Radar images and other data acquired by the Magellan spacecraft make an exciting new venture possible: the geologic mapping of the planet Venus. Such mapping will form a basis for determining the planet's geologic history and understanding its geologic processes. The Venus Geologic Mapping (VMAP) program has been instituted to fulfill these goals and to conduct complete systematic scientific investigations of Venus. These complete studies will result in a set of published geologic quadrangle maps of all of Venus. VMAP is the most ambitious mapping program in terms of size and complexity yet attempted by planetary geologists. The VMAP program is made possible because of the complete coverage of Venus with radar images, altimetry, and data on physical-electrical properties.
The Magellan spacecraft was launched from Kennedy Space Center in Florida on 4 May 1989, entered orbit around Venus on August 10, 1990, and acquired the first radar images 6 days later. The radar is virtually unaffected by the thick Venusian atmosphere, and the radar images have a higher resolution (120-360 m) than has been achieved by Earth-based or other spacecraft missions. Magellan images, altimetry, and data on physical-electrical properties have revealed unparalleled details of mountain ranges, high plateaus, volcanoes, vast volcanic plains, lava flows, and areas of extensively deformed crust.
The map series consists of 62 quadrangles at 1:5,000,000 scale to be published in the Miscellaneous Investigations Map Series of the USGS Astrogeology Research Program (fig. 1). This VMAP program has been sponsored initially by NASA's Venus Data Analysis Program (VDAP), followed by NASA's Planetary Geology and Geophysics Program (PGG). These programs are administered by NASA Headquarters and coordinated by the USGS Astrogeology Research Program (Appendix A). Initial map proposals in 1992 were reviewed by the VDAP review panel; subsequent new proposals have been reviewed by the Lunar and Planetary Geoscience Review Panel. Mapping progress, workshop organization, and science issues are overseen by the VMAP Steering Group (Appendix A). USGS staff are responsible for base-map production and distribution, map editing, geographic name assignments, and geologic map production.
The basic products of the VMAP program are the 1:5,000,000-scale geologic maps of quadrangles, which will provide meaningful descriptions of the geology of the quadrangles and support various interpretive topical studies. Adherence to established mapping principles should preserve the value of the map despite the inevitable progress in geologic interpretation. Of course, such principles can be applied through various creative approaches. Geologic mapping is no mere mechanical exercise, because it requires interpretation of the distributions of backscatter and landforms portrayed by radar images as laterally continuous rock units formed by processes consistent with the image and other data; a geologic history, based on relative ages, is established using superposition and intersection relations among the rock units.
This handbook has been produced to guide geologic mappers in the VMAP program to meet the following challenges: (1) to gain an understanding of Magellan data and their bearing on the interpretation of geologic terrain, (2) to facilitate the application of conventional and special planetary mapping techniques to Venus, and (3) to promote continued usage of USGS guidelines for map publication. To first order, geologic mappers will employ standard photogeologic techniques on synthetic aperture radar (SAR) backscatter mosaics; refinements will result from analyses of high-resolution images, altimetry, and data on physical-electrical properties. Mappers will no doubt encounter specific mapping problems not addressed here that will require their own creativity and judgment to resolve (at times with assistance from others engaged in geologic studies of Venus).
In addition to this handbook, geologic mappers should refer to the guides to Magellan image interpretation by Ford and others (1989, 1993) and Michaels (1992), the special issues of JGR-Planets, "Magellan at Venus" (1992, v. 97, nos. E8 and E10), and "Suggestions to Authors of the Reports of the United States Geological Survey," edited by Hansen (1991). These publications will provide more complete and detailed technical information, illustrative figures, and style guidelines than are supplied in this handbook and will assist in understanding Magellan radar data and in producing geologic maps of Venus. Other relevant publications are offered in our "Recommended Reading" list.
We anticipate that mapping techniques and guidelines will evolve. We will appreciate suggestions for updates and additions to be included in future versions.
Figure 1. Index of 1:5,000,000-scale quadrangles of Venus.
Table of Contents
|Planetary Geologic Mapping|
|USGS Astrogeology Research Program||NASA|