1. Tectonics of Io By Dave Parmelee 4/21/05

2. Io Statistics 3 rd largest moon of Jupiter Discovered in 1610 by Marius and Galileo Radius = 1,815 km Density = 3.57 g/cm 3 Surface gravity is 18.3% of Earth’s 421,600 km from Jupiter 5.20 AU from Sun Mean surface temp: ~135K

3. More info Core composed of Fe and FeS extends half way to surface No visible impact craters  young surface Most volcanically active body in the Solar System Generates twice as much heat as Earth Surface characterized by mountains and paterae –Patera – a topographic depression >1 km in diameter that is not obviously impact-generated (also referred to as a caldera)

4. Volcanism on Io At least 120 active volcanoes on Io Volcanic features: plumes, lava lakes, lava flows Signature volcano landform is the patera Volcanoes on Io do not form tall mountains –Low viscosity lava –Pyroclasts blow away

5. Source of heat Gravitational pull from Jupiter creates intense tidal bulges on Io. Orbital resonance due to Europa prevents this system from reaching equilibrium.

6. Continual resurfacing Intense tidal friction  High levels of volcanism  Continual resurfacing –Between 1979 and 1996, about a dozen areas on Io the size of Connecticut were resurfaced –Resurfacing on Io is estimated to occur at a rate of 1 cm/year –Resurfacing rates play a critical role in the formation of Io’s mountains

7. Tectonic features Tectonics on Earth  horizontal Tectonics on Io  vertical 149 mountains discovered on Io; 165 estimated to exist –Almost all of these are tectonic, not volcanic A statistical correlation exists between location of mountains and paterae, suggesting a link between tectonism and volcanism on Io

8. Model 1: Subsidence-induced stress Constant resurfacing buries the old crust, causing horizontal compression (boiled egg analogy) σ subsidence = E/(1-υ) * z/R This causes pervasive fracturing in the lithosphere below a depth of ~4 km Stress is relieved by thrust faulting

9. Model 2: Thermal-induced stress Theory—resurfacing rates fluctuate over time and space; a decrease leads to heat build-up in the lower lithosphere, causing thermal expansion σ thermal = EαΔT/(1-υ) Requires dramatic fluctuations in resurfacing rates to cause a stress of any significance

10. Comparison of stresses In reality, it is probably a combination of the stresses that drives crustal uplift, although unless the lithosphere is extremely thin or resurfacing rates are much slower than estimated, subsidence-related compressional stress plays a larger role than thermal expansion. (Figure from Jaeger et al. 2003.)

11. Lithosphere thickness The base of the lithosphere on Io is assumed to be at the 1500 K isotherm By estimating the volume of all of Io’s mountains and making it ΔV for the entire lithosphere, we can find the depth of the 1500 K isotherm –This is a minimum value The value comes out to ~12 km –At this thickness, subsidence causes the primary stress, but stress due to thermal expansion is significant

12. Focusing mechanism necessary A lithosphere that is pervasively fractured and undergoes thrust faulting would tend to uplift as parallel mountain ranges However Io’s mountains are isolated and randomly distributed Therefore a stress-focusing mechanism is necessary to focus stress in the lithosphere at a point and prevent uplift of parallel ridges

13. The role of hot spots Large number of volcanic centers supports idea of many hot spots Association of many mountains with paterae (volcanic centers) suggests connection between hot spots and tectonic uplift Theory—Asthenospheric diapirs act to focus compressive stress in the lithosphere directly over the diapir head

14. Conclusions There is still much speculation regarding tectonism on Io The primary cause of stress is probably subsidence-related horizontal compression Thermal expansion also causes a non-negligible stress Evidence supports but does not prove the theory that lithospheric stress is focused by diapirs

15. This picture is cool