Tectonics of Io

By Dave Parmelee

4/21/05

Io Statistics

• 3rd largest moon of Jupiter• Discovered in 1610 by

Marius and Galileo• Radius = 1,815 km• Density = 3.57 g/cm3

• Surface gravity is 18.3% of Earth’s

• 421,600 km from Jupiter• 5.20 AU from Sun• Mean surface temp: ~135K

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)

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

Source of heat

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

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

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

Model 1: Subsidence-induced stress

• Constant resurfacing buries the old crust, causing horizontal compression (boiled egg analogy)

σsubsidence = E/(1-υ) * z/R

• This causes pervasive fracturing in the lithosphere below a depth of ~4 km

• Stress is relieved by thrust faulting

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

σthermal = EαΔT/(1-υ)

• Requires dramatic fluctuations in resurfacing rates to cause a stress of any significance

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.)

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

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

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

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

This picture is cool