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[38.03] Europa Cycloids: Key to the Chronology of Tectonics and Implications for Nonsynchronous Rotation

G.V. Hoppa, B.R. Tufts, R. Greenberg, T.A. Hurford, D. O'Brien (LPL, Univ. of Arizona)

Cycloidal features on Europa appear to form as tensile cracks in response to diurnal tides of a global ocean [1,2]. Modeling formation of specific features shows where they likely formed relative to the direction of Jupiter. Imagery of the southern hemisphere of Europa suggests that Cilicia and Delphi Flexus are the youngest ridges in the region. These cycloids are also younger than other prominent tectonic features including Astypalaea Linea. Modeling of Cilicia and Delphi Flexus suggest that these features would have formed as cracks 65^\circ and 75^\circ to the west respectively (see also [3]). Cross-cutting relationships show that a third prominent (but unnamed) cycloidal ridge is older than Delphi, but still younger than Astypalaea. Modeling of this feature suggests that it could have formed 50^\circ (modulo 180^\circ) to the west. However the cross-cutting relationships suggest that it formed before Cilicia and Delphi; thus likely 230^\circ west of its current position.

Tectonic features that are even older according to stratigraphy (e.g. Astypalaea) must date from previous cycles of nonsynchronous rotation. That result implies that an upper limit to the number of cycloids or global scale lineaments that form regionally over a nonsynchronous rotation period may be as few as 1 or 2. Moreover, that rate of crack formation is reasonable , because once a crack forms the tidal stress is relieved in its region.

This chronology may also be used to constrain formation rates of ridges and the nonsynchronous rotation period of Europa. Assuming the lower limit nonsynchronous rotation period of 12,000 yrs [4], the growth from a crack to the observed double ridges (100 m high and 2 km wide) associated with cycloidal features must take place as quickly as 2000 yrs. Alternatively, the fastest possible nonsynchronous rotation period for Europa would be 170,000 yrs, assuming estimates of double ridges formation time of 30,000 yrs [5]. What is the slowest possible nonsynchronous rotation period? Given that only ~1 or 2 cracks form per rotation in the Astypalaea region, at least 500 rotations would be required for the tectonic resurfacing that the cratering record implies happened within the past 10⁸ yrs. Therefore the nonsynchronous rotation period must be <200,000 yrs, as well as >12,000 yrs.

References: [1] Hoppa G.V. and B.R. Tufts (1999) \textit{LPSC XXX}, cdrom. [2] Hoppa, G.V. et al. (1999) \textit{Science} 285, 1899-1902.[3] Greenberg, R. et al. (2000) \textit{LPSC XXXI}, cdrom. [4] Hoppa et al. (1999) \textit{Icarus} 137, 341-347. [5] Greenberg, R. et al. (1998) \textit{Icarus} 135, 64-78.