2.2. Annual Slipface Reversal
 Although dune slipface orientations show the predominant wind directions that influence the dune field, they do not indicate the season or time of day in which these winds blow. In addition, the slipfaces do not indicate the age of the dune field. In fact, without evidence for recent dune movement, it is difficult to prove conclusively that these slipfaces were not formed by ancient winds that no longer blow, and that the dunes are not largely dormant. The paucity of erosional features on the dunes supports the idea that the dunes are not stabilized, relict features (see, for example, Figures 41c and 41d of Malin and Edgett ; are they slipface adjustments or erosional scars?). In addition, movement of dunes the size of those in the Proctor dune field could take a century or more to identify using data with the currently available image resolution, and thus their lack of observed movement in MOC and Viking images [e.g., Malin and Edgett, 2001] cannot be used to conclude that they are stabilized.
 However, there is evidence for slipface reversal in dunes on the eastern edge of the Proctor dune field that clearly indicates dune activity within the time span of the MGS mission (see Figure 2d and discussion from Paper 1). The eastern portion of the dune field consists of relatively smaller dunes with visible interdune areas, in contrast to the huge dunes atop a 50 m thick mound of sand found in the central and western-central portions of the dune field [see Fenton, 2003, Figure 12]. Because the eastern dunes are smaller than those in the center of the dune field, they have a smaller reconstitution time, thus the eastern part of the dune field may be a younger offshoot of the main accumulation of sand. At some point, winds from the southwest (the primary winds) blew sand from the main dune field to the northeast, where it was deposited upon encountering east-northeasterly winds (the tertiary winds), which only influence the eastern part of the dune field. Here the accumulation of dark sand is largely balanced between the primary and tertiary winds, producing reversing transverse dunes common to the dune field.
 In Paper 1, bright material was described on slipfaces of double-sided barchans at the eastern edge of the dune field. This bright material was attributed to the erosion of nearby underlying bright bedforms, which have a rounded appearance consistent with deflation and abrasion. It is only off the eastern edge of the dune field that the bright bedforms have this rounded appearance, and it is only at the eastern edge of the dune field that the dark barchans display bright slipfaces. This bright material cannot be residual frost because it is visible even in summertime images, when the surface is far too warm to support either CO2 or H2O frost.
 The slipface containing bright material switches sides of the double-sided barchans, as shown in Figure 4. Figure 4 compares two MOC Narrow Angle images of the same area at different times of the year. Figure 4a shows mid-fall frosted dune surfaces with bright material on northeast slopes. Figure 4b shows an overlapping image from the following year during the late spring, with fully defrosted dune surfaces bearing bright material on western slopes. Note that although the solar azimuth is similar in Figures 4a and 4b, the solar incidence angle is much lower (i.e., the sun is higher in the sky) in Figure 4b leading to fewer shadows and an increased emphasis on albedo contrast. The albedo contrast in Figure 4b is enhanced by the lack of frost cover present in Figure 4a. Figures 4c and 4d illustrate the slipface brinks and accumulations of bright material with colors corresponding to the formative wind directions (yellow is primary, magenta is tertiary). Figure 4e shows both slipface directions, emphasizing that they truly are on opposite sides of the barchans. The slipfaces on typical terrestrial reversing dunes switch sides with opposing (usually seasonally shifting) winds, erasing the old slipface from the preceding season [e.g., Sharp, 1966; Lindsay, 1973]. In contrast to observations of terrestrial dunes, the dunes in Proctor Crater display both slipfaces at all times, indicating that neither slipface is fully erased by opposing winds. The survival of opposing slipfaces may be caused by some amount of internal cementation of the dune (providing resistance to wind erosion), or by the fact that the slipfaces are too large to be reworked in a single Martian year.
 In Figure 4c, one dune in the upper right corner of the image has two slipface brinks drawn on it. The more southwesterly and larger slipface corresponds to that in Figure 4a, and the smaller and more northeasterly brink corresponds to that in Figure 4b. This shift in slipface position probably indicates movement of this slipface toward the northeast between mid-fall and the following spring, although the dune itself has not shifted position. This movement indicates a shift of 13 to 37 m, depending on where along the brinks the movement is measured. Such a shift indicates strong and persistent seasonal winds. From one image to the next, no other slipfaces moved and no dunes changed position, suggesting that movement of this type is rare on a timescale of less than one Martian year.
 Figure 4 indicates that between mid-fall and late spring, the prevailing winds change direction and that both winds influence the dunes. Bright material is likely blown from the stoss (upwind) slope onto the slipface along with any loose dark sand from the dune surface. If the dark sand on the dunes is mobile then an explanation must be found for why mobile bright material does not mix with the dark sand as it moves back and forth over the dune. Rather, the bright material remains on the surface, unmixed and unburied. It is possible that the dark sand in the dune is somewhat indurated, and that only the bright sand is moving back and forth as the seasons progress. However, the thermal inertia of the Proctor Crater dunes is consistent with loose, coarse sand, not with indurated material, implying that the wind may be able to move the dark sand (see Paper 1 and references within). Because of its thermal inertia, some amount of dark sand most likely moves back and forth over the dune (along with bright material) as the winds shift. The bright material is likely sand that is more easily mobilized by the wind than the dark sand, and thus it is the last to settle onto the slipfaces as the winds decrease. This may indicate that the bright saltating material is made of smaller or less dense particles, causing them to saltate under lighter winds than the coarse basaltic grains comprising the dark sand.
 And alternative explanation is that the bright material is dust that settles in the wind shadow created by the dune (i.e., the slipface). In this case, suspended dust carried by the wind settles on the downwind side of obstacles: boulders, craters, and in this case, dunes. Like the bright sand hypothesis, the bright dust indicates accumulations on the downwind side of the dunes. However, an explanation must then be found for why the bright dust only accumulates on the eastern edge of the dune field throughout the Martian year. For this reason, we find the bright sand hypothesis to be more consistent with known theory.
 Six MOC Narrow Angle images pass over the eastern edge of the Proctor Crater dune field, imaging slipfaces at different seasons. Following the hypothesis that the bright material accumulates on the downwind side of the dunes, the dunes in this region are influenced currently by the primary and tertiary winds. Figure 5 shows each inferred wind direction as a function of Ls, labeled by the MOC NA frame in which it was found. The red lines mark the three mean and standard deviation dune slipface directions. Reading from this plot, the primary slipfaces are active throughout fall and winter, and the tertiary slipfaces are active during spring and summer. MOC frame M07/02777 from Ls = 206.68° appears to have bright material on both slipfaces, and likely indicates a transition time between the two seasonal wind regimes. As discussed next, the modeled winds reflect the activity of these slipfaces.
Figure 5. The Ls acquisition time of MOC NA frames showing accumulations of bright material on oppositely oriented slipfaces. Downwind dune orientations are indicated on the vertical axis. Horizontal red lines correspond to the three mean dune slipface orientations (shown in Figure 2a) and respective standard deviations. In (southern) fall and winter, the primary slipfaces have accumulations of bright material; in spring and summer, the tertiary slipfaces have such accumulations.
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