Seasonal Recession of Martian South Polar Cap: 1992 HST Observations1

Home , Geographical pole, Solis Lacus

ICARUS 123, 87–100 (1996) ARTICLE NO. 0143

PHILIP B. JAMES Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606 E-mail: [email protected]

R. TODD CLANCY Space Science Institute, Boulder, Colorado 80301

STEVEN W. LEE LASP, University of Colorado, Boulder, Colorado 80309

LEONARD J. MARTIN Lowell Observatory, 1400 West Mars Hill Rd., Flagstaff, Arizona 85721

AND

JAMES BELL Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853

Received April 17, 1995; revised January 22, 1996

INTRODUCTION The seasonal recession of the south polar cap of Mars has been studied using the Wide Field Planetary Camera images The seasonal recession of the martian south polar cap from Hubble Space Telescope obtained during the summer of during the spring season in the southern hemisphere has 1992 when the angular size of Mars was between 5 and 6 arc been extensively documented using data sets compiled by sec. The observations cover the interesting period between ground based astronomers and by orbiting spacecraft. and 284, when the recession is rapid and the asymme- Hubble Space Telescope was able to observe the latter 259 ؍ LS try in the cap is large. These will be the only HST observations part of this season between late May and early July, 1992; of this season during the 1990s because of the solar elongation the elongation of Mars had just exceeded 50Њ, the minimum constraint on telescope pointing. allowed for HST observations, and the angular size of the The well known ‘‘Mountains of Mitchel’’ outlier, which has been observed by Viking and by many ground based observers planet was very small, between 5.2 and 5.8 sec of arc. There were four sequences obtained between LS ϭ 255 and 285; is clearly present in the ﬁrst set of Hubble ,260 ؍ near LS images. The latitude of the cap edge, the displacement of these were alternately centered near central meridians 300 the center of the cap from the geographical pole, and the and 75 in order to monitor potential dust activity in the albedo of the polar deposits determined from these images Hellespontus and Solis Lacus regions, respectively. All of are consistent with observations of these quantities by these sequences consisted of two WFPC images using ﬁlters Viking in 1977; the data are not compatible with a recession centered at 413 and 673 nm. Table I summarizes the rele- as advanced as that deduced from the 1956 telescopic vant parameters of Mars at the times of these four se- observations.  1996 Academic Press, Inc. quences. An orthographic, unprojected image with central meridian of 75Њ is shown in Fig. 1. The recession of the south polar cap between L ϭ 1 Based on observations with the NASA/ESA Hubble Space Telescope S obtained at the Space Telescope Science Institute, which is operated by 170 and 340 was observed in detail during the 1977 the Association of Universities for Research in Astronomy, Incorporated, Extended Viking Mission (James et al. 1979), a period under NASA contract NAS5-26555. which included two major martian dust storms. Together

87 0019-1035/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved. 88 JAMES ET AL.

TABLE I with extensive telescopic data pertaining to this season, Observational Parameters particularly for the favorable oppositions of 1956, 1971, 1973, 1986, and 1988, the Viking data lead to the following Date Central Diameter Sub-Earth Phase scenario for cap recession: between L ϭ 180 and 230 (1992) Filter L meridian arcsec latitude angle S S the south cap is fairly symmetric about and centered on 5-30.15 F413M 259 300 5.25 Ϫ22.85 34.5 the geographical south pole, while later in the spring 5-30.16 F673N 259 302 5.25 Ϫ22.85 34.5 between LS ϭ 230 and 270 the center of the cap shifts 6-11.87 F413M 267 073 5.41 Ϫ20.57 35.9 to the residual cap, which is centered at about Ϫ86.5Њ 6-11.88 F673N 267 075 5.41 Ϫ20.57 35.9 latitude and 30Њ longitude. The factors which control 6-27.96 F413M 277 305 5.63 Ϫ16.99 37.6 6-27.96 F673N 277 307 5.63 Ϫ16.98 37.6 the residual cap position have not been satisfactorily 7-09.63 F413M 284 075 5.82 Ϫ14.02 38.7 determined. The residual cap was composed of carbon 7-09.67 F673N 284 089 5.82 Ϫ14.01 38.8 dioxide ice during the years it was observed by Mariner 9 (Paige et al. 1990) and Viking (Kieffer 1979, James

FIG. 1. An unprojected Hubble image of Mars acquired on June 11, 1992 using the 413 nm ﬁlter. The south polar cap is displaced roughly along the 30Њ meridian to the north of the geographic pole during this season. For the central meridians of this set of images (75Њ) and the Hellespontus set (300Њ), the south polar cap does not extend to the limb of the planet. HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 89

FIG. 2. Hubble images of the martian south polar region acquired on May 30, 1992 corresponding to solar areocentric longitude (LS) ϭ 259. The images were acquired through relatively narrow bandpass ﬁlters centered at 413 nm (left) and 673 nm (right). The images are projected in a polar stereographic projection, with 0Њ longitude at the top of the ﬁgure and longitude increasing in a counterclockwise direction. Latitude circles are shown for every 5Њ, and longitude lines for every 30Њ. The images have been stretched in such a way that the cap interior is saturated.

et al. 1979). Some data (Jakosky and Barker 1984) sug- is variable during this season and is possibly a sensitive gest that the CO2 may completely disappear in some indicator of interannual variability in the CO2 cycle (James years (e.g., 1969), and one might speculate that the re- et al. 1987). In particular, Earth based data from the 1956 cession pattern noted above might be modiﬁed in the opposition showed a cap which was smaller than that ob- absence of the residual cap. Jakosky and Haberle (1990) served by Viking or during other oppositions by 4.5Њ during have shown that atmospheric perturbations may cause the seasonal period spanned by the HST data. So data the cap to jump between the two stable conﬁgurations. from this season are especially relevant to the variability Thus, interannual variations in the south polar cap reces- issue. Because of the very small angular size of Mars during sion have important implications for general martian this period, there are no Earth based data of this particular climate as well as for the CO2 and H2O condensate cap recession for comparison. cycles. The HST data set is intrinsically similar to spacecraft Analyses of observations of the cap recession obtained observations in that the polar cap is clearly resolved so during different years have suggested that the cap recession that investigations can deal with the isolated observations 90 JAMES ET AL.

FIG. 2ÐContinued

inherent with HST schedules. Measurements using Plane- functions. Therefore, we are limited to considerations of tary Patrol photographs, which were more or less continu- the size and location of the cap relative to the geographic ously acquired during oppositions, rely on statistics to pole as well as to reflectance at fixed values of photomet- eliminate the large uncertainties which would accompany ric angles. single measurements (James and Lumme 1982). On the other hand, the HST observations do not generally have sufficient resolution to record most of the structure within PROCESSING the cap which was seen by Viking. This is partly due to the small angular diameter of Mars at this time and, even The south cap is close to an illuminated limb in all of more crucially, to problems resulting from the spherical these images. Accurate location of cap features therefore aberration in the HST primary mirror. In addition, the requires accurate determination of the sub-Earth point on cap in Earth based or HST images is seen near the the images. The latter is complicated by the large phase limb, at large emission angle, as opposed to views from angles during the period of these observations. The proce- spacecraft, which are often acquired at small emission dure which was adopted for locating the center of the angles, thus potentially increasing uncertainties due to images utilized the fact that the size of the planet in the atmospheric obscuration and to unknown surface phase images is well determined from the known angular diame- HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 91

FIG. 3. Hubble images of the martian south polar region acquired on June 11, 1992 corresponding to solar areocentric longitude (LS) ϭ 267. The images were acquired through relatively narrow bandpass ﬁlters centered at 413 nm (left) and 673 nm (right). The images are projected in a polar stereographic projection, with 0Њ longitude at the top of the ﬁgure and longitude increasing in a counterclockwise direction. Latitude circles are shown for every 5Њ, and longitude lines for every 30Њ. The images have been stretched in such a way that the cap interior is saturated.

ters and from the image scale, taken here to be 0.044 of signals from many other elements of the picture. In arcsec/pixel (James et al. 1994—henceforth referred to as addition, the raw images suffer from numerous blemishes I). A circle with a diameter equal to the diameter of Mars which result from dust particles in the optical system; these in pixel space was adjusted to fit the illuminated limb of are not completely removed by flat fielding and must be the planet as accurately as possible, and the center of the excised painstakingly by hand. The resulting images were circle was taken as the sub-Earth point. The resulting val- then deconvolved by applying 50 iterations of the Richard- ues of the sub-Earth point on the images were used to son–Lucy algorithm as described in I. map them into stereographic projections centered on the An investigation of the effects of Lucy deconvolution south pole of the planet. on the boundaries of a sharp albedo feature on a planet Inasmuch as these images were acquired before the re- was reported in I. Such boundaries are diffuse, even after placement of the Wide Field Planetary Camera (WFPC) many iterations of Lucy. The boundary of a large feature, with a unit designed to compensate for spherical aberra- such as a polar cap, which would appear in a perfect tion, the raw data at each pixel consisted of a superposition image as a naturally smooth, sharp boundary separating 92 JAMES ET AL.

FIG. 3ÐContinued

extensive regions of uniform intensity, is located on the We have investigated the latter problem by convolving deconvolved image at the point where the intensity is small, artificial features on a flat background with the point the mean of the two regions. In practice, the utility of spread function of the planetary camera and then decon- this result is limited by the various assumptions indicated volving with the Richardson–Lucy algorithm using the above, none of which is valid in practice. Especially during same point spread function. The sizes of these artificial the late spring season viewed here, the edge of the south ‘‘caps’’ ranged from 4 to 20 pixels in diameter. We found cap has large irregularities and the albedo is not constant that the correct location of the actual edge was still given within the cap (see I). The ideal of uniform intensity on by the preceding prescription. However, the conclusion in the perfect image will only be achieved at zero phase with I to the effect that the deconvolution procedure preserves a perfectly Lambertian surface, approximations which are the correct photometry does not hold for caps 12 pixels in clearly false; since the cap is viewed near the limb, at diameter or smaller. This effect must be accounted for large emission angles, the deviation of the actual phase in considering absolute photometry and will be discussed function from the Lambertian approximation may be further below. important. The cap is small in this case, only a few pixels Paige (1985) used Viking data to compute the ‘‘Lam- in diameter, thereby negating the ‘‘extensive’’ assumption bertian’’ albedo of the south polar cap (the albedo of a mentioned above. polar cap assumed to be formed from frost which is a HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 93

FIG. 4. Hubble images of the martian south polar region acquired on June 27, 1992 corresponding to solar areocentric longitude (LS) ϭ 277. The images were acquired through relatively narrow bandpass ﬁlters centered at 413 nm (left) and 673 nm (right). The images are projected in a polar stereographic projection, with 0Њ longitude at the top of the ﬁgure and longitude increasing in a counterclockwise direction. Latitude circles are shown for every 5Њ, and longitude lines for every 30Њ. The images have been stretched in such a way that the cap interior is saturated.

Lambertian reﬂector) as a function of photometric angles. likely be revealed when there are known irregularities in This albedo remained fairly constant for all but the largest the cap edge, e.g., in the vicinity of the Mountains of Mit- emission angles, supporting the assumption that a Lam- chel, or where there are large variations in the albedo near bertian representation for the phase function for the sur- the edge of the cap. face cap is reasonable. Inasmuch as the result was only for the core of the cap, it does not imply that the entire cap has uniform albedo; the Viking images clearly show that DATA AND INTERPRETATION this is not the case. It may also be true that the photometric properties of the inner, residual cap differ from those of The polar stereographic projected images for the four the seasonal frost. Despite these potential difﬁculties we pairs are presented in Figs. 2–5. In each case the violet, have assumed herein that the stated prescription—namely 413 nm image and the red, 673 nm image are on the left that the true albedo boundary is at the mean intensity, and right, respectively; the 0Њ meridian is at the top of halfway between the interior of the cap and its surround- the image, and longitude increases in a counterclockwise ings—is valid here. Deviations from the rule will most direction. The images have been stretched so that the as- 94 JAMES ET AL.

FIG. 4ÐContinued

sumed cap boundary is defined by the maximum intensity The same effect has been observed in our images of the pixels; this, of course, results in saturation of the cap inte- north polar cap for large emission angles. The net result rior in the figures, though there was no saturation in the is that, although the qualitative shape and location of the raw images. Large variation can be seen in the pixel size south polar cap is consistent with Viking observations, within the image; the map projection was chosen to avoid especially at 413 nm, the images are not very reliable for having the image extend over the limb, which is defined quantitative measurements of the cap boundary except by the largest pixels. The cap did not impinge on the limb for longitudes near the central meridians. We therefore for any of the images. Also, because the images were taken considered the location of the cap boundary at the two sub- near summer solstice, the terminator fell roughly 10Њ out- Earth longitudes, 300Њ and 75Њ, and at 0Њ roughly halfway side the area included in the figures. between the central meridians of the two sets. It is immediately evident that there is significant darken- The two major sources of error in the cap edge measure- ing of the cap near the limb at both 673 and 413 nm. ments are the uncertainty in centering and the uncertainty Comparison of the 413 nm images with sub-Earth latitudes in the procedure used to locate the cap edge (see I). These of 300Њ and 75Њ (Figs. 2 and 4 compared to Figs. 3 and 5) translate into an error ranging from Ϯ1.7Њ to Ϯ2.1Њ in the reveals significant darkening away from the central meridi- edge of the cap on the central meridian, the variation ans, and the effect is even more pronounced at 673 nm. produced by the change in sub-Earth latitude. Thus devia- HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 95

FIG. 5. Hubble images of the martian south polar region acquired on July 9, 1992 corresponding to solar areocentric longitude (LS) ϭ 284. The images were acquired through relatively narrow bandpass filters centered at 413 nm (left) and 673 nm (right). The images are projected in a polar stereographic projection, with 0Њ longitude at the top of the figure and longitude increasing in a counterclockwise direction. Latitude circles are shown for every 5Њ, and longitude lines for every 30Њ. The images have been stretched in such a way that the cap interior is saturated. tions as large as those observed for the 1956 data should resolution. The quoted Viking cap edge was determined be easily observable. as the most northerly point at which frost was seen, Except for the first image pair at the central meridian, namely at the north edge of the outlier; the edge of the there is close agreement within the estimated error be- central cap in the Viking mosaic is, on the other hand, tween the cap edges determined from these data and quite consistent with the HST value. Figure 8 is an those measured on the Viking images from 1977; the two enlargement of this region on the violet HST image from sets of data are compared in Fig. 6. The only disagreement the LS ϭ 259 pair which has been stretched to bring out is in the first case (LS ϭ 259) at central meridian of 300Њ. structures having a lower albedo contrast in this region. The well known cap extension popularly referred to as This image clearly shows the cap extension corresponding the Mountains of Mitchel is known to be most prominent to the Mountains of Mitchel in its anticipated position between longitudes 300Њ and 330Њ at this seasonal date; at roughly Ϫ75Њ, 320Њ. As the width of this feature in a Viking image showing this feature is shown in Fig. 7. pixel space is only about six pixels, the cautions regarding It can be seen that, in this region of the cap, the frost the effect of deconvolution on absolute photometry of cover is far from uniform at a scale less than the image small features clearly applies here, and it is not surprising 96 JAMES ET AL.

FIG. 5ÐContinued

that the use of the ‘‘half intensity’’ algorithm to locate albedo along the central meridian, and the derived normal the cap fails here. reflectance is characteristic of the entire planet. Several We are also interested in the albedo of the south polar assumptions and corrections are necessary before the raw cap, which figures in the global energy balance. DN values CCD response at the two wavelengths can be converted were converted into reflectances using the conversion to albedos. formulas and parameters given in the WFPC Handbook (MacKenty et al. 1992). The reflectance of the ‘‘polar 1. The sensitivity of the WFPC system was degraded cap’’ is defined here by the maximum DN values reached by contamination deposited on the surfaces of the field in a plot through the center of the cap. As a reference, flatteners as the CCD chips operated at their cold tempera- the surface albedos of light and dark areas on the central tures. The effect of the contamination was a strong function meridian were also determined at 673 nm. For the images of wavelength, being most important in the UV and much with central meridian of 300Њ the dark region corresponds less noticeable in the red portion of the spectrum. The to the vicinity of crater Huygens and the light region to effect of the contamination on filter transmission was mea- eastern Arabia; the dark and light regions for the 75Њ sured as a function of time for several commonly used meridian correspond, respectively, to Solis Planum and filters and was reported in the final Science Verification eastern Tharsis. At 413 nm there was little variation in Report of the WFPC Team (Light 1991). Although the HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 97

FIG. 6. The location of the polar cap boundary is plotted as a function of LS for the 1992 Hubble Space Telescope images (discrete points) and 1977 Viking mosaics (continuous lines). The cap edge at 0Њ longitude is indicated by (*) for the HST data and by a solid line for the Viking observations; at 75Њ by (᭝) and a dotted line; and at 300Њ by (X) and a dashed line.

effect was not measured for either of the filters used here, nm). Observations with the wide band (300–3000 nm) al- we interpolated the results from filters which were ob- bedo channel in the Viking IRTM gave values from 0.4 to served and corrected our observed intensities using the 0.6 for the bolometric Lambert albedo (Paige 1985). The known time since the most recent decontamination, which Viking observations suggested that the cap albedo was not restored the system to its original throughput. constant in time during that year, though possible deposi- 2. Some assumption must be made concerning the sur- tion of dust during the two 1977 dust storms may imply face phase function in order to extract albedos from re- that these observations might not apply to years with differ- flectances. We assumed that the non-frosted surface could ent dust histories. There was also a good deal of spatial be adequately represented by a Minnaert function with inhomogeneity within the cap. The values obtained in this parameter k ϭ 0.65 (Pierazzo and Singer 1993), while we analysis of HST images are consistent with these previous represented the polar cap as a Lambertian reflector. No determinations. Geometric albedos obtained from the correction was made for atmospheric scattering. HST data for the non-frosted surface units are very consis- 3. We used the results of the numerical experiments tent with determinations from Earth based observations mentioned above to correct the derived polar cap albedos (McCord et al. 1971, Singer 1982) within the 20% absolute for the effects of the spherical aberration and subsequent calibration uncertainty of observations. Lucy–Richardson deconvolution. The cap albedos observed in both filters are also consistent with previous observations within the calibration un- The results are summarized in Table II. Previous deter- certainties. However, the albedo values decrease monoton- minations of the Lambert albedo of the south polar cap ically through the observation period, and the overall have included astronomical determinations by Dollfus variation is outside of the uncertainty expected for relative (1965), who found an albedo of 0.67 in 1958 at 0.58 nm, comparisons (roughly 10%). Since this is often a dusty time and Lumme and James (1984) who found a Lambert albedo on Mars, changes in the apparent cap albedo could be of 0.79 at 620 nm using 1971 data. The former measurement produced by atmospheric obscuration; unfortunately, the was made in mid-spring in the southern hemisphere while pair of UV filters used in our previous HST observations the latter was in early spring. Viking images from early to set limits on dust (see I) were not available in this set spring in the southern hemisphere in 1977 led to a determi- of observations. The lack of appreciable changes in the off nation (James et al. 1979) of 0.40 to 0.62 for frost in the cap albedos in both colors suggests that there was not much Viking red filter (590 nm) and 0.21 to 0.49 in violet (450 variation in the global atmospheric opacity during this pe- 98 JAMES ET AL.

FIG. 7. A mosaic of Viking Orbiter Imaging frames which encompasses the south polar cap of Mars. These images were acquired in 1977 at an areocentric solar longitude of 263. The displacement of the cap from the geographic pole and the Mountains of Mitchel in the vicinity of (Ϫ75Њ, 325Њ) are readily apparent in this mosaic, the resolution of which is about 1 km per pixel compared to a nominal resolution in the HST images of about 55 km per pixel at the sub-Earth point.

riod. The red/violet albedo ratio for the cap, which would It is more likely that errors in the cap albedo were intro- be affected by either dust or condensate clouds, does not duced by our treatment of the effects of the deconvolution change much, suggesting little variation in the opacity over algorithm on the shrinking cap which is located increasingly the cap during the period of the observations. Because the close to the limb. Our corrections were based upon a small, emission angles of the cap increased through this series of square cap immersed in a large region of constant albedo; targets, this decrease could be caused by deviations from edge effects may become more important as the cap gets the assumed Lambertian phase function. However, al- smaller. Therefore, although the data are ostensibly consis- though such a variation would be consistent with the limb tent with a seasonal decrease of the cap albedo, consider- darkening mentioned above, one would expect a decrease able uncertainty remains. in the red/violet ratio, which is not observed, on the same The south polar cap observations made by the HST basis. The lack of variation in the violet albedo outside Planetary Camera clearly conﬁrm that the qualitative ap- the cap indicates that our treatment of the contamination pearance of the cap and its temporal behavior are similar problem (which would only affect the violet ﬁlter anyway) to those observed by Viking in 1977. The seasonal recession is correct. in the size of the cap and its displacement from the geo- HST OBSERVATIONS OF MARTIAN SOUTH POLAR CAP 99

FIG. 8. An extreme stretch of a portion of the 413 nm image in Fig. 1 designed to bring out structure in the Mountains of Mitchel region. Although the details of the structure are clearly not resolved, the existence of an outlier is apparent in this image. graphic pole are clearly evident in the deconvolved, pro- the Viking observations. The quantitive size of the polar jected images. The major cap outlier referred to popularly cap, measured by its northward extent at selected longi- as the Mountains of Mitchel is clearly visible at LS ϭ 259 tudes is, within the uncertainties in the measurements, but has disappeared by LS ϭ 277; this is consistent with also consistent with the Viking data from 1977 during the corresponding period in LS and with the Planetary Patrol data from 1971. In particular, the variance of the cap reces- TABLE II sion from the 1977 curve at 0Њ longitude, included in all Albedos on Central Meridian four sets of images, is much less than the measurement uncertainty of the HST data points. The HST images do Feature L ϭ 259 L ϭ 267 L ϭ 277 L ϭ 284 S S S S not provide evidence for significant deviations in the be- 413 nm disk 0.071 0.064 0.070 0.074 havior of the cap in these three years despite significantly 413 nm cap 0.42 0.40 0.34 0.30 different atmospheric temperatures and clarity during 1992 673 nm dark 0.27 0.23 0.25 0.25 as opposed to the earlier years (Clancy et al. 1993, Encrenaz 673 nm bright 0.40 0.38 0.39 0.42 et al. 1995, I). Variations in the regression as large as those 673 nm cap 0.76 0.68 0.63 0.56 673/413 cap ratio 1.81 1.70 1.85 1.87 that occurred in 1956 are clearly ruled out by the data. The limb darkening of the polar deposits and its dependence on 100 JAMES ET AL. wavelength are clearly evident in the images. Cap albedos JAKOSKY,B.M.,AND R. M. HABERLE 1990. Year to year instability of determined from the images are reasonably consistent with the Mars south polar cap. J. Geophys. Res. 95, 1359–1365. earlier measurements. JAMES,P.B.,AND K. LUMME 1982. Martian south polar cap boundary: Most of the uncertainty in interpretation of the images 1971 and 1973 data. Icarus 50, 368–380. results from the deconvolution process which was necessi- JAMES, P. B., K. M. MALOLEPSZY, AND L. J. MARTIN 1987. Interannual variability of Mars’ south polar cap. Icarus 71, 298–305. tated by the spherical aberration of the HST primary mir- JAMES, P. B., G. BRIGGS,J.BARNES, AND A. SPRUCK 1979. Seasonal ror. We believe that aberration free images, such as will be recession of Mars’ south polar cap as seen by Viking. J. Geophys. Res. acquired by WFPC2, would not suffer from these defects. 84, 2889–2922. Unfortunately, due to the solar elongation constraint on JAMES, P. B., R. T. CLANCY,S.W.LEE,L.J.MARTIN,R.B.SINGER, the pointing of the Hubble Space Telescope this season E. SMITH,R.A.KAHN, AND R. W. ZUREK 1994. Monitoring Mars with the Hubble Space Telescope: 1990–1991 Observations. Icarus will not be observable again until late 2001. The south polar 109, 79–101. cap will be monitored at much better resolution during KIEFFER, H. H. 1979. Mars south polar spring and summer temperatures: the perihelic oppositions of the next decade as well as by A residual CO2 frost. J. Geophys. Res. 84, 8263–8288. Mars orbiters. LIGHT,ROBERT 1991. Camera throughput and contamination effects. In Final Orbital/Science Verification Report (S. M. Faber, Ed.). Space Telescope Science Institute, Baltimore. ACKNOWLEDGMENT LUMME, K., AND P. B. JAMES 1984. Some photometric properties of martian south polar cap region during the 1971 apparition. Icarus Support for this work was provided by NASA through Grant 2379 58, 363–376. from the Space Telescope Science Institute. MACKENTY, J. 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