There is an undeniable fascination in photographing the familar objects of the solar system from an unfamiliar vantage point. The best examples, of course, are the photographs of Earth from deep space.
We had hoped to photograph Earth from the surface of Mars, even though we knew it would appear only as a pinpoint of light. Unfortunately, we discovered that our camera lacked the sensitivity to discern such a faint object. The problem was intensified by the fact that Earth always appears dose to the Sun, just as Venus does when viewed from Earth. This is the general situation when viewing a planet with orbital radius smaller than that of your own planet.
We were restricted to viewing two objects beyond Mars, Phobos, and the Sun. Figures 176 through 178 show the Sun imaged at different elevation angles. As it rises higher and the path length of light through the atmosphere decreases, the solar disc becomes brighter. Figure 176 is taken at 5:48 a.m. The Sun, about 5° above the horizon, is barely distinguishable. The speckled appearance of the sky is an artificial effect, caused by extreme enhancement of detail in a very bland scene. Figure 177 was taken at 6:12 a.m.; the elevation angle of the Sun is about 10°. Figure 178 was taken at 7:00 a.m.; the Sun is approximately 20° above the horizon. The irregular shape and secondary bright spots are due to spurious reflections of light from the outer camera housing.
Phobos is one of two natural satellites of Mars. An irregular, cratered object about 22 km in diameter, it circles Mars approximately 1000 km above the surface every 11 hours. Although visible as no more than a bright object several pixels in diameter (fig. 179), images of Phobos proved very valuable in determining the opacity of the night sky. Color and IR reflectance values of the integrated disc were clues to the satellite's chemical composition. The spectral data best fit that of carbonaceous chondrite, a variety of meteorite believed to represent primitive solar system material.
Possibly the most unusual of all Viking Lander pictures records the passage of Phobos' shadow during a solar eclipse (fig. 180). On Earth the apparent size of the Moon is exactly the same as that of the much larger but more distant Sun. Consequently, direct sunlight is completely blocked out during a total solar eclipse. On Mars, Phobos covers only a quarter of the solar disc. However, passage of the penumbral shadow causes a general drop in light level that is instrumentally detectable. Figure 180 is a repeated line scan image looking back across Viking Lander 1. The colors have been distorted purposely to enhance detail. The blue and white horizontal stripes correspond to test chart patches. The brownish stripes in the middle represent the martian surface visible above the spacecraft. Note a decrease in light levels in the sky midway through the imaging event. The darkening, caused by the passage of the penumbral shadow of Phobos, is present over approximately 100 vertical lines. Moving at about 2 km sec, the shadow took about 20 sec to pass over the Lander. It should be obvious that successful acquisition of this picture requires extremely accurate calculations regarding the orbit of Phobos about Mars. Not only was the passage of the small shadow across the Lander correctly calculated, the time of the event also was predicted within a few seconds.