.743S HIGH-DISPERSION SPECTROSCOPIC OBSERVATIONS OF MARS .147. J. II. THE WATER-VAPOR VARIATIONS Ap 67 Ronald A. Schorn 19 Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California Hyron Spinrad Berkeley Astronomy Department, University of California Roger C. Moore The RAND Corporation, Santa Monica, California Harlan J. Smith McDonald Observatory, University of Texas Lawrence P. Giver Berkeley Astronomy Department, University of California Received May 31, 1966; revised August 8, 1966 ABSTRACT Nineteen high-dispersion spectrograms of Mars taken at McDonald and Lick Observatories during the 1964-1965 apparition have been examined for Doppler-shifted rotational lines of H20 near X8200. The presence of H20 in the atmosphere of Mars appears to be confirmed, but indications are that its concentration varies with time and location on the planet. Typical amounts of H20 are of the order of 10-20 u précipitable water vapor. The spectroscopic results are compared with photographic patrol studies during this apparition The amount of water in the vapor phase seems to depend upon the size of the Martian polar cap, implying a physical connection between the two. I. INTRODUCTION In 1963 Kaplan, Münch, and Spinrad (1964; hereinafter referred to as “KMS”) detected weak absorption features near X8200 Â on a high-dispersion spectrogram of Mars obtained at Mount Wilson. They attributed these features to Doppler-shifted Martian H20 lines. From the equivalent widths of these features and laboratory measure- ments of Rank, Fink, Foltz, and Wiggins (1964) they derived an H20 abundance of 14 ± 7 /x of précipitable water. Although various indirect estimates of the amount of H20 had been made before the KMS investigation, spectroscopic results had been uniformly negative up to that time. The situation as it stood in 1963 is well summarized in KMS. We will only say here that, because of the lack of positive results in previous attempts, the Mount Wilson work must be considered an exploratory effort. At the same time Dollfus (1963), by a photometric comparison of the intensities of the 1.4 ju H20 band in the spectra of Mars, the Moon, and stars as observed from the Jungfraujoch, derived a Martian H20 abundance of about 200-/1 précipitable water. The appearance of two independent positive detections of water vapor, in amounts differing by an order of magnitude, clearly indicated the need for additional observations. The importance of the amount of water vapor, if any, in the atmosphere of Mars to the study of biological, geological, and meteorological problems led us to plan an exten- sive observing program for the 1964-1965 apparition. We also planned to investigate the weak A8700 band of C02 for better values of C02 abundance and, consequently, a better estimate of the Martian surface pressure. The C02 studies are reported elsewhere by Spinrad, Schorn, Moore, Giver, and Smith (1966; hereinafter referred to as “Paper I”)* For positive detection and more reliable abundance estimates, a large number of good 743 © American Astronomical Society • Provided by the NASA Astrophysics Data System .743S 744 SCHORN, SPINRAD, MOORE, SMITH, AND GIVER Vol. 147 .147. J. spectra were needed. Detection of the faint Martian H2O lines near both quadratures, Ap when the Doppler shifts would have opposite signs, would greatly reduce the possibility 67 that weak terrestrial lines would confuse the picture. Furthermore, we wanted to ob- 19 serve Mars over as long a period of time as possible to see if the H2O abundance varied with time and position on the planet. The observed behavior of the Martian pole caps, the wave of darkening, and white clouds, combined with the small amount of H2O believed to exist in the atmosphere of Mars, naturally suggests that the local humidity might vary greatly. The Martian H2O lines observed by KMS were slightly stronger near the south pole than the north, which also suggested that we might be able to detect variations in humidity. Since the largest possible dispersion and image scale were required, only a few existing telescopes were capable of carrying out our observing program. The Lick Observatory 120-inch and the McDonald Observatory 82-inch telescopes were the instruments used. Because of the greater demands on the Lick telescope, most of the observations were made at McDonald. Although the 120-inch coudé spectrograph is somewhat superior to the 82-inch coudé spectrograph in scale and speed, experience showed that the substan- tially lower terrestrial humidity at McDonald made these two instruments about equally effective for the detection of Martian H2O lines. The 82-inch telescope underwent a substantial modification in 1964, made possible by a grant from the NASA Office of Space Sciences, and the coudé spectrograph was not ready until December. Observations before that time were perforce made with the 120- inch. By the time observing conditions at Lick became uncertain, due to the usual poor winter weather there, the McDonald coudé was available. Use of both these instruments insured fairly good coverage through the whole apparition except, of course, for a few months around opposition, when the Doppler shift of Mars was too small for our pur- poses. II. THE OBSERVATIONS The observations reported here are summarized in Table 1. Nineteen well-exposed spectra are listed. The first column gives the date, and the second indicates the instru- ment used—“L” for the Lick 120-inch telescope and “M” for the McDonald 82-inch telescope. The third column gives the planetocentric longitude of the sun, La. = 0 corresponds to the beginning of spring in the northern hemisphere of Mars; L8 = 90° to the beginning of summer, etc. The fourth and fifth columns give the average Doppler shifts in angstrom units of the suspected Martian lines as measured by Spinrad (AXjy) and Schorn (AXä), respectively. A minus sign means that Mars is approaching Earth; a plus sign means that it is receding. The number of lines used in determining each aver- age is given in parenthesis following the average. The sixth colunn gives AX, the X8200 Â Doppler shift of Mars derived from the tables of Niehaus and Petrie (1961). The seventh column is the estimated abundance of précipitable water in microns. The last column gives comments regarding the visibility of the suspected H20 lines over the disk of Mars. To both measurers it appeared in December, 1964, that the “Martian” lines were visible only in the northern hemisphere of Mars. We should point out that these spectra represent only one-third of the nights sched- uled for this program. Exceptionally cloudy weather at both Lick and McDonald was to blame for half the wasted nights, the rest being due to a variety of factors. Successful Mars H2O spectra were obtained only when the terrestrial humidity was quite low. In addition, the hypersensitizing routine used, described in Paper I, while producing a gain in speed of a factor of 20 to 50 when applied to IV-N emulsions, was somewhat un- predictable. A number of underexposed, overexposed, and/or mottled plates were produced, which were unsuitable for our use. Furthermore, since neither Lick nor McDonald possessed an infrared exposure meter, exposures made through the common layers of thin cloud were uncertain. One more difficulty can be laid to our attempt to record the X8700 CO2 band (as described in KMS) at the same time as the X8200 H2O © American Astronomical Society • Provided by the NASA Astrophysics Data System .743S No. 2, 1967 SPECTROSCOPIC OBSERVATIONS OF MARS 745 .147. J. band. The sensitivity of the IV-N emulsions used varied so much over this region that Ap it was difficult simultaneously to expose both bands properly. 67 All exposures were made with ammonia-hypersensitized IV-N emulsions. The Lick 19 spectrograms were obtained with the 160-inch-focal-length camera of the 120-inch telescope coudé-focus spectrograph, at a dispersion of 4.09 Â/mm at X8200 Â. The pro- jected slit width was about 15 p and the slit was positioned, with the aid of an image rotator, so that it passed through the north and south poles of Mars. Typical exposure times were 2 hours. The McDonald plates were made with the 160-inch-focal-length camera of the 82-inch-telescope coudé-focus spectrograph at a dispersion of 4.14 Â/mm at X8200 Â. An image rotator was not available at McDonald but all exposures made TABLE 1 Recent H20 Observations Date Obs AXfl(Â) AXß(Ä) AXcf (Â) w*(n) Remarks 9/24/64 L 5° -0 296 <15) 9/25/64. L 6 - 298 <151 10/24/64 L 20 - 368 <15 No Martian lines visible 11/17/64 L 31 - 413 <15j 12/24/64. M 47 -0 348 (4) -0 428 (6) - .438 ^15 12/26/64 M 48 - 468 (4) - 431 (4) - 437 ^15 12/27/64. M 49 - 377 (5) - 431 (5) - .437 ^15 12/28/64 M 49 - 439 (7) - 426 (5) - .436 ^45 12/29/64 M 49 - 431 (4) - .438 (3) - 436 ^15 12/31/64: M 50 - .455 (2) - 517 (3) - 434 ^15 N. hemisphere only 1/15/65 M 57 - 368 (5) - 406 ~15 1/16/65 M 57 - 397 (4) - 403 ~15 1/19/65 M 59 - 409 (4) - 394 ^15 1/23/65 M 60 - 377 (1) - 410 (6) - 379 ^15 1/24/65 M 61 - .327 (1) - 381 (2) - .376 ^15^ 5/14/65 M 110 + 340 (7) + 358 ^10 Probably both hemi- spheres 5/18/65 L 112 + 307 (3) + 391(4) + 361 ~10 Perhaps more in S. hemisphere 5/24/65 M 114 + 331 (7) + 363 ~10 Probably both hemi- spheres 6/10/65 L 122 +0 334 (4) +0 335 (5) +0 359 25 In S.