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[85] ALBEDO VARIATIONS on the surface of Mars can be attributed to aeolian phenomena or to the deposition and sublimation of volatiles in the polar regions. No evidence exists for the seasonal wave of darkening once proposed by some observers.


Very few surface changes were noted during the Viking primary mission in 1976, a finding consistent with the predictions of relatively low wind velocities during northern summer. However, with the approach of southern summer. wind activity increased, and many albedo changes were noticed that were similar (but not identical) to those recorded by Mariner 9 in 1972 at a comparable season .


Comparisons of specific albedo boundaries in the Mariner 9 and Viking pictures showed that, in many cases, subtle changes in outline and/or contrast had occurred: in a few areas, e.g., Syria Planum, the albedo boundaries were dramatically different.


The most conspicuous wind markers on the planet are light, crater associated streaks whose pattern is that of the wind flow expected during southern summer (when surface winds are strongest and major dust storms occur). Although some new light streaks formed between 1972 and 1976. and a few old ones disappeared, most light streaks were essentially unchanged in outline and direction. These bright streaks probably are deposits of dust, as storm fallout, which accumulates in the lee of craters and other topographic obstructions to the wind flow.


Viking observations confirm that the average lifetime of ragged dark streaks, usually interpreted as erosion scars, is shorter than that of the light streaks. Many dark streaks changed conspicuously in both outline and direction, not only between 1972 and 1976, but also during the Viking mission itself.



Mixed-Tone Streaks in Memnonia. These distinctive mixed-tone streaks appear to consist of a central, tapered light streak bordered by two dark side-lobes. Similar streaks have been produced in wind tunnel simulations. If the wind tunnel results are valid, the dark lobes represent areas of wind erosion, and the bright central portion is a region of deposition where bright fine particles accumulate. Numerous ragged dark streaks are also visible, and are interpreted as zones of erosion associated with topographic obstacles- in this case including crater ramparts, ejecta blankets, and possible lava flow fronts. [41B51; 13°S, 139°W]


Mesoagaea Area. Only subtle albedo changes have occurred in this complex mixed-tone streak since the Mariner 9 coverage. The complex streak seems to be the result of deflation of low albedo material from the crater at upper right. The numerous bright streaks outside the large mixed-tone streak are interpreted as accumulations of dust storm fallout. Craters and isolated hills seem to produce similar bright streaks. Bright areas within the main dark streaks could be either deposits of dust storm fallout or shadow zones behind topographic obstacles where dark material from the upwind crater was not deposited. Thus, the conspicuous bright, hill-associated streak within the main dark streak could be a normal bright streak or a "shadow" streak. [88A81-88; 8°N, 192°W]


Frost Streaks in the Annual South Polar Cap. These bright, streamlined albedo features are associated with craters near the retreating margin of the annual frost cap. The features become more prominent (relative to the background) as the cap edge approaches their location, and disappear shortly after the margin passes. The bright streaks are interpreted as accumulations of carbon dioxide frost in the lee of craters, and suggest that winds may be effective in redistributing frost in the polar regions of Mars. The wind direction indicated by the streaks suggests that the streaks are laid down during southern winter. [161B26; 61°S, 71°W]



Contrast Reversal in the Cerberus Region. These frames show the contrast reversal of crater-and-hill associated streaks. Through a red filter (left, about 0.58 µm), the streaks have a 'normal" appearance and are brighter than their surroundings; through a violet filter (right about 0.45 µm), they- appear darker. Laboratory measurements indicate that such contrast reversal is a common property of well-sorted, very fine samples of iron oxide materials. Not all bright streaks on Mars show such contrast reversal in the violet. [ Left 53B56, Right 53B57; 12° N, 202° W ]

Aeolian Activity on Pavonis Mons. Persistent aeolian activity was observed on the flanks of the three large Tharsis volcanoes. Early Viking imaging revealed that since the last Mariner 9 coverage in 1972, each of the three large Tharsis volcanoes (Ascraeus, Pavonis, and Arsia), had developed a more or less complete dark albedo ring on its flanks. The dark albedo ring was especially well-developed on Pavonis Mons, as seen here, where it was 20 km wide and situated at altitudes between 20 and 25 km.

(a) The boundaries of the dark albedo ring are ragged, and the upper boundary is composed partly of coalescing, ragged dark streaks trending downhill. (b) Another view shows the same area taken after the 1977 global dust storm. The observed changes are best explained by the erosion by downslope winds of bright albedo material probably storm fallout. Aeolian activity has been observer! up to the summits of the Tharsis volcanoes, proving that Martian winds are strong enough to transport fine particles even at the very low pressures at the tops of the Tharsis volcanoes. [Left 52A15, Right 416A45; 0° N, 113° W ]






Spectacular Albedo Changes in Syria Planum. These four views of Syria Planum show both long-term and short-term changes in the surface markings. (a) A Mariner 9 view of the area in southern summer is shown. (b) A Viking Orbiter 1 view of the same area almost three Martian years later was taken shortly after the start of the global-scale storms. (c) and (d) These were taken in mid to late southern summer. The changes observed in the bright streaks in the last three frames are attributed to strong north-to-south winds during the global dust storms. [(a) Mariner 9 DAS 08585544, (b) 294A69, (c) 416A49, (d) 439A48; 12° S, 110° W]




Changes in Wind Streaks on the Slopes of Arsia Mons. The changes in both dark and light streaks shown occurred after the global dust storms. In the Iower image, winds blowing down the long slopes of the volcano redistributed some of the light coating of dust deposited during the global dust storms, forming both dark streaks (erosion of dust) and bright streaks (deposition of additional dust). [574A46, 648A03; 9° S, 124° W]



Erosion of Dust from Large Areas Following the Global Dust Storms. (a) Shortly after the global dust storms, wind erosion downwind of craters produced dark streaks: a combination of global circulation and local winds blowing down slopes formed streaks pointing in two directions from the same craters. (b) Later winds, also blowing downslope, stripped dust from much wider areas to form the large dark markings. [603A08, 639A67; 31°S, 117°W ]