The ice content of Kuiper belt objects
The ice content of Kuiper belt objects
复制标题
柯伊伯带天体的冰含量
作者：
M. Fulle
罗塞塔首次测量了太阳系原始天体中非挥发性物质的体积密度1。冥王星和冥卫一2的模型假设岩石密度为2，770至3，260 kg m-3，烃与硅酸盐的质量比h/s = 0.2。然而，这个值，首先提出的彗星1 P/哈雷3，是由一个低的尘埃-冰比，后来被细化到更大的值4，意味着更大的h/s比以及偏见。软质氢化碳合金（原太阳星云中碳氢化合物的最佳陆地类似物）具有与冰相似的体积密度，因此柯伊伯带天体（KBO）的低体积密度可能是由于丰富的冰或碳氢化合物。彗星67 P/Churyumov-Gerasimenko（67 P）的组成由铁硫化物的体积丰度c1（体积密度ρ1 = 4，600 kg m-3）、镁和铁橄榄石和辉石的体积丰度c2（ρ2 = 3，200 kg m-3）、烃类的体积丰度c3（ρ3 = 1，200 kg m-3）和冰的体积丰度c4（ρ4 = 917 kg m-3）确定。体积丰度取决于冰的孔隙度和形成彗星和KBO的卵石中的原始成分（介于太阳和CI-南极端壳1之间），并提供h/s =（c3ρ3）/（c2ρ2）>> 0.2（表1）。我们假设彗星和KBO中的非挥发性物质具有相似的组成，由罗塞塔提供的c2/c1和c3/c1比率确定（表1）。这与67 P6和1 P/Halley 3中的元素C/Fe比率一致，彗星可能起源于KBO的碎片，或者彗星和KBO可能共同起源于类似的卵石层7。彗星和KBO在冰的丰度和成分上有所不同，这取决于它们在吸积过程中与太阳的距离以及它们的演化。在这里，我们推断冰丰度c4在KBO，在那里的岩石静压力消除了所有的空隙之间的卵石引力的波动7和随后的演变。因此，平均KBO堆积密度为
To the Editor — Rosetta has measured the bulk density of non-volatiles in a primitive Solar System object for the first time1. Models of Pluto and Charon2 assume rock densities ranging from 2,770 to 3,260 kg m–3 and a hydrocarbons-to-silicates mass ratio h/s = 0.2. However this value, first suggested for comet 1P/Halley3, is biased by a low dust-to-ices ratio that was later refined to larger values4, implying larger h/s ratios as well. Soft hydrogenated carbon alloys5 (the best terrestrial analogues of hydrocarbons in the protosolar nebula) have a bulk density similar to ices, so that the low bulk density of Kuiper belt objects (KBOs) can be due either to abundant ices or to hydrocarbons. The composition of comet 67P/Churyumov–Gerasimenko (67P) was fixed1 by the volume abundances c1 of Fe-sulfides (bulk density ρ1 = 4,600 kg m–3), c2 of Mg and Fe olivines and pyroxenes (ρ2 = 3,200 kg m–3), c3 of hydrocarbons5 (ρ3 = 1,200 kg m–3), and c4 of ices (ρ4 = 917 kg m–3). The volume abundances depend on the ice porosity and on the pristine composition (between the solar and CI-chondritic end-cases1) in the pebbles forming comets and KBOs, and provide h/s = (c3ρ3)/(c2ρ2) >> 0.2 (Table 1). We assume that the non-volatiles in comets and KBOs have a similar composition, which is fixed by the ratios c2/c1 and c3/c1 provided by Rosetta (Table 1). This is consistent with the elemental C/Fe ratio in 67P6 and in 1P/Halley3, with a possible origin of comets as fragments of KBOs, or with a probable common origin of comets and KBOs from similar pebbles7. Comets and KBOs differ instead in the abundance and composition of ices, which depend on their distance from the Sun during accretion and on their evolution. Here we infer the ice abundance c4 in KBOs, where the lithostatic pressure eliminates all the voids among the pebbles following gravitational collapse7 and the subsequent evolution. Therefore the average KBO bulk density is