Ridges and tidal stress on IoRidges and tidal stress on Io

Astr688 term projectAstr688 term project

Huaning LiHuaning Li

Dec 7,2004Dec 7,2004

Tidal effects on IoTidal effects on Io

Reasons: Reasons: Proximity to Jupiter Proximity to Jupiter Orbital eccentricity which is forced by its Orbital eccentricity which is forced by its

resonance with Europa and Ganymederesonance with Europa and Ganymede

Results: Results: Tidal heating: heat source for volcanism which Tidal heating: heat source for volcanism which

modifies Io’s surface on time-scales of modifies Io’s surface on time-scales of months and years months and years

Tidal stress: a cycle of expansion and Tidal stress: a cycle of expansion and contraction occurs over a orbital period (42 contraction occurs over a orbital period (42 hrs)hrs)

ObservationObservation

Galileo images cover Latitude: -50° to 50° Galileo images cover Latitude: -50° to 50° Longitude: 45° to 170°WLongitude: 45° to 170°W

28% of the available high resolution images show 28% of the available high resolution images show ridges: 29 out of 105ridges: 29 out of 105

High resolution images didn’t target at plains High resolution images didn’t target at plains where ridges are mostly located. where ridges are mostly located.

Ridges are quite common on IoRidges are quite common on Io

Possible ridge formation processesPossible ridge formation processes

I. Formed by winds on IoI. Formed by winds on Io Atmosphere pressure Atmosphere pressure Earth 10Earth 1055 pa, Mars 600 pa, Io 10 pa, Mars 600 pa, Io 10-5-5 pa pa Threshold friction speed vs. Wind speedsThreshold friction speed vs. Wind speeds ~20 km/s » 320 m/s ~20 km/s » 320 m/s

II Formed by volcanic and tectonic activitiesII Formed by volcanic and tectonic activities The ridges are not oriented symmetrically with respect The ridges are not oriented symmetrically with respect

to nearby volcanic features to nearby volcanic features

III Formed by tidal stressIII Formed by tidal stress There is evidence on Europa. Greenberg et al. 1998There is evidence on Europa. Greenberg et al. 1998 Tidal stress resulted in observable surface features due Tidal stress resulted in observable surface features due

to the fact that Europa has an icy lithosphere. to the fact that Europa has an icy lithosphere.

Ridges around Chaac pateraRidges around Chaac patera

Possible ridge formation processesPossible ridge formation processes

I. Dunes formed by winds on IoI. Dunes formed by winds on Io Atmosphere pressure Atmosphere pressure Earth 10Earth 1055 pa, Mars 600 pa, Io 10 pa, Mars 600 pa, Io 10-5-5 pa pa Threshold friction speed vs. Wind speedsThreshold friction speed vs. Wind speeds ~20 km/s » 320 m/s ~20 km/s » 320 m/s

II Volcanic and tectonic activitiesII Volcanic and tectonic activities The ridges are not oriented symmetrically with respect The ridges are not oriented symmetrically with respect

to nearby volcanic features to nearby volcanic features

III Tidal stressIII Tidal stress There is evidence on Europa that tidal stress formed There is evidence on Europa that tidal stress formed

observable ridges on the icy lithosphere . observable ridges on the icy lithosphere . Greenberg et al. 1998Greenberg et al. 1998

X

X

Greenberg et al. 1998Greenberg et al. 1998

Modeled tidal stress field on EuropaModeled tidal stress field on Europa

Bond lines: tidal tensionBond lines: tidal tensionCrossed lines: ridgesCrossed lines: ridges

Correlation!Correlation!* Principal stresses near the equator tend to be oriented north-south and * 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. east west, as are the azimuths of the ridges observed near the equator. * Stresses at high latitude are generally oriented at more oblique angles, * Stresses at high latitude are generally oriented at more oblique angles,

which also apply to the single set of ridge far from the equator.which also apply to the single set of ridge far from the equator.

Black linesBlack lines: the azimuth of the : the azimuth of the observed ridgesobserved ridges

Open circlesOpen circles: the location of : the location of images which do not images which do not show ridges show ridges

Gray linesGray lines: the modeled : the modeled background stress field.background stress field.

thick line – tensionthick line – tension thin line - compression thin line - compression

Modeled Tidal stress field on IoModeled Tidal stress field on Io

Comparison and conclusionComparison and conclusion

Io doesn’t have the same ridge formation as Io doesn’t have the same ridge formation as Europa because:Europa because:

Ridges are not on icy lithosphere but in a Ridges are not on icy lithosphere but in a relatively volatile layerrelatively volatile layer New material is not brought into the crustNew material is not brought into the crust from belowfrom below

More high resolution observations and theoretical More high resolution observations and theoretical modeling are needed for further consideration of modeling are needed for further consideration of the correlation between ridge azimuth and tidal the correlation between ridge azimuth and tidal stress orientation on Io.stress orientation on Io.