Micrometer-sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties
Micrometer-sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties
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DOI：
10.3847/1538-4357/aa8c7f
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发表时间：
2017-10-20
影响因子：
4.9
通讯作者：
Fraser, H. J.
中科院分区：
文献类型：
--
作者：
Gaertner, S.;Gundlach, B.;Fraser, H. J.
模型和观测表明，雪线上和雪线以外的冰粒聚集在行星形成的最早阶段占主导地位，因此许多实验室研究都是针对这一点。然而，原行星盘中的压力-温度梯度意味着冰不断地被加工，经历不同固相和气相之间的相变。关于冰粒子本身的性质是否决定了碰撞结果，以及碰撞实验如何有效地再现原行星环境中的条件，仍然存在悬而未决的问题。以前的实验经常在碰撞结果上产生明显矛盾的结果，只在温度依赖性设置上达成一致，大约在210 K以上。通过利用尼姆罗德中子散射仪器的独特功能，我们表征了碰撞实验中使用的冰颗粒的体积和表面结构，并研究了这些结构如何在约30毫巴的恒定压力下随温度变化。我们的冰粒是在液氮下形成的，当它们从103 K加热到247 K时，会经历结晶冰相、升华、烧结和表面预熔化的变化。扩散表面层的厚度从大约10埃增加到大约30埃（大约2.5到12个双层）证明在高于大约210 K的温度下分子迁移率增加。由于没有其他变化与碰撞结果的温度趋势，我们得出结论，在这些温度下的碰撞实验中，表面预熔化现象起着关键作用。因此，压力-温度环境对碰撞结果的影响可能比以前认为的更大。
Models and observations suggest that ice-particle aggregation at and beyond the snowline dominates the earliest stages of planet formation, which therefore is subject to many laboratory studies. However, the pressure-temperature gradients in protoplanetary disks mean that the ices are constantly processed, undergoing phase changes between different solid phases and the gas phase. Open questions remain as to whether the properties of the icy particles themselves dictate collision outcomes and therefore how effectively collision experiments reproduce conditions in protoplanetary environments. Previous experiments often yielded apparently contradictory results on collision outcomes, only agreeing in a temperature dependence setting in above approximate to 210 K. By exploiting the unique capabilities of the NIMROD neutron scattering instrument, we characterized the bulk and surface structure of icy particles used in collision experiments, and studied how these structures alter as a function of temperature at a constant pressure of around 30 mbar. Our icy grains, formed under liquid nitrogen, undergo changes in the crystalline ice-phase, sublimation, sintering and surface pre-melting as they are heated from 103 to 247 K. An increase in the thickness of the diffuse surface layer from approximate to 10 to approximate to 30 angstrom (approximate to 2.5 to 12 bilayers) proves increased molecular mobility at temperatures above approximate to 210 K. Because none of the other changes tie-in with the temperature trends in collisional outcomes, we conclude that the surface pre-melting phenomenon plays a key role in collision experiments at these temperatures. Consequently, the pressure-temperature environment, may have a larger influence on collision outcomes than previously thought.