Thermal infrared observations and thermophysical characterization of OSIRIS-REx target asteroid (101955) Bennu
Thermal infrared observations and thermophysical characterization of OSIRIS-REx target asteroid (101955) Bennu
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DOI：
10.1016/j.icarus.2014.02.005
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发表时间：
2014-05-15
期刊：
影响因子：
3.2
通讯作者：
Ziffer, J.
中科院分区：
文献类型：
--
作者：
Emery, J. P.;Fernandez, Y. R.;Ziffer, J.
近地小行星(NEA)在过去几年中得到了越来越多的关注，因为它们提供了关于太阳系形成和演化的见解，它们构成的潜在危险，以及它们对机器人和人类航天任务的可及性。在近地天体中，碳质小行星尤其令人感兴趣，因为它们可能包含地球如何获得水和有机材料供应的线索，而且还没有航天器详细研究过任何一个。(101955)Bennu在近地天体中很特别，因为它不仅将由航天器访问，而且OSIRIS-REX任务还将把Bennu的风化层样本带回地球进行详细的实验室研究。本文介绍了热红外光度分析和光谱分析，验证了本奴是碳质的，其表面覆盖着细粒(亚厘米)风化层的假设。斯皮策太空望远镜在2007年对本努进行了观测，利用红外光谱仪(IRS)获取了5.2-38微米波长范围内的光谱，并在16和22微米的10个不同经度上拍摄了图像，还利用红外阵列相机(IRAC)在3.6、4.5、5.8和8.0微米处对10个不同经度的本努进行了成像。假设球体具有已知的自转周期和自旋极取向，热物理分析返回的有效直径为48410m，与从斯皮策观测方向的雷达形状模型计算的有效直径一致(492+/-20m，Nolan，MC，Magri，C.，Howell，E.S.，Benner，L.A.M.，Giorgii，J.D.，Hergenrother，C.W.，Hudson，R.S.，Lauretta，D.S.，Margo，J.-L.，Ostro，S.J.，Schees，D.J.[2013]。可见几何反照率为0.046+/-0.005(使用H-v=20.51，Hergenrother，C.W.等人。[2013]。伊卡洛斯226,663-670)。在热分析中加入雷达形状模型，并考虑表面粗糙度，得到的圆盘平均热惯量为310+/-70Jm(-2)K-1 S-1/2，明显低于其他几个同等大小的近地天体。热惯量随转动相(+/-60Jm(-2)K~(-1)S(-1/2))的变化可能较小。光谱分析在表面矿物学方面没有定论；发射率光谱具有较低的信噪比，没有检测到光谱特征。热惯量表示风化岩的平均颗粒尺寸在几毫米到大约一厘米的范围内。这种中等的粒度也与7.5-20微米光谱范围内的低光谱对比度相一致。如果是真的，热惯量的旋转变化将与平均颗粒尺寸仅约一毫米的变化一致。本努表面的热物理性质在纵向上似乎相当均匀。对尘埃彗发的搜索未能检测到任何延长的排放，将本努4750公里范围内的尘埃上限定为10(6)克。比较了三种常用的热模拟方法，并讨论了在解释这类模型的结果时应注意的问题。我们预测，OSIRIS-REX航天器将发现一个低反照率表面，具有丰富的亚厘米大小的颗粒，在经度上分布相当均匀。(C)2014 Elsevier Inc.保留所有权利。
Near-Earth Asteroids (NEAs) have garnered ever increasing attention over the past few years due to the insights they offer into Solar System formation and evolution, the potential hazard they pose, and their accessibility for both robotic and human spaceflight missions. Among the NEAs, carbonaceous asteroids hold particular interest because they may contain clues to how the Earth got its supplies of water and organic materials, and because none has yet been studied in detail by spacecraft. (101955) Bennu is special among NEAs in that it will not only be visited by a spacecraft, but the OSIRIS-REx mission will also return a sample of Bennu's regolith to Earth for detailed laboratory study. This paper presents analysis of thermal infrared photometry and spectroscopy that test the hypotheses that Bennu is carbonaceous and that its surface is covered in fine-grained (sub-cm) regolith. The Spitzer Space Telescope observed Bennu in 2007, using the Infrared Spectrograph (IRS) to obtain spectra over the wavelength range 5.2-38 mu m and images at 16 and 22 gm at 10 different longitudes, as well as the Infrared Array Camera (IRAC) to image Bennu at 3.6, 4.5, 5.8, and 8.0 mu m, also at 10 different longitudes. Thermophysical analysis, assuming a spherical body with the known rotation period and spin-pole orientation, returns an effective diameter of 484 10 m, in agreement with the effective diameter calculated from the radar shape model at the orientation of the Spitzer observations (492 +/- 20 m, Nolan, MC., Magri, C., Howell, E.S., Benner, L.A.M., Giorgini, J.D., Hergenrother, C.W., Hudson, R.S., Lauretta, D.S., Margo, J.-L., Ostro, S.J., Scheeres, D.J. [2013]. Icarus 226, 629-640) and a visible geometric albedo of 0.046 +/- 0.005 (using H-v = 20.51, Hergenrother, C.W. et al. [2013]. Icarus 226, 663-670). Including the radar shape model in the thermal analysis, and taking surface roughness into account, yields a disk-averaged thermal inertia of 310 +/- 70 J m(-2) K-1 S-1/2, which is significantly lower than several other NEAs of comparable size. There may be a small variation of thermal inertia with rotational phase (+/- 60 J m(-2) K-1 s(-1/2)). The spectral analysis is inconclusive in terms of surface mineralogy; the emissivity spectra have a relatively low signal-to-noise ratio and no spectral features are detected. The thermal inertia indicates average regolith grain size on the scale of several millimeters to about a centimeter. This moderate grain size is also consistent with low spectral contrast in the 7.5-20 mu m spectral range. If real, the rotational variation in thermal inertia would be consistent with a change in average grain size of only about a millimeter. The thermophysical properties of Bennu's surface appear to be fairly homogeneous longitudinally. A search for a dust coma failed to detect any extended emission, putting an upper limit of about 10(6) g of dust within 4750 km of Bennu. Three common methodologies for thermal modeling are compared, and some issues to be aware of when interpreting the results of such models are discussed. We predict that the OSIRIS-REx spacecraft will find a low albedo surface with abundant sub-cm sized grains, fairly evenly distributed in longitude. (c) 2014 Elsevier Inc. All rights reserved.