Global imaging of Mars by Hubble space telescope during...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. E8, PAGES 18,883-18,890, AUGUST 25, 1996

Global imaging of Mars by Hubble space telescope during the 1995 opposition

Philip B. James, • James F. Bell III, 2 R. Todd Clancy, Leonard J. Martin, 5 and Michael J. Wolff •

3 Steven W. Lee, 4

Abstract. Hubble space telescope (HST) imaging of Mars near the 1995 opposition resulted in excellent synoptic-scale images of the planet during the spring season in the northern hemisphere. Because this season coincides with the aphelion position of Mars in its orbit, it is therefore the most difficult for ground based observation because of the relatively small angular size of Mars. This is the first sequence of images fully utilizing the capability of the new Planetary Camera to produce global synoptic images of the planet. The images reveal bright, discrete clouds associated with topographic features superimposed on a zonal band of condensate clouds between latitudes -10 ø and 30ø; the maximum violet optical depth of the cloud band is about 0.3. In a few instances, the appearance of clouds beyond the morning terminator can be used to infer cloud heights of roughly 8 km. A large, dark albedo feature in the Cerberus region, observed for many years by ground-based observers, has almost disappeared in the 1995 HST images. Other aspects of Mars, such as the north polar cap, appear much as they did during previous oppositions. Although cloudy regions were observed by spacecraft during this season, the HST images uniquely reveal the global extent of significant optical depth clouds.

Introduction

Changes in the surface and atmosphere of Mars have been observed for many years [Slipher, 1962; Antoniadi, 1930; Martin et al., 1992]. It has been difficult to telescopically distinguish repetitive seasonal variability from interannual variability because of the large variations in the apparent angular size of Mars during its 780 day synodic cycle, which imposes a 15 to 17-year periodicity in scientifically useful observation of a particular Martian season, specified by the areocentric solar longitude L s. Spacecraft observations have revealed details of the seasonal variations on

Mars during a few Martian years, but the limited time spanned by these observations precludes conclusions on the amount of interannual variability in the climate. Also, most spacecraft observations have lacked the synoptic character and repetition frequency necessary to study atmospheric phenomena.

•Department of Physics and Astronomy, University of Toledo, Toledo, Ohio.

2Department of Astronomy, Cornell University, Ithaca, New York.

3Space Science Institute, Boulder, Colorado. 4Laboratory for Atmospheric and Space Physics, University of

Colorado, Boulder, Colorado. 5Lowell Observatory, Flagstaff, Arizona.

Copyright 1996 by the American Geophysical Union.

Paper number 96JE01605. 0148-0227/96/96JE-01605509.00

Hubble space telescope (HST) observations are also limited in seasonal coverage by solar pointing constraints and by scheduling realities. While HST observations are infrequent relative to ground-based monitoring programs, they at least partially fill the temporal gap between Earth- based and spacecraft data, and they provide a substantial increase in spatial resolution over Earth-based observational studies [James et al. , 1994].

The observations of Mars discussed here were acquired using the Wide Field/Planetary Camera 2 of HST on February 24 and 25, 1995. There are three sets of images centered at sub-Earth longitudes of 34.5 ø , 151.8 ø , and 275.9 ø , the angular size of the planet at this time, shortly after opposition, was 13.5 arcsec, and the sub-Earth latitude was 17.7 ø , providing considerable overlap in the north polar region. All three sequences include exposures using five filters with bandpasses centered at 255, 336, 410, 502, and 673 nm.

These HST observations occurred during late spring in the northern hemisphere of Mars (L s =63.5ø). Viking observations during the late 1970s [Jakosky and Farmer, 1982; Jakosky and Haberle, 1992] showed that the concentration of water vapor in the Martian atmosphere was starting to build to its summer maximum in the northern hemisphere during this season, and some of the most prominent Martian discrete clouds, in the Thatsis and Elysium regions, have first been observed at this time [Smith and Smith, 1972; Hunt et al., 1980]. This season is less well known than other Martian seasons because concurrent oppositions of Mars are aphelic, with the maximum angular size of the

18,883

18,884 JAMES ET AL.' HUBBLE OBSERVATIONS AT 1995 MARS OPPOSITION

planet roughly 14 arcsec. The potemial importance of phenomena during this season with respect to the annual and longer term cycles of dust and water on the planet makes acquisition of better aphelic data a high priority.

Atmospheric Phenomena

The HST images show cloudiness on all sides of the planet. Discrete bright clouds appear in violet and UV images along the afternoon (eastern) limbs of each image sequence, near major volcanoes and in the western portion of the Valles Marineris canyon system. The clouds, composed of water ice crystals, appear more prominem at shorter wavelengths because, for reasonable particle size distributions, their reflectance is relatively independent of wavelength compared to that of the surface, which is dominated by minerals containing oxidized iron which are highly absorbing short of 500 nm. The morning terminators are particularly heavily clouded; in one image, Ascraeus Mons protrudes above the cloud cover, and in another, Elysium Mons appears though the haze. Clouds also form a southern autumn polar hood, with strongest brightenings in the Hellas and Argyre basins.

The most saliem feature of the Martian atmosphere during the period of these observations was the appearance of a planet encircling belt of clouds seen in Figure 1. The discrete clouds are superimposed on this band of clouds, which extends from about -10 ø latitude to -I-30 ø latitude with

maximum opacity occurring at about +20 ø. The 410-nm opacity of the clouds at 20 ø latitude is generally between 0.25 and 0.35 except in the vicinity of 65 ø W longitude, where the opacity is between 0.4 and 0.5 over a broad range of latitudes corresponding to the Valles Marineris canyon

system (Figure 2). Inasmuch as the optical depths of the brightest discrete components such as the W clouds (discussed below) have been estimated by various authors to be =0.5 [Curran et al., 1973; Christensen and Zurek, 1984; Akabane et al. , 1987; James et al. , 1994], the widespread cloud belt is a more significant reservoir of water than the individual bright clouds. For a mean particle size of 2 [Curran et al., 1973] this corresponds to up to = 1 precipitable microns of water. Since the average amount of water vapor in this latitude band is about 10 precipitable/zm at the time of these observations [Jakosky and Farmer, 1982], the clouds contain an appreciable fraction of the local water at this time and therefore represent an important water reservoir.

The historical record indicates that this amount of

cloudiness is more than is usually seen on Mars but is not unprecedented; longitudinal bands of clouds were first noted by the late E.C. Slipher of Lowell Observatory on photo- graphs taken in 1954 [Slipher, 1962]. However, he had been observing Mars since 1905, and these cloud "belts" were considered by him to be unusual events. The Viking spacecraft saw a fairly cloudy Mars both during their arrival and after achieving orbit during early northern summer [Briggs et al., 1977]; the high volcanoes were also protrud- ing above a cloud layer on some of the Viking images. The Viking Orbiter cameras also recorded numerous clouds above the Martian limb and terminator [Jaquin et al., 1986]. Similar clouds have been occasionally reported by visual observers since at least 1890, although some misinterpreted these phenomena to be mountains [Martin and Zurek, 1993; Holden, 1890; Douglass, 1895].

General circulation models [Pollack et al., 1981] predict a strong Hadley circulation with ascending branch, favoring cloud formation, in the northern subtropics near summer

90

HST 410 nm 24- 25 February 1995

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-60 180 90 0 270 t 80

Figure 1. Mosaic of cylindrical projections of the three violet (410 nm) images of Mars, separated by 120 ø of longitude, obtained on February 24-25, 1995. Where the images overlap, the pixel with the smaller emission angle is chosen, but there are no phase function corrections for the enhancement of cloud brighiness toward the limb. The belt of clouds at low latitudes (-10 ø to 30 ø) with superimposed bright clouds in the Tharsis, Valles Marineris, and Elysium regions is evidem, as is brightening associated with a south polar hood in the southern hemisphere.

JAMES ET AL.' HUBBLE OBSERVATIONS AT 1995 MARS OPPOSITION 18,885

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1995 the cloud to the west of Ascraeus Mons was consider-

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ably brighter than the Alba cloud. The Olympus Mons cloud was brighter in 1995 than it was in 1991. The locations of the clouds to the west of the major topographical highs is consistent with the predictions of circulation models which predict topographically induced positive vertical velocities in these regions [Haberle et al., 1993; Webster, 1977].

The historical record suggests that these clouds are much

0.•01 -..•11/ :, I weaker before L S = 60 ø [Smith and Smith, 1972]. Howev- ß er, our HST images of the Tharsis region at the 1993

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Figure 2. Optical depth as a function of latitude for several different longitude slices through the equatorial cloud belt. These opacities were calculated via a model based upon the discrete ordinate radiative transfer code of Stamnes et al.

[1988] using phase functions and surface reflectivities as discussed by James et al. [1994].

solstice. The reduced aphelion insolation favors water vapor saturation and condensation at relatively low levels in the atmosphere. Depending on how rapidly the ice particles precipitate to lower levels, where the meridional velocity is directed to the north, these clouds could be a significant impediment to the transport of water from the northern to the southern hemisphere during this season and could therefore be significant factors in the _n•t water transfer

between hemispheres [Clancy et al., 1996]. The discrete clouds associated with the Tharsis volcanoes

and Valles Marineris, historically called W clouds due to their appearance in an inverted telescope image, were prominent in the L s = 60 ø images acquired in both 1991 [James et al., 1994] and 1995 (Figure 3a). There is significant diurnal variation in the clouds during a Martian day, and the relative intensities of the components of the W complex are known from ground-based astronomy to shift from year to year; data relevant to day-to-day variations with fixed local time are not common, but ground-based images suggest that such changes occur slowly, if at all. Interannual variation is most likely responsible for differences in the clouds observed in HST images acquired at L s = 60 ø in 1991 and 1995' in 1991 the clouds near Alba Patera

and the western portion of Valles Marineris (i.e., Tithonius Chasma) were the most prominent components, while in

Figure 3. Cylindrical projections of HST F410M images of the Tharsis and western Valles Marineris regions of Mars in (a) February 1995 at L s = 63.7 ø and (b) January 1993 at

20.2 . The parent images were centered at 150 ø and Ls = o 165 ø longitude, respectively, resulting in local times over Tharsis of 1500 and 1600 UT. The bright, discrete clouds associated with the major volcanoes and other features in the region clearly stand out against the background clouds in the equatorial belt discussed in the text. Contours showing the large topography variations in the region are superimposed on the images. The gray levels in the two maps are not directly comparable, having been acquired with two different instruments, WFPC2 and WFPC1, respectively; estimates suggest that the clouds in Figure 3a have significantly larger opacity.


opposition, which occurred at L$ = 20 ø also show a large number of bright discrete clouds, including (especially) Olympus Mons, in the W complex at that time (Figure 3b). The clouds at this season are shifted to the east of the

topographic features relative to those in the 1995 images. This shift seems consistent with the change predicted by GCMs in the zonal circulation between equinox and solstice, with weak surface easterlies yielding to a westerly jet at the relevant latitudes, although such a shift was not seen in a previous simulation of the effects of topography on the local circulation [Webster, 1977].

Quantitative comparison of the optical depths of the clouds in Figures 3a and 3b is complicated by the fact that Figure 3a was acquired using WFPC2 while Figure 3b was produced from an aberrated WFPC1 image which had undergone a deconvolution process. In addition, the brightest clouds in Figure 3a are actually saturated in the image, preventing a determination of opacity for them. The optical depth of the Alba Patera cloud in Figure 3a is estimated at 0.46, twice its value of 0.23 in the 1993 observations and much larger than the very small background opacity in that region. The maximum optical depth determined from the 1993 images was 0.48 for the Olympus Mons cloud [Zheng, 1994]. These data suggest that the L s = 63 ø clouds in 1995 are more substantial than those at L s = 20 ø in 1993 by roughly a factor of 2.

Figure 4 presents an expanded view of a predawn cloud viewed near the morning limb of Mars on February 24, 1995. The brightness of this 336-nm image is dominated by Rayleigh and cloud scattering, particularly near the limb, due to the low ultraviolet reflectance of the Mars surface

[James et al. , 1994; Clancy et al. , 1996]. Several such discrete predawn clouds were observed at equatorial and high southern latitudes in the ultraviolet and violet images. The particular predawn cloud of Figure 4 extends over

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[ig•re 4. A mag•fied, stretched image of the western of •e plaet show•g a cloud beyond $e te•ator illu•- nated by vi•e of its alti•de, c•culated to be 8 •. •s predawn cloud is located over Sinus Meridian.

7ø-11 ø N latitude, 3550-360 ø W longitude region, which is just north of Sinus Meridiani. Smaller, less bright clouds are also barely visible at lower latitudes (00-5 ø N) in this figure. This region, at an elevation of 0-1 km, exhibits only modest regional elevation gradients [U.S. Geological Survey, 1993]; thus these clouds are probably not orographic in nature.

The predawn illumination of this cloud allows for a model-dependent derivation of the cloud top height, based on the altitude dependence of the local time of sunrise. We assumed a longitudinally uniform distribution of the cloud brightness and height and used the longitudinal (hence, local time) offset, about 4.5 ø, of the cloud 336-nm brightness to calculate a cloud top height of 8 km. The uncertainty in this cloud top height derivation is primarily a result of the assumption of cloud uniformity. The weaker clouds at 00-5 ø N lead to very similar cloud top altitudes. However, higher altitudes (> 10 km) are possible if the formation of such discrete clouds is restricted to a local time of 30 min before sunrise.

Cloud altitudes of 5-10 km are consistent with the recent

study of Clancy et al. [1996], which employed microwave temperature and water vapor profiling of the Mars atmosphere to conclude that low-altitude water vapor saturation conditions are characteristic of the aphelion Mars atmosphere. However, in this context the significance of the cloud height determination from the cloud of Figure 4 is not entirely conclusive, since any diurnal variation in this cloud height is undetermined. If this cloud is orographically controlled, it may exhibit substantial diurnal variation [Blumsack et al., 1973].

Seasonal North Polar Cap

The 1995 HST images provided the opportunity to view the entire north polar cap at values of L s identical to Mariner 9 views in 1972 [$oderblom et al., 1973]. The sizes and shapes of the caps in the two years are very similar at the resolution of the HST images. (The image scale is 23 km/pixel at the sub-Earth point. At the north pole, the scale is about 75 km/pixel, while 40 km/pixel is typical at the edge of the cap. The clear resolution of Crater Korolev, about 80 km in diameter, establishes two pixels as an upper limit for the sizes of objects which can be resolved if there is sufficient albedo contrast.) The frost coverage in the interior of the cap is clearly patchy as in 1972, though the lower resolution of the HST images relative to the Mariner 9 frames does not permit a detailed comparison of bright and dark areas. The crater Korolev (73,196) is a conspicuously bright, frosted outlier in the L s = 63.5 ø HST images as in the Mariner 9 frames, and both mosaics show a dark embayment near 80 ø longitude.

The portion of the cap between 260 ø and 360 ø longitude, imaged in January 1995 at L s = 39.7 ø, is nearly circular with a boundary at 66.5 ø latitude. At L s = 63.5 ø the perimeter of the cap is more uneven and noncircular in appearance (Figure 5). In particular, the "peculiar polygo- nal" outline alluded to by the Mariner 9 team [Soderblom et al., 1973] is apparent, with the cap assuming a distinctly hexagonal appearance reminiscent in both shape and latitude of the polar hexagonal wave feature on Saturn [Godfrey, 1988]. The perimeter of the cap does not seem to correlate

JAMES ET AL.: HUBBLE OBSERVATIONS AT 1995 MARS OPPOSITION 18,887

Figure 5. A mosaic of the north polar region on Mars composed of polar stereographic projections of the three separate images in the February 1995 sequence. The asymmetric pattern of the cap at L S = 63.5 ø is very similar to that observed by Mariner 9 in 1972 [Soderblom et a/.,1973]; the appearance of crater Korolev as a bright outlier and of a dark embayment at roughly 80 ø longitude also duplicates the Mariner 9 observations. Zero degrees longitude is at the top of the figure, and longitude increases clockwise; latitude circles are separated by 5 ø .

with local topography, though there is large uncertainty in the latter in the polar regions. Nor is it obviously related to variations in surface properties, such as the circumpolar dune fields which are evident as dark arcs within the

boundary of the cap. It seems more likely that the shape might be related to standing waves forced by larger scale topography which affect the amounts of CO 2 deposited during the winter. Simulations [Hollingsworth et al., 1996; Barnes et al., 1996] suggest that standing waves with wavenumber three, rather than six as suggested by the shape of the feature, are most likely to be excited by the actual Martian topography, however.

Surface Phenomena

Low albedo regions of Mars have long been known to change as a function of time. At one time these changes were ascribed to vegetation [Slipher, 1962], but it is now thought that the changes are produced by small amounts of bright, windblown dust alternately scoured from and deposited upon dark bedrock areas during dust storms. Substantial changes were noticed in Syrtis Major during the period surrounding the 1977 planet encircling dust storms [Kahn et al., 1992], and it was on this basis that we chose to monitor the Syrtis major region frequently during our HST observations. One crater on the western boundary of Syrtis was observed to darken noticeably in 1991, but there were no changes in the configuration of this regional albedo

feature comparable to those observed by Viking orbiters [James et al., 1994].

At the other extreme, Cerberus, a prominent dark albedo feature approximately 1500 by 500 km in size which forms the southeastern boundary of the bright Elysium region, has essentially disappeared since the time of the Viking mission (Figure 6). An image-based analysis of the Cerberus feature [Chaikin et al., 1981] indicated it had changed little between the time of Mariner 9 and Viking. In fact, it appears to have remained fairly constant in location and appearance in telescopic observations obtained since the early 1900s [Slipher, 1962]. However, the feature faded gradually during the period, leading up to the 1995 opposition in February when it appears as only three small discrete spots. More recent Earth-based observations (D. Parker, private communication, 1995) suggest that this region, which is on the slope of the Elysium bulge, has been continuously fading since 1984 except for temporary reversals in 1988 and 1993. While it is true that one of the brightest Martian clouds forms near Elysium Mons during this season, the cloud formation has not affected the appearance of Cerberus in the past, and we see no evidence for increased cloudiness in this particular region in our blue-U¾ HST images. The dark albedo features in Arcadia Planitia (northeast of Cerberus) and Terra Cimmeria (south of Cerberus) appear much as they did at the time of Viking, suggesting that there has been no redistribution of dust on the regional scale. Therefore we suggest that more local transport of bright dust from the southern flanks of Elysium into the Cerberus region is the cause of the albedo change.

In order to analyze the derailed spectral behavior of different surface units, the raw images were calibrated to absolute flux units. The HST photometric conversion values are based on observations of well-calibrated standard stars

observed prior to obtaining our Mars images. In order to compare these data with previous ground-based and spacecraft observations and with photometric modeling results, the flux data were converted to radiance factor by ratioing the flux values to the expected flux from a Lambert surface at Mars' distance and illuminated normally by the Sun. No corrections were made for limb darkening in these data. These calibrated images were coregistered into three- dimensional image cubes (spatial x spatial x spectral), and five-color spectra of individual units were extracted for analysis. The shape of these spectra is consistent with previous ground-based and HST spectra of Mars [McCord and Westphal, 1971; Bell et al., 1990; James et al., 1994], providing confidence in the quality of the flux calibration performed.

All surface regions observed are very red, even the cloud and ice regions. The brightest regions are also the reddest, exhibiting 673 nm/410 nm (R/V) ratios of 5.3 to 5.8. The clouds and ices have R/V = 3.8 to 4.5, and the darkest regions have R/V values of 3.2 to 3.5. The least red region occurs in the darkest part of Syrtis Major, near the Nili and Meroe volcanic calderas (R/V = 2.5). In general, the bright regions in the Syrtis hemisphere exhibit very similar spectral shape at the three longest wavelengths, indicating that even at this high (ground-based) spatial scale, the effects of the global redistribution of bright windblown dust act to mute spectral differences from region to region. The dark regions exhibit larger spectral differences. For example, the spectrum of Syrtis is distinctly convex between 410 and 673


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Figure 6. Comparison of the appearance of the Cerberus albedo feature in HST and Viking observations. Maps are in simple cylindrical projection. The IRTM phase-corrected albedo map is from the Mars Consortium data collection [Pleskot and Miner, 1981]; the brightness scale to the fight refers only to this IRTM map. The HST map was produced from a 673 nm image obtained in February 1995. A Minnaert correction has been applied (k = 0.65), and data at emission angles greater than 75 ø have been eliminated. It is evidem that much of the Cerberus albedo feature is not visible at the time of the HST observations.

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nm, consistera with the spectrum of a relatively poorly oxidized basalt [Adams and McCord, 1969]. However, the spectra of Sirenum and Acidalia, which are both as dark or darker than Syrtis, are much redder and are indicative of regions of increased alteration.

Figure 7 presems a two-dimensional histogram of R versus V reflectance from units in the entire hemisphere imaged by HST which is centered on Syrtis Major. (Two dimensional histograms are plots of one spectral parameter versus another and can be used to define spectral units within a multispectral image cube [McCord et al., 1982].) Five spectral units were identified and spatially mapped: (1) low R and low to intermediate V unit that occurs in dark

regions like Syrtis Major and Mare Tyrrhenum. These regions are the least altered and represem the closest material to "bedrock" visible in our data. (2) intermediate R and low V unit that occurs in the dark northern plains in Utopia and in small isolated low albedo regions. The spectrum of this unit is consistera with a higher degree of alteration than unit 1 but with no evidei•ce for an inflection

near 502 nm that would be indicative of a greater degree of crystallinity. (3) high R and low V unit that occurs in association with the bright regions in Arabia and Isidis near 30 ø N. This region is highly altered and was relatively free of clouds at this time, thus explaining the high R and low V

Figure 7. The 410-nm image of the Syrtis Major hemisphere (upper left) and the 673-nm image of the same hemisphere (upper right) were coregistered to produce the two-dimensional histogram image in the lower panel. The histogram displays the 673-nm radiance factor (ordinate) versus the 410-nm radiance factor (abscissa). Darker gray values indicate a greater frequency of occurrence of image pixels at that particular x,y value. The broad spectral units outlined are discussed in the text.

JAMES ET AL.: HUBBLE OBSERVATIONS AT 1995 MARS OPPOSITION 18,889

spectral behavior. (4) intermediate to high R and intermediate V unit occurring in central Arabia and southern Isidis. The spectra of these regions are consistent with a high degree of alteration and a modest amount of cloud cover. (5) high R and high V unit occurring primarily in Isidis. This region had substantial cloud cover during this time, and the spectra of Isidis are consistent with a highly altered surface possibly containing some crystalline ferric oxide phases. There is apparently a high degree of mixing between all of these units, based on the many pixels that fall along distinct mixing lines between the unit boundaries.

These preliminary analyses indicate that the HST multispectral images can be well-calibrated and that their high signal to noise ratio facilitates the mapping of diagnostic differences in spectral behavior on Mars at fairly high telescopic spatial resolution. These data were limited by the acquisition of only five spectral bands; our subsequent HST data, using nine bandpasses, should improve the robustness of the conclusions substantially.

Summary

These HST images reveal more extensive clouds on Mars than was typical in spacecraft observations of the 1970s. In addition, the dark albedo region in Cerberus which has been observed for many years by ground-based astronomers and by spacecraft has essentially vanished. Though other aspects of the planet, such as the north polar recession, are similar to previous years, these observations highlight the fact that Mars is a dynamic planet and that the very limited observational archive does not necessarily include all possible Martian phenomena. The ability of HST to monitor changes in Martian climate and surface features is also reinforced by these data. The unique capability of HST for Mars observations is its ability to acquire synoptic-scale images of Mars with scientifically useful spatial and spectral resolution throughout much of the planet's synodic cycle. Thus the HST images have been particularly useful in identifying the global nature of the aphelion clouds. The major limitation of HST in monitoring Mars is the limit on the frequency of observations due to the small amount of time which can be

allocated to any one project on the facility.

Acknowledgments. This work was supported by GO grant 5832 from the Space Telescope Science Institute.

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(Received January 8, 1996; revised May 14, 1996; accepted May 20, 1996.)

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