Fractional crystallization is an essential process proposed to explain worldwide compositional abundances of igneous rocks. It requires crystals to precipitate from the melt and segregate from its residual melt, or experience crystal fractionation. The compositional abundances of volcanic systems show a bell curve distribution suggesting that the process has variable efficiencies. We test crystal fractionation efficiency in convective flow in low to intermediate crystallinity regime. We simulate the physical segregation of crystals from their residual melt at the scale of individual crystals, using a direct numerical method. We find that at low particle Reynolds numbers, crystals sink in clusters. The relatively rapid motion of clusters strips away residual melt. Our results show cluster settling can imprint observational signatures at the crystalline scale. The collective crystal behavior results in a crystal convection that governs the efficiency of crystal fractionation, providing a possible explanation for the bell curve distribution in volcanic systems.
The surface of Europa contains several types of roughly elliptical features, collectively called lenticulae. Lenticulae may have positive relief (domes) or negative relief (pits), may disrupt the crust (chaos), or discolor the surface (spots); some lenticulae have attributes of both domes and chaos (dome/chaos). We map the location, dimensions and shapes of all lenticulae and their interactions with other lenticulae and lineaments.We find that (1) pits and domes have similar sizes; (2) chaos are larger than the other lenticulae; (3) pits are clustered within the trailing antijovian quadrant and the leading subjovian quadrant whereas domes, dome/chaos, and chaos terrains are more uniformly distributed; (4) the areal density for all lenticulae is not uniform; (5) lenticulae do not divert the path of younger lineaments such as ridges. Taken together, these observations are consistent with conceptual models in which lenticulae are created by intrusion of liquid water bodies, or convection within, the ice shell. Additionally, the observations are consistent with the notion that each type of lenticula is a surface expression of dynamics within the ice shell at a different stage of lenticulae evolution.The similar size and shape of pits and domes suggests that one may evolve into the other. Because domes are more numerous and more uniformly distributed than pits, they are more likely to represent the end stage of this evolution, assuming the endstage leaves a longer-lasting surface expression. Models also predict that larger features are more likely to disrupt the crust, which is consistent with dome/chaos and chaos being larger than pits and domes. We find no examples of lineaments offsetting pits but lineaments do cross some chaos. Pits also have a preferred northwestsoutheast elongation, whereas domes, dome/chaos, and chaos do not have a preferred orientation. If lenticulae orientation is influenced by crustal stress, then pits may have formed during a shorter time interval than the other features. As a result, pits may sample a shorter, more recent time period than domes, dome/chaos, and chaos, consistent with pits being the earliest stage in the evolution of lenticulae. We find no strong evidence that lineaments are deflected by lenticulae, implying either that the stresses created by lenticulae are too small to influence lineaments, or that the complete evolution of lenticulae occurs on a time scale that is short compared to the time between the formation of lineaments at a given location.
Radar sounding is a geophysical method capable of directly imaging subsurface interfaces within the ice shell of the icy moons, including Jupiter's moon, Europa. For this reason, both the European Space Agency's JUpiter ICy moons Explorer and the National Aeronautics and Space Administration's Europa Clipper missions have ice penetrating radar sounders in their payloads. In addition to the ice-ocean interface and shallow water lenses, liquid water in the eutectic zone of Europa's ice shell could also be a target for radar sounding investigations. However, the wide range of possible configurations for eutectic-zone water bodies and the overlying ice make their absolute echo strength difficult to predict. To address this challenge, we employ a suite of simple water configurations and scattering models to bound the eutectic detectability in terms of its effective reflectivity. We find that, for each configuration, a range of physically plausible eutectic parameters exist that could produce detectable echoes, with effective reflectivity values greater than-50 dB at HF or VHF frequencies.
Crosscutting relationships of tectonic lineaments on Europa record the history of surface deformation.We mapped the displacement and orientation of older features crosscut by two types of lineaments: bands and double ridges. These measurements allow us to determine both the strike-perpendicular and strike-parallel displacement along investigated features. Double ridges record both ridge-perpendicular contraction and expansion, with a mean of 0.16 ± 0.06 km (needs space) of contraction based on the analysis of 16 double ridges. Bands record expansion, with a mean of 3.33 ± 0.27 km for the six bands analyzed, but with perpendicular displacement less than their apparent morphologic widths of 3-24 km. The implied global surface strain for double ridges (including those that expand) and bands is 2.22 ± 0.76% contraction and 7.60 ± 3.7% expansion, respectively. Double ridges thus may accommodate part of the surface expansion recorded by bands. Most current models for double ridges do not predict contraction. The models that satisfy the observations for bands are "slow-spreading" models, cryovolcanism, and folding.
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