Evidence for subduction in the ice shell of Europa

Journal name:
Nature Geoscience
Volume:
7,
Pages:
762–767
Year published:
DOI:
doi:10.1038/ngeo2245
Received
Accepted
Published online

Abstract

Jupiters icy moon Europa has one of the youngest planetary surfaces in the Solar System, implying rapid recycling by some mechanism. Despite ubiquitous extension and creation of new surface area at dilational bands that resemble terrestrial mid-ocean spreading zones, there is little evidence of large-scale contraction to balance the observed extension or to recycle ageing terrains. We address this enigma by presenting several lines of evidence that subduction may be recycling surface material into the interior of Europas ice shell. Using Galileo spacecraft images, we produce a tectonic reconstruction of geologic features across a 134,000 km2 region of Europa and find, in addition to dilational band spreading, evidence for transform motions along prominent strike-slip faults, as well as the removal of approximately 20,000 km2 of the surface along a discrete tabular zone. We interpret this zone as a subduction-like convergent boundary that abruptly truncates older geological features and is flanked by potential cryolavas on the overriding ice. We propose that Europas ice shell has a brittle, mobile, plate-like system above convecting warmer ice. Hence, Europa may be the only Solar System body other than Earth to exhibit a system of plate tectonics.

At a glance

Figures

  1. Study region.
    Figure 1: Study region.

    a, False-colour image of Europas northern trailing hemisphere. Pink-hued, tabular zones (arrowed) are convergent bands and potential sites of subduction. Area of (b) outlined by wide dashed lines. Area of Fig. 2 outlined by smaller dashed lines. b, Regional extent of tabular zones (white dashed areas) in visible light images. Named lineae also shown. E15RegMap01 images (228 m pix−1) superposed on a global Mercator basemap (1 km pix−1). Images from Images from NASA/JPL/University of Arizona

  2. Interpretation of prominent geologic units and tectonic reconstruction.
    Figure 2: Interpretation of prominent geologic units and tectonic reconstruction.

    a, Tabular zones (yellow) represent contractional deformation where subduction may have occurred and are referred to as subsumption bands. All other mapped features predate these bands (mapping in ArcGIS). Original, uninterpreted image shown in Supplementary Fig. 2. b, Original configuration of geologic features after tectonic reconstruction via the removal of transform and dilational offsets (see Supplementary Fig. 4 for the entire reconstruction sequence). A zone of missing surface area (white space) ~99 km wide occurs on the NW side of the southern subsumption band and is interpreted to have subducted. Images from NASA/JPL/University of Arizona

  3. Plate boundary morphologies.
    Figure 3: Plate boundary morphologies.

    a, Subsumption band interiors show hummocky material (black arrow) and smoother material (grey arrow) juxtaposed along sharp boundaries (dashed line). Older features terminate abruptly at the band edges (white arrow). b,c, Highly convergent transpressional boundaries (b) and predominantly transform transpressional boundaries (c) have a complex internal structure and a margin of mottled material several kilometres wide. d, The transtensional southern transform has a smooth interior. Ragged edges mark an array of left-stepping, en echelon fractures, consistent with right-lateral motion. Fault motion sense shown by black arrows in bd, which have identical scales. Locations shown in Supplementary Fig. 2. Images from NASA/JPL/University of Arizona

  4. Conceptual model for subduction.
    Figure 4: Conceptual model for subduction.

    Recycling of surface area through the subduction of a cold, brittle, outer portion of the ice shell into its warmer interior, where it is ultimately subsumed. Potential cryolavas are forced to the surface through the overriding plate. The higher density of the subducting outer ice layer obviates buoyancy-induced topographic relief at the site of subduction. Plate collision results in contractional deformation in the overriding plate along the collisional margin, creating the tabular subsumption bands.

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Author information

  1. Present address: ConocoPhillips Company, 600 N. Dairy Ashford, Houston, Texas 77079, USA.

    • Simon A. Kattenhorn

Affiliations

  1. Department of Geological Sciences, University of Idaho, 875 Perimeter Drive, MS 3022, Moscow Idaho 83844-3022, USA

    • Simon A. Kattenhorn
  2. Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel Maryland 20723, USA

    • Louise M. Prockter

Contributions

S.A.K. was responsible for the tectonic reconstructions of the study area. L.M.P. was responsible for the morphological analyses of the boundaries. Both authors contributed to the interpretation and analysis and to the preparation and finalization of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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