High-dispersion spectroscopic observations of Mars

XCARUS 18, 43--57 (1970)

High-Dispersion Spectroscopic Observations of M a r s IV. The Latitude Distribution of Atmospheric Water Vapor R O B E R T G. T U L L The University of Texas at Austin, Austin, Texas 78712

Received April 2, 1970; revised May 1, 1970 Spectra of Mars, centered on the 8200-A band of H20, have been obtained using the coud6 spectrograph of the 107-inch telescope at McDonald Observatory with reciprocal dispersion of 1.9 A/mm. The plate scale (4.4 arc sec/mm) and angular resolution (3~-6 ~) were sufficient to measure the strength of the Doppler-shifted H20 lines at five points across the disk. The spectra wer9 obtained in March and April 1969, when the apparent diameter of Mars was 10~ to 16~ during midsummer of the northern hemisphere; the Doppler shift at 8200 A varied from -0.42 to -0.28 A. On the first plate, obtained on 1969 March 27, the abundance reached a maximum of about 48 microns precipitable H20 at 30 ° to 40° north latitude and decreased to about 20 microns at 30 ° south latitude. The second plate, taken on 1969 April 28, showed the same north-south decrease in abundance but the total amounts were about 2/3 of the March abundances. INTRODUCTION T h e quest for w a t e r on the planet Mars has a history paralleling the searches for atmospheric o x y g e n a n d for life (de Vaucouleurs, 1954). Seasonal variations in the surface markings, a n d especially in the polar caps, have long suggested the presence of water, and indeed, K u i p e r (1949) and Dollfus (1951) f o u n d evidence from the infrared spectra and polarization of the polar caps t h a t t h e y are either composed of or are overlain with w a t e r ice or frost. L e i g h t o n a n d M u r r a y (1966) p o i n t e d out t h a t a m i x t u r e o f COs a n d H 2 0 ices could explain t h e infrared and polarization data, a n d later N e u g e b a u e r et al. (1969) showed t h a t the t e m p e r a t u r e of the south polar cap is near t h a t o f C02 ice in a n a t m o s p h e r e o f 6-mb pressure, giving almost certain evidence t h a t the bulk of the polar cap is COs ice. W a t e r v a p o r was discovered in t h e Martian a t m o s p h e r e at Mr. Wilson Observ a t o r y b y Spinrad et al. (1963), a n d K a p l a n et al. (1964), who d e t e c t e d weak, Doppler-shifted a b s o r p t i o n lines of t h e 8200-A b a n d in high-dispersion spectra

during the 1963 apparition. The presence o f H 2 0 was confirmed and f u r t h e r analyzed b y Schorn et al. (1967), Owen a n d Mason (1969), a n d Schorn et al. (1969). Schorn et al. (1967) indicated t h a t the w a t e r v a p o r concentration appears to v a r y with t i m e a n d with location on the planet. T h e concentrations f o u n d to d a t e h a v e varied from a b o u t 10 to 35 microns precipitable H 2 0 in the vertical column of air. T h e 107-inch (2.7-m) t e l e s c o p e a n d coud6 spectrograph of t h e U n i v e r s i t y of T e x a s ~ McDonald O b s e r v a t o r y a t Mr. Locke b e c a m e operational on March 8, 1969. T h e spectrograph has two cameras of 6 a n d 14 ft (1.8 a n d 4.2 m) focal length, and gratings of 204 × 306 m m ruled area. T h e telescope a n d spectrograph are described elsewhere (Smith, 1968; Tull, 1969). W i t h the high spatial a n d spectral resolution possible w i t h this i n s t r u m e n t , a new a t t a c k on the question o f the spatial distribution of w a t e r v a p o r on t h e p l a n e t a p p e a r e d feasible, a n d early tests indicated t h a t a d e q u a t e exposures on a m m o n i a t e d I V - N plates could be o b t a i n e d in the observing t i m e available, i.e., 6 or 7 hours per night. 43

44

R O B E R T G. TULL

TABLE I OBSERVATIONAL PARAMETERS Date 1969

Plate

"Exposure, Minutes

3/27

16

425

Emulsion

Mars apparent diameter

L, •

Projected slit width

Image rotator used

Radial velocity (km/sec)

10.~7

132°

3 4 ? 9 !98°-302 °

22/L

No

-15.5

15.~0

148°

2 4 ° 2 231°-319°

29ft

Yes

-11.1

15'.'8

150°

22?0

29/~

Yes

--10,3

Phase angle

Central ~meridian ~

Ammoniated 4/28

45

360 IV-N

5[2

47

360

193°-281°

a Planetocentric longitude of the sun. Planetographie longitude on the system of the American EphemeriS. (Moore et al., 1966),! a total of 29 lines were found b y visual inspection of the Spectra of Mars at the 8200-A water three plates, near the expected positions vapor band were obtained during March, of Martian H20 lines; 17 of these were April, and May, 1969, using the long-focus found on all three plates. The displacecamera. Three plates were exposed to ments of these lines from the centers of sufficient density to allow measurement of the corresponding telluric H20 lines were line positions, and two of these were of measured on a Mann measuring engine at high quality, allowing equivalent width the J e t Propulsion Laboratory. Table I I I measurements. The third was obtained lists the rest wavelengths of HuO lines, when the terrestrial humidity was fairly and in the second column, the number of high and the Martian radial velocity was plates on which a line was detected with low. The observational parameters for the expected Doppler displacement. The these plates are listed in Table I. The third column lists, for reference, a strength image rotator had not yet been aligned, class on a scale of 1 to 10, based on laboraand hence could not be used for the March tory measurements by Farmer (1969) 27 exposure. Exposures were on am- corrected to 225°K. The only line of moniated ~ IV-N plates (Barker, 1968). strength class/>4 which was not detected Table I I gives the spectrograph para- on all three plates (except for two cases meters. Portions of the three spectra are marked with asterisks in Table III) is at shown in Fig. 1; Fig. 2 shows a further 8193.113 A; notes made during visual enlargement of plate 16, s h o w i n g the inspection of plate 16 indicated an abMartian H20 lines Doppler-shifted to the normal depression of the continuum on violet at 8176, 8189, 8193, and 8197 A. the violet wing of this line, indicating the probable presence of an unidentified absorption or blemish on this plate. Reduction8 Five lines showing :no significant Change A f t e r eliminating lines blended with in displacement as the planet's Doppler known Fraunhofer or weak telhiric lines shift changed wereeliminated from further analysis,due to suspected blending with TABLE II previously unidentified telluric lines. The SPECTItOG~AP~P~X~ETE~S displacements of t w o additional lines were poorly correlated from plate to plate. Camera F.L. 425 cm ill4 Visual inspection revealed defects, e.g., Coll'nnator F.L. 805 cm fl32.5 anomalous plate-grain clumpings, which Grating B (1200grooves/ram, 1 R . A. S e h o r n h a s c o m p i l e d a n u n p u b l i s l ~ e d 0.6 p Blaze 204×306 ram) list of interfering lines based on the listed reference and on a letter from C. C. Kiess. Dispersion 1.9~/mm Thanks are due to him for the use of this list. OBSERVATIONS

HIGH-DISPERSION SPECTRA OF M A R S . IV.

45

FIG. 1. P o r t i o n s o f t h r e e s p e c t r a o f Mars. M o s t of t h e s t r o n g lines are tellurio H 2 0 . Some o f t h e D o p p l e r - d i s p l a c e d a b s o r p t i o n s d u e t o M a r t i a n H 2 0 are m a r k e d c~. Original d i s p e r s i o n w a s 1.9 A r a m .

would affect equivalent widths, for a number of other lines. Column 4 of Table I I I indicates which lines remained, after eliminating these questionable candidates, for determination of water vapor abundance on each plate. The strong line at A8189 A was omitted because of suspected contamination from an unidentified Fraunhofer line (Owen, 1967), although there is evidence that such a contaminating line does not exist (Owen and Mason, 1969; Hunten et al. 1967) ; the line does not show evidence of anomalous strength or displacement in this study. The mean measured displacement and the standard deviation of the mean, for the lines indicated in column 4 of Table III, are given in Table IV. A residual systematic error exists between the mean measured displacement and the ephemeris value on all three plates, in the sense that the measured displacements are 3~/o to 8~/o too small. A second measurement of plate 16 under higher magnification re-

duced the error. The effect may be due to a systematic shift of the apparent core of weak lines observed on the wings of strong lines ; a quantitatively similar systematic error was observed in measured separations of weak and strong telluric HsO line pairs, and weak and strong Fraunhofer line pairs, on the same plates under essentially identical conditions of lighting, magnification, and true line separations as used in t ~ ~ . . l i ~ e ~ m a ~ m ~ m e ~ t s .... Figure 3 Shows~the me~tt~ed"~splacements for the lines of column 4, Table III, plotted against date of observation; the solid curve represents the expected Doppler displacement at 8200 A. The agreement with the expected values appears to be satisfactory in view of the above discussion. The plates were traced in the density mode on a Joyce-Loebl mierodensitometer at the J e t Propulsion Laboratory, after careful checks to ensure repeatability of the measurements and lack of backlash in

46

ROBERT G. TULL

FIO. 2. The region 8176-8200 -~. This is a further enlargement of a portion of the plate obtained on March 27, 1969; the region 81898200 .~ overlaps the first spectrum in Fig. 1, showing additional Martian H20 lines at the positions marked ~.

the wedge and pen motion. The spectra were wide enough (2.4 and 3.4 mm) to allow scans along four or five parallel strips of the spectrum with acceptably high signal-to-noise ratio. Table V gives the positions, measured from the center of the spectrum, and the widths and lengths, of the projected microdensitometer slit used for each spectrum. The measured central

depths of the strong Fraunhofer lines N a I 8182 _~, NaI 8194 A, and F e I 8220 A were compared with the tracings in the Utrecht (Minnaert et al., 1940) and Delbouflle and Roland (1963) photometric atlases of the solar spectrum to determine the position of the zero point of intensity and the height of the continuum. This method defined an H and D curve which was linear over the continuum densities encountered. Equivalent widths of weak Fraunhofer lines, measured with respect to the continuum intensity defined in this way, were compared with Moore et al. (1966). For plate 16, the comparison was favorable and no correction of the measured equivalent widths was necessary. For plate 45 the comparison indicated that measured equivalent widths for weak Fraunhofer lines ( W h < 1 0 mA) were systematically too large b y a factor of 1.6; measured equivalent widths of the H 2 0 lines were, therefore, reduced by this factor. On plate 16 the Mars H 2 0 lines were fully resolved, facilitating the measurement of equivalent widths. However, the continuum noise was relatively high, individual fluctuations commonly being equivalent to absorptions of a few milliangstroms. There was some contamination from skylight, since the exposure was not terminated until about 2 hours after sunrise; however, the tracings of the skylight indicated no absorptions or emissions near any of the measured absorption lines which could be expected to produce errors greater than 1 or 2 milliangstroms in the measured equivalent widths. Both tracings of this plate, made with different projected microdensitometer slit widths, were measured and reduced for comparison. On plates 45 and 47, none of the Mars H 2 0 lines are completely separated from the telluric lines, although they are clearly present visually on both plates and on the microdensitometer tracings on plate 45; see Figs. 4, 5, and 6. On the tracings of plate 47, made with a relatively wide microdensitemeter slit, the Mars lines appear only as local flattenings of the violet wings of the telluric lines; no

HIGH-DISPERSION

47

SPECTRA OF MARS. IV.

TABLE I I I H20 LINES ~NVESTIGAT:ED

Rest ~ (A)

Number of plates

8141.936 8152.498 8153.703 8158.019 8161.434 8164.54 8168.820 8169.995 8176.975 8181.848 8186.371 8189.272 8193.113 8197.704 8218.114

3 2 3 3 3 1a

Lab. strength class

2 2 2 3 5 7

Useful lines Plate No. 16

45

47

×

×

×

Number of plates

l~est ~ (A)

1 1

---

3

10

×

×

×

8223.990 8226.962 8227.986 8229.762 8231.289 8256.515 8259.692 8272.042 8279.600

3 3

4 4

× ×

x x

x ×

8282.024 8289.535

3 2

3

9

8300.408

3

1

8

3 3

10 4

8318.139 8321.242

1 2

×

×

×

×

×

×

2 3 3 3 1~ 3 2 2 3

Useful lines Plate No.

Lab. strength class

16

45

47

3 9

×

×

x

4 3 8

x ×

x

--

8

×

×

x

×

×

x

3 3

3 7 2 3

2

a Blended with the telluric H20 line A8164.157 A on plates 16 and 45. b Flaws on plates 45 and 47 prevented detection of this line. TABLE IV H20 LINE Posr~IoNs :Error Plate

Measured A~t (A)

km/sec

A

16 45 47

--0.400 =t=0.008 (s.d.) --0.280 =i=0.006 --0.272 ::t:=0.006

+0.91 +0.88 +0.33

+0.025 +0.024 +0.009

a t t e m p t was m a d e to m e a s u r e their e q u i v a l e n t widths. On the tracings of plate 45 m a d e with t h e n a r r o w e r slit, the M a r t i a n lines are b e t t e r resolved t h a n t h e y are w h e n t r a c e d with t h e wider slit; only those tracings were measured. Measu r e m e n t of t o t a l a b s o r p t i o n in the Martian c o m p o n e n t was done b y fitting a dispersion profile to the telluric c o m p o n e n t , t h u s defining a " c o n t i n u u m " for t h e M a r t i a n line. I n some cases in which the red wing of t h e telluric line is unaffected b y blends, it was f o u n d useful to use t h a t wing, folded over onto the violet wing, to define the c o n t i n u u m . I n plate 16, where m a n y of the M a r t i a n line profiles

Number of lines measured

11 10 9

h a v e well-defined minima, there was no s y s t e m a t i c difference in equivalent w i d t h s of the lines measured from the t w o tracings, in regions of high c o n t i n u u m density, while measured equivalent widths t e n d e d to be u n d e r e s t i m a t e d with the narrower slit in regions of low c o n t i n u u m density. This is evident in Fig. 11 showing the average ratio o f measured/atlas strengths of F r a u n hofer lines measured at different positions across the s p e c t r u m ; with the wider slit t h e v a r i a t i o n across the s p e c t r u m was flatter t h a n with the n a r r o w e r slit, where the measured equivalent widths fell to 80% near the s o u t h e r n edge of the spect r u m , a n d 5 0 % near the n o r t h e r n edge,

48

ROBERT G. TULL -5

[

l

l

l

l

l

l

l

l

l

l

l

l

l

+/

+//

-I0

+E o

~> - 1 5

I

-2C I0

I 20 March

I

I 30

I

I

I

9

I

I

I

19

29

April Date, 1969

t

I

I

I

9

19 May

FIG. 3. M e a s u r e d r a d i a l v e l o c i t y of Mars, o b t a i n e d f r o m t h e m e a s u r e d D o p p l e r displacem e n t s of t h e H 2 0 lines, as a f u n c t i o n of d a t e of o b s e r v a t i o n . T h e s m o o t h c u r v e gives t h e E p h e m e r i s velocity. T h e r a d i u s of e a c h circle is equal to the standard deviation of the mean; t h e lower p o i n t o n 3/27, a n d t h e p o i n t o n 5/2, were o b t a i n e d f r o m t h e s t r o n g e s t lines, w i t h relatively high microscope magnification. The r e m a i n i n g t w o p o i n t s were o b t a i n e d f r o m all o b s e r v e d lines u s i n g low m a g n i f i c a t i o n .

of the values from the atlas (Moore et al., 1966). ANALYSIS

Measured equivalent widths were converted to the products S . X with the aid of tables given by Jansson and Korb (1968), where S is the normalized line strength and X is the total abundance of

the molecule along the optical path, assuming Voigt line profiles in an atmosphere at T = 2 2 5 ° K , P = 6 mb, and a ratio of Lorentz to Doppler line widths equal to 0.04. New laboratory measurements of H20 line strengths by Farmer 2 (1969), corrected to 225°K, were then used to determine, for each line, the total abundance X; the weighted means and probable errors of these values, expressed in microns of precipitable H20, are given in Table VI. The possibility of determining the effective temperature of the water vapor from these data was investigated by computing X using the values of S corrected to 200 °, 250 °, and 296°K; within this temperature range, the standard deviation in X appears to come to a minimum, indicating t h a t 200 ° < T < 250°K, but the method is not sufficiently sensitive to determine a unique temperature without more precise equivalent widths. The distribution of photographic density across a spectrum is a function of the surface brightness distribution over the planetary disk, the seeing variations and guiding errors of the telescope, and the characteristic curve of the emulsion, as well as such effects as photographic turbidity and Eberhard effect. The surface brightness distribution across the planetary disk is a function of the solar zenith angle, the phase angle, the surface 2 C. B. F a r m e r h a s r e c e n t l y e x t e n d e d his original list of 24 m e a s u r e d H 2 0 line s t r e n g t h s t o 43, in t h e r a n g e 8141.936 t o 8321.587 A. I a m d e e p l y i n d e b t e d t o Dr. F a r m e r for c a r r y i n g out the additional laboratory measurements to include lines i n v e s t i g a t e d in t h i s s t u d y .

TABLE V MICRODENSITOI~IETER P~RAm~TEI~S

107 ~ coud~ p l a t e P r o j e c t e d m i c r o d e n s i t o m e t e r slit

16 0.03 X 0.8

16 0.06 X 0.8

45 0.03 × 0.8

45 0.06 X 1.0

47 0.06 × 1.25

--1.10 --0.60 0 +0.60 +1.10

--0.90 --0.30 +0.30 +0.90

--1.50 --0.75 0 +0..75 +1.50

--1.125 --0.375 +0.375 +1.125

--0.625 +0.625

(ram) D i s t a n c e f r o m c e n t e r of s p e c t r u m

(mm)

49

H I G H - D I S P E R S I O N SPECTRA OF MARS. IV.

MARS H20 8176.975

o"

®

March 27, 1969

8 leL84R

@

®

8197.704

@

•

8226.962

o~

•

• -0.5 mm "*-+O.gmm

"2 sky F I G . 4.

MARS H20 8176.975

d' f

o

o"

April 28, 1969 8197.704

8181.848 ®

d'

F I G . 5.

•

8226.962

0~A • t

50

ROBERT G. TULL

Moy 2, 1969

MARS H20 8176.975

i

t

8181.848

!

8197.704

I

t

8226.962

t

v

t

f

A

FIG. 6. FITS. 4-6. M i c r o d e n s i t o m e t e r t r a c i n g s o f four o f t h e s t r o n g e s t lines. T h e e x p e c t e d locations o f t h e D o p p l e r - s h i f t e d M a r t i a n c o m p o n e n t s are i n d i c a t e d . T r a c i n g s o b t a i n e d f r o m several different parallel strips o f t h e s p e c t r a are s u p e r i m p o s e d .

TABLE VI RESULTS Microdensitometer slit width (mm)

X/R (Effective)

16

0.03

+0.66 +0.44 +0.06 --0.31 --0.54

4.16 4.35 2.98 1.83 2.46

~0.85 (p.e.) ±0.48 ±0.43 ~0.40 &0.39

3.76 2.69 2.38 2.96 3.90

33 48 37 18 19

±7 =t=5 ~5 ±4 ±3

16

0.06

+0.60 +0.26 --0.15 --0.47

4.42 3.58 3.40 4.20

-;-0.34 ±0.63 -t-0.56 +0.61

3.28 2.37 2.58 3.54

40 45 39 35

±3 ~8 ±7 ±5

+0.71 +0.44 +0.15 --0.17 --0.43

3.04 2.48 1.84 1.45 1.23

~0.63 ±0.23 +0.26 +0.35 ±0.14

3.33 2.47 2.29 2.54 3.19

27 30 24 17 I1

~6 ±3 ~3 :t:4 +1

Plate

45

0.03

HIO abundance total path ( x 10s° molecules/cmI)

Air mass Microns Me precipitable HzO (Effective) verticalcol.

p.e.

HIGH-DISPERSION SPECTRA OF MARS. IV.

scattering law, the surface albedo, atmospheric absorption, illumination of the atmosphere b y direct sunlight and b y the surface, and illumination of the surface b y the atmosphere. In the 8200-A region the atmosphere of Mars is transparent, and hence absorption and scattering b y the Martian atmosphere will be neglected. The surface brightness is proportional to eosqi, where i is the angle of incidence of sunlight; we will assume q - 1 (Lambert scattering), a simplification which is justified on the basis that the probable errors in the data are too large to warrant attempts to duplicate the relative surface brightness to greater precision. The solid curve of Fig. 8 shows the function cosi along the projection of the spectrograph slit on the disk of Mars on April 28, when the slit was maintained pole-to-pole b y means of an image rotator. The northern limb was bright, while the southern limb was beyond the sunset terminator. On March 27 the image rotator was not used. For the coud~ spectrograph of the 107-inch telescope, the projected position angle of the slit in the absence of the image rotator is P. A. =

277 ° ~- 8 ° - 15.t h

referred to the top end of the slit, where (t,8) are the hour angle and declination. During the 7 hours of exposure, the projected slit rotated 105 °, as indicated in Fig. 13. The surface brightness along the slit was obtained b y numerically integrating the brightness over that part of the planetary disk which fell on the slit as the image rotated. This is shown as the solid curve in Fig. 7. As indicated in Fig. 13, half of the slit fell entirely in the northern hemisphere, and the other half fell almost entirely in the southern hemisphere; the slit lay nearly along the equator at the beginning of the exposure, and rotated to a position 20 ° past the central meridian at the end of the exposure. The southern half of the slit intersected the sunset terminator, while the northern half lay across the morning bright limb, throughout most of the exposure. The theoretical surface brightness functions along the slit are, therefore, both asymmetrical, with

51

the brightness maximum occurring in the northern hemisphere. The observed brightness distribution was obtained from microdensitometer scans across the spectra, corrected to intensity with the aid of the H and D curve determined as outlined above. The observed brightness at any point (x, 0) on the disk, in polar coordinates relative to the center of the disk, is represented b y the convolution of the theoretical brightness distribution with a Gaussian function representing the light contamination from other points on the disk due to seeing, guiding, and scattered light. We make a one-dimensional approximation, thus neglecting light contamination from directions perpendicular to the slit--which is justified again on the basis of the limited precision of the data. The dashed curves of Figs. 7 and 8 show the result of convolution of the Lambertian surface brightness distribution with a Gaussian function with the standard deviations a given in Table VII. The last column of the table gives, for comparison, the estimated diameter of the seeing disk for a point source during the observations; the agreement between these estimates and twice the Gaussian seeing function is satisfactory, considering that the seeing estimates did not attempt to account for guiding errors. The solid dots in Figs. 7 and 8 represent the measured intensity distribution across the spectrum, and show a reasonable fit to the theoretical convoluted distribution. The major differences between the measured and theoretical curves can be understood in terms of the lower albedo in the southern hemisphere. At any point (x, 0) on the apparent disk, the effective air mass me(x,O ) along the total atmospheric path may be represented b y the convolution of the Gaussian seeing function with the true air mass, mt(x,O), weighted b y the distribution of surface brightness, where mr(x, 0) is the total path length through the atmosphere, relative to the thickness of the atmosphere at the zenith, at the point (x,0) on the disk,

mr(x, O) = sec z¢ + A see z¢ + see z o + A

sec z o ;

2

ROBERTG. -

i,

I

TULL

I

I

I

i

.5

1.0

1.5

"°

- 1.5

- 1.0

-.5

0

SOUTH

X / Ro"

NORTH

FIG. 7. Intensity distribution perpendicular to dispersion, March 27, 1969. The heavy curve is the theoretical intensity based on an assumed Lambert scattering law. Dots are the observed distribution. The dashed curve was obtained by a convolution of the theoretical curve with the illustrated Gaussian seeing spread function (light curve), chosen as a best fit to the wings of the observed distribution. The departure of the observed points from the dashed curve is due to the lower albedo of the Martian southern hemisphere. A one-dimensional approximation was used. i

I

I

I

I

I

I

1.0

/./"7

\

f //

.5

\\ \

;'7/ - I.~

- 1.0

-.5

0

SOUTH

\'\

I'., i., .S

1.0

1.5

X/Ro~

NORTH

FzG. 8. Same as Fig. 7, for April 28, 1969; ze a n d z o are the local zenith distances of the E a r t h a n d t h e Sun, respectively, a n d the corrections A secz were c o m p u t e d TABLE VII GAuss~

Plate

16 45

SEEING A~TD G ~ G

: F~NCTIONS

Date 1969

March 27 April 28

Seeing

0.28R (175.} 0.35R (2fb)

2"-3" 2,,_5,,

assuming a scale height o f 10 kin, from equations g i v e n b y A b r a m o w i t z a n d S t e g u n (1.964) (Barker, 1969). I n t h e case o f a r o t a t i n g image, the c o n v o l u t i o n is s u m m e d o v e r a finite n u m b e r of a n g u l a r positions o f the slit, giving a q u a n t i t y
53

HIGH-DISPERSION SPECTRA OF MARS. IV.

tracings, while only 6 lines were used for the final reduction of plate 45. Figures 11 and 12 indicate the variation in the measured strengths of weak Fraunhofer lines across the spectra of 3/27/69 and 4/28/69; it is evident that no significant departures from unity occur, with the exception of the point at X / R ~ +0.66 on 3/27/69. No correction was made for this departure in the final analysis, b u t it tends to indicate that the corresponding point on Fig. 13 is artificially depressed. Figures 13 and 14 show the distribution of abundance of H~O in microns of precipitable water in a vertical column above the surface as a function of position on the planet. For plate 45, taken with an image rotator maintaining the slit approximately pole-to-pole on the planet, the

densitometer slit is a function of the gradient of the intensity distribution along the length of the slit and of the seeing function, and is found from a similar convolution. Figures 9 and 10 give the effective air mass and the effective value of x along the slit for the two observations. RESULTS

Table VI gives the total H 2 0 abundance along the path, the effective air mass Me, and the effective position Xe as a function of slit position, for each reduction, followed b y the amount of H 2 0 in a vertical column of the atmosphere, together with the probable error of the mean. On plate 16, 10 and 11 lines were used from the two I

I

i

I

I

7.O

/I

6.0

5.0

~.o . . ~ r-

4.0

.5

_> t,y, ~_ u) cn

,.,

U.

/

x

3.0

-I.0

I 1 - 1.5

- 1.0

-.5

I 0 X / R d,

I .5

I 1.0

I 1.5

Z.O

FIG. 9. T h e effective a i r m a s s , Me, a n d effective p o s i t i o n o n t h e disk, Xe, as a f u n c t i o n o f p o s i t i o n X ( p e r p e n d i c u l a r t o d i s p e r s i o n ) i n t h e s p e c t r u m , m e a s u r e d f r o m t h e c e n t e r of t h e a p p a r e n t disk, i n u n i t s o f t h e M a r t i a n r a d i u s ; i n t e g r a t e d o v e r t h e 105 ° r o t a t i o n o f t h e i m a g e March 27, 1969.

54

ROBERT G. TULL

I~

1,0

~

I

I

I/

t

I 4.0

\ I.-

ul

"'

o

u_

~ 3.0 t~

,,, X

,,~

-.5

~

-1.0 I - 1.5

I - 1.0

I -.5

I 0

I .5

I 1.0

I 1.5

2.0

X/Rd' FzG. 10. Same as Fig. 9, April 28, 1969. I m a g e n o t r o t a t i n g in this case; slit was aligned pole-topole.

effective positions are expressed directly in areocentric latitude. Two sets of points are given for plate 16 (Fig. 13), representing separate reductions of two sets of tracings of the plate. Each point represents the integrated distribution over the 105 ° rotation of the image, along an arc whose effective distance from the center of the disk is given as the abscissa. The solid 1.5

i

rotation of the image, for an assumed curve represents an integration over the north-south HsO distribution given in Table VIII, equal to 1.5 times the distribution of Fig. 14. DISCUSSION

Schorn et al. (1967) have shown evidence t h a t the water vapor content of the Martian atmosphere varies with season

1.0

T

_~1.0

1

i

+_

.5

O

I -.8

I -.6

i -.4

i -.2

I i 0 .2 Unit = R@

i .4

i .6

i .8 I

F I e . 11. Variation o f relative strengths of F r a u n h o f e r lines across t h e spectrum, March 27, 1969, f r o m two separate m i c r o d e n s i t o m e t e r tracings. Open circles: tracing of 5 F r a u n h o f e r lines w i t h a 30-micron p r o j e c t e d slit. Solid dots : tracing of 14 F r a u n h o f e r lines w i t h a 60-micron p r o j e c t e d slit. The ordinate is t h e ratio of t h e s u m of m e a s u r e d e q u i v a l e n t widths to t h e s u m of published solar e q u i v a l e n t widths.

i

-.8 -.6

I

I

-.4 -.2

i

i

J

i

i

0

.2

.4

.6

.8

U n i t = Ro~

FIQ. 12. V a r i a t i o n of r e l a t i v e s t r e n g t h s across t h e spectrum, April 28, 1969, for 11 F r a u n h o f e r lines t r a c e d w i t h a 30-micron proj e c t e d slit. Published e q u i v a l e n t widths for t h e s e lines range f r o m 2 to 9 m A . T h e o r d i n a t e is as in Fig. 11, corrected for t h e e q u i v a l e n t w i d t h calibration for this plate.

HIGH-DISPERSION SPECTRA OF MARS. IV. I

I

I

I

I

I

u

55

I

NCP

N Poior c o p . _ ~

~" ~

~,~j)/Evefling

Ter mlnoto,

60 Noon Meridion/'

~/O~SP

,

l

(~

,

0 ¢J

e

n

20

0 - h(

I -.8

I -.6

I S -.4

I I -.2 0 Unit = R d'

I .2

I .4 N

I .6

I .8

1.0

F I o . 13. D i s t r i b u t i o n of a t m o s p h e r i c w a t e r v a p o r o n M a r s o n M a r c h 27, 1969 ( M a r t i a n d a t e

August 4). The results from reductions of two separate microdensitometer tracings are shown: open circles, mean obtained from 11 lines, using a 30-micron projected microdensitometer slit; solid dots, mean obtained from 10 lines, using a 60-micron slit. Vertical bars are probable error of the mean. Horizontal bars represent the effective length and position of the microdensitometer slit. Results are integrated over a 105° rotation of the image in 7 hours of exposure with the image rotator; the solid curve represents an integration over the image rotation of an assumed north-south I-I20 distribution, given in Table VIII, i.e., 1.5 × the distribution found on April 28, 1969. The inset diagram shows the positions of the slit at beginning and end of the exposure. a n d w i t h l a t i t u d e on Mars. T h e p r e s e n t o b s e r v a t i o n s clearly show t h e l a t i t u d e v a r i a t i o n , a n d a p p e a r to h a v e o c c u r r e d a t a season (mid-to-late s u m m e r ) w h e n t h e a b u n d a n c e of I-I~O is n e a r a m a x i m u m . Q u a n t i t i e s as high as 48 microns precipitable H~O h a v e n o t p r e v i o u s l y b e e n r e p o r t e d o n Mars in t h e m o d e r n spectroscopic d a t a . T h i s result is a consequence of t h e high s p a t i a l resolution of t h e s p e c t r a ; if t h e a b u n d a n c e is a v e r a g e d across t h e w i d t h of the spectrum, weighted by the intensity distribution, we find a t o t a l m e a n a b u n d a n c e of 37 ~-6 microns on M a r c h 27 (plate 16). Owen a n d Mason (1969) obt a i n e d plates a t t h e coud~ s p e c t r o g r a p h o f t h e S t r u v e 82-inch telescope a t Mc-

D o n a l d O b s e r v a t o r y during F e b r u a r y a n d M a r c h 1969, s h o r t l y before p l a t e 16 was e x p o s e d ; t h e y deduced an a v e r a g e of 35 ± 15 microns of precipitable H~O a v e r aged o v e r t h e w i d t h of t h e s p e c t r u m , b a s e d on l a b o r a t o r y m e a s u r e s o f four lines b y R a n k et al. (1964). R e d e t e r m i n i n g the abundance from the equivalent widths g i v e n b y Owen a n d Mason for eight lines a n d using F a r m e r ' s (1969) l a b o r a t o r y measurements, their abundance averaged o v e r t h e w i d t h of t h e s p e c t r u m , for t h e t o t a l optical p a t h , b e c o m e s 98 m i c r o n s of p r e c i p i t a b l e H~O, which f u r t h e r reduces to 33 ± 14 m i c r o n s on division b y a n air m a s s of 3, in excellent a g r e e m e n t w i t h t h e a v e r a g e o v e r t h e w i d t h o f t h e M a r c h 27

56

ROBERT G. TULL i

,

i

,

NPolorc o p ~

I

I

,3/" EveniallTerminator

Equmlor/v Noolt Meridlon /

60

I

I

|

NCP

~sP

0¢u -r' 40

.~ 2 0

a

60"

'

40*

I

S

I

I

20 =

I

I

0 Lotitude on Mors

I

I

20"

N

I

40"

I

I

60"

FIG. 14. The distribution of H20 vapor as a function of Martian latitude on April 28, 1969 (Martian date August 21 ), from measurements of six lines using a 30-micron projected microdensitometer slit. The slit was maintained pole-to-pole with the image rotator. The symbols are otherwise the same as in Fig. 13. s p e c t r u m in the present s t u d y within the limits o f error. P r e l i m i n a r y inspection of three spectra o b t a i n e d on S e p t e m b e r 15, 16, a n d 17, 1969, when the Doppler displacement at 8200 A was +0.273 .~ (+10.0 km/sec), TABLE V I I I ~!SSUMED N-S DISTRIBUTIONFOR PLATE 16 ( = sin Latitude)

X/R

Microns precipitable HaO

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 --0.6

45 46 48 42 35 28 21 13

reveals no detectable M a r t i a n H 2 0 lines. These spectra were o f lower d e n s i t y a n d were o b t a i n e d w h e n the terrestrial a t m o s pheric w a t e r v a p o r c o n t e n t was higher t h a n d u r i n g the earlier o b s e r v a t i o n s ; however, t h e y t e n d to indicate t h a t t h e a b u n d a n c e was less in September, b y a f a c t o r o f a t least 2, t h a n it was in April, t e n d i n g to confirm seasonal v a r i a t i o n s r e p o r t e d b y S c h o r n e t ¢d. (1967). A firm u p p e r limit has n o t been d e t e r m i n e d for these more recent plates. ACKNOWLEDGMENTS

The reduction and analysis of the plates was carried out at the Jet Propulsion Laboratory of the California Institute of Technology during my appointment as a Summer Senior Scientis~ in 1969. I t is a pleasure to thank Dr. William Pickering for the use of the facilities and the financial support; Dr. C. B. Farmer for use of

HIGH-DISPERSION SPECTRA OF MARS. IV.

~

57

d a t a prior to publication and for helpful dis- MINNAERT, M., MULDERS, G. F. W., AND HOUTGAST, J. (1940). "Photometric Atlas cussions; Dr. R. A. Schorn and Dr. J. S. Margolis of the Solar Spectrum from A3612 to ~8771." for m a n y helpful discussions; and Dr. Harlan J. D. Schnabel, Kampert and Helm, Amsterdam. Smith for making telescope time available to = MOORE, C..E., MINNAERT, M. G. J., ~ HOUTme at McDonald Observatory. GAST, J. (1966), "The Solar Spectrum 2935/~ ~ t o 8770A." Second Revision of Rowland's REFERENCES • '~Preliminary Table of Solar Spectrum WaveABRAMOWITZ, M., AND STEGUN, I. (1964). lengths,"/VBS Monograph 61. "Handbook of Mathematical Functions" NEUGEBAUER, G., MONCH~ G., CHASE, S. C. JR., HATZENBELER, H., MINER, E., AND SCHONBS Applied Math. Series, Sec. 7.1.26. FIELD, D. (1969). Mariner 1969: Preliminary BARKER, E. B. (1968). Improved chemical method for hypersensitization of infrared results of the infrared radiometer experiment. emulsions. J. Opt. Soc. Am. 58, 1378-1382. Science 166, 98-99. BARKER, E. B. (1969). "A Study of the Variations OWEN, T. (1967). Water vapor on Venus A dissent and a clarification. Astrophys. J. in CO2 Abundance and Surface Pressure of Mars." PhD Dissertation, University of Letters l50, 121-123. Texas at Austin, Texas. OWE~, T., AND MASON, H. P. (1969). Mars: DELBOUILLE, L., A~-D ROLAND, G. (1963). Water vapor in its atmosphere. Science 165, "Photometric Atlas of the Solar Spectrum 893-895. from A7498 to A12016." Institute d'Astro- RANK, D. H., FINK, U., FOLTZ, J. V., AND physique de l'Universitd de Liege. WmGINS, T. A. (1964 I. Intensity measureDOLT.FUS, A. (1951). La polarisation de la ments on spectra of gases of planetary lumiere renvoyde par les diffdrentes regions interest--Hs, HsO, and CO2. Astrophys. J. de la surface de la planete Mars et son inter140, 366-373. prdtation. Compt. Rend. Acad. Sci. 233, 467- SCHORN, R . A., SPINRAD, H., MOORE, R. C., 469. SMIZH, H. J., AND GIVER, L. P. (1967). HighFARMER, C. B. To be published. dispersion spectroscopic observations of Mars. HUNTEN, D. M., BELTON, M. J. S., AND SPINRAD, II. The water-vapor variations. Astrophys. J. H. (1967). Water vapor on Venus--reply. 147, 743-752. Astrophys. J. Letters 150, 125-126. SCHORN, R. A., FARMER, C. B., AND LITTLE, S. J. JA~SSON, P. A., AND KORB, C. L. (1968). A (1969). High-dispersion spectroscopic studies of Mars. I I I . Preliminary results of 1968-69 table of the equivalent widths of isolated lines with combined Doppler and collision broadwater-vapor studies. Icarus 11,283-288. ened profiles. J. Quant. Spectrosc. Radiat. SMrrH, H. J. (1968). McDonald observatory's Transfer 8, 1399-1409. 107-inch reflector. Sky and Telescope 36, KAPLAN, L. D., MONCH, G., ASD SPINRAD, H. 360-367. (1964). A n analysis of the spectrums of Mars. SPII~tAD, H., MilCH, G., A~D KAPLAN, L. D. Astrophys. J. 130, 1-15. (1963). The detection of water vapor on Mars. KUIPER, G. P. (1949). "The Atmospheres of the Astrophys. J. 137, 1319-1321. E a r t h and Planets." Univ. of Chicago Press, TULL, R. G. (1969). Planetary spectroscopy with Chicago, Illinois. the 107-inch telescope. Sky and Telescope 38, LEmHTON, R. B., AnD MURRAY, B. C. (1966). 156-160. Behavior of carbon dioxide and other volatiles VAUCOULEtraS, G. DE (1954). "Physics of the on Mars. Science 153, 136-144. Planet Mars." Faber and Faber, London.