Ridges and tidal stress on Io
Astr688 term project
Huaning Li
Dec 7,2004
Tidal effects on Io
Reasons:
Proximity to Jupiter
Orbital eccentricity which is forced by its
resonance with Europa and Ganymede
Results:
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)
Observation

Galileo images cover Latitude: -50° to 50°
Longitude: 45° to 170°W

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.
Ridges are quite common on Io
Possible ridge formation processes
I. Formed by winds on Io


Atmosphere pressure
Earth 105 pa, Mars 600 pa, Io 10-5 pa
Threshold friction speed vs. Wind speeds
~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
III Formed by tidal stress


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.
Ridges around Chaac patera
Possible ridge formation processes
I. Dunes formed by winds on Io X


Atmosphere pressure
Earth 105 pa, Mars 600 pa, Io 10-5 pa
Threshold friction speed vs. Wind speeds
~20 km/s » 320 m/s
II Volcanic and tectonic activities X

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 .
Greenberg et al. 1998
Greenberg et al. 1998
Modeled tidal stress field on Europa
Bond lines: tidal tension
Crossed lines: ridges
Modeled Tidal stress field on Io
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
thin line - compression
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.
Comparison and conclusion


Io doesn’t have the same ridge formation as
Europa because:
Ridges are not on icy lithosphere but in a
relatively volatile layer
New material is not brought into the crust
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.