Tectonics of Io

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Tectonics of Io

By Dave Parmelee 4/21/05
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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 Earths
• 421,600 km from Jupiter
• 5.20 AU from Sun
• Mean surface temp 135K

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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 gt1 km in
  diameter that is not obviously impact-generated
  (also referred to as a caldera)

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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

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Source of heat

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

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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 Ios mountains

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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

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Model 1 Subsidence-induced stress

• Constant resurfacing buries the old crust,
  causing horizontal compression (boiled egg
  analogy)
• ssubsidence E/(1-?) z/R
• This causes pervasive fracturing in the
  lithosphere below a depth of 4 km
• Stress is relieved by thrust faulting

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Model 2 Thermal-induced stress

• Theoryresurfacing rates fluctuate over time and
  space a decrease leads to heat build-up in the
  lower lithosphere, causing thermal expansion
• sthermal Ea?T/(1-?)
• Requires dramatic fluctuations in resurfacing
  rates to cause a stress of any significance

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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.)

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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 Ios mountains
  and making it ?V 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

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Focusing mechanism necessary

• A lithosphere that is pervasively fractured and
  undergoes thrust faulting would tend to uplift as
  parallel mountain ranges
• However Ios 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

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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
• TheoryAsthenospheric diapirs act to focus
  compressive stress in the lithosphere directly
  over the diapir head

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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

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This picture is cool