The mountains of Io: Global and geological perspectives from Voyager and Galileo

Abstract

Abstract. To search for local and global scale geologic associations that may be related to the internal dynamics of Io, we have completed a global catalog of all mountains and volcanic centers. We have identified 115 mountain structures (covering -3% of the surface) and 541 volcanic centers, including paterae (calderas and dark spots) and shield volcanoes. The average length of an Ionian mountain is 157 km, with the longest being 570 km. The mean height of Ionian mountains is 6.3 km, and the highest known mountain is Bo6saule Montes (17.5 +3 km). Five basic morphologic types of mountains have been identified; mesa, plateau, peak, ridge, and massif. Very few mountains bear any physical similarity to classic volcanic landforms, but many resemble flatiron mountains on Earth and are interpreted as tilted crustal blocks. This would be consistent with the hypothesis that most mountains are thrust blocks formed as a result of compressive stresses built up in the lower crust due to the global subsidence of volcanic layers as they are buried over time [Schenk and Bulmer, 1998]. More than one mechanism may be responsible for all Ionian mountains, however. The proximity of some mountains to paterae may indicate a direct link between some mountains and volcanism, although it is not always clear which came first. In contrast to earlier studies, a pronounced bimodal pattern is observed in the global distribution of both mountains and volcanic centers. The regions of highest areal densities of volcanic centers are near the sub-and anti-Jovian regions, but are offset roughly 90 ø in longitude from the two regions of greatest concentration of mountains. This anticorrelation may indicate the overprinting of a second stress field on the global compressive stresses due to subsidence. The bimodal distribution of volcanic centers and mountains is consistent with models of asthenospheric tidal heating and internal convection developed by Tackley et al. [2001]. Over regions of mantle upwelling, compressive stresses in the lower crust induced by global subsidence might be reduced, encouraging volcanism and discouraging mountain building. In regions of mantle downwelling, these compressive stresses in the lower crust might be increased, discouraging volcanism and encouraging mountain building. Alternatively, the global pattern may be related to possible (but undocumented) nonsynchronous rotation of Io, which would produce two regions each of compression and extension in the crust. Evidence of layering and of mass wasting, including landslides, block sliding, debris aprons and downslope creep, on Ionian mountains suggests that the crust of Io is essentially a layered stack of partially consolidated volcanic lavas and plume deposits, becoming more consolidated with depth. The lower crust especially may also be ductily deformed, punctuated by volcanic intrusions and faulting at paterae, and broken into blocks, some of which have been uplifted to form mountains.

Three-Dimensional Simulations of Mantle Convection in Io

“…Unpublished global surveys by Schenk and Hargitai (1998) and Schenk et al (2000) also support such a distribution (Fig. 10a).…”

The Thermal State of Io

“…The actual three-dimensional distribution of tidal heating in Io will lead to slower mantle flow driven largely by lateral gradients in melt fraction. This may explain the anti-correlation between mountains and calderae (Schenk et al 2001), since calderae will be located preferentially over upwellings while mountains will tend to be found over downwellings, where compressional stresses are highest. This slow flow will probably contribute little to the heat flow, but may be important for recycling lithospheric material back into the mantle.…”

Lineations and structural mapping of Io's paterae and mountains: Implications for internal stresses

“…Many appear to be heavily influenced by mass wasting. The average length of the 115 mountains on Io is 157 km and the mean height is 6.3 km, with the highest peak, Boösaule Montes, rising 17.5± 3 km above the surface (Schenk et al, 2001). A possible formation mechanism for these high mountains includes deep thrust faulting and rotation of blocks due to compression from loading of the crust by erupted material (Schenk and Bulmer, 1998).…”

“…This has enabled accumulation of vast stores of internal heat (Peale et al, 1979) and has made Io the most volcanically active object in the solar system (Davies, 2001), evidenced by abundant lava flows and lava fountains (Radebaugh et al, 2001). Io also contains some of the largest mountains in the solar system, supported by a silicate-rich crust interlayered with sulfurous frosts (Schenk et al, 2001). Chalcophile elements and mafic lavas give Io its distinctive red, black, and yellow colors (Kargel et al, 1999).…”