Abstract

The reflectance spectra of active asteroids (AAs) measured in the visible and near-UV ranges exhibit unusual features, which are apparently caused by the light scattering in an exosphere formed under active processes on an asteroid. To estimate the prospects for a quantitative interpretation of these features, we numerically simulated reflectance spectra of an AA enveloped by an exosphere composed of aggregate submicron particles of various composition and morphology, as well as homogeneous submicron particles. We assumed that the sizes of aggregates’ constituents correspond to those of grains in agglomerates of cometary and interplanetary dust. It has been shown that the scattering on aggregates of submicron grains produces interference features at wavelengths shorter than 0.6 μm, and the positions of these features are determined by both the sizes of these grains (rather than the aggregates themselves) and the real part of their refractive index. The structure of an aggregate and variations (up to ±20%) in the sizes of constituents weakly influence the position of these features. The shape of the spectrum at longer wavelengths also depends on the sizes of grains in the aggregates and serve as an additional criterion for estimating this parameter. Calculations performed for aggregate particles absorbing in a short-wavelength range (which is typical of many materials that one may expect to find on AAs) show that the absorption significantly weakens the interference details appeared in this range. Hence, the attempts to detect strongly absorbing particles in the exosphere and to estimate their properties by these spectral features cannot yield reliable results, as opposed to the simulations for weakly absorbing particles. The presence of homogeneous weakly absorbing submicron particles in the exosphere of an AA results in a steady growth of the intensity at wavelengths shorter than 0.4−0.5 μm. Spectral measurements at the wavelengths shorter than 0.35 μm may help to estimate more reliably the properties of weakly absorbing particles, both aggregate and homogeneous, in the exospheres of AAs.

中文翻译：

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关于利用紫外和可见反射光谱特征估计活跃小行星外层粒子特性的前景

摘要

在可见光和近紫外范围内测量的活动小行星 (AA) 的反射光谱表现出不寻常的特征，这显然是由小行星在活动过程中形成的外逸层中的光散射引起的。为了估计对这些特征进行定量解释的前景，我们数值模拟了 AA 的反射光谱，该 AA 被散逸层包围，散逸层由各种成分和形态的聚集亚微米粒子以及均匀的亚微米粒子组成。我们假设聚集体成分的大小对应于彗星和行星际尘埃团块中颗粒的大小。已经表明，亚微米颗粒聚集体上的散射会在短于 0.6 μm 的波长处产生干涉特征，这些特征的位置由这些颗粒的大小（而不是聚集体本身）和折射率的实部决定。聚集体的结构和成分大小的变化（高达 ±20%）对这些特征的位置影响微弱。较长波长处的光谱形状也取决于聚集体中颗粒的大小，并作为估计该参数的附加标准。对在短波长范围内吸收的聚集体颗粒进行的计算（这是人们可能期望在 AAs 上发现的许多材料的典型特征）表明吸收显着削弱了出现在该范围内的干扰细节。因此，与弱吸收粒子的模拟相反，尝试检测外逸层中的强吸收粒子并通过这些光谱特征估计它们的特性无法产生可靠的结果。AA 外逸层中均匀的弱吸收亚微米粒子的存在导致波长小于 0.4–0.5 μm 的强度稳定增长。波长小于 0.35 μm 的光谱测量可能有助于更可靠地估计 AAs 逸散层中弱吸收粒子（聚集体和均匀粒子）的特性。AA 外逸层中均匀的弱吸收亚微米粒子的存在导致波长小于 0.4–0.5 μm 的强度稳定增长。波长小于 0.35 μm 的光谱测量可能有助于更可靠地估计 AAs 逸散层中弱吸收粒子（聚集体和均匀粒子）的特性。AA 外逸层中均匀的弱吸收亚微米粒子的存在导致波长小于 0.4–0.5 μm 的强度稳定增长。波长小于 0.35 μm 的光谱测量可能有助于更可靠地估计 AAs 逸散层中弱吸收粒子（聚集体和均匀粒子）的特性。