[100-101] The deeply textured rocks at the Lander 2 site yield attractive graphic patterns, especially when they are accentuated by low Sun and deep shadows (figs. 116 and 117).

The blocks are so distinctive that one might expect that their origin could be easily determined. However, this turns out not to be the case. The abundant pits are similar to vesicles that occur in terrestrial volcanic rocks derived from gas charged lavas. As the gas rises to the surface, some of the bubbles are essentially frozen in the solidifying lava. Although this is the favored interpretation for the martian rocks, the pits are larger and more widespread than is typical for terrestrial situations. Alternate interpretations are that the pits mark the former presence of easily eroded clasts or crystals, or that the rock results from partial cementation in an upper soil zone in a manner analogous to that for some tropical regions on Earth.

All these arguments proceed by analogy: the features on Mars resemble those on Earth that we know to be formed by particular processes. Unfortunately, it is difficult to quantify the likelihood of one analogous comparison as opposed to another. In reasoning by analogy we are constrained by our own experience. The number of analogies at our disposal is a function of our knowledge of terrestrial landform. If processes on Mars produce unique forms, then dependence upon analogies may blind us to that uniqueness.

 

[102-103] Figures 118 to 121 make up two pairs of pictures with different illumination angles. Because of their roughly textured and pitted surfaces, the blocks change dramatically in appearance as the lighting varies from direct to oblique.

 

[104-105] Figure 122 is a 100° panorama taken in the late afternoon. The lower part of the meteorology boom appears to the left; the shadow of the upper mast, including sensors, is visible immediately to the right. Figure 122 includes a region from 30° to 50° in camera elevation. Figure 123 is from 40° to 60°, so that a rock that appears in the lower far right of figure 122 is located in the upper left of figure 123.

[106-107] These three figures show the effects of erosion in the vicinity of the descent engine exhaust impact point. A thin veneer of fine grained sediment has been swept away to reveal polygonally fractured "rock." In fact, the polygonal fragments probably represent cemented soil fragments. This inference is strengthened by the observation that chemical analyses of unconsolidated soil closely resemble analyses of the platy chips. Refer to figure 56 for documentation of a comparable effect at the Lander 1 site.

This indurated soil zone, termed duricrust, has probably formed by upward migration and evaporation of groundwater with attendant precipitation of cementing compounds. Although duricrust was speculatively identified at the Viking 1 site, it was not until we received these Viking 2 pictures that we felt confident of the interpretation.

The prominent structure in figures 124 and 125 is the magnet cleaning brush. Mounting brackets for the surface sampler assembly appear at the left in figure 126.

[108-109] Two views of rocks and sediment at the Viking 2 site. Fillets of sediment that partly bury some blocks are shown well in figure 127. The grazing early morning light accentuates parallel grooves in the large block at the center of figure 129. This stratification might have been caused by flow in a viscous volcanic lava.

Approximately vertical illumination in figure 128 brings out differences in soil texture. The finer grained material appears to be a patchy wind deposit superposed on coarser accumulations of cemented soil fragments. The surface sampler has been unable to acquire any of these coarser fragments which apparently disintegrate when scooped up.

[110-111] Three views of the same area were taken in late afternoon (fig. 130), early morning (fig. 131), and midday (fig. 132). The appearance of the scene changes dramatically with the change in illumination. Although many of the blocks have the pitted appearance characteristic of the Viking 2 site, some are clearly different in morphology and, perhaps, chemical composition. Look, for example, at the smoothly polished block that appears a little to the right of center in figure 132. Its singular appearance is more subtly revealed in figures 130 and 131. In the first case, a smooth face reflects the evening light.

Orthogonal blocks occur throughout the scene. Some wind faceting may have occurred, but most of the blockiness is probably the result of erosion of a volcanic lava that was traversed by several sets of mutually perpendicular joints.

[112-113] Note two boulders that occur close together a little to the left and above center in figure 133. They have matching faces which suggest that a single boulder has been split in two and the two parts separated, perhaps by frost heaving. Several of the large vesicular boulders in this scene display parallel stratification. A trench dug by the surface sampler is visible in the lower right.

Oblique lighting in figure 134 accentuates pits so large they look like small craters. Indeed, if Mars lacked a shielding atmosphere that destroyed small meteoroids, an impact origin for the pits probably would be favored.

The spacecraft shadow in the lower part of figure 135 creates an attractive graphic design. Note that the right part of figure 135 is the same field of view as the left center part of figure 134.