Bedrock exposures are among the most provocative features at the Viking 1 landing site. Bedrock, of course, refers to a body of solid rock that underlies a layer of soil or unconsolidated sediment. On the Moon, bedrock is almost everywhere covered by a thick layer of impact debris. Only at the Apollo 15 site were astronauts Dave Scott and Jim Irwin able to chip off samples of bedrock from layers exposed in the wall of the Hadley Rille.
Figures 52 and 53 show a knobby exposure of bedrock broken by vertical fractures. Some of the protuberances are so deeply eroded that they almost form detached boulders. Indeed, we believe that many of the boulders at this site form by this mechanism of in situ weathering.
The area of bedrock in figures 54 and 55 is more clearly delineated. Horizontal rock surfaces have been swept clear of sediment. The fact that this rock weathers in a different fashion than the rock displayed in figures 52 and 53 suggests a compositional difference between the two exposures of bedrock. Both rock types are believed to be representative of volcanic lavas that inundated the entire region early in martian history.
Finding bedrock at this landing site was a surprise. Among other things, it indicates that martian geologic history is more complex than lunar history. If a thick layer of ejected deposits once covered this region, it has been subsequently removed by wind, water, or maybe even ice in the form of glaciers.
At the right edge of figure 53 is a block with a marking that, generously interpreted, is the letter "B." An enlargement of the same block is shown in an inset.
This block was first photographed approximately five days after landing. The marking elicited no comment from scientists, but one of the media representatives excitedly reported graffiti on Mars. A press conference was hastily arranged, at which time geologists explained that there are many surface markings and erosional stains on rocks that superficially resemble organized symbols. No doubt, some viewers with vivid imaginations remain unconvinced.
After the letter "B" was first discovered, scientists amused themselves by searching for other letters. Commonly, the letters they discerned happened to be the first letter of their first or last names. Psychologists take note!