Io: General Overview
21 February 1996
- S compounds dominate the surface of Io, as volcanic processes dominate the crustal evolution.
- The surface is very young - no craters seen.
- The height of many of the features on the surface (>9 km high mountains and scarps 150-1700 m high) suggest that sulfur is not the only component in the Io crust - a stronger material such as silicate is required.
- Focussed in equatorial zone
- 10 cm/yr resurfacing rate - very high!
- Mountains: These are the oldest features, more rugged surface than other units. Up to 9-10 km high, suggesting a strong material composition. No chains, don't look like volcanic constructs. Are they just tops of a widely varying topography that is now buried under sulfur?
- Plains: Three types
- intervent (40% of area mapped) probably frozen fallout from plume activity. Composed of a mix of S and SO2, same composition as other plains units.
- layered - smoother than intervent, with boundary scarps. These are probably erosional limits bounded by faults.
- Volatile related collapsed layered units
- Vents: pit craters, shields, fissures, crater cones. 200 calderas w/ dia > 20 km, whereas Earth has maybe 15.
- Other: Plumes (consistent, ejecting at 0.5-1 km/s, some over 300 km high and 1,400 km across).
- Scarps associated with calderas and flow fronts
- normal faults and graben trending NW, NE (due to tidal f]exing). Most evident where they disrupt old mountain features.
- Irregular, fretted (Erosional) scarps: Theory suggests scarp due to SO2 frost sapping. Change of phase for at least S and possibly SO2. (SO2 solid down to 1 km, while at 1 km S is molten, but between 2-4 km, both phases may be molten)
- Surface dominated by S compounds, but internal structure probably has much silicate
- Surface composition is a thin veneer of sulfur-rich compounds overlying a silicate-rich planet
- Basins alternate with topographic swells or high points, suggesting tidal heating is responsible. However, there is a question as to whether high topography is associated with high heat flow or low heat flow.
- Two models suggested: 1) thermal uplift (heat=thinning of crust=uplift, so high topo=hi T) or 2) continental crust (crust=lithosphere and continents of silicates float on lo density roots so high topo=lo T)
- Thermal uplift is most attractive because it explains the distribution of S02 - basins would be cold traps for SO2 frost. This model links tidal heating, volcanism, heat flow vs. topography and SO2 distribution.
- Prediction: SO2 should be longer lived in basins (colder) so we should see changes in bright SO2 deposits only in the highlands (more volcanic activity covers other deposits).
- Tidal heating
- Jovian magnetosphere - if Io has some conductivity, currents should be induced. Internalized energy = power source
- Not enough to account for the large energy output - so large that silicates must almost certainly be a constituent in magma (900oK eruptions uggested at Pele). Are we looking at a particularly active episode of tidal energy dissapation?