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III. Session: Passive Radio Observations of Venus, Saturn, Mercury, Mars, and Uranus

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III. Session: Passive Radio Observations of Venus, Saturn, Mercury, Mars, and Uranus 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.
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