Planetary observations at a wavelength of 6 cm

Planet.

SpaceSci.

1966. Vol. 14.

pp.1017

PLANETARY

to 1022.

Pergatnon Press Ltd.

Printed in Northern Icehttd

OBSERVATIONS OF 6cm

AT A WAVELENGTH

M. P. HUGHES Harvard Radio Astronomy Station, Fort Davis, Texas, U.S.A. (Received 19 May 1966) Abstract-Radiometric measurements of Venus at a wavelength of 6 cm made in the period from November 1965 through March 1966 indicate that the disk temperature reached a minimum of 630 f 30°K soon after inferior conjunction. An observation of Mars near opposition in 1965 gave a disk temperature of 190 i 60”K, the same as that reported by Kellermann. The disk temperature of Jupiter remained essentially constant at 291 f 25°K from March 1965 through March 1966. Saturn was found to have a disk temperature of 190 rt 45”K, which is slightly higher than anticipated. INTRODUCTION

This paper describes observations that were made of Venus, Mars, Jupiter, and Saturn during 1965 and 1966. The receiving system consisted of an 85-ft parabolic antenna and a modified Dicke receiver which used a parametric r.f. amplifier(l). At a wavelength of 6 cm the response pattern of the antenna closely approximated a gaussian with a half-power beamwidth of 10.8 arc min in both right ascension and declination. A reference horn produced a narrow beam displaced 1.8” West of the main receiving beam. The parametric amplifier operated in the degenerate mode, and was followed by a balanced mixer and a zero-frequency i.f. amplifier having a bandwidth of 100 MC/S. With a receiver output time constant of 10 set, the minimum detectable signal was 1 x 1O-26MKS flux units. An argon-discharge noise-tube provided an internal calibration signal of 0.7°K. OBSERVATIONAL

PROCEDURE

AND DATA ANALYSIS

Scans were made in both declination and right ascension across each of the four planets. For all observations the output time constant of the receiver was set at 2 sec. The signals were recorded on a chart recorder and were also digitized and printed out every 10 set by a digital integrating device with a finite time constant of 10 sec. During scans in declination, the antenna was driven North or South at a rate of 6 arc min per minute of time while tracking the planet in right ascension. North-going and South-going scans were averaged together, allowance being made for the effect of the time constant. All scans in right ascension were made as drift scans with the antenna stationary. The averages were processed on a digital computer by a program which convolved the data with a gaussian, 5 arc min to half amplitude, to remove the high-frequency noise components. The program then fitted a gaussian curve to the convolved data by the method of least squares. The amplitude of the gaussian was corrected for the effect of the convolution, converted to a temperature in degrees Kelvin, and taken to be the antenna temperature for the planet. The program also removed any slope present in the baseline and plotted the data. An example of the effect of the convolution is shown in Fig. 1, where an observation of Saturn has been replotted from the computer analysis (a) before convolution, and (b) after convolution. 1017

1018

M. P. HUGHES

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For observations of Venus and Jupiter, where the antenna temperatures exceeded 0.2%; the internal calibration signal was recorded at the beginning of each scan. For Mars and Saturn, where the antenna temperatures were less than 0*05”K and receiver noise was the limiting factor in accuracy, the internal calibration signal was recorded once every tenth scan. Daily observations of one or more of the radio sources 3C123, M87, and 3C348 served to relate antenna temperatures of the planets to flux densities. The flux densities adopted for the calibrators at 6 cm were respectively 16.9, 70.0, and 13-O x 10sN MKS

PLANETARY

OBSERVATIONS

AT A WAVELENGTH

1019

OF 6cm

units(2). On the assumption that the radiation from the planets was thermal in origin, the standard radiation formulae were applied to convert the flux densities of the planets to equivalent disk temperatures, To. The radiation from Jupiter was strong enough to be observed reliably with the following on-off method. A series of short scans in right ascension and declination was taken to find the position of maximum antenna temperature from the planet. An on-off observation of Jupiter then followed this sequence: NCNJSCSJWCWJECE, where C and J represent 2-min observations of the internal calibration signal and of Jupiter; and N, S, W and E represent similar observations of the background 30 arc min north, south, west, and east of the planet’s position. The antenna temperature for Jupiter was then determined by comparing the deflection due to Jupiter with the calibration.

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FIG. 2. EQUIVALENTDISK TEMPERATL~RE OF VENUS AS A FUNCTIONOF PHASEANGLE FOR THE PERIODNOVEMBER1965-MARCH 1966.

VENUS

Venus was observed between Eastern elongation in 1965 and Western elongation in 1966. Figure 2 shows the equivalent disk temperatures of the planet during this period, plotted as a function of phase angle. Between four and fourteen individual scans were averaged to determine each point in this diagram. The spread in data following inferior conjunction precludes any detailed analysis, but there is some evidence for a small phase effect with minimum disk temperature occurring soon after inferior conjunction. This result is in agreement with observations at 3.15 cm by Mayer et ~1.‘~)and at 10 cm by Draket4) which are indicative of retrograde rotation. The most reliable measurements were made near inferior conjunction in January and February 1966 when the signal-to-noise ratio was greater than 30: 1. At this time the disk temperature was 630 f 30°K. This value is plotted as an open circle in Fig. 3a, which shows the microwave spectrum of the planet. The disk temperature remains essentially constant near 600’K for wavelengths greater than 3 cm.

M. P. HUGHES

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F1o.3. MICROWAVE SPECTRA OF THE PLANETS. OPEN CIRCLES DENOTE DISK TEMPERATURES REPORTED IN THIS PAPER. ARROWS INDICATE INFRA-RED TEMPERATURES’~-‘I. (kIiJ?R MICROWAVEDATA: VENUS,PROM BARRR~AND STAELII@; MARS,PROMDENT e~~l.'~J;JUPITER, PROM ROBERT+~~; SATURN, PROM DAVIESAND WILLIAMS, m ADDITIONAL POINT AT 11.3cm PROMDAVIES,BEARD,AND COOPER(~*~.

Thirteen scans were made over the planet on 4 April 1965. The flux density of Mars was found to be 0.45 f 0.15 x 1O-z6MKS units, indicating a disk temperature of 190 & 60’K. Measurements of the Martian disk temperature in the wavelength range 0.0012-21.3 cm have been summarized by Dent et al .@). On the assumption that the only source of heat for Mars is insolation, these authors corrected the disk temperatures to a mean solar distance of 1524 a.u. by multiplying the reported temperatures by a factor inversely proportional to the square root of the planet’s solar distance. Values normalized in this manner can be compared directly with one another. In the wavelength range 0*12-21.3 cm the mean of the normalized temperatures is 192°K.

PLANETARY

OBSERVATIONS

AT A WAVELENGTH

OF 6cm

1021

The planet’s solar distance at the time of the measurements reported here was 1.650 a.u., which leads to a normalized disk temperature of 198 & 62°K. This value is close to the mean temperature in the microwave range and in good agreement with the temperature found at 6 cm by KeIlermannu33), whose measurements have a smaller error. Figure 3b shows the microwave spectrum of Mars with the value reported here plotted as an open circle. The temperature of the planet in the radio range is,somewhat less than the infra-red value and changes little with wavelength. TABLE

FLUX

DENSITY AND

Method of observation

Date 1965 Mar. 1965 Apr. 1965 Nov. 1965 Dec. 1965 Dec. 1966 Mar. 1966 Mar.

1. THE

31 1 5 3 10 8 16

on-off Scans Scans Scans on-off Scans Scans

APPARENT

Antenna polarization position angle 76” 76” 8: 8:: 0”

DISK TEMPERATURE

OF J~PXTER AT

Flux density (MKS

x

Observed 4-1 4.1 8.0 8.9 9.2 62 5.8

+ f f f f * f

0.5 o-5 0.5 0.5 0.5 0.5 0.5

10-*B)

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f i i f f f C

6cm

1 1 0.6 0.6 0.5 0.8 0.8

Apparent disk temperature (“K) 265 f 30 265 f 30 302 i 20 300~20 310 f 20 301 f 25 293 & 25

JUPITER

Table 1 lists the results of observations taken during 1965 and 1966. System errors for the first two measurements in the list may be greater than for the rest, since the antenna characteristics have not been fully investigated for polarizations other than North-South. Column 6 lists the effective black-body disk-temperature of Jupiter, calculated on the assumption that the radiation at 6 cm is unpola~zed. The average of these values, 291 i 25”K, is plotted as an open circle in Fig. 3~. The non-thermal component of the flux is clearly apparent at 6 cm, since the effective disk temperature is more than twice the infra-red temperature of 130°K (‘). The flux density of Jupiter, normalized to a geocentric distance of 4.04 a.u., is given in Column 5 of Table 1; the average, 9-l x 1O-26MKS units, is very close to the value found at 6 cm by Roberts and Komesaroff(l*). Over the period of the obse~ations, there was no evidence for a significant change in the flux from Jupiter. SATURN

During November 1965, thirty-eight scans were made across Saturn with the antenna feed polarized North-South. On the assumption that the rings do not contribute to the flux, the average disk temperature determined from these measurements was 190 f 45°K. This value is plotted as an open circle in Fig. 3d. The disk temperature reported here lies above the empirical curves published by Davies and Williams(rl). This result suggests either that there is a polarized non-thermal component of radiation as reported by Rose et a1.(15),or that the temperature of the planet’s atmosphere increases with depth. Further observations of the disk temperature in the decimeter range and of the polarization at all wavelengths are necessary to resolve this question. Acknowledgements-1 wish to thank Dr. A. Maxwell for helpful suggestions concerning the presentation of the data. observations tion.

I am also indebted to Mr. R. F. Rinehart and Mr. J. H. Taylor for their assistance with the and computation. This work was supported linancially by the U.S. National Science Founda-

1022

M. P. HUGHES REFEXENCl3S

1. M. P. HUOHES,E. MOLEY, D. R. PARENTIand J. J. WHELEHAN,IEEE Trans. Antennas Propag. 13, 432 (1965). 2. A. MAXWELL and R. F. RINEHART, Private communication. 3. C. H. MAYER, T. P. MCCULLOUGH and R. M. SLCIANAKER, La Physique des Pla&es, lime CON.int. d’astrophysique, Lic’ge, p. 357 (1963). 4. F. D. DRAKE, Astr. J. 69,62 (1964). 5. E. PBTTIT, The Solar System, Vol. III. Planets and Satellites (ed. Kuiper G. P. and Middlehurst B. M.), p. 400. University of Chicago Press (1961). 6. W. M. SINTON, La Physique des Plan&es, lime CON. int. d’astrophysique, L&e, p. 300 (1963). 7. D. H. MENZEL, W. W. COBLENTZ and C. 0. L-LAND, Astrophys. J. 63,177 (1926). 8. A. H. BARRENand D. H. STAELIN, Space Sci. Rev. 3,109 (1964). 9. W. A. DENT, M. J. KLEIN and H. D. ALLJZR,Astrophys. J. 142,1685 (1965). 10. J. A. ROBERTS, Planet. Spuce Sci. 11,221(1963). 11. R. D. DA~IIXSand D. WILLIAMS, Planet. Space Sci. 14,15 (1966). 12. R. D. DAVIES, M. BEARD and B. F. C. COOPER, Phys. Rev. Lett. 13,325 (1964). 13. K. I. KELLERMANN, Nature, Lo&. 206,1034 (1965). 14. J. A. ROBERTSand M. M. KOMESAROFF,Icarus 4,127 (1965). 15. W. K. ROSE, J. M. BOLOGNA and R. M. SLOANAKER, Phys. Rev. Lett. 10,123(1963). PekXOMe--P~EOMeTpHqecKIle EIaMepeHElH BeHepbI Ha AJlHHe BOJIHbI B 6 CM, IlpOHk?BeAeHHbIe B nepuon ~na~~@cH C ~os6pff 1965 r. A0 KOHqa MapTa 1966 r. YKaablBaIOT, 4TO TemnepaTypa AHcKa AocTmna MmHMyMa B 630 * 30°K BCKOpe uocne HmKHerO CoegHHeHHa. HaBmOgemm Mapca ~6nKaa II~OTHBOCTORFIE~H B 1966 r. cHa6Amo TemepaTypy HncKa B 190 f 6O”K, maqe roBopR Ty-me, KoTopaR 6ma 06HapyHteHa KennepmaHoM. TemepaTypa AmKa IOnuTepa 0cTaBanacb no cyqecTBy HemMeHHoti Ha 231 f 26°K C MapTa 1965 l’. BlIJIOTb A0 KOHIJaMapTa 1966 T. OKa2aJIOCb, ¶TO y CaTypKa TeMnepaTypa AmKa paBmeTcrr 191 f 40”K, ¶TO cnerKa Bmue, 9eM upennonaram.