GEOMORPHOLOGY OF MOUNTAINS ON IO PROVIDES INSIGHTS INTO MOUNTAIN-PATERA RELATIONSHIPS

Io, the solar system’s most volcanically active body, also has distinctive mountains, most of which are not volcanoes. The majority of Io’s lavas emanate from low-relief paterae and form extensive flows rather than edifices, whereas the ~140 Ionian mountains are high-standing massifs, rising several km above the plains. The mountains appear to be tectonic in origin, and the connection between them and Io's volcanism is unclear. We used a geomorphologic approach to try and address this question.

Seismic activity and gravity are the predominant erosional forces on Io. Fresh or newly-uplifted mountains should maintain high peaks and craggy bedrock topography, with little evidence for slope failure. More degraded mountains should lack jagged peaks, have fewer steep slopes, and show well-developed debris aprons. Of the 140 identified mountains, 71 were imaged at sufficient resolution for geomorphic classification.

We developed an erosion index (EI) ranging from 1 (“fresh” mountains, with sharp ridges, smooth and steep slopes) to 5 (very degraded mountains, lacking bedrock escarpments, dominated by hummocky and furrowed surfaces and more gentle slopes). The index is qualitative in nature, 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.

Few Ionian mountains are pristine. Fewer than 15% had EI of 1 or 2, and only 4% had EI=1. The majority of the mountains were moderately to substantially degraded (EI 3 to 5). We found a statistically significant (p=0.001) inverse correlation 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.

Statistically (p=0.03) the more degraded mountains tend to be closer to paterae. Mountains with EI=5 are on average only 16 km away (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 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.