Post contributed by Oliver White, Search for Extraterrestial Intelligence Institute.
Io is the innermost Galilean satellite of Jupiter, and with over 400 active volcanoes, it is the most volcanically active body in the Solar System, a consequence of tidal heating from friction generated within its interior as it is pulled between Jupiter and other Galilean satellites. Constant volcanic resurfacing means that the oldest surface is likely not more than a million years old (Williams et al., 2011). No instrumentation specifically designed to measure topography has ever been deployed to Io, but White et al. (2014) constructed a global digital elevation model (DEM) covering ~75% of Io’s surface from all available stereo coverage in Voyager and Galileo imaging, and controlled it using Galileo limb profiles (Image 1). This map represents a continuous topographic dataset that has revealed topographic variations not otherwise apparent in Voyager and Galileo imaging and limb profiles, and which may be correlated with geologic units (e.g. Williams et al., 2011). While not providing coverage across the entire globe, the map stands as the most comprehensive continuous topographic data set of Io’s surface, at least until a spacecraft arrives at Io with a dedicated photogrammetric camera or a laser altimeter on board.
Image 1: Global stereo DEM overlain on a mosaic of Voyager and Galileo images in simple cylindrical projection at 2 km/pixel. Gaps in the DEM represent masked noise or absence of stereo coverage. A broad smoothing filter has been applied to the plains areas of the DEM post-mosaicking but not to comparatively high relief features such as mountains, layered plains, and some paterae (volcanic craters). The width of the map at the equator is 11,445 km.
White et al. (2014) used this map to identify a longitudinal arrangement of basins and swells that, as far as can be determined due to limitations in coverage, extends from 80°W to 345°W in longitude and from 30°S to 55°N in latitude, and displays a longitudinal wavelength of 130° and an amplitude reaching 1.5 km. The locations of the basins correspond well to increased spatial densities of mountains and the swells to increased spatial densities of volcanic centers, as determined by Schenk et al. (2001), Kirchoff et al. (2011), and Hamilton et al. (2013). This observation supports the hypothesis that in lower areas, the compressional stresses associated with burial, subsidence, and compression of volcanic deposits predominate and mountain formation occurs preferentially over volcanism, while elevated areas are experiencing upwelling of asthenospheric material that creates extension which offsets the compressive regime, resulting in enhanced volcanic activity.
Image 2: (a) Detail of the stereo DEM containing the depression at 25°S, 135°W. Ground tracks of profiles in (b) are indicated. (b) Topographic profiles across the depression. The cross marks the crossing point of the two profiles.
The stereo DEM has also revealed the topographic signatures of various regional-scale features. These include an ~1000 km diameter, 2 km deep depression at 25°S, 135°W (Image 2), the boundaries of which correlate well to a unit of undivided flow material mapped by Williams et al. (2011), and which may represent flows from an adjacent patera filling a preexisting topographic depression, or alternatively flows that have buried their source patera located at the nadir of the depression.
Image 3: (a) Detail of the stereo DEM containing the topographic “arc” at 42.5°S, 169.5°W. Locations of mountains and layered plains are indicated. (b) Details of Dorian and Tohil Montes in Galileo color imaging, highlighting the lineated fabric of these mountains.
A “topographic arc” comprising separate units of mountain and layered plains is identified at 42.5°S, 169.5°W (Image 3), the mountainous portions of which display a lineated fabric aligned parallel to the arc that potentially indicates a shared uplift origin for the mountains as a consequence of a regional compressive regime.
Image 4: (a) Stereo DEM and geomorphological map of Tvashtar Paterae, both overlain on Galileo image 527345700 (0.185 km/pixel) and with the ground track of the profile in (b) indicated. Bright feature at center is saturation of the Galileo image caused by intense thermal emission from the lava-fountaining episode that was observed in November 1999. (b) Topographic profile taken across Tvashtar Paterae, with the locations of the geomorphological units in the right-hand map of (a) highlighted.
Tvashtar Paterae, located at 63°N, 123.5°W, represents a rare example of high-resolution stereo data (DEM pixel scale of 0.3 km/pixel) covering a complex of comparatively small-scale paterae, and the data are sufficient to allow definition of elevations for each of the separate geomorphological units of which the complex is composed (Image 4).
Further reading:
Hamilton, C.W., C.D. Beggan, S. Still, M. Beuthe, R.M.C. Lopes, D.A. Williams, J. Radebaugh, and W. Wright (2013) Spatial distribution of volcanoes on Io: Implications for tidal heating and magma ascent. Earth Planet. Sci. Lett., 361, 272-286, doi:10.1016/j.epsl.2012.10.032.
Schenk, P.M., H. Hargitai, R. Wilson, A. McEwen, and P. Thomas (2001) The mountains of Io: Global and geological perspectives from Voyager and Galileo. J. Geophys. Res., 106, 33,201–33,222, doi:10.1029/2000JE001408.
Kirchoff, M.R., W.B. McKinnon, and P.M. Schenk (2011) Global distribution of volcanic centers and mountains on Io: Control by asthenospheric heating and implications for mountain formation. Earth Planet. Sci. Lett., 301, 22-30, doi:10.1016/j.epsl.2010.11.018.
White, O.L., P.M. Schenk, F. Nimmo, T. Hoogenboom (2014) A new stereo topographic map of Io: Implications for geology from global to local scales. J. Geophys. Res. Planets, 119, 1276-1301, doi:10.1002/2013JE004591.
Williams, D.A., L.P. Keszthelyi, D.A. Crown, J.A. Yff, W.L. Jaeger, P.M. Schenk, P.E. Geissler, and T.L. Becker (2011) Geologic map of Io: U.S. Geological Survey Scientific Investigations Map 3168, scale 1:15,000,000, 25 p.
Planetary Geomorphology Image of the Month 
Haruki Chou
/ December 1, 2017images much too small.
why page so narrow?