferometric techniques have been isotherms on the illuminated hemi sphere will be con applied with great success to Jupiter [Radhakrishnan centric circles about th e subsolar point. Because and Roberts, 1961; Morris and Berge, 1962] and to practically nothing is known about the temperature Venus [Clark and Kuzmin, 1965]; a lunar occultation ?n the unlit hemisphere, it is usually assumed that it of Jupiter has been observed at 74-cm wavelength IS at constant temperature, T/). These statements with the Australian 2lO-ft radio telescope (Kerr, 1962]; may be put on a quantitative basis by writing the brio-ht- and microwave emission of Venus has been measured ness temperature distribution as '" with high planetary 'resolution during the fly-by mission of Mariner 2 [Barath et aI. , 1964]. (5) 2. Mercury
l\;!ercury is the smallest of the planets, having a 0 .0 5 MERCURY radIUS of 2420 km, and a mean distance from the sun 8 .0 AND 8.7 kMc/ S of 0.387 A.U. It has been generally believed from T . II SECO NDS ~ "- visual observations that Mercury's sideral rotation W I2 U. 0: period is 88 days, the same as its sideral period of ...::> .,.. . . ""0: /'eo . revolution about the Sun; hence it is in synchronous w ...... 0. . ~ . ~ rotation and always keeps the same side toward the w . .-. ... ~ . ... . Sun. Infrared radiometric observations have led to .. z ""z a determination of the sub solar point temperature of ...w approximately 6lO OK, but no measurement of the z temperature of the dark hemisphere is available "" [Pettit, 1961]. It has been generally assumed that -0.0 5 Mercury has a very tenuous atmosphere - possibly no atmosphere - and that the temperature of the dark 4 portions of the planet is near 0 oK. Recent obser EAST MIN UTES OF TIME WEST vations, however, including radio observations, suggest that these assumptions and values may be in error FIGU RE 1. The first radio detection 0/ Mercury. Note that It was necessa ry to average m an y c urves before the Me rcu ry s i"nal was a p as we shall show. pa rent {Howard , Barrett , and Had doc k. 1962]. ..' 1566 1.0 where 0 is the polar angle of a point on the surface The ratio of the block-body temperature to measured from the planet-Sun axis, and Tss is the tem the sub-solar pOint temperature, TBo/To, vs. perature of the subsolar point at the depth of origin 0 .9 the plonetocentric angle (i) between the sun of the radio emission. The exponent n is unknown but and ear th for various temperature distributions elementary physical considerations for a smooth, on the sunlit surface, T = To cos" 8. O.S nonrotating sphere with no atmosphere would require n = 1/4. Howard, .Barrett, and Haddock [1962] con sidered n = 0, 1/4, and 1 with To = 0 oK in the analysis 0 .7 of their results. It must be emphasized that (5) is taken to be the distribution of the radio brightness 0 .6 temperature, although its functional dependence was assumed from considerations of the surface tempera c >- 0 .5 ture distribution. It can be expected that the bright ness temperature will differ from the surface tempera ture because of the effects of emissivity and escape 0.4 of the radio energy from below the surface. The coefficients of (5) will, therefore, depend on the physi 0.3 cal parameters of the surface and subsurface material, but the functional dependence on 0 can be expected 0 .2 to follow closely that of the surface temperature. The mean value of the radio brightness temperature, 0 .1 TB , is found by integrating (5) over the hemispherical surface of the planet that is visible from Earth, 0 O' 60' 90' ISo" TB- =-R21 JT(O, cfJ) cos ex dA, (6) PLANETOCENTRIC ANGLE BETWEEN SUN AND EARTH {i J 7T M
FIGURE 2. The integral of (9) as a function of phase angle for where RM is the radius of Mercury and ex is the angle n = 0,1/4, and 1 [Howard, Barrett, and Haddock, /962]. between the normal to dA and Earth. As written, (6) appears to be independent of the phase angle of the planet, but the limits of the integral will involve the phase angle. Since the temperature distribution as is low but, as has been pointed out previously, the sumed in (5) is independent of cfJ, T(O, cfJ) = T(O), and initial observations by Howard, Barrett, and Haddock substitution of (5) in (6) gives [1962] were not of sufficient precision to reveal a phase vanatIOn. When interpreted in terms of (8), (9), T-B=To+"[f2 1 i (Tss-To) cos" 0 cos ex dA, (7) and figure 2 with n = 1/4 and To = 0 OK, the results 7T M S.H. indicated a sub solar brightness temperature of 1100 where the integration is now restricted to that portion ± 300 OK, which is considerably in excess of the sub of th e sunlit hemisphere which is visible from Earth solar surface temperature of 610 OK as determined from and, therefore, the limits of the integral are functions infrared observations [Pettit, 1961]. The disagree of phase angle. If we adopt the notation of Howard, ment worsens if allowance is made for the surface Barrett, and Haddock [1962], (7) can be written emissivity to differ from unity. If a subsolar tempera ture of 610 OK is adopted, then the observations imply (8) a brightness temperature of approximately 270 OK for the unilluminated hemisphere. Thus the first observa Y(n, i)= ~ 2 1 cos" 0 cos ex dA. (9) tions, in spite of their low precision, were suggestive 7T M S .H. of a temperature of several hundred degrees for the unilluminated surface of Mercury. For n = 1 it is possible to carry out the integration of Mercury was observed on 10 days in May and June (9) in closed form to get 1964, by Kellermann [1965] at 11.3-cm wavelength, using the Australian 21O-ft radio telescope. The observations covered a range of phase angles from 29° to 145° but failed to show any change of the brightness temperature as a function of phase, within the limits of precision of the observations. Furthermore, the where i is the phase angle, i.e., the planetocentric angle results showed that the mean brightness temperature between the Sun and Earth and restricted to values at 11.3 cm was in good agreement with the results of between 0 and 7T. Figure 2 shows Y(n, i) as a function Howard, Barrett, and Haddock at 3.45-cm and 3.75-cm of phase angle for n = O. 1/4, and 1. wavelengths. Table 2 shows Kellermann's results. Equations (8) and (9) and figure 2 predict that a large Note that the mean brightness temperature for a phase phase effect should be observed in the radio observa angle of 29° is only 220 ± 160 OK, even though at this tions if the dark-side temperature of Mercury To angle 94 percent of the planetary disk was illuminated. 1567 T ABLE 2. Mean brightness temperature ture were considered briefly by Field [1962]; the prob· of Mercury at J J .3·cm wavelength as a lem of internal heating caused by radioactivity was function of planetary phase angle [Kellermann, 1965 ] considered by Walker [1961]. Walker's conclusion was that for radioactive heating similar to that in Dale Phase angle. i Mean bri ghtnc.§ s chondritic meteorites the temperature of the unil· temperat ure. TH luminated hemisphere would be only 29 OK, far short Degree OK of the requirements for the radio measurements. May 8. 1964 145 290 ± 30 9 142 320 ± 40 Two effects related to the motion of the planet have JO 140 220 ± 70 been neglected in the discussion above. Both will 11 137 320 ± 70 27 98 330 ± 80 have to be considered as more accurate radio measure· 28 96 275 ± 80 ments are made. These effects are the varying dis· June 2 84 300 ± 90 6 74 250 ± 100 tance between Mercury and the Sun, and the libration 18 38 410 ± 150 20 29 220 ± 160 of Mercury. The first can amount to ± 20 percent in distance, an effect that will perhaps be reduced as r /2 when translated to temperature at the surface of the planet [Pettit, 1961], and the second will provid~ a A measurement of Mercury emission at 1.53-cm source of heating, averaged over 88 days, for a portIOn wavelength has been reported by Welch and Thornton of the unilluminated hemisphere. Present observa· [1965]. These workers report the mean brightness tions are insensitive to these effects, but they may be· temperature to be 465 ± 115 as determined by oK come discernible in future observations. measuring the ratio of Mercury emi ssion to Jupiter emission and 'assuming a temperature of 155 oK for 3. Venus the brightness te mperature of Jupiter at 1.53 cm. The Venus is second only to Jupiter in the number of assumed Jupiter temperature is consistent with the studies of it made at radio wavelengths. This is due, value 144± 23 measured at 0.835 cm [Thornton oK, of course, to the unexpected intensity of the Venusian and Welch, 1963]. The average illumination of the radiation plus the increased interest in the nearby disk of Mercury during the 1.53-cm observations was planets as a result of the implications of contemporary 0.25. Welch and Thornton assume a subsolar tem space technology. A review of the radio observations perature of 620 n = 1/2 in (9) and co mp~t e t~ a t the OK, of Venus, made before 1964, and the interpretations mean briahtness temperature from the IllumInated thereof has recently been published by Barrett and di sk would be approximately 100 OK if the dark side Staelin' [1964]; therefore, the present review will temperature is 0 Since the observed value is oK. concentrate on subsequent observations. 465 OK , this result implies a s ubstantial contribution The spectrum of radio emission from Venus is well from the unilluminated hemis phere of Mercury. If established as a result of many measurements at (9) is used to compute the brightness temperature of 0.4·, 0.8·, 3·, and lO·cm wavelengths. The spectrum the dark hemi sphere, under the assumptiQ.n of a sub shows emission that is characteristic of brightness solar temperature of 620 OK, n = 1/2, and TB = 465 OK , temperatures of approximately 600 OK between 3 cm the result is approximately 440 OK, again demonstrating and 10 cm, and 350 to 400 OK between 0.4 em and 0.8 that a large microwave brightness temperature appears cm as shown in figure 3. This spectrum has led to to be required to match the observations. co~sideration of many different physical mechanisms The existence of an appreciable temperamre for of emission and absorption, but two have been most the unilluminated hemisphere was only suggested widely discussed. In one model it is assumed that by the first observations, but is also indicated by the l1.3-cm and 1.53-cm observations. A temperature of 800 - 300 OK would satisfy all of the radio observations on the basis of the simple model represented by (5), 700 but as more refined observations become available, .'" w a U:ore complete model that includes thermal damping Q: ::::J I- ~. and microwave attenuation in the planetary subsurface
FR EQUENG Y (Gels) the radio observations of Mars have been performed only at wavelengths near 3 cm. During the favorable FIGURE 4, The Venus measurements of Welch and Thornton [1965) opposition of September 1956, Mars was observed on (triangles) and Staelin and Barrett [1965) (dots). as shown in 7 days between September 9 and 21 by Mayer, Mc Cul table 3, lough, and Sloanaker [1958b, c], using the U.S. Naval The rectangles at 10 Gc/sec and 35 Cc/sec represent the many previous measurements at these frequencies. Research Laboratory 50-ft antenna, operatin g at 3.15- 1570 cm wavelength. The average brightness temperature deduced from the observations was 218 ± 76 oK . Mars was again observed there at 3.14-cm wavelength by Giordmaine, Alsop, Mayer, and Townes [1959]6 weeks after the November, 1958, oppositi on. The mean brightness temperature was 211 ± 20 OK, in excell ent agreement with the 1956 observations. The temperature of the center of the Martian di sk in the 8-13 f1- infrared "window" has been determined to be 288 OK by Sinton and Strong [1960], and a diurnal temperature variation of approximately 100 OK was detected. In view of these results, it can be said that the 3-cm radio observations are in good agreement with the values to be expected from the infrared R. A. (DIAL READINGS) results. The radio brightness temperatures will be less than the planetary surface temperatures, as FIGURE 5. A drift scan of Saturn made with a maser radiometer and the University of Michigan 85-ft radio telescope [Cook et al., determined by the infrared, for three reasons: (a) The 19601· infrared temperature of 288 OK refers to the center of the disk, whereas the 3-cm observations represent an amplifier and the 85-ft antenna of the University of average over the disk and, therefore, will include appre Mi chi gan; and Saturn was easily seen on each drift ciable fractions of the surface wi th temperatures less scan across Saturn, as show n in fi gure 5. The mean than 288 OK; (b) the microwave emissivity of Mars will brightness te mperature was determined to be 106 be less than unity, thereby lowering the brightness ± 21 OK, in good agreement with the infrared radio temperature; and (c) the microwave emission may be metric temperature at the cloud-top level. This expected to originate in the subsurface material of result suggests a thermal origin for the 3.75-cm radia Mars where the temperature may be less than that of tion , a conclusion that appears to be valid for all of the surface. A suitable theory that allows for these the planets at short centimeter wavelengths. effects has not been developed specifically for Mars; Three series of observati ons have been conducted in fact, at the present time, it is not warranted in view at wavelengths of approximately 10 cm which giv e of the limited number of radio observations and the results slightly different from the 3.75-cm results. lack of any major di sagreement with accepted ideas Drake [1962] measured Saturn emi ssion at lO-cm about the planet. wavelength, usin g th e 85-ft antenna at the National A very unusual series of observations of Mars at Rad io Astronomy Observatory, and determined a 21.3-cm wavelength has been reported by Davies brightness temperature of 196 ± 44 OK, almost double [1964], using the 250-ft antenna of Jodrell Bank. The the 3.75-cm value. This result was suggestive of mean brightness temperature was reported to be non thermal radiation, but could al so be explained by 1150 ± 50 OK, far in excess of any thermal temperature th ermal processes simply by assuming that the lO-cm that can be expected from the surface or subsurface radiation was originating from a deeper layer in the of the planet. This result is reminiscent of the well atmosphere than the 3.75-cm radiation and that the known nonthermal radiation from Jupiter, which is atmospheric temperature was so mewhat hotter at th e usually attributed to Jovian radiation belts similar level of origin. A surprising series of observations to the terrestrial Van Allen belts. It seems likely, was reported by Rose, Bologna, and Sloanaker [1963], however, that Mars was probably observed at deci using an 84-ft radio telescope at the U.S. Naval Re meter wavelengths with negative results following the search Laboratory at 9.4-cm wavele ngth. The mean scientific excitement created by the Jupiter decimeter brightness temperature was 177 OK, in good agreement observations; but these negative results, if they exist, with Drake's value; however, the 9.4-cm radiation have not been published. Nevertheless, it seems was fOUIld to be strongly polarized. If it is assumed premature to speculate about the origin of the strong that the 3.75-cm results are strictly thermal emission, Martian decimeter radiation until it has been con to evaluate the nonthermal contribution to the observed firmed by further observations. 177 OK, the thermal component, or 106 OK, should be subtracted from the 9.4-cm result. It then follows 5. Saturn from the observations that the nonthermal component of the 9.4-cm radiation is 51 ± 22 percent polarized. As might be expected from the values giv en in Furthermore, the observations showed that the electric table 1, Saturn is well within the range of detectabilit y vector of the emission is parallel to the axis of rotation, of present radio-astronomy techniques and has been wh ereas the Jupiter decimeter e mission shows the observed at a number of centimeter wavelengths. A electric vector to be parallel to the equator. These measurement of Saturn was reported by Drake and observations would imply that the magnetic axis of Ewen [1958] at 3.75-cm wavelength, but no brightness Saturn is nearly perpe ndi cular to the axi s of rotation temperature was deduced and the detecti on is open if the analogy with Jupiter emission is made. It to question. In 1960, a series of observations was now appears that th e observati ons were in" error, conducted by Cook et a1. [1960], using a maser pre- the polarization of Saturn's ·radiation is time-variant,
1571
786-228 0-66-5 or the polarization is an extreme function of frequency. emission from the planet. The upper limit set by this conclusion is based on measurements recently these observations on the mean brightness tempera reported by Davies, Beard, and Cooper [1964]. These ture at 11.3-cm wavelength is 320 oK. The infrared workers observed Saturn with the Australian 21O·ft temperature of Uranus is approximately 100 OK; thus radio telescope at 11.3-cm wavelength. The mean the 11.3-cm observations indicate an upper limit to brightness temperature was determined to be 182 the non thermal radiation e mitted by Uranus. ± 18 oK, in excellent agreement with both the 10-cm and 9.4-cm observations, but no polarization was detected within an upper limit of 6 percent. The agreement between observations near 10-cm wave 7. References length furnishes an argument against the polarization Barath, F . T ., A. H. Barrett, J. Copeland, D. E. Jones, and A. E. Lilley being strongly frequency-dependent; further measure (1964), Mariner 2 microwave radiometer experiment and results, ments are clearly needed to resolve the apparent dis Astron. J. 69, No.1, 49-58. crepancy, especially at longer wavelengths. Barrett, A. H. (1960), Microwave absorption and emission in the atmosphere of Venus, 1. Geophys. Res. 65, No.6, 1835-1838. A recent measurement of Saturn emission at 1.53-cm Barrett, A. H. (1961), Microwave absorption and emission in the wavelength has been reported by Welch and Thornton atmosphere of Venus, Astrophys. 1. 133, No.1, 281-293. [1965]. As in their Mercury measurements, the ratio Barrett, A. H. (1962), Microwave spectral lines as probes of planetary of Saturn to Jupiter brightness temperatures was atmospheres, Mem. Soc. Roy. Sci. Liege 7,197-219. Barrett, A. H., and D. H. Staelin (1964), Radio observations of Venus determined to be 0.94 ± 0.15. If the brightness tem and the interpretations, Space Sci. Rev. 3, No.1, 109-135. perature of Jupiter is taken to be 155 OK, this result Burke, B. F., and K. L. Franklin (1955), Observations of a variable leads to a value of 146 ± 23 OK for Saturn's brightness radio source associated with the planet Jupiter, 1. Geophys. Res. temperature at 1.53-cm wavelength. This result is 60, No.2, 213-217. Clark, B. G., and A. D. Kuzmin (1965), The measurement of the polar· higher than the 3.75-cm temperature, which, if veri ization and brightness distribution of Venus at 10.6 wavelength, fied by further measurements at both wavelengths, Astrophys. J. 142, No.1, 23-44. may be attributed to absorption by ammonia in the Cook, 1. 1., L. G. Cross, M. E. Bair, and C. B. Arnold (1960), Radio atmosphere of Saturn. detection of the planet Saturn, Nature 188, No. 4748, 393-394. Davies, R. D. , M. Beard, and B. F. C. Cooper (1964), Observations Note that in all determinations of the mean bright of Saturn at 11.3 centimeters, Phys. Rev. Letters 13, No. 10, ness temperature it has been assumed that the size 325-327. of the radio-emitting region is equal to the optical disk Davies, R. D. (1964), Paper presented at the XIIth General Assembly of Saturn. This assumption implies that the rings of the International Astronomical Union, Hamburg, Germany, August 25-September 3. of Saturn are not responsible for the emission. This Drake, F. D., and H. I. Ewen (1958), Broad·band microwave source assumption could be checked by observations of high comparison radiometer for advanced research in radio astronomy, accuracy extending over a long period of time, but the Proc. IRE 46, No.1, 53-60. measurements would be quite difficult unless a large Drake, F. D. (1962), Microwave spectrum of Saturn, Nature 195, No. 4844, 893-894. portion of the emission originated in the rings. Field, G. B. (1962), Atmosphere of Mercury, Astron. 1. 67, No. Finally, mention should be made of attempts to 9,575-576. observe decameter burst-like radiation from Saturn, Gibson, J. E., and H. H. Corbett (1963), The brightness temperature again, analogously with Jupiter. The confidence level of Venus at 1.35 cm, Astron. 1. 68 No.1, 74. Giordmaine, 1. A., L. E. Alsop. C. H. Mayer, and C. H. Townes of such observations is necessarily low unless Saturn (1959), Observations of Jupiter and Mars at 3·cm wavelength, is a strong and frequent emitter at decameter wave· Astron. 1. 64, No.8, 332. lengths, because of the possible confusion with emis· Heiles, C. E., and F. D. Drake (1963), The polarization and inten· sion of terrestrial origin. At this time, several possible sity of thermal radiation from a planetary surface, Icarus 2, No.4, 281-292. events were noted which could be associated with Howard, W. E., A. H. Barrett, and F. T. Haddock (1962), Measure Saturn, but the observers have not yet claimed a ment of microwave radiation from the planet Mercury, Astrophys. reliable detection at decameter wavelengths. A 1. 136, No.3, 995-1004. recent paper on this subject is that of Smith et aI., Jones, D. E. (1961), The microwave temperature of Venus, Planet. Space Sci. 5, No.2, 166- 167. [1965]. Kellermann, K. I. (1965), ll·cm observations of the temperature of Mercury, Nature 205, No. 4976, 1091-1092. Kerr, F. 1. (1962), 210·foot radio telescope's first results, Sky and Telescope 24, No.5, 254-260. Mayer, C. H. , T. P. McCullough, and R. M. Sloanaker (1958a), 6. Uranus Observations of Venus at 3.15-cm wavelength, Astrophys. 1. 127, Table 1 shows that detection of radio emission trom No. 1,1- 10. Mayer, C. H., T. P. McCullogh, and R. M. Sloanaker (1958b), the remote planets: Uranus, Neptune, and Pluto is a Measurements of planetary radiation at centimeter wavelengths, very difficult task. An attempt to detect Uranus at Proc. IRE 46, No.1, 260-266. 11.3-cm wavelength has recently been reported by Slee Mayer, C. H., T. P. McCullough, and R. M. Sloanaker (l958c), [1964], using the Australian 210-ft radio telescope. Observations of Mars and Jupiter at a wavelength of 3.15 cm, Astrophys.1. 127, No.1, 11-16. Although Uranus was not detected, it appears that a Morris, D., and G. L. Berge (1962), Measurements of the polarization weak radio source was detected near the position of and angular extent of the decimeter radiation from Jupiter, Astro· Uranus on February 24, 1964. The presence of the phys. J. 136, No.1, 276-282. Pettit, E. (1961), Planetary temperature measurements, The Solar source was checked by making observations on May 2, System, 3, Planets and Satellites, ed. G. P. Kuiper and B. M. 1964, when Uranus was in a new position. Observa· Middlehurst, ch. 10, 400-441 Univ. of Chicago Press, Chicago, tions on May 3, 1964, failed to show any evidence of Ill.). 1572 Pollack, J. B., and C. Sagan (1965), Polarization of thermal emission Discussion Following Barrett's Paper of Venus, Astron. J . 70, No.2, 146. Radhakrishnan, V., and J. A. Robe rts (1961), Polarization and angular D. O. MuhLeman: Have the data points in the plane extent of th e 960·Mc/sec radiation from Jupiter, Phys. Rev. Letters 4, No. 10, 493- 494. tary spectra you presented been corrected to the same Rose, W. K., J. M. Bologna, and R. M. Sioanaker (1963), Linear polari. phase angle? zation of the 3200·Mc/sec radiation from Saturn, Phys. Rev. ALan H. Barrett: No . Letters 10, No.4, 123- 125. D. O. MuhLeman: Wouldn't such a correction de Sinton, W. M., and J. Strong (1960), Radiometric observations of Mars, Astrophys. J. 131, No. 2, 459- 469. crease the scatter in the points? Slee, O. B. (1964), A search for radio emi ssion from Uranus at 11.3 ALan H. Barrett: Other uncertainties in the data cm, Astrophys. J. 14 0 , No.2, 823-824. are certainly larger than the di splacements in the Smith, A. G., G. R. Lebo, F. N. Six, Jr., T. D. Carr, H. Bol1hagen, points caused by phase effect. J. May, and J. Levy (1965), Decameter·wavelength observations J. A. Roberts: Is the radar rotation period of Mer of Jupiter, the apparitions of 1961 and 1962, Astrophys. J. 141, No.2, 457-477. cury sufficiently great to explain the high temperature Soboleva, N. S., and Yu. N. Pariiskii (1964), The possibility of ob· observed on the dark side of the planet? serving the polarization of thermal radio emission of planets, ALan H. Barrett: I have not made these calculations. Astron. lh. 41, No . 2, 362-365. (Engli sh transl., Soviet Astron. F. D. Drake: If the surface of Mercury is like that of AJ 8, No.2, 282-284.) Staelin, D. H., and A. fI. Barrett (1965), Radio measurements of the Moon, then despite the somewhat greater rotation Venus near ]·cm wavelength, Astron. J. 70, No.5, 330- 33l. period we would expect the radio emission behavior Thornton, D. D. and W. J. Welch (1963), 8.35 mm radiation from to be very much like that of the Moon. As is well Jupiter, Icarus 2, No.3, 228-232. known, at the wavelengths at which Mercury has been Tolbert, C. W. , and A. W. Straiton (1964a), 35·Gc/s, 70·Gc/s and 94·Gc/s Cytherean radiation, Nature 204, No. 4965, 1242-1245. observed, the dark side of the Moon is nearly as bright Tolbert, C. W., and A. W. Straiton (1964b), An investigation of 35 as the bright side, and so we would expect the same Gc, 70 Gc, and 94 Gc Cytherean radiation, Electrical Engineering behavior of Mercury. Research Laboratory, Universit y of Texas, Report No. T-D2, J. H. Thomson: R. D. Davies has now determined October 15. Troitsky, V. S. (1954), Theory of lunar radio emission , Astron. lh. that the apparently very high brightness temperatures 31, No.6, 511- 528. of Mars at 21 -cm wavelength observed at 10drell Bank Vetukhnovskaya, Yu. N., A. D. Kuzmin, B. G. Kutuza, B. Ya. in 1963 were seriously affected by a confusing cosmi c Losovsk ii , and A. E. Salomonovich (1963), Measurement of radio radio source, with the result that the Martian bright emission from the night side of Venus in centimeter wavelength range, Izv. Vysshikh Uchebn. Zavedenii , Radiofiz. 6, 1054- 1056. ness temperature appeared very much higher than it Walker, J. C. G. (1961), The thermal heat budget of Mercury, Astro· actually was. After correction for this confusing phys. J. 133, No. 1, 274- 280. source, the equivalent blackbody disk temperature Welch, W. J., and D. D. Thornton (1965), Recent planetary observa· for Mars at the 21-cm wavelength in 1963 was 320 tions at wavele ngths near 1 cm, Astron. J. 70, No.2, 149- 150. ± 95 oK. Similar measurefl1ents during the 1965 opposition produced an equivalent blackbody disk temperature of 225 oK. 8. Additional Related Reference Basharinov, A. E. , Yu. N. Vetukhnovskaya, A. D. Kuzmin, B. G. Kutuza, and A. W. Salomonovich (1964), Measurements of the brightness temperature of Venu s at 8 mm, Astron. lh. 41, No.4, 707-710. (E ngli sh transl., Soviet Astron. AJ 8, No.4, 563-565, 1965.) (69Dl2-593)
Mars and Venus at 70-cm Wavelength H. E. Hardebeck
Center for Radiophysics and Space Research, Cornell University, Ithaca, N.Y.
Observations of Venus and Mars at a frequency of 430 Mc/s have been made with the Arecibo telescope. An observing technique was used in which the source was observed, and some days later, when the planet had left its previous position, the position was reobserved to determine the flux produced by the background radio sources. The equivalent blackbody temperature measured for Venus at 430 Mc/s was 518±40 oK. The upper limit on its flux density at 1 A.U_ was 0.05 X 10- 26 MKS units at 195 Mc/s. The upper limit at 430 Mc/s was 0.024 X 10- 26 MKS units.
(Paper 69Dl2-594)
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