 The two photographs in EXAMPLE 3 (below), taken by the NASA Mars rover Phoenix, were part of a visual argument proposed by the NASA scientists who directed the Phoenix mission. The first photo provides an initial view of a dig made by the rover; the second a view of the same dig four Martian days (sols) later. When we look at the bottom left hand corner of the first photograph and compare it to the bottom left hand corner of the second photograph, we can see white crust in the first photograph but not the second. According to the NASA scientists, the only way to plausibly explain this change is by understanding the crust as water ice which evaporated when the dig exposed it to the sun. 