GEOMORPHOLOGY OF MOUNTAINS ON IO PROVIDES INSIGHT INTO MOUNTAIN-PATERA RELATIONSHIPS. C. H. Seeger1 and R. Cox2, Geosciences Department, Williams College, Williamstown, MA 01267; 1christina.seeg@gmail.com, 2rcox@williams.edu.

Introduction: Io’s surface is peppered with moun-tains, most of which are tall and geomorphologically dramatic. Despite the amount of volcanism on Io, however, the ~140 identified Ionian mountains are not volcanoes. They are high-standing uplifts, rising an average of 6.3 km above the plains [1]. They exhibit a range of morphologies, from flatiron-like ridges expos-ing tilted strata that are clearly tectonic in origin [2], to more enigmatic massifs that appear to have undergone significant mass wasting (Fig. 1).

Figure 1: Examples of mountains categorized by geomor-phologic state (Erosion Index, EI), with 1 being the freshest and 5 being the most degraded.

How mountain formation relates to Io's low-viscosity, plains-forming volcanism is unclear. Mapping the dis-tribution of mountains and volcanic centers suggests both local correlation (because many mountains and paterae appear as pairs) and global anticorrelation (be-cause the densest concentrations of mountains are lon-gitudinally offset 90 degrees from the densest concen-trations of volcanic centers) [3, 4]. Previous spatial analyses, however, have not considered the morpho-logic differences among mountains. Analyzing moun-tain geomorphology and relative amounts of erosion may provide a more nuanced insight into relationships between volcanism and mountain distribution.

Mountain Geomorphology and Erosion: Seismic shaking and gravity are the predominant erosional forces on Io. Fresh or newly-uplifted mountains should show high peaks and craggy bedrock topography, with little evidence for slope failure. More degraded moun-tains, which have been subject to more slope failure events—either because of their greater age or their proximity to tectonically active sites—should lack jag-ged peaks, have fewer steep slopes, and show well-developed debris aprons. Consider the morphological differences between rugged Himalayan peaks and the aged Appalachian Mountains: Ionian mountains exhib-it a similar range of morphologies.

We categorized mountains based on their geomor-phologic characteristics, and then tested for statistical relationships to mountain-patera spacing. Of the 140 identified mountains, 77 were imaged by NASA’s Gal-ileo spacecraft at sufficient resolution for geomorphic classification.

Erosion Index (EI). We developed an index ranging from 1 (“fresh” mountains, with sharp ridges and smooth, steep slopes) to 5 (very degraded mountains, lacking bedrock escarpments, dominated by hum-mocky and furrowed surfaces and more gentle slopes) (Fig. 1). The EI is qualitative, being based on visual examination of Galileo images; but we demonstrated that it is reproducible by having a group of 27 people independently classify a varied set of 25 mountains at a range of image resolutions [5].

Pixel brightness distribution. We used the ArcGIS Zonal Histogram tool to measure the distribution of pixel values in each mountain image. Mountains with numerous sharp scarps have higher proportions of ex-treme pixel-brightness values (strong contrast between bright and dark areas), whereas those with more de-graded slopes have higher proportions of mid-range

EI Geomorphology Example

1 = freshest

Very fresh: sharp ridges, smooth and steep slopes Scale bar 50 km

2

Mostly fresh: many ridges, some slightly softened; some lobes at northern end Scale bar 50 km

3

Intermediate: one defined, sharp ridge, but much material slumped into wrinkled lobate apron Scale bar 30 km

4

Mostly degraded: one softened ridge; most material slumped into large lobes Scale bar 30 km

5 = most

degraded

Very degraded: lacks cliffs or sharp features; hummocky and furrowed Scale bar 30 km

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grey pixels. A statistically significant correlation be-tween pixel-brightness values and EI further corrobo-rates the geomorphological classification scheme.

Relationship to Paterae: Statistically (p=0.03), more degraded mountains tend to be closer to pate-rae. Mountains with EI=5 are on average only 16 km away from a patera (range 0-68 km), whereas the freshest mountains (EI=1) are on average 98 km from the nearest patera (range 30-190 km). If we assume that the freshest mountains are those most recently formed, the observation that they tend to be the ones most distant from paterae suggests that mountain uplift is either decoupled from patera formation, or precedes it. On the other hand, the association be-tween paterae and degraded mountains suggests that proximity to paterae increases the rate of erosion.

Scarcity of Fresh Mountains: Io’s rapid resur-facing rate means that Ionian mountains must all be quite young. It is therefore quite surprising that so few of them are geomorphologically fresh. We found

a statistically significant (p=0.001) inverse correla-tion between EI and relief: more eroded mountains are substantially shorter, with average elevation of 8 km for EI=1 mountains, decreasing to an average of 4.6 km for those with EI=5 [5]. Fewer than 15% of Ionian mountains have EI of 1 or 2, and only 4% have EI=1. The majority of the mountains are moder-ately to substantially degraded (EI 3 to 5). This sug-gests that mass-wasting processes must be very effi-cient on Io, and overall mountain erosion rates may be quite high.

References: [1] Schenk P. M. et al. (2001) JGR,

106, 33201–33222. [2] Kirchoff M. R. and McKin-non W. B. (2009) Icarus, 201, 598-614. [3] Kirchoff, M. R. et al. (2011) Earth and Planet. Sci. Lett, 301, 22-30. [4] Jaeger, W. L. et al. (2003), J. Geophys. Res., 108, E8. [5] Seeger, C.H. (2016) BA Thesis, Williams College.

1675.pdfLunar and Planetary Science XLVIII (2017)