Here are the surfaces of many solar system bodies at the same resolution for the sake of comparison. At 280m per pixel, each image is 39 km, or about 24 miles, to a side. This is roughly the size of a typical county in the United States. Each surface is the product of several factors acting over the age of the solar system. The one factor most widespread is that of impact cratering. Impacts hit every major body in the solar system rather hard from about 4.5 billion years ago to 3.8 billion years ago, and since then at a modest, relatively constant rate. Other factors altering surfaces of worlds include: 1) Emplacement of lava flows, either from the surface melting, or molten lava from below venting out and flowing over older solid surface. This often leaves a visible vent or crater, often seen with volcanos on Earth. 2) Wrinkling due to the global contraction of the surface or subsurface. 3) Local buckling or folding due to large plates sliding about, and into one another. 4) Large cracks, faults or canyons due to the global expansion of the surface or subsurface. 5) Wind or water erosion. 6) Precipitation of rain, snow or dust, either as part of a climactic event, or residue from volcanic or impact events. Ice can build up to considerable extent in polar regions. 7) On Earth, the effects of life. Plants and humans dominate in their effects on Earth. The surface of Mercury is rather old, going back to sometime in the middle of the initial early bombardment. While not quite saturated with impact craters, as the most ancient surfaces would be, it is still heavily cratered. Virtually no major forces have acted upon the surface of Mercury since then. Venus has had its surface completely re-paved in the latter portions of its history. It is thought that a massive, global, volcanic event about 700 million years ago erased all or virtually all traces of any surface that existed before that. It also has a thick atmosphere that vaporizes incoming meteors that might otherwise generate small craters. A relatively small number of large impact craters do exist, but none in this image, which consist of old plains of hardened lava, with a newer (bright, in this radar image) lava flow coming from the right, where a volcano lies just out of the picture. Some bright wrinkles on the darker lava plain indicate where the crust buckled under expansion, possibly due to climactic variation under the dense carbon dioxide atmosphere. Earth experiences more variety of surface re-working than any other solid body in the solar system. This image, of Cleveland, Ohio, shows Lake Erie in the upper left. The paler areas on land are the city itself, and show the way that the human presence manifests itself, even as seen from space. The darker areas around that are rural areas, cleared for agriculture in the past two centuries. The area was scraped flat by glaciers, which also created the lake, about ten thousand years ago. Plate tectonics, glaciation, and erosion by wind and water re-work the surface on short time scales, and leave no trace of impact cratering except in very few examples of extremely recent (geologically speaking) impacts. The surface of Mars is intermediate in age. The original crust was paved over with lava in many places, but is still roughly three billion years old over large portions of the planet. Water erosion seems to have been very active in the distant past, but is not now. Whether the channels in the picture above were due to long-term effects of a wet climate, or the catastrophic effects of one or a few releases of large volumes of water is not known. Wind and dust cause a slight degree of erosion even to the present day, but much less so than on Earth. A few impact craters indicate that this surface is in the billions of years old, while some areas on Mars are much younger, having been paved over with fresh lava in the past hundreds of million years, and are almost crater-free. Io is the most volcanic body in the solar system, due to heating generated by the tidal strain Jupiter places on Io. Material ejected from volcanos falls on and flows over large portions of the surface, which changes over very short time scales. No impact craters have been detected anywhere on Io. This image has a diagonal stripe at 280m resolution in approximate color, set in true color at lower resolution. It depicts a lava flow from one of two vents at a site called Prometheus. The variation in background color owes to different layers of older flows and "snowing out" of residue forced upward in earlier explosions. Europa The image of Ganymede shows the work of tectonic forces. The surface of Ganymede apparently slid apart in this location, with the upper half of the image moving upward, and the lower half moving downward. The horizontal lines across the image are the evidence for this, and were perhaps formed by subsurface liquid water or icy slush rising up to fill the cracks, and then re-hardening. The reason for the crust spreading apart in the first place may be due to expansion of the entire globe as liquid water froze. Tidal forces from Jupiter may also be involved. Impact craters lie on top of the horizontal lines, and were therefore created later. The number of craters indicates that the tectonic events in the area happened early in Ganymede's history. The large crater which dominates the picture also has a partial ray pattern around it, indicating that it brought up brighter material from below which fell back down onto the surface. Callisto's surface here shows two forces at work. Most obvious is impact cratering, which has saturated the area and shows that Callisto's surface has not undergone any other massive change since the beginning of its history. However, closer examination shows that there are no very small craters on the surface.