1. Ridges and tidal stress on Io Astr688 term project Huaning Li Dec 7,2004

2. Tidal effects on Io Reasons: Reasons: Proximity to Jupiter Proximity to Jupiter Orbital eccentricity which is forced by its resonance with Europa and Ganymede Orbital eccentricity which is forced by its resonance with Europa and Ganymede Results: Results: Tidal heating: heat source for volcanism which modifies Io’s surface on time-scales of months and years Tidal heating: heat source for volcanism which modifies Io’s surface on time-scales of months and years Tidal stress: a cycle of expansion and contraction occurs over a orbital period (42 hrs) Tidal stress: a cycle of expansion and contraction occurs over a orbital period (42 hrs)

3. Observation Galileo images cover Latitude: -50° to 50° Galileo images cover Latitude: -50° to 50° Longitude: 45° to 170°W Longitude: 45° to 170°W 28% of the available high resolution images show ridges: 29 out of 105 28% of the available high resolution images show ridges: 29 out of 105 High resolution images didn’t target at plains where ridges are mostly located. High resolution images didn’t target at plains where ridges are mostly located. Ridges are quite common on Io

5. Possible ridge formation processes I. Formed by winds on Io I. Formed by winds on Io Atmosphere pressure Atmosphere pressure Earth 10 5 pa, Mars 600 pa, Io 10 -5 pa Earth 10 5 pa, Mars 600 pa, Io 10 -5 pa Threshold friction speed vs. Wind speeds Threshold friction speed vs. Wind speeds ~20 km/s » 320 m/s ~20 km/s » 320 m/s II Formed by volcanic and tectonic activities The ridges are not oriented symmetrically with respect to nearby volcanic features The ridges are not oriented symmetrically with respect to nearby volcanic features III Formed by tidal stress There is evidence on Europa. Greenberg et al. 1998 There is evidence on Europa. Greenberg et al. 1998 Tidal stress resulted in observable surface features due to the fact that Europa has an icy lithosphere. Tidal stress resulted in observable surface features due to the fact that Europa has an icy lithosphere.

6. Ridges around Chaac patera

7. Possible ridge formation processes I. Dunes formed by winds on Io I. Dunes formed by winds on Io Atmosphere pressure Atmosphere pressure Earth 10 5 pa, Mars 600 pa, Io 10 -5 pa Earth 10 5 pa, Mars 600 pa, Io 10 -5 pa Threshold friction speed vs. Wind speeds Threshold friction speed vs. Wind speeds ~20 km/s » 320 m/s ~20 km/s » 320 m/s II Volcanic and tectonic activities The ridges are not oriented symmetrically with respect to nearby volcanic features The ridges are not oriented symmetrically with respect to nearby volcanic features III Tidal stress There is evidence on Europa that tidal stress formed observable ridges on the icy lithosphere. There is evidence on Europa that tidal stress formed observable ridges on the icy lithosphere. Greenberg et al. 1998 Greenberg et al. 1998 X X

8. Greenberg et al. 1998 Modeled tidal stress field on Europa Bond lines: tidal tension Crossed lines: ridges

9. Correlation! * Principal stresses near the equator tend to be oriented north-south and east west, as are the azimuths of the ridges observed near the equator. * Stresses at high latitude are generally oriented at more oblique angles, which also apply to the single set of ridge far from the equator. Black lines: the azimuth of the observed ridges Open circles: the location of images which do not show ridges Gray lines: the modeled background stress field. thick line – tension thick line – tension thin line - compression thin line - compression Modeled Tidal stress field on Io

10. Comparison and conclusion Io doesn’t have the same ridge formation as Europa because: Io doesn’t have the same ridge formation as Europa because: Ridges are not on icy lithosphere but in a Ridges are not on icy lithosphere but in a relatively volatile layer relatively volatile layer New material is not brought into the crust New material is not brought into the crust from below from below More high resolution observations and theoretical modeling are needed for further consideration of the correlation between ridge azimuth and tidal stress orientation on Io. More high resolution observations and theoretical modeling are needed for further consideration of the correlation between ridge azimuth and tidal stress orientation on Io.