RICE UNIVERSITY INFRARED PHOTOMETRY AND RADIOMETRY OF THE PLANETS by Kendall Ray Armstrong A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Thesis Director's signature: Hous ton, Texa s May, 1971 ABSTRACT INFRARED PHOTOMETRY AND RAD I OME'TRY OF THE PLANETS by Kendall R. Armstrong The first observations of the planets Venus, Jupiter, and Saturn have been made in the far infrared at wavelengths between 304 and 3004 using the Rice University Flying Infrare Telescope and crystal transmission filters cooled to liquid He temperatures. Wide band brightness temperatures have been found for the three planets which vary significantly from their calculated and observed effective temperatures. Photometric observations at wavelengths less than 254 obtained at the University of Arizona Catalina Observatory complete the near infrared spectrum of Venus, Mars, Jupiter, Saturn, Uranus, Vesta, the Galilean satellites,- and Titan. Results at 10.24 and 224 indicate tenuous atmospheres on the satellites Jupiter I and Titan. Millimeter radio- matry gives a 1.2 mm brightness temperature of 103(±32)K for Uranus. It is shown from high resolution scans that the high 54 brightness temperature of Jupiter originates below the visible cloud tops in the region of the north equatorial belt. High resolution photometry of the Saturn system shows that the rings radiate a significant fraction of the flux observed at 10.2b and at 22b radiate over half the flux observed from the dish-rings system. TABLE OF CONTENTS Page 1.0 INTRODUCTION 1 2.0 THE NEAR INFRARED PHOTOMETRY 3 3.0 FAR INFRARED RADIOMETRY 8 3.1 The Far Infrared Filters 8 3.2 The Observations 13 4.0 OTHER OBSERVATIONS 21 4.1 Millimeter Radiometry 21 4.2 High Resolution Photometry of Jupiter and Saturn -22 5~ 0 DISCUSSION 2 5 5.1 The Planets 27 5.1. a. Mercury 27 5.1. b. Venus 23 5.1. c. Mars 33 5.1. d. Jupiter 34 5.1. e. Saturn 39 5.1. f. Uranus and Neptune 45 5.2 The Satellites 46 5.2. a. Asteroid Vesta 46 5.2. b. The Galilean Satellites 49 5.2. c. Titan 52 6.0 CONCLUSIONS 54 FIGURE CAPTIONS 59 ACKNOWLEDGEMENTS 63 REFERENCES 65 1 1. INTRODUCTION The. infrared spectrum from approximately 5U to 300p. is the most important region of the spectrum in the astronomical study of planetary atmospheres and is a powerful tool in understanding the composition of the atmospheres and planetary surfaces. Reflection spectroscopy in the region below 3U has been used to determine much of the composition and physical state of the upper atmosphere of the planets, but since it depends directly on incident solar radiation rather than radiation originating from the planet itself, does not give the information necessary to construct physical models of the planets' atmospheres and interiors. Planetary radiation in the thermal region of the spectrum has bean absorbed and redistributed by the planet before being reradiated, and observations of the brightness temperature over the complete spectrum serve as the boundary conditions, as wall as dictate which processes are most important, for the problem in radiation transfer which must be solved in the construction of planetary models. Questions such as the He : H2 abundance ratio for Jupiter and Saturn can be answered by remote observation only through measurements in the far infrared region because of the spectroscopic inactivity resulting from the symmetric, 2 homonuclear molecular structure of He. The observation of the far infrared spectra of Venus, Jupiter, and Saturn between 30|J. and 300P is the most important result of this research. Two factors have made these observations possible: the use of a flying infrared telescope mounted in an aircraft and flown above the tropo¬ sphere to overcome the strong water vapor absorption band in the far infrared region; and the development of a system of He cooled crystal filters which have well defined, high transmission pass bands. These observations fill in the most significant portion of the gap between 25H and milli¬ meter wavelengths for which ground-based astronomy is impossible. Numerous ground based photometric observations have been reduced to complete the thermal spectra of the planets and extend the known spectra of the Galilean satellites and Titan to 251-i. Radiometric observations of the planets at 1.2 mm and'high spatial resolution photometry of Jupiter and Saturn are also used in the interpretation of the planetary spectra. 3 2.0 THE NEAR INFRARED PHOTOMETRY Ground based observations in the near infrared are limited by the atmosphere to a few specific windows in the electromagnetic spectrum through which incident radiation is transmitted with relatively little atten¬ uation. These windows occur at wavelengths less than about 25b betv/een the strong rotational absorption bands of several atmospheric gases, mainly H^O, CO^ and Og (Howard and Garing, 1962). Between 25p and 1mm the lower atmosphere is made almost opaque by the rotational band of water vapor at 100P. The KLMNQ photometry used for this research was obtained in five windows centered near 2.2, 3.4, 5.0, 10.2 and 22p. Observations of the planets and satellites were made with the 28-inch, 60-inch and 61-inch telescopes at the Catalina Observatory during the regular photometric observing program described by Johnson and Mitchell (1962). Briefly, each observation is initiated by entering onto paper tape, from a punched observing card and from the photometer, the following data: the identification number of the object; the right ascension and declination of the object, the date and time of the observation; the integration time; the identification number of the filter 4 being used? and the amplifier gain setting. Integrations (usually 15 seconds) of alternate deflections from the object and sky are punched onto the tape and continued until the observer feels sufficient accuracy has been obtained. After each block of integrations the time at the end of the observation and the quality of the observation ("good" or "bad") as judged by the observer are entered onto the paper tape. The observations for each night are fed into a digi¬ tal computer and are reduced according to a standard procedure (Johnson and Mitchell, 1962). The computing program checks the assumed atmospheric extinction co¬ efficient for each filter passband from observations of 1 standard stars during the night, and if warranted, new coefficients are computed from observations of one or more of the standard stars. The observed relative magni¬ tudes are transformed to their values in the absence of atmospheric extinction, and then converted to fluxes using the absolute calibration of the filters (Johnson, 1965a; Low ejt al., 1970) . The planets Venus, Mars, Jupiter, Saturn, and Uranus? the asteroid Vesta; and the satellites J I (Io), J II (Europa), J III (Ganymede), J IV (Callisto) . 5 and Titan have been observed under the program outlined above. Most of the observations were made during the period January-May, 1969; a few observations at 5p and 22\x were made in February, March, and November, 1968. Measurements at 2.2li and 3.4|i are included in the observations made after January, 1970. Because the planetary observations were made during the regular stellar observing program, the beam diameter of the telescope was usually smaller than or approximately the same size as the visible disks of Venus, Jupiter and Saturn. In such cases errors of 20-30% may be intro¬ duced when calculating brightness fluxes, since the exact shape of the beam and its position on the planetary disk is not well known. Even in the absence of uncertaint¬ ies in beam size and pointing, the surface brightness would not be expected to remain constant from one observ¬ ation to the next because of the curvature of the plane¬ tary surface, limb darkening effects, and real temperature variations across the planetary surface. Only on five nights from February 22 to March 8, 1967, when a beam diameter of 180 arcsec was used, do the brightness temp¬ eratures obtained correspond to the total flux emitted by the visible planetary hemisphere. 6 All data obtained at 5n during the first five months of 1969 have been disregarded in the computation of brightness temperatures because of a leak near 12p in the 5|i filter used at that time. The basic filter had half-power points at 4.57|_i and 5.42^ with less than .02% transmission from 2.Op to 4.0ji and from 5.8p to 7.Op. Barium Fluoride was used to provide blocking beyond 13n; however, a leak was discovered at 12u which passed ap¬ proximately 11% of the flux transmitted by the N filter. Sapphire, which has good blocking beyond 7{j., was substi¬ tuted for the BaF2 to eliminate this leak. The monochromatic flux received from the projected disk of a. spherical planet of radius R in radiative equi¬ librium with solar radiation at an orbital distance r from the earth, with a telescope beam having a radius 3$. < R at distance r, is given by „ „ TTR^ ,a\ 2 TT&rtf2 „ -2-2 -1 -1-1 F = B w cm 1 str d-1) \ \ 2 ("a' TTR2 I- ' where is the monochromatic intensity of flux emitted from the surface of the planet and "a is the average orbital radius of the planet. The monochromatic surface brightness is then, 2 2 2 1 Wx = TTBX = Fx (f) (|) (f) W cm-V (1-2) where to is the planetary diameter in radians and a is 7 the telescope beam width, with the understanding that tu/a = 1 for a > UJ. The results of the KLMNQ photometry for each of the planets and satellites are summarized in Table 1.