Geology has come a long way - less than 100 years after geology first became a profession that one could do for a living, geologists have plied their trade to the edge of the Solar System. Mapping the geology of the Solar System offers many challenges to the planetary geologist: making base maps from images taken by a spacecraft cruising past a planet at 15 km/s is like a land surveyor setting his surveying equipment up on a speeding truck and trying to get good information, and geologic field work, for the most part, has to be done robotically (at least for the near future). Familiar concepts take on new meaning in the context of the exotic landscapes, environments, and chemistries of other planets: for example, lavas can be freezing cold (those that erupted on Ariel were probably 100 oC below zero); on Io, volcanic activity is so intense that the geology there changes almost as fast as our weather does: sea level, a common reference level on Earth, is not an option on oceanless planets: weathering on many airless bodies comes from solar or interstellar weather, such as solar winds, radiation effects, and meteoroid storms. Applying our geologic knowledge, with its inherent terrestrial (Earthlike) bias, to other bodies with very different conditions is clearly a challenge.However, many of the traditional principles used by geologists for unraveling Earth's geologic history are general enough to be applicable to other planetary bodies. The methods are still valuable, even if the clues and the stories to be deciphered are much different. Studies of impact craters are of particular value in understanding and comparing the early processes throughout the Solar System. Although impact craters have not been acknowledged as important on Earth by most geologists until recently, the principles of superposition and accumulation are familiar to geologists, and application of these traditional principles to craters allows them to use craters to help establish a chronology of major geologic events for many of the planets. After the initial phase of planetary formation from the accretion of planetesimals, impacts have occurred nearly randomly, so by determining the rate at which craters accumulate, we can determine approximate ages of surfaces by analysis of the number of craters that have accumulated.
This map is intended as a comparison to U.S. Geological Survey map I-2447, "Mapping the Solar System". Some of the bodies shown on that sheet are not represented here because there is insufficient data to derive even a rudimentary geologic map. The geologic maps that are shown here do not represent the true complexity of those bodies for one of two reasons: some bodies are undoubtedly more detailed, but we do not have enough data to map them better; while for other bodies more detailed information is known than could be presented at this scale. Where spacecraft data are not of sufficient resolution to allow global mapping of the geology, low-resolution shaded relief airbrush map data are shown. The large gaseous planets do not seem to have distinct mappable surfaces, therefore they are shown here for reference only. Although an adequate overview of the geology of each planet and satellite shown cannot be given in the space available here, the interested reader can explore more information from the various printed and on-line sources available.
1) For other information such as mass, gravity, and orbit data, see companion sheet "Mapping the Solar System" (USGS map I-2447).
2) Exploration phases: 1, reconnaissance-telescopic study, spacecraft flybys.
2, exploration-orbiters, landers, initial sampling, rovers, global mapping, gravity mapping, and so forth.
3, intensive study-detailed, methodic, global sampling, interior studies, manned exploration.
3) Geologic activity observed: v-volcanism; t-tectonism; g-geothermal; e-erosion; ?-possible activity.
4) Only indicated for bodies with geologically significant atmosphere.
5) Value for radius of Callisto that is printed on map I-2447 is in error. Correct radius is 2404 km.
6) Varies due to eccentric orbit.