Geomorphology and topography of Ménec Fossae and Thrace Macula on Europa: Insights on formation processes

Events which deposit fresh material onto Europa’s surface may be irregular and catastrophic, such as large-scale impacts, or localized, and potentially extant, processes in which faults, fractures, or brine transport bring subsurface liquid onto the surface. This liquid may originate from shallow reservoirs within the ice shell or directly from the subsurface ocean [1]. Emplacement may be a slow, extrusive process, or quicker, potentially with the formation of cryovolcanic plumes (e.g. [2,3,4]). Plume deposits or any other surface material can also be transported to high altitudes by another mechanism: when hypervelocity interplanetary micrometeoroids, or larger objects, impact the surface of an atmosphereless planetary body like Europa, these can generate impact ejecta with high enough velocities to reach altitudes of hundreds of kilometres [5]. Most of these ejecta particles are gravitationally bound, moving on ballistic trajectories lasting up to hundreds of seconds and producing an almost isotropic dust exosphere around Europa [6,7]. Subsurface oceans, in the Jovian and in other planetary systems, can therefore be characterized using the particles they emit, via either indirect or direct routes: 1) Detection and analysis of ejecta particles lofted by micrometeoroid impacts from those parts of the surface that recently interacted with subsurface water (e.g. certain landforms, such as chaos terrains on Europa, or plume deposits); 2) Direct sampling of plume particles in space (where plumes are present, as perhaps possible on Europa).

The SUrface Dust Analyzer (SUDA), onboard the Europa Clipper spacecraft, is designed to measure the composition and trajectories of such impact ejecta particles and/or plume material. SUDA is an impact ionisation time-of-flight mass spectrometer, which uses the ions generated by hypervelocity impacts of dust grains onto the instrument target to generate mass spectra of the impinging particles. Depending on the altitude, SUDA will be able to detect up to tens of ejected surface particles per second during each flyby, each likely to contain a wide variety of organic and/or inorganic compounds, and via trajectory reconstruction, map them to their origins on the surface [8].

At global scales, landform evolution on atmosphereless bodies is primarily driven by impact gardening and tectonics. The tectonic regime that dominates within Europa’s ice shell is likely to be extensional, a hypothesis supported by numerous lines of evidence, such as the widespread presence of dilational bands that represent >40% of the total surface area [9]. Several types of geological features generated within such a tectonic regime, including chaos terrains and fossae, might affect the emplacement of buried material and subsequent exposure of fresh volatiles on Europa’s surface.

Chaos Terrains are, on Europa, geologically very young and extensively disrupted surface features, interpreted as reflecting recent interaction with subsurface material [10]. Leading-hemisphere chaos regions have recently been shown to be compositionally distinct from their surroundings, probably indicating contributions from endogenous sodium chloride sourced from the subsurface ocean [11,12]. Fossae are long, narrow depressions (troughs). The term is used for topographic features that occur on extraterrestrial planetary surfaces, whose exact origin is uncertain, although they are thought to be the result of extensional tectonic processes [13].

Here we investigate two neighboring features on Europa, Ménec Fossae and Thrace Macula (a chaos terrain), for which we produced high-resolution photoclinometrically-derived [14] digital terrain models (DTMs, Fig. 1) and geomorphological-structural maps (Fig. 2). Our preliminary results suggest that this area of Europa has undergone transtensional (strike-slip paired with extension) tectonic activity, as indicated by the orientation and relationship of the faults and fossae. Such tectonic style has possibly created a pathway facilitating the ascent of subsurface material, especially volatiles, that play an important role in the formation of chaos terrain. These results will help ascertain the most likely regions on Europa in which to find fresh material, representative of the subsurface ocean, and be used as input data for dust ejecta trajectory models that will ultimately assist the mass spectrometer Surface Dust Analyzer (SUDA), onboard the upcoming Europa Clipper mission, in compositionally mapping Europa’s surface [15].

 

Figure 1. Regional DTMs, Ménec Fossae (top) and Thrace Macula (bottom)

 

Figure 2. Geomorphological map of Ménec Fossae

 

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