Transcript Tectonics of Io

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
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