[135]

 

Figure 187

 

Figure 188

Figure 196

[136-137] Figure 192, taken early in the Viking 1 mission, was nicknamed "The American Flag Picture" even before it was taken. During the final months of preparation before the landing NASA managers took exception to our decision to feature only the Mars surface in the first few color pictures. In particular, they emphasized the popular appeal of a color picture looking back across the spacecraft, the American flag in the foreground and the martian horizon in the distance. We temporized; they insisted. Figure 192 was the result. The photograph is both pictorially attractive and scientifically useful. The bright ridge in the distance, part of the crater rim described in figures 77, 78, and 79, catches the morning Sun. Difficulties in precisely balancing the colors are indicated by the violet hue of the blue color chip on the test chart. The relative differences in reflectance that yield a color image are illustrated by the three constituent images taken in blue light (fig. 189), green light (fig. 190), and red light (fig. 191).

Figure 193 is a view looking back across Lander 2. Dark boulders are prominent against the yellowish brown soil, standing like regularly deployed sentinels out to the far horizon.

In the course of the martian year an important surface change was observed at the Viking 2 site. During the winter months a thin layer of frost formed. This is documented by comparing two black and white pictures, the first taken early in the mission in late martian summer (fig. 194), and the second taken months later in late martian winter (fig. 195). Patches of receding frost are visible in the latter picture. During the height of winter, frost probably covered the entire surface. Figure 196 is a color image of the frost, looking across the spacecraft. The colors purposely have been distorted slightly to enhance color contrast.

The frost persists at temperatures higher than those required for frozen carbon dioxide (dry ice). It may be water ice or a carbon dioxide clathrate, a crystalline mixture of H₂O and CO₂.

[139] The region in front of Lander 2 was photographed many times in color. In figure 197 the surface sampler shroud contrasts sharply with the remainder of the scene. Dust clings to the surface of footpad 3 and the lower part of the leg strut.

Figure 198 illustrates the operational versatility of the Viking cameras. By generating special commands we were able to combine operational modes in ways never envisioned by the original designers. In this picture the 0.12° color diodes were stepped at 0.04° intervals instead of the usual 0.12° intervals. The result is a "high resolution" color image in which spatial resolution approaches that of a black and white picture for which both aperture and stepping angle are 0.04°. The red and green patches near the rock in the upper right occur because the three blue, green, and red images that comprise the color picture were taken on successive days. In the intervening period the sampling shovel disturbed the soil. False colors result primarily from differences in shadow distribution before and after the sampling event.

Figure 199 can best be assigned to the special effects category. It illustrates one of the many ways in which black and white images can be combined and printed in color to yield unusual patterns. In this instance an early morning image is assigned the color yellow and a picture taken in the afternoon is assigned the color blue. Although colors produced in this way are completely false, important details in the scene are sometimes favorably accentuated.

Figures 200 and 202 are a related pair, nicknamed the "sundown" and "sunset" pictures. Early in the mission, when it became clear that there was a large amount of scattering particles in the lower atmosphere, we realized the potentially dramatic appeal of low Sun pictures. However, the first priority was to document the landscape with high resolution and color images. Following several weeks of successful operation on the martian surface we relaxed to the point of attempting exotic but high risk pictures. The sundown picture was taken on sol 30, just 5 min before the sunset picture. The Sun is to the right, only 4° above the horizon. When the sunset picture was taken the Sun was 2° below the local horizon. The banding in the sky is an artifact produced by the camera. The brightness actually changes continuously, moving away from the Sun, but the natural continuum is divided into incremental brightness levels by camera electronics. If the number of quantified brightness levels were much larger, then the closely spaced steps would more closely approximate the continuous gradation of the natural scene. The slightly different color from band to band is also an artifact, caused by the fact that the brightness boundaries for the three color channels are not coincident. The general tendency for the sky to become bluer in the direction of the Sun is real, caused by dominant short wave length forward scattering of light in that direction. No planets or stars are visible in the evening sky. The occasional pinpoints of color, ...

[140] ....especially noticeable in figure 203, are artifacts caused by noise in data transmission and by mismatching during image enhancement.

Initially dismissed as little more than oddity, the sunset picture has been reproduced many times, especially in its more exotic "stretched" version (fig. 203). Admittedly, this version is more an art object than a scientific product. The colors have been specially enhanced to bring out the detail in the foreground. However, the visual impact of this computer painted scene is universal: the martian twilight envelopes the viewer.

Later in the mission the sunset picture was matched with a sunrise picture at the Viking 2 site (fig.201) Again, the Sun is several degrees below the horizon. The relatively pronounced blue shading in the sky is probably a result of early morning ice crystal fog.