维基百科

彗星尘

彗星尘是指起源于彗星的宇宙尘埃，它可以提供彗星起源的线索。当地球通过彗星尘埃轨迹时，它会产生流星雨。

物理性质 编辑

大小 编辑

彗星活动产生的尘埃大部分为次微米级[1]至微米的大小[2][3]。然而，这一部分是短暂的，会因为辐射压导致它们被吹出太阳系[4][5]，或因为坡印廷–罗伯逊效应而螺旋向内[6][7]。

下一个尺寸等级是"蓬松的"（"fluffly"）[4][5]或"群集类型"（"cluster-type"）[8]，是前述颗粒的聚合物。它们的大小通常为20-100微米，但可以观察到尺寸不是任意的[9]，这是由于多孔的聚合物容易断裂 [10] or compact.[8][11][12]。

更大的粒子是微流星体[13][14]，不再是尘埃了[15][16]。在国际天文学联合会（IAU）没有定义的情况下[17][18]，各小组设计了自己的粉尘定义：小于100微米[19]、50[20]、40[21] 30,[22]、和20微米[23] 还有<10微米[24][25][26][16]。其中一些尘埃/微流星体的定义是近似的或模棱两可的[27][28][29]，有些还重叠或自相矛盾（冲突）[30][23][22]。

IAU于2017年发布了一份正式声明。流星体为30微米至1米，尘埃较小，且不鼓励使用"微流星体"一词（尽管不是微流星体）[31]。国际流星组织 （IMO）虽然注意到了新的定义[32]，但仍在其网站上显示先前的定义[33]，即0.001cm[34]。美国流星学会（英语：American Meteor Society）（AMS）也没有给出严格的定义[35][36]。

成分 编辑

粉尘的成分通常为球粒陨石。其单体含有镁铁质硅酸盐，如橄榄石和辉石[37]。硅酸盐富含高冷凝温度的镁橄榄石和顽火辉石[27]。当这些颗粒迅速凝结时，它们往往会形成非常小的颗粒，而不是合并成液滴。

与球粒陨石一样，颗粒含有 Fe（Ni）硫化物[38][39]和嵌入金属和硫化物的玻璃（glass with embedded metal and sulfides，GEMS）[38]。

存在不同数量的有机物（CHON）[40][41][42]。虽然有机物在宇宙中非常丰富，并且被广泛预测存在于彗星中，但在大多数望远镜中，它们的光谱都是模糊的。仅在哈雷飞越期间通过质谱法确认有机物的存在[43][44]。一些有机物多以多环芳香烃（PAHs）的形式存在[45][19][46][47][48]。

可能会发现非常小的太阳前颗粒（presolar grains，PSG）夹杂物[27][48]。

尘埃和彗星的起源 编辑

 
显微镜下看见的彗星尘埃颗粒。

彗星起源的模型有[49]：

  1. 星际模型，
  2. 太阳系模型，
  3. 原始碎石堆，
  4. 在天王星-海王星区域周围的尘埃盘中星子的聚集，
  5. 被原恒星风吹出的冷外壳物质。

彗星尘埃的密度、化学成分、体积等特性，可以区分这些模型。例如，彗星和星际尘埃的同位素比率非常相似，表明有一个共同的起源。

在（1）星际模型说冰是在太阳之前的致密云中的尘埃颗粒上形成的。冰和尘埃的混合物在没有明显化学修饰的情况下聚集成彗星。J、梅奥·格林伯格在20世纪70年代率先提出这个想法[50][51]。

在（2）根据太阳系模型，星际云中形成的冰是原太阳周围气体和尘埃吸积盘中首先蒸发的一部分。蒸发的冰后来再溶解并组装成彗星。因此，这个模型中的彗星与那些直接由星际冰形成的彗星的成分不同。

在（3）彗星形成的原始碎石堆模型说，彗星是在木星形成的区域凝聚形成的。

星尘号在维尔特二号彗星（81P/Wild）的尘埃中发现了结晶硅酸盐，这意味着尘埃形成于超过玻璃转化温度（>1000K）以上的区域，是在环绕着年轻恒星盘面的内侧高温地区，然后混合著太阳星云从恒星内侧向外辐射很远的距离，或是在发展中的红巨星或超巨星内凝结的尘埃粒子向外流动。星或超巨星流出的尘埃粒子，在太阳星云中径向混合。维尔特二号彗星尘埃的成分与新形成恒星周围吸积盘外部区域的尘埃成分相似[52]。

彗星和它的尘埃使我们能够研究太阳系主要行星轨道之外的区域。彗星的区别在于它们的轨道：周期超过200年的长周期彗星，有长的椭圆轨道，随机倾斜于太阳系平面。周期小于200年的短周期彗星通常与太阳系平面的倾角小于30度，以与行星轨道相同的逆时针方向围绕太阳旋转。

彗星在其轨道上运行时时会经历一系列不同的情况。对于长周期彗星来说，大多数时间它离太阳太远，太冷了，冰不会蒸发。当它通过类地行星区域时，蒸发速度会很快，足以吹走小颗粒，但最大的颗粒可能会抵制夹带，留在彗核上，开始形成尘埃层。在太阳附近，加热和蒸发的速度将非常快，以至于没有尘埃可以保留下来。因此，覆盖彗核的尘埃层厚度可以表明彗星接近太阳的频率与近日点的距离。如果彗星有厚厚的尘埃层堆积，它可能曾频繁的通过近日点，但不会过度的靠近太阳。

尘埃层的厚堆积对所有短周期彗星可能是一个很好的描述，因为人们认为在短周期彗星核的表面上堆积的尘埃层厚度在米量级。随着时间的推移，尘埃层的积累将改变这颗短周期彗星的物理特性。尘埃层既抑制了太阳对彗星冰的加热（阳光无法穿透尘埃层，并且导热不良），又减缓了下方彗核的气体损失。处于短周期彗星典型轨道上的彗核会迅速降低其蒸发率，以至于无法检测到彗发或彗尾，天文学家可能会将其视为低反照率的近地小行星。

进一步组合和主体 编辑

主条目：吸积

参考资料 编辑

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