The first step was to survey all available images to identify all mountains on Io. Voyager-1 and Voyager-2 imaged Io in 1979, but only 45% of the surface was covered at high resolution (275°-360° longitude). Galileo, in orbit around Jupiter, has now viewed the remaining areas and will photograph selected areas at 50m/px resolution in late 1999. Together, these data provide coverage of >90% of the surface of Io at an average resolution of 2.8 km/px (ranging from 0.3 to 10 km/px).
It was necessary to process these images to make them useful for mapping. The images taken by Galileo and Voyager required different calibration. In the Voyager images it was needed to remove the resaux that blemished the images, while in the Galileo images radiation induced noise needed removal. In the case of some compressed Galileo images the artifacts caused by the compression needed correction as well. To properly map Io from the Voyager and Galileo images it was necessary to update the control net: this was done by selecting 501 match points used to register all the images simultaneously. The ISIS JIGSAW program was then used to update the pointing vectors for each image. We used these corrected images to map Io’s mountains.
Orthographic images (tiles) at a common resolution of 1 km and width of 15° were made for each mountain in order to measure and compare Io’s mountains at uniform map scale and free of viewing angle distortion. We used all available images of the same area taken at different viewing angles and lighting conditions. These different views allowed us to characterize mountain morphology with much greater confidence than using one image. These tiles were used for each measurement of length, width, area and height.
Several techniques were used to measure heights of mountains. We measured the length of the shadow cast by a mountain’s highest peak to estimate the height of the mountain; 70% of the mountains were measured by shadow height measurement. Similarly we were able to place lower limits of the heights of those mountains tall enough to project into sunlight even though their base was in shadow. Using an automated stereo correlation program , heights were estimated for those mountains viewed in stereo images. In many cases it was possible to use multiple techniques to estimate mountain heights.
We mapped a total of nearly 100 individual mountainous structures on Io. These fall into several distinct morphologic types. Most of the mountains can be classified as one of 4 major types. Plateaus (Fig. 1c), the most common type, have broad, uneven surfaces while mesas (Fig. 1d) have smooth, flat surfaces. Both of these types frequently feature a basal scarp. Ridges are characterized by linear or curvilinear ridge crests. Several mountains are characterized by two parallel ridges (Fig. 1b). Among the highest mountains are the massifs which generally have steep slopes several kilometers high and rugged peaks (Fig. 1a). Though almost the entire surface is covered by volcanic material, only few of the mountainous features have an obvious volcanic morphology or heights exceeding 2 km. The highest mountain on Io, the southern massif of Boösaule Montes, has an estimated height of 14-16 km. Up to 30 % of the mountains show evidence for erosion or mass wasting, 20% for formation of bright deposits. At least 10 % of the mountains studies are pervasively striated, which may be evidence of layering or fracturing.
Roughly 3 percent of the surface of Io is covered by mountains. The distribution of the mountains is not uniform across the surface (Fig. 2). Although the distribution might be random, at least two concentrations of mountains appear to occur: these are observed at 20°, 45° and at -20°, 225°, which are roughly antipodal to each other (Fig. 2b). One of these concentrations is centered near the large volcanic plume Pele. The region with the lowest concentration of mountains is associated with Colchis Regio, a region of smooth bright plains.
The greatest concentrations of mountains are also roughly 90° offset from the current sub- and anti-Jovian hemispheres (Fig. 3). They may also be 90° offset from the regions of greatest concentration of volcanic centers on Io (Fig. 3a, b). Although more work remains to be done, these results suggest that volcanic activity and mountain formation may be anticorrelated.
Our morphologic study suggests that there may several mechanisms responsible for the formation of mountains. The apparent anti-correlation of mountain distribution with volcanic centers places important constraints on possible mountain formation mechanisms. Although generally anti-correlated, the style of volcanism and the distribution of each morphologic type of mountain is not uniform across the surface of Io, and we have not yet taken this into account. Much work remains to be done in analyzing this extensive data set, which we anticipate will be useful in developing and evaluating tests for models proposed to explain mountain formation on Io.
 Schaber, G. G. (1982) Geology of Io. in: Satellites of Jupiter, pp. 556-597, Univ. Arizona Press, Tucson.
 Carr, M., presented at Io in the Galileo Era conference, Flagstaff, AZ, 1997.
 Keszthelyi, L and McEwen, A. (1997) Magmatic Differentiation of Io Icarus, 130, 437- 448.
 Schenk, P. M. and Bulmer, M. H. (1998) Origin of Mountains on Io by Thrust Faulting and Large-Scale Mass Movements Science, 279, 1514-1517.
 Schenk, P. M. et al. (1997) Geology and topography of Ra Patera, Io, in the Voyager era: Prelude to eruption Geophys. Res. Lett., 20, 2467-2470. "
The work continued after these preliminary
results and was published in the Journal of Geophysical research (Vol 106,
E12, Pp 33201-33222) it issue dated to December 25, 2001, in the aricle
The Mountains of Io: Global and geological perspectives from Voyager and
Galileo, by Paul Schenk, Henrik Hargitai, Ronda Wilson, Alfred McEwen and
Peter Thomas. For this article, the original database was revised, corrected
and new data was added.
Since that time, new images were taken and bringing up new ideas. We are intended to include the newest data in this online database, so this site is not only a electronic version of the revised 2001 database, but it is continousely updated.
Fig. 1.: The first manuscript of the abstract quoted above
Fig. 2.: The offline database on LPI's server in 1998 (sample page)