The smallest particles in Saturn's A and C Rings
The smallest particles in Saturn's A and C Rings
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土星 A 环和 C 环中最小的粒子
DOI：
10.1016/j.icarus.2013.08.015
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发表时间：
2013
期刊：
影响因子：
3.2
通讯作者：
M. Hedman
中科院分区：
文献类型：
--
作者：
R. Harbison;P. Nicholson;M. Hedman
宇宙飞船对土星主环的无线电掩星表明，粒子大小分布呈幂律，最小可达1厘米量级（Marouf，EA，泰勒，GL，Zebker，HA，Simpson，RA，Eshleman，VR [1983]. Icarus 54，189-211; Zebker，HA，Marouf，EA，泰勒，GL [1985]. Icarus 64，531-548.）。在紫外和近红外波长之间缺乏光学深度变化表明缺乏微米尺寸的颗粒。在这两种制度之间，颗粒大小分布在很大程度上是未知的。粒径分布翻转的临界点一定存在，但其位置和形状在现有研究中尚不清楚。利用卡西尼号上的VIMS仪器在近红外波段进行的一系列太阳掩星，我们能够测量A环和C环中粒子的前向散射光。与环形颗粒的衍射模型，和以前的无线电工作作为约束的粒度分布的斜率，我们估计使用截断幂律粒度分布的最小粒度。C环的最小粒度为4.1-1.3+ 3.8 mm，假设幂律指数为q= 3.1，最大粒度为10 μ m。A环信号显示出类似水平的散射通量，但由于存在违反均匀环假设的自重力尾流和需要多级散射的较高光学深度，建模变得复杂。如果q< 3，我们的A环模型要求最小粒径小于1毫米（假设q= 2.75时< 0.34 mm，或更陡的q= 2.9时为0.56-0.16+ 0.35 mm），以与VIMS观测结果一致。这些结果似乎与先前的光学（Dones，L.，Cuzzi，JN，Showalter，MR [1993]. Icarus 105，184-215）和红外线（French，RG，Nicholson，PD [2000]. Icarus 145，502-523）的工作，这意味着在A环中几乎没有小于1厘米的粒子。但是，由于浅幂律，这些颗粒提供的光学深度相对较小（消光在0.03和0.16之间，或吸收在0.015-0.08之间）。
Radio occultations of Saturn’s main rings by spacecraft suggest a power law particle size-distribution down to sizes of the order of 1 cm (Marouf, EA, Tyler, GL, Zebker, HA, Simpson, RA, Eshleman, VR [1983]. Icarus 54, 189–211; Zebker, HA, Marouf, EA, Tyler, GL [1985]. Icarus 64, 531–548.). The lack of optical depth variations between ultraviolet and near-IR wavelengths indicate a lack of micron-sized particles. Between these two regimes, the particle-size distribution is largely unknown. A cutoff where the particle-size distribution turns over must exist, but the position and shape of it is not clear from existing studies. Using a series of solar occultations performed by the VIMS instrument on-board Cassini in the near-infrared, we are able to measure light forward scattered by particles in the A and C Rings. With a model of diffraction by ring particles, and the previous radio work as a constraint on the slope of the particle size distribution, we estimate the minimum particle size using a truncated power-law size distribution. The C Ring shows a minimum particle size of 4.1-1.3+ 3.8 mm, with an assumed power law index of q= 3.1 and a maximum particle size of 10 m. The A Ring signal shows a similar level of scattered flux, but modeling is complicated by the presence of self-gravity wakes, which violate the assumption of a homogeneous ring, and higher optical depths, which require multiple-order scattering. If q< 3, our A Ring model requires a minimum particle size below one millimeter (< 0.34 mm for an assumed q= 2.75, or 0.56-0.16+ 0.35 mm for a steeper q= 2.9) to be consistent with VIMS observations. These results might seem to contradict previous optical (Dones, L., Cuzzi, JN, Showalter, MR [1993]. Icarus 105, 184–215) and infrared (French, RG, Nicholson, PD [2000]. Icarus 145, 502–523) work, which implied that there were few particles in the A Ring smaller than 1 cm. But, because of the shallow power law, relatively little optical depth (between 0.03 and 0.16 in extinction, or 0.015–0.08 in absorption) is provided by these particles.
影响因子：
3.2
作者：
A. Bodrova;J. Schmidt;F. Spahn;N. V. Brilliantov
通讯作者：
N. V. Brilliantov