Mercury's high density implies that there is a high concentration of heavy metal (probably iron) within the planet. Conversely, Mars has a lower concentration of iron than Mercury.
The outer Galilean satellites, Ganymede and Callisto, have much lower densities than the other bodies. Their densities are not much greater than that of water! Ice is exposed in young impact craters supporting the fact that there are thick layers of ice on these bodies.
Based on DECREASING crater coverage and INCREASING evidence of geologic activity (volcanoes, etc.) with proximity to Jupiter you can conclude that the inner satellites are more geologically ACTIVE.
In fact, Jupiter and its satellites may have formed in as a "mini" Solar System. In this theory, Jupiter was the central source of heat and gravity (remember its composition is very close to that of our Sun!), the inner satellites have relative high densities (like the inner planets of the Solar System) and the outer satellites have low densities (like the "Gas Giants" of our outer Solar System) and were not hot enough to "burn off" their ice layers (and still aren't).
The "ringed planet" images at the bottom of the page don't look very different from one another. The 2 sets of "star" clusters, however, are less bright in the quarter-resolution image. This is because the pixels in these images are too small and closely spaced for our eyes to easily see the loss in resolution EXCEPT for features that had very few pixels to begin with. That's why our original example "asteroid" (made up of a small number of large pixels) looks very different from its quarter-resolution counterpart (shown above on the right).
Hearing Galileo's Whisper Across the Solar System
DSN stations are located at California, Spain and Australia because these sites are spread roughly evenly around the Earth which allows a spacecraft very far from Earth to always be seen by at least one DSN station even though the Earth is rotating once per day.