MIXING FRACTION OF INNER SOLAR SYSTEM MATERIAL IN COMET 81P/WILD2
MIXING FRACTION OF INNER SOLAR SYSTEM MATERIAL IN COMET 81P/WILD2
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COMET 81P/WILD2 中太阳系内部物质的混合分数
DOI：
10.1088/0004-637x/694/1/18
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
A. Butterworth
中科院分区：
文献类型：
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作者：
A. Westphal;S. Fakra;Z. Gainsforth;M. Marcus;R. Ogliore;A. Butterworth
通过红外观测推断彗星彗发中存在结晶硅酸盐，这一直是一个长期存在的谜团。如果彗星由原始星际物质组成，那么结晶硅酸盐是意想不到的，因为星际硅酸盐几乎完全是无定形的。加热到 >1100 K 可以使硅酸盐退火至结晶度，但尚未发现足够强的原行星热源可以将外星云中的材料加热到如此高的温度。这个难题导致人们提出大规模混合在原行星盘中很重要。关于星尘样品中难熔的富含钙铝的夹杂物和高浓度惰性气体的报告强调了这种混合的必要性。然而，到目前为止，来自星尘样本的证据很大程度上都是轶事，而且不可能对混合分数进行定量限制。在这里，我们报告了基于同步加速器的 X 射线微探针测量了已知彗星（木星家族彗星 81P/Wild2）材料中铁化学态的相对浓度。我们发现这颗彗星富含硫化铁。基于硫化物浓度的元素 S/Fe 比率，S/Fe >0.31(2σ)，高于大多数球粒陨石。我们还发现含铁硅酸盐至少有 50% 是结晶的。基于这些测量，我们估计 81P/Wild2 中内部星云物质的分数 ψ。根据结晶含铁硅酸盐分数的下限，我们发现 ψ>0.5。如果观察到的太阳系内部的硫消耗早于大规模混合或与大规模混合同时发生，那么我们对 S/Fe 比率的下限给出了 ψ 的上限~0.65。该测量可用于测试早期太阳系的混合模型。
The presence of crystalline silicates in the comae of comets, inferred through infrared observations, has been a long-standing puzzle. Crystalline silicates are unexpected if comets are composed of pristine interstellar material, since interstellar silicates are almost entirely amorphous. Heating to >1100 K can anneal silicates to crystallinity, but no protoplanetary heating sources have been identified that were sufficiently strong to heat materials in the outer nebula to such high temperatures. This conundrum led to the suggestion that large-scale mixing was important in the protoplanetary disk. Reports of refractory calcium–aluminum-rich inclusion-like objects and large concentrations of noble gases in Stardust samples underscore the need for such mixing. However, the evidence from the Stardust samples until now has been largely anecdotal, and it has not been possible to place quantitative constraints on the mixing fraction. Here we report synchrotron-based X-ray microprobe measurements of the relative concentrations of the chemical state of iron in material from a known comet, the Jupiter-family comet 81P/Wild2. We find that the comet is rich in iron sulfides. The elemental S/Fe ratio based on the sulfide concentration, S/Fe >0.31(2σ), is higher than in most chondritic meteorites. We also found that Fe-bearing silicates are at least 50% crystalline. Based on these measurements, we estimate the fraction ψ of inner nebular material in 81P/Wild2. With the lower bound on the crystalline Fe-bearing silicate fraction, we find that ψ>0.5. If the observed S depletion in the inner solar system predated or was contemporaneous with large-scale mixing, our lower bound on the S/Fe ratio gives an upper bound on ψ of ∼0.65. This measurement may be used to test mixing models of the early solar system.