Bland Philip A, Travis Bryan J
Department of Applied Geology, Curtin University of Technology, G.P.O. Box U1987, Perth, Western Australia 6845, Australia.
Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719, USA.
Sci Adv. 2017 Jul 14;3(7):e1602514. doi: 10.1126/sciadv.1602514. eCollection 2017 Jul.
Carbonaceous asteroids may have been the precursors to the terrestrial planets, yet despite their importance, numerous attempts to model their early solar system geological history have not converged on a solution. The assumption has been that hydrothermal alteration was occurring in rocky asteroids with material properties similar to meteorites. However, these bodies would have accreted as a high-porosity aggregate of igneous clasts (chondrules) and fine-grained primordial dust, with ice filling much of the pore space. Short-lived radionuclides melted the ice, and aqueous alteration of anhydrous minerals followed. However, at the moment when the ice melted, no geological process had acted to lithify this material. It would have been a mud, rather than a rock. We tested the effect of removing the assumption of lithification. We find that if the body accretes unsorted chondrules, then large-scale mud convection is capable of producing a size-sorted chondrule population (if the body accretes an aerodynamically sorted chondrule population, then no further sorting occurs). Mud convection both moderates internal temperature and reduces variation in temperature throughout the object. As the system is thoroughly mixed, soluble elements are not fractionated, preserving primitive chemistry. Isotopic and redox heterogeneity in secondary phases over short length scales is expected, as individual particles experience a range of temperature and water-rock histories until they are brought together in their final configuration at the end of convection. These results are consistent with observations from aqueously altered meteorites (CI and CM chondrites) and spectra of primitive asteroids. The "mudball" model appears to be a general solution: Bodies spanning a ×1000 mass range show similar behavior.
碳质小行星可能是类地行星的前身，然而尽管它们很重要，但众多尝试对其早期太阳系地质历史进行建模的努力尚未达成一个解决方案。一直以来的假设是，水热蚀变发生在具有与陨石相似物质特性的岩石小行星中。然而，这些天体最初会以火成碎屑（球粒）和细粒原始尘埃的高孔隙率聚集体形式吸积，孔隙空间大部分被冰填充。短寿命放射性核素使冰融化，随后无水矿物发生水蚀变。然而，在冰融化的那一刻，没有地质过程使这种物质固结成岩。它会是一种泥浆，而非岩石。我们测试了去除固结成岩这一假设的影响。我们发现，如果天体吸积未分选的球粒，那么大规模的泥浆对流能够产生按大小分选的球粒群体（如果天体吸积的是经空气动力学分选的球粒群体，那么不会发生进一步分选）。泥浆对流既能调节内部温度，又能降低整个天体的温度变化。由于系统被充分混合，可溶性元素不会分馏，从而保留了原始化学组成。由于单个颗粒在对流结束时最终聚集在一起之前经历了一系列温度和水 - 岩相互作用历史，预计在短长度尺度上次生相中的同位素和氧化还原异质性会存在。这些结果与水蚀变陨石（CI 和 CM 球粒陨石）的观测结果以及原始小行星的光谱一致。“泥球”模型似乎是一个通用的解决方案：跨越 1000 倍质量范围的天体表现出相似的行为。
