Structure and Evolution of Kuiper Belt Objects and Dwarf Planets
Structure and Evolution of Kuiper Belt Objects and Dwarf Planets
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柯伊伯带天体和矮行星的结构和演化
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
A. Coradini
中科院分区：
文献类型：
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作者：
W. McKinnon;D. Prialnik;S. Stern;A. Coradini
柯伊伯带天体（KBO）是由混合星际和太阳星云起源的挥发性冰、碳质物质和岩石的混合物增生而来的。发生这种吸积的外海王星区域可能比今天的径向更紧凑。这和在太阳星云时期的气体阻力的影响，认为更快速的KBO吸积比通常认为的。KBO的早期演化主要是由于放射性衰变加热的结果，最重要的潜在来源是26 Al，而大型天体的长期演化则由U，Th和40 K的衰变控制。回顾了几项研究处理KBO模型的演变，计算的一维数值代码，解决热量和质量平衡方程。结果表明，根据参数（主要是岩石含量和孔隙电导率），KBO内部可能已经达到相对较高的温度。该模型表明，KBO可能会失去冰的非常挥发性的物种在早期的演变，而冰的挥发性较低的物种应保留在寒冷的，较少改变的地下层。最初，无定形冰可能在KBO内部结晶，释放出被困在无定形冰中的挥发物，一些物体可能也失去了部分挥发物。一般来说，外层受内部演化的影响远小于内部，在没有其他影响（如碰撞）的情况下，预测分层的成分和改变的孔隙度分布。因此，柯伊伯带天体不太可能是“太阳系中最原始的天体”，但它们确实包含了早期太阳系如何吸积和动态演化的关键信息。对于大型（矮行星）KBO，长期的辐射加热可能会导致差异化的结构-岩石核心，冰地幔，富含挥发性冰的“地壳”，甚至海洋。海洋和（潜在的）火山活动持续到今天强烈地依赖于身体的大小和熔点的抑郁症所提供的盐，氨等的存在（我们审查的情况下，特别是冥卫一）。KBO的表面颜色和成分类别通常以“先天与后天”来讨论，即，一般的原始成分与表面处理，但KBO的真正性质也取决于它们是如何进化的。现在在柯伊伯带发现的大范围的KBO，一些物体上的深水冰沉积物，冥王星和2003年EL 61分化的证据，以及与单一原始组成和可变孔隙度不相容的密度范围，强烈暗示KBO之间存在显着的内在组成差异。形成区（吸积率），身体的大小，和动力（碰撞）的历史的相互作用可能会产生KBO成分类（及其光谱相关），让人想起内太阳系中的不同类别的小行星，但其成员广泛分布在KBO动力子群。
Kuiper belt objects (KBOs) accreted from a melange of volatile ices, carbonaceous matter, and rock of mixed interstellar and solar nebular provenance. The transneptunian region, where this accretion took place, was likely more radially compact than today. This and the influence of gas drag during the solar nebula epoch argue for more rapid KBO accretion than usually considered. Early evolution of KBOs was largely the result of heating due to radioactive decay, the most important potential source being 26Al, whereas long-term evolution of large bodies is controlled by the decay of U, Th, and 40K. Several studies are reviewed dealing with the evolution of KBO models, calculated by means of one-dimensional numerical codes that solve the heat and mass balance equations. It is shown that, depending on parameters (principally rock content and porous conductivity), KBO interiors may have reached relatively high temperatures. The models suggest that KBOs likely lost ices of very volatile species during early evolution, whereas ices of less-volatile species should be retained in cold, less-altered subsurface layers. Initially amorphous ice may have crystallized in KBO interiors, releasing volatiles trapped in the amorphous ice, and some objects may have lost part of these volatiles as well. Generally, the outer layers are far less affected by internal evolution than the inner part, which in the absence of other effects (such as collisions) predicts a stratified composition and altered porosity distribution. Kuiper belt objects are thus unlikely to be “the most pristine objects in the solar system,” but they do contain key information as to how the early solar system accreted and dynamically evolved. For large (dwarf planet) KBOs, long-term radiogenic heating alone may lead to differentiated structures — rock cores, ice mantles, volatile-ice-rich “crusts,” and even oceans. Persistence of oceans and (potential) volcanism to the present day depends strongly on body size and the melting-point depression afforded by the presence of salts, ammonia, etc. (we review the case for Charon in particular). The surface color and compositional classes of KBOs are usually discussed in terms of “nature vs. nurture,” i.e., a generic primordial composition vs. surface processing, but the true nature of KBOs also depends on how they have evolved. The broad range of albedos now found in the Kuiper belt, deep water-ice absorptions on some objects, evidence for differentiation of Pluto and 2003 EL61, and a range of densities incompatible with a single, primordial composition and variable porosity strongly imply significant, intrinsic compositional differences among KBOs. The interplay of formation zone (accretion rate), body size, and dynamical (collisional) history may yield KBO compositional classes (and their spectral correlates) that recall the different classes of asteroids in the inner solar system, but whose members are broadly distributed among the KBO dynamical subpopulations.