NotesTopographic evidence for shield volcanism on Io
Abstract
A shield volcano has been identified on Io, based on photoclinometrically determined slope values and planimetric presentation. The slope values (typically 10°) and 2.5 km height of the volcano imply that it is composed of a material with the mechanical properties of basaltic rock. The height of the volcano may also indicate a minimum value of ∼40 km for the thickness of the local lithosphere.
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Cited by (15)
Volcanism on Io: New insights from global geologic mapping
2011, IcarusCitation Excerpt :Most shield volcanoes (e.g., Ra Patera) on Io typically have very shallow slopes of 0.1° and maximum elevations of ∼2 km. Steeper-sided shield volcanoes are rare; only 3 have been identified with surface slopes greater than 1°, two at Zamama (Schenk et al., 2004; Williams et al., 2005) and one within the Pele ring deposit (Moore et al., 1986). Diffuse deposits (DD, Fig. 2b) mantle underlying topography in a manner characteristic of fine-grained fragmental material, and typically occur on, near, or around active volcanic centers.
We produced the first complete, 1:15 M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo–Voyager image mosaics produced at a spatial resolution of 1 km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red–brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO2 + contaminants) occur mostly in the equatorial antijovian region (±30°, 90–230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red–brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10 km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45–75°N, ∼60–120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005–33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201–33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22–30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO2. The much greater areal extent of gas-derived diffuse deposits (red + white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra) + yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200 m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions.
Stereo and photoclinometry derived topography of shield-like volcanoes on Io indicate little relief (<3 km) and very low slopes (0.2° to 0.6°). Several shield volcanoes appear to be associated with broad rises of 1 to 3 km, but only 5 shield volcanoes have been identified with steep flank slopes (between 4° and 10°). These steep slopes are restricted to within 20–30 km of the summit, but where discernable, most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree. Despite their abundance, ionian shield volcanoes are among the flattest in the Solar System. The steepest volcanoes on Io are most comparable to large venusian shield volcanoes. Using simplistic Bingham rheologies we estimate the viscosity and yield strengths of ionian lavas. Yield strengths are estimated at 101–102 Pa, lower than most basaltic lavas. Viscosity estimates range from 103 to 105 Pa s, although these are probably upper limits. Actual values may have been as low as 100 Pa s. Viscosity is sensitive to flow velocity, which is poorly known on Io. The best constraint on flow velocity comes from observations of the 1997 Pillan eruption, which bracket the eruptive phase to 132 day maximum, and more probably less than 50 days. Low slopes, long run-out distances and our estimated rheologic properties are consistent with (but not proof of) a low silica, low viscosity, high temperature composition for ionian lavas, supporting arguments for low-silica lava compositions such as basalt or komatiite. We cannot eliminate sulfur on rheologic grounds, however.
We have used Galileo spacecraft data to produce a geomorphologic map of the Culann–Tohil region of Io's antijovian hemisphere. This region includes a newly discovered shield volcano, Tsũi Goab Tholus and a neighboring bright flow field, Tsũi Goab Fluctus, the active Culann Patera and the enigmatic Tohil Mons-Radegast Patera–Tohil Patera complex. Analysis of Voyager global color and Galileo Solid-State Imaging (SSI) high-resolution, regional (50–330 m/pixel), and global color (1.4 km/pixel) images, along with available Galileo Near-Infrared Mapping Spectrometer (NIMS) data, suggests that 16 distinct geologic units can be defined and characterized in this region, including 5 types of diffuse deposits. Tsũi Goab Fluctus is the center of a low-temperature hotspot detected by NIMS late during the Galileo mission, and could represent the best case for active effusive sulfur volcanism detected by Galileo. The Culann volcanic center has produced a range of explosive and effusive deposits, including an outer yellowish ring of enhanced sulfur dioxide (SO2), an inner red ring of SO2 with short-chain sulfur (S3–S4) contaminants, and two irregular green diffuse deposits (one in Tohil Patera) apparently produced by the interaction of dark, silicate lava flows with sulfurous contaminants ballistically-emplaced from Culann's eruption plume(s). Fresh and red-mantled dark lava flows west of the Culann vent can be contrasted with unusual red–brown flows east of the vent. These red–brown flows have a distinct color that is suggestive of a compositional difference, although whether this is due to surface alteration or distinct lava compositions cannot be determined. The main massif of Tohil Mons is covered with ridges and grooves, defining a unit of tectonically disrupted crustal materials. Tohil Mons also contains a younger unit of mottled crustal materials that were displaced by mass wasting processes. Neighboring Radegast Patera contains a NIMS hotspot and a young lava lake of dark silicate flows, whereas the southwest portion of Tohil Patera contains white flow-like units, perhaps consisting of ‘ponds’ of effusively emplaced SO2. From 0°–15° S the hummocky bright plains unit away from volcanic centers contains scarps, grooves, pits, graben, and channel-like features, some of which have been modified by erosion. Although the most active volcanic centers appear to be found in structural lows (as indicated by mapping of scarps), DEMs derived from stereo images show that, with the exception of Tohil Mons, there is less than 1 km of relief in the Culann–Tohil region. There is no discernable correlation between centers of active volcanism and topography.
Topography of Small Martian Valleys
1993, IcarusSmall valleys are common in the ancient highlands of Mars. Valley widths are typically a few kilometers and are independent of length for valleys longer than about 50 km. Topographic profiles across several representative small valleys, produced using one-dimensional photoclinometry, have smoothly curved shapes and valley-side slopes of 10°-20°. Profiles of unusual valleys from Maumee Valles, the flanks of Alba Patera, and "softened" terrain also have slopes of 10°-20°, but the profiles of Maumee Valles are more V-shaped than those of the typical small valleys, while the profiles of the "softened" valley have proportionately longer upslope convexities. Depths from 15 to 800 m are measured for the small martian valleys and most valleys have depths between 20 and 250 m. The mean depth-to-width ratio from 135 measurements of 45 different valleys is 0.08 (standard deviation 0.04). When valley profiles with uneven rims are excluded, the mean depth-to-width ratio is 0.07 (standard deviation 0.03). Applying the lower ratio to the widths of a larger sample of small valleys yields depths that are almost all between 40 and 300 m. The majority of the small valleys we have studied are probably the result of groundwater sapping, and in this context the measured valley depths imply that liquid water was often less than a few hundred meters below the surface in the vicinity of these valleys at the time early in martian history when the valleys formed.
Least-squared fits to the brightness profiles across a disk or “limb darkening” described by Hapke's photometric function are found for the simpler Minnaert and lunar-Lambert functions. The simpler functions are needed to reduce the number of unknown parameters in photoclinometry, especially to distinguish the brightness variations of the surface materials from that due to the resolved topography. The limb darkening varies with the Hapke parameters for macroscopic roughness , the single-scattering albedo (w), and the asymmetry factor of the particle phase function (g). Both of the simpler functions generally provide good matches to the limb darkening described by Hapke's function, but the lunar-Lambert function is superior when viewing angles are high and when is less than 30°. Although a nonunique solution for the Minnaert function at high phase angles has been described for smooth surfaces, the discrepancy decreases with increasing and virtually disappears when reaches 30° to 40°. The variation in limb darkening with w and g, pronounced for smooth surfaces, is reduced or eliminated when the Hapke parameters are in the range typical of most planetary surfaces; this result simplifies the problem of photoclinometry across terrains with variable surface materials. The Minnaert or lunar-Lambert fits to published Hapke models will give photoclinometric solutions that are very similar (>1° slope discrepancy) to the Hapke-function solutions for nearly all of the bodies and terrains thus far modeled by Hapke's function.
The surface of Io: A new model
1989, IcarusThe spectral properties of Io can be duplicated by combinations of basalt and condensates of SO2 and its dissociation products, polysulfur oxide and S2O, without elemental sulfur. The arguments concerning elemental sulfur on Io are reviewed and it is concluded that this material probably is not present in spectrally significant amounts. These results imply that the hot spots and plumes probably are silicate volcanoes and SO2 fumaroles and the dark caldera are fresh mafic silicate extrusives. It is suggested that the exposed surface consists of mafic silicates partially covered with thin deposits of SO2, polysulfur oxide, and S2O. A model for the distribution and state of SO2 that is consistent with observations is one in which most of the spectrally active frost occurs as thin, ephemeral, partial coatings on the topmost particles of the regolith. The composition of the torus can be explained without requiring unusual Na compounds on the surface. Several features of Io's spectrum are also displayed by Europa, although weakly. It is suggested that they have the same source as on Io: endogenic SO2, polysulfur oxide, and S2O, rather than magnetospherically imbedded sulfur from Io. The leading-trailing side asymmetries on both bodies can be explained by preferential magnetospheric sputter erosion of the condensates, rather than ion imbedding.