Analysis of condensates formed at the viking 2 lander site: The first winter

ICARUS 4"], 173--183 (1981)

Analysis of Condensates Formed at the Viking 2 Lander Site: The First Winter STEPHEN D. W A L L Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91106 Received S e p t e m b e r 15, 1980; revised M a y 26, 1981 A thin light-colored g r o u n d covering appeared on the surface o f M a r s near the Viking 2 lander from L~ = 230° to L s = 16°, a total o f 249 Mars d a y s , during the l a n d e r ' s first winter on the surface. This paper presents a reduction o f applicable lander imagery during the period. Imaging s e q u e n c e s , relative surface albedo, spectral reflectance estimates, a n d limited photometric data are presented a n d c o m p a r e d with previous laboratory m e a s u r e m e n t s . Photometric data are best fit by an average Minnaert k = 1.1 (blue), k = 1.0 (green), a n d k = 0.95 (red). A p p e a r a n c e and disappearance rates, spectral reflectance, and photometric data all tend to confirm an earlier proposal that the covering was a c o m b i n a t i o n of H 2 0 a n d COs, w h i c h fell already c o n d e n s e d onto dust particles brought n o r t h w a r d by the s e a s o n ' s first major dust storm. U n d e r this a s s u m p t i o n , the covering thickness is e s t i m a t e d to be b e t w e e n 0.5 and a few millimeters.

deposits at the Viking 1 site and first noted the appearance of a thin light-colored ground covering at the Viking 2 site interpreted as an H20 or CO2 condensate. The covering existed for more than 200 sols, disappearing just after spring equinox. A similar coating appeared at about the same seasonal time in the second winter. Images from the second winter have not been processed by the Viking Project as of early 1981. However, indications are that the covering was more extensive. The presence of the condensate at the Lander 2 latitude (48°N) has been interpreted as an extension of the north polar cap (James, 1979). Earth-based spectral reflectance data for the north polar cap resulted in some controversy over its composition (e.g., Kieffer, 1970a,b); later data indicate a composition of water ice (Farmer et al., 1977; Kieffer et al., 1977). Mariner 7 photometric data for the south polar cap has also produced controversial results (Pleskot and Kieffer, 1977; Pang and Hord, 1971). The purpose of this paper is to present data acquired by the lander cameras which relate to the first winter's ground cover and to discuss implications of the data to theo-

INTRODUCTION

After their initial characterization of the local Martian surface (Mutch et al., 1976a,b), the most important task of the Viking landers has been to monitor the surroundings for significant changes, either on the surface or in the atmosphere as observed from below. Visual observations have included repeated images using the same solar lighting geometry (available twice yearly) to minimize the effects of shadows and photometry; images repeated over periods of several sols (Martian days); and many solar and twilight observations to characterize atmospheric changes. The Viking meteorology experiment has also acquired atmospheric data frequently during most of the landed mission. Several changes have been documented: Pollack et al. (1979) discussed optical depth changes and ice fog observations at both sites; Guinness et al. (1979) reported a significant shift in the soil color toward red and a general brightening after the first year's dust storms and aeolian movement of dust at times not associated with the storms. Jones et al. (1979) reported the appearance of two small slumps in the dust 173

0019-1035/81/080173-11502.00/0 Copyright (~ 1981by AcademicPress. lnc. All rights of reproduction in any form reserved.

174

STEPHEN D. WALL

ries which have been position and origin. decalibration methods In the second section, sons with previously discuss implications.

p r o p o s e d as to comData reduction and will first be outlined. I will m a k e comparipublished data and

DATA AND DATA REDUCTION

The m o s t difficult problem in the analysis o f imagery during this period is a lack of clear, uncontaminated, decalibratable data. During the first winter several hundred images were acquired; h o w e v e r , a t m o s p h e r i c opacity varied by a factor of 4 as two major dust storms passed o v e r the lander, and correct c a m e r a gain settings could not be accurately predicted. As a result m a n y images contain either saturated data or an insufficient n u m b e r o f quantization levels. In addition, lander engineers restricted use of the c a m e r a s to prevent failure of the scanning m e c h a n i s m s as ambient temperatures reached u n e x p e c t e d lows. The intended method for decalibrating imagery involves the use o f images of reference test charts located on top o f the lander to remove the effects o f changing surface illumination. Unfortunately, in m o s t cases such images were either not taken or were incorrectly taken, so other less accurate means have been used, as described below. Despite the p r o b l e m s m a n y of the sequences are useful for analysis. Qualitatively, one can see indications o f changes by comparing color images o f selected surface areas which were repeated at 10 to 12sol intervals. Five sequences taken through the winter are listed in Table I. Sequence c, which looks o v e r the lander deck to the west-southwest, is shown in Fig. 1. Several cautions are n e c e s s a r y a b o u t the absolute color and color variations seen in this figure. First, the lander c a m e r a s ' ability to sense color is complicated by spectral " l e a k s " which cause the c a m e r a to sense infrared radiation in its red and blue channels (Patterson et al., 1977; H u c k et al., 1977). For example, the test charts on the lander deck include a blue color chip which

is interpreted as purple by the c a m e r a because its high infrared reflectance is sensed by the red channel " l e a k . " Because the c a m e r a also has infrared channels, these leaks can be c o m p e n s a t e d if infrared imagery is acquired simultaneously with visual color imagery. H o w e v e r , for reasons discussed above, infrared imagery was not

TABLE

I

VIKING 2 LANDER WINTER MONITORING SEQUENCES Sequence

Local lander time a

VL-2 sol

Image CEID b

a

1212-1217 1211-1216

257 305

21D224, 21D225, 21D226 21E048, 21E049, 21E050

b

1000 1325-1329 1300-1303 1032 1259-1303 0944 1259-1303 IZ~;9-1303 1259-1303 1211-1216 1211-1216

046 218 221 233 245 257 269 293 317 329 353

22B126 22D139, 22D169, 22D180 22D209, 22D220 22D249, 22E033, 22E073, 22E088, 22EI28,

c

1100 1100 1112 1100 1100 1100 1100 1059 1231 1130 I 125 I 124 1115 1038 1145

193 200 214 233 281 305 329 353 377 389 400 406 410 414 778

2213038 22I)068 22D116 22D181 22E005 22E045 22E085 22E 125 22E 169 22E211 22E230 22E247 22F011 22F030 22H220

d

1100 1409-1414 1409-1414

198 257 305

211)049 21D232, 21D233, 21D234 21E056, 21E057, 21E058

e

1125-1130 1330-1334 1330-1333 1329-1333 1329-1333 1329-1333 1104

219 221 245 269 293 317 449

22D150, 22D173, 22D213, 22D253, 22E037, 22E077, 22F140

22D140, 22D141 22D170, 22D171 22D210, 22D211 22D250, 22E034, 22E074, 22E089, 22E129,

22DI51, 22D174, 22D214, 22D254, 22E038, 22E078,

22D251 22E035 22E075 22E090 22E130

22D153 22D175 22D215 22D255 22E039 22E079

a Local lander time is the time since local midnight. Sol 0 began at midnight prior to Lander 2 touchdown. Where a range of times is given, three consecutive singlet (blue, green, red) images have been combined into a color image. b Single CEIDs (Camera Event Identifiers) are color triplets. Where three are listed they are singlets, as described above.

CONDENSATES AT VIKING 2 LANDER SITE always acquired during the winter season, and an approximate compensation was used to produce the pictures in Fig. 1. This approximation is referred to as RADCAM color and retains some misinformation due to incorrect interpretation of spectral information. Second, variations in atmospheric conditions produce variations in the coloration of the light reaching the surface and thus affect surface color as perceived by the camera. The magnitude of these variations can be judged by comparing the gray chips on the test chart through the sequence in Fig. 1. Quantitative data reductions presented later in this paper are corrected for atmospheric color variations, but the pictures in Fig. 1 are not. The third caution is necessary because images taken on sols 281-353 were taken with incorrect camera gains. Subsequent RADCAM color balancing emphasizes electronic noise and distorts the color in the sol 281 and 305 images, and much of the image is saturated in the sol 353 image. Variations in color photographic processing, which would be a fourth caution, have been eliminated in Fig. 1 by combining all imagery onto a single negative. Spectral reflectance estimates, broadband surface albedo change, and limited photometric data have been reduced from the imagery. The lander cameras are essentially radiometers with mechanical scanning mechanisms, described in detail by Huck et al. (1975). A photosensor array (PSA) provides a selection of two spatial resolutions (0.04 or 0.12°) and six spectral bandwidths ranging from 0.4 to 1.1 p,m. Voltage produced at the output of this array, referred to as PSA voltage, is linearly proportional to the radiance incident on the selected photosensor. A technique devised by Park and Huck (1976) has been used here to derive spectral radiance estimates using PSA voltages from pairs of color and infrared images. In brief, this technique expresses the output of each channel as a linear integral function of the surface spectral radiance and the channel

175

spectral sensitivities. These six equations are solved simultaneously to determine the coefficients in a linear combination of natural cubic spline basis functions which represent an estimate of the surface radiance. If it were seen by the camera, the estimate would yield the same channel voltages as were actually obtained, and thus the estimate is not distinguishable by the camera from the actual radiance. However, there is no guarantee of the estimate's uniqueness. Spectral resolution varies with wavelength but is approximately 0.1/xm. The principal error source is an undersampling of the actual radiance which may spectrally alias short-period features and not electronic noise or other data uncertainty; therefore, confidence intervals are not meaningful and are not assigned. A spectral radiance estimate was calculated for the surface area under consideration and for a gray patch on the reference test chart visible in Fig. 1. The two estimates were ratioed and multiplied by the known reflectance of the test chart patch to produce an estimate of the surface spectral reflectance. The test chart patches are Lambertian reflectors to within _+7% for the incidence angles used and their reflectances are known to _+0.04 (Wall et al., 1975). However, the illumination of the surface is by direct sunlight and skylight only, whereas the test charts are also illuminated by light reflected from the lander body. The total illumination of the test chart is therefore unknown, and the spectral reflectance estimates should be regarded as relative only. As verification that no significant amount of dust has collected on the chart (Guinness et al., 1979; Bragg, 1977) the red channel PSA voltages from the four lowest reflectance gray patches are plotted against their measured reflectances in Fig. 2. The red channel response is most sensitive to accumulation of light red dust on dark gray patches. A linear relationship with zero y intercept indicates a clean chart and proper camera operation. Dust on any single patch would cause that patch's re-

200 D0M

VL-2SOL-193

FRAME- D0311

Dl16

214

Z30

D181

233

261

E005

201

203

E045

305

2ee

E086

329

319

E126

~

333

ElilO

377

340

E211 E230 E247

JIM 400 408

318 361

Fl6. 1. Portions of winter monitoring sequence c. Details for individual images are listed in Table I. Aerocentric longitude, L~, and VL-2 sol are shown. Patch labeled ~ A " was used for spectral reflectance estimates (Fig. 3) and broadband albedo (Fig. 4). Patch "'B" was used as reference for broadband albedo.

230

Ls(dq) - 2 2 6

CONDENSATES AT VIKING 2 LANDER SITE sponse to deviate f r o m the linear response, and an e v e n coating o f dust o v e r all patches would cause the y intercept to increase from zero. Figure 3 shows spectral reflectance estimates for the surface area " A " s h o w n in Fig. 1 before, during and after visual observation of c o n d e n s a t e in that area. The first reflectance estimate, taken from images 22C009 and 22C010 on sol 55 (Ls = 132°), c o m p a r e s well with the estimates o f H u c k et al. (1977) taken in front of the lander on sol 26. The second estimate is taken from the same patch in images 22E247 and 22E248 on sol 406 (L~ = 340°), w h e n the patch was obviously c o v e r e d (22E247 is the 12th image in Fig. 1). Blue reflectance has increased by a p p r o x i m a t e l y a factor o f 4 from the sol 55 estimate. T h e r e is m u c h higher red and infrared reflectance, in general a g r e e m e n t with c o n d e n s a t e color ratios calculated b y Guinness et al. (1979). Their color ratios, obtained with individual c a m era channel data, indicate r e d / b l u e ratios o f a b o u t 2.4 for u n c o v e r e d soil and a b o u t 1.5 1.0

0.9 0.8

>•

0.1

0.6

d ,~, 0.5 0.4 0.3

0.2

0.1 I

I

0.1 0.2 IESTCHARTREFLECTANCE

0.3

FIG. 2. C a m e r a red c h a n n e l r e s p o n s e to the test chart s h o w n in Fig. 1, as a function o f chart reflectance. This graph indicates that the chart is clean and that c a m e r a r e s p o n s e is linear.

I

177

I

I

I f ~

0.4

\

/

/

I

/.

,~-\

//

/ _g 0.2

//

,

r

I

I

0.4

0.5

0.6

0.7

I

t

0.8

0.9

1.0

X ,#m

FIG. 3. Relative spectral reflectance e s t i m a t e s o f the surface patch identified in Fig. 1 on VL-2 sols 44, 406, and 559. Sol 44 images were taken before c o n d e n s a t e accumulation, sol 406 images while it was on the ground, and sol 559 images after its disappearance.

for the condensate, both f r o m a sol 377 image. The reflectance estimates in Fig. 3 have m u c h higher r e d / b l u e ratios (7.8 for u n c o v e r e d soil on sol 559 and 3.0 for condensate on sol 406). The d i s c r e p a n c y is due at least in part to the c a m e r a ' s infrared radiation leaks which m a s k the true spectral differences. In any case, the condensate is clearly tinted red. 1 The third estimate in Fig. 3 shows reflectance of the same patch on sol 559 (L s = 53 °) f r o m images 22G190 and 22G191, well past the last visual o b s e r v a t i o n o f c o n d e n s a t e s on sol 449. Blue reflectance matches that prior to the c o n d e n s a t e period, but red and infrared reflectance are still m u c h higher than in the sol 55 estimate. Implications of these changes will be discussed in the next section. Spectral reflectance estimates can only be meaningfully p r o d u c e d w h e n both color 1 N A S A - J P L press-release color p h o t o g r a p h s o f the c o n d e n s a t e were not carefully color balanced and often do not support this conclusion. A d i s c u s s i o n of the accuracy of lander color products is contained in H u c k et al. (1977).

178

STEPHEN D. WALL

and infrared images exist, for reasons discussed above. To provide a measure of surface albedo change on a more frequent basis, relative b r o a d b a n d albedo was calculated with data from Fig. 1 and some additional images. Blue, green, and red channel PSA voltages from the surface patch " A " shown in Fig. 1 were ratioed to those from a vertical portion of the lander antenna support '~B." As with most o f the lander, the support is painted with paint similar to that used on the test charts; h o w e v e r , neither the reflectance nor p h o t o m e t r y of this specific area has been measured. The test chart was not used as reference because it is saturated in the sol 353 image. The vertical support was c h o s e n to minimize the probability of dust accumulation. H o w ever, varying amounts o f lander-reflected illumination m a y be an important error source. The quantity BA/BB + GA/GB + RA/RB

is taken as a relative measure o f the broadband albedo of the patch " A . " B, G, and R are blue, green, and red PSA voltages, respectively, and the subscripts A and B refer to surface patch " A " and the reference patch " B , " respectively. This quantity is plotted in Fig. 4 versus VL-2 sol n u m b e r and the equivalent aerocentric longitude, Ls. L~ = 0 ° at spring equinox and VL-2 sol = 0 at local midnight preceding t o u c h d o w n o f L a n d e r 2. T o g e t h e r with albedo change are plotted several other pertinent data. A sensor located just a b o v e the footpad o f leg 2 m e a s u r e s t e m p e r a t u r e a few centimeters a b o v e the surface. The diurnal minima of these t e m p e r a t u r e s exhibit cyclic b e h a v i o r with periods of a few sols. The minima of these cycles, called lower diurnal minimum t e m p e r a t u r e s , are graphed on the lowest vertical axis. Accuracy is + 5 ° K at 160°K (Martin Marietta Corporation, 1976, p. 4.8.1.5-3). The next graph in this figure shows ambient pressure as m e a s u r e d b y the m e t e o r o l o g y instrument, 153 c m a b o v e the nominal landing surface and 71 c m a b o v e the lander deck

~2 5]

:

:

LOWER BOUND

? t. 5, ]i i

l

.5[

I

•

L

12' 11, :

A~ 8 ENT PRESSUR

i

:

2

"

2i

15ol L

150

290

700

230

25{)

260

300 VL 2 SOL 290

350

~29

350

AEROCENTRIC I s' ~eq

FIG. 4. A timeline of relevant data during the condensate period. Ls = 0° at spring equinox. CO2 saturation pressure shown is saturation pressure at the lower diurnal minimum temperature for that sol (see text). Optical depth is PM measurement at VL-2 from Pollack el al. 11979).

(Henry, 1978; Chamberlin et al.. 1976; Hess et al., 1977). Plotted on this same axis are the CO2 saturation pressures at the lower diurnal minimum t e m p e r a t u r e s discussed above. When ambient pressure equals CO2 saturation pressure at the ambient t e m p e r a t u r e one would expect formation of solid COs. H o w e v e r , errors of _+5°K in the ambient t e m p e r a t u r e translate into errors of _+3 m b a r in COs saturation pressure, so the time the two pressures were first equal is uncertain by 40-50 sols. The next graph shows afternoon optical depths at VL-2 from Sun diode m e a s u r e m e n t s , as calculated by Pollack et al. (1979). Photometric data for the condensate were acquired by imaging the surface and the lander magnet-cleaning brush support in color at 45-min intervals through the

CONDENSATES AT VIKING 2 LANDER SITE morning o f sol 382. The condensate existed only in patches on the surface by that time. The patches were identified and surface normals were c o m p u t e d by assuming the surface to be flat and lander tilt with respect to that surface to be negligible. Incidence, emission and phase angles were c o m p u t e d for each patch. As the day progressed solar incidence angle changed from 64 to 56° , and phase angle varied from 45° to near 0°. Emission angle was varied from 41 to 86° by selecting condensate patches at different distances from the lander. All condensate patches were assumed to have equivalent properties. Three images and 16 condensate patches were used; the images are listed in Table II. Data reduction was performed by assuming that the camera response to the condensate was the sum o f a c o m p o n e n t due to diffuse skylight and a c o m p o n e n t due to direct sunlight. Thus the decalibrated PSA voltage can be written as VToT/VRE

F =

Rsu N + RsuY,

(1)

where VTOT is the observed PSA voltage, and Rsu N and RsK v are the contributions from direct sunlight and diffuse skylight, respectively. Vary is the PSA voltage from the support arm. Thus .q = R s K v / R s u N = RSKy/[(VToT/VREr) - RsKv]

(2)

where 0 --< ~ --< 1 can be used as a measure o f atmospheric scattering, and (3)

R s u s = VToT/VREv( 1 + ~7)

Skylight contribution to the condensate

179

PSA voltage was r e m o v e d by measuring a shadowed portion o f a condensate patch and an adjacent sunlit portion. Equation (2) was used to calculate a value o f ~. Variation of optical depth was assumed negligible, and the calculated value o f ~1 was used to correct all data using Eq. (3). Corrected data were fit to the Minnaert photometric function model, B = Bo(g)bto~O)/z taut-l,

where B = RsuN is the observed radiance, /x0 and /z are cosines of incidence and emission angles, respectively, and Bo(g) and k(g) are unknown functions of g, the phase angle. This equation can be rewritten as log (B/z) = log Bo(g) + k(g) log (/z/z0) and a graph o f log (B/z) against log (/z/z0) will have slope k(g). Such a plot is shown in Fig. 5, with data points grouped by phase angle. There is no significant variation in k with phase angle, and fits by inspection give k = l.l_+ 0.2, k = 1.0_+ 0.1, a n d k = 0.95 + _ 0.2 for blue, green, and red data, respectively. These values are in good agreement with Veverka's (1973) reduction o f photometric data for terrestrial *'settling s n o w " and " n e w s n o w " taken by Knowles Middleton and Mungall (1952). Pleskot and Kieffer (1977) found 1.21 -< k < 1.31 for the Martian south polar cap in the spectral region 2-14 /zm, and James et al. (1979) found 0.74 < k < 1.82 in the red and violet, also for the south polar cap. H o w e v e r , Pang and Hord (1971) reported k ~ 3 at wavelengths 0.2-0.3 /zm over that same south polar region.

TABLE II CAMERA EVENTS USED IN PHOTOMETRIC SEQUENCES

CEID

Local time

Incidence angle (o)

21E191 21E194 21E 195

1100 1300 1302

64 56 56

DISCUSSION In this section, I will c o m m e n t on the implications o f the reduced data to a previously proposed mechanism for origin and chemical composition o f the ground covering and on several possible structural compositions. Although the limited data cannot definitely choose any of these, they do

180

STEPHEN D. WALL

-1.1

~ ~ ~'

=- -2.0

3.01

,:" I

-4O t " 5.0. .

. .

-4.0

-3.0 •

-2.0

-1.0

0

log (#~0) la) BLUE 0

- ~ -2.0 I

[

~~ "

o -3.0 I -4.01

-50 l 5.0

O< g~;.]O (deg)

~

10< g<~0 20< g~< 30

•

30< g~<40 . 4.0

. .. 40< g~ 50 3.0 2.0 -l.O log (,uu O} (b} GREEN

0

0

, ~ -2.0

-3.0 -4,0! -B'-°~;6

-4.'0

-3~

:~;~

11o

o

log (/z~ 0 ) (c) RED

Fie. 5. Photometric function dcrived from sol 382

photometric sequence listed in Table II. Data points are separated by phase angle, as indicated.

show certain preferences which will be discussed. The timeline o f Fig. 4 provides a reasonably clear picture o f the condensate formation period. Accumulation began shortly after sol 200 (Ls = 230°). Although meteorology data indicate that CO2 saturation pressure was not reached until after sol 250, uncertainty in the saturation pressure would allow this event to have actually occurred near sol 200, as discussed above. It is certainly reasonable to infer that carbon dioxide played a key role in the deposition. Optical depths show the onset o f the first dust storm near sol 170 (Ls = 210°). Then, as the temperature fell and pressure rose, the broadband albedo of p a t c h " A " in Fig. 1 quickly doubled. Other sequences (not shown) show light patches on the surface by sol 257. A fairly complete ground c o v e r is evident by sol 269. Although incorrect gain settings make images on sols 281,

293, and 305 difficult to interpret, most albedo features are obscured in sol 305, 317, and 329 images. A careful search of imagery at the time of heaviest ground c o v e r reveals no obscuration of structural detail such as pebbles or rock vesicles by the condensate. No buildup in the lee o f rocks is seen, though there is a suggestion on sol 233 o f possible redistribution by wind. Lack o f obscuration o f surface relief places an upper limit on thickness at the camera's resolution limit, which is a few millimeters near the lander. A trench dug by the surface sampler never showed more than a small amount o f accumulation. This does not seem consistent with upwelling o f ice from below the surface, a mechanism which has been proposed. All images after sol 329 show slowly disappearing patches o f the ground cover in areas protected from the Sun by shadows of rocks. Several small rocks in sequence c also remained covered until past sol 377. The last remnant o f the covering was seen in image 22F140 on sol 449 (Ls = 16°), 249 sols after the first appearance. Although several authors have suggested pure CO2 ice formation at the L a n d e r 2 latitude of 48~N (Hess et al., 1977; Kieffer, 1976), it is clear that at least the patch identified in Fig. 1 survived much too long to be pure CO2 ice. Daytime temperatures reached well above 180°K and CO2 saturation pressure was above 20 mbar past sol 375. Jones et al. (1979) conclude that a mixture of COals) and H20(s) best explains the deposition and disappearance rates. Neither photometric function nor spectral reflectance from 0.4 to 1.0 p.m is capable o f distinguishing solid CO2 from solid H20 (Wood e t al., 1967; Wood et al.. 1968; Clark, 1981). The condensate spectral reflectance estimate of Fig. 3 is much more reminiscent o f the sol 559 reflectance than of opaque H20(s), opaque COz(s), or the preexisting surface. This strongly indicates that the high red and near-infrared reflectance is produced by dust redder than

CONDENSATES AT VIKING 2 LANDER SITE the preexisting surface which fell during the dust storm activity. Possible physical structures include a layer of redder dust onto which an optically thin layer of condensate grew; a layer of condensate grown onto the preexisting surface which was later covered by the redder dust; or a single layer of mixed condensate and dust. These structures will be referred to as "frost on dust," "dust on frost," and " m i x t u r e , " respectively. Clark (1981) has investigated the reflectance properties of frost grown on minerals and minerals deposited onto frost. He shows, for example, that grains of frost -<30 /zm in diameter grown onto 125-~m grains of Mauna Kea red cinder cause a 25% increase in 0.8-/zm-wavelength albedo when the frost layer is only 50/xm thick. By analogy with the present data, the 5% increase in 0.8-/xm albedo in Fig. 3 would require a maximum of 10/zm of pure frost if one assumes the "frost on dust" structure. However, Smith et al. (1969) show that thin (<70/zm) cryodeposits are quite specular, and that Lambert (k ~ 1) behavior is exhibited only by deposits of 250/zm thickness. Since the photometric data (Fig. 5) show Lambertian behavior of the observed ground cover, the "frost on dust" structure seems unlikely. Mineral grains deposited on frosts are also quite efficient at changing albedo. Clark (1981) shows that montmorillonite grains deposited onto frost produce a discernable change in albedo with a fractional areal coverage of 0.002, and that a coverage fraction o f 0.1 produces a 20% albedo change at a wavelength of 0.8/.tm. A similar effect is seen using Mauna Kea red cinder. If, as he suggests, the same masking occurs at shorter wavelengths, the amount of overlying dust required to produce the observed red albedo would probably also mask the condensate's blue albedo. Figure 3 shows an obvious increase in blue reflectance when the condensate was present. Thus, the data also argue against a " dus t on frost" structure.

181

A mixed structure consisting of redder dust imbedded in water or carbon dioxide solids could occur in several ways. During the dust storm these three could have simply mixed locally, but depending on the ratio of HzO/CO2 such a mixture might require more H20 than was available in the local atmosphere. The amount of precipitable water in the atmospheric column has been estimated at a few micrometers (Jones et al., 1979). A more likely mechanism predicted by Pollack et al. (1977) suggests that dust particles - 1/.~m in radius might be moved from the south by dust storms. As the particles move northward they collect water vapor and increase in size. Then, as temperatures decrease, carbon dioxide condenses onto the particles and they grow to perhaps 25 /zm in radius and settle quickly to the surface. In the early spring the carbon dioxide sublimes away, leaving water ice and dust on the surface until much later. This mechanism could loosely be called " s n o w . " Other authors have proposed snow as a method of sweeping water or carbon dioxide from the atmosphere (James et al., 1979; Pang and Hord, 1971). However it formed, a mixture of redder dust particles with water and carbon dioxide ices would allow the observed higher blue reflectance and the small increase in red and near-infrared reflectance, at the same time allowing a layer thick enough to exhibit Lambertian photometry. If one assumes this mixed structure, the only upper limit on thickness is that imposed by the lack of surface detail obscuration, a few millimeters. Clark (1980) suggests that a frost-dust mixture would have to have at least 0.5 mm thickness to produce enough internal scattering to increase the albedo. Thus the constraints on thickness are a lower limit of 0.5 mm and an upper limit of a few millimeters. CONCLUSION

Relative surface albedo, spectral reflectance estimates and a limited photometric function have been reduced from Viking 2

182

STEPHEN

lander data taken during a period in the first winter when a light ground covering appeared on the surface. The covering first appeared near VL-2 sol 200 (L~ = 230°). During the deposition, surface broadband albedo more than doubled and blue reflectance increased by a factor o f 4. Red and near-infrared reflectance also increased. The deposition began just after the first major dust storm reached the lander site and within 50 sols o f the ambient pressure falling below CO2 saturation pressure. Within measurement errors the three events may have been coincident. At its peak the ground c o v e r apparently covered most o f the surface, but imagery during this time is very poor. A photometric sequence yields Minnaert constants for the covering o f k ~ 1.0 which are not any clear function of phase angle. Later in the winter the covering began to disappear leaving receding patches in the shadows o f rocks. One patch remained visibile until sol 449 (L~ = 16°) for a total of 249 sols, long after daytime temperatures reached above 181YK. Spectral reflectance taken after this time shows that red and near-infrared reflectance remained higher than before the covering arrived. Comparison of lander data with earlier laboratory measurements o f CO2 and H20 frosts and snow shows that reflectance estimates do not resemble either COz or H20 solids. E x c e p t for its higher blue reflectance, the condensate reflectance most closely resembles that o f the surface after the covering disappeared. The covering may have been colored by dust which fell before it, by dust mixed with it, or by dust on top of it, but the data most strongly support a mixture of dust with H20(s) and CO~(s). If this structure is assumed thickness is estimated to be between 0.5 and a few millimeters. These results, taken together with lander meteorology data, tend to confirm an earlier proposal that the coveting was a combination of water and carbon dioxide which fell to the surface already condensed onto dust particles

D. W A L L

brought to the lander site by the two large dust storms.

ACKNOWLEDGMENTS A n y analysis o f spacecraft data has a large cast to thank for its existence. I would specifically like to thank K. Jones, F. Fanale, J. Veverka, J. Stephens, and S. K. Park for helpful guidance in this work. D. Vane and S. Ritke aided in the data reduction. J. Pollack and S. L. Hess kindly supplied optical depth and meteorology data. C. E. Carlston, the Viking Lander Imaging T e a m , and the Viking Flight T e a m designed the imaging s e q u e n c e s and maintained the lander through the crucial winter period. Helpful reviews by R. Clark and L. Pleskot are also acknowledged. This work is J P L Planetology Publication N u m ber 314-81-70 and w a s supported by the Viking Project, Langley R e s e a r c h Center, and by the Mars Data Analysis Program at Jet Propulsion Laboratory, California Institute of Technology, under Contracts NAS7-100 and N A S 1-9000 with the National Aeronautics and Space Administration.

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