主带原始小行星上的水蚀变：可见光谱学的结果
Aqueous alteration on main belt primitive asteroids: results from visible spectroscopy
摘要：
本工作重点研究了作用于主带并在蚀变小行星上产生水化矿物的水蚀变过程。水蚀变对于揭示太阳系历史最早期发生的过程特别重要，因为它可以提供关于小行星热演化和小行星带中水源定位的信息。我们提出了新的光谱观测属于原始类C，G，F，B和P的80颗小行星的可见光区域。我们结合目前的观测与文献中提供的小行星的可见光光谱，共600原始主带小行星。我们的分析表明，水蚀变序列从P型物体开始，几乎没有改变，并通过F，B，C和G小行星增加。大约50%的观测到的C-型小行星在可见光中表现出吸收特征。由于水合硅酸盐，这意味着超过70%的硅酸盐将具有3 $μ$ m的吸收带，因此在其表面上具有水合矿物。这一过程在位于2.3至3.1天文单位之间的原始小行星中占主导地位，这比以前提出的日心距离要小。在50 - 240公里大小的原始小行星中，水蚀变过程占主导地位，而对于小于50公里的天体，水蚀变过程的影响较小。水蚀变过程和小行星的轨道参数之间没有相关性。比较了水化硅酸盐和CM 2碳质陨石的$\sim$ 0.7 $μ$ m带参数，发现陨石的带中心比小行星的带中心波长长。这种中心位置的差异可能归因于不同的矿物丰度，以及地球上可用的CM2可能不能代表整个水蚀变小行星群的事实。
英文摘要：
This work focuses on the study of the aqueous alteration process which acted in the main belt and produced hydrated minerals on the altered asteroids. The aqueous alteration is particularly important for unraveling the processes occurring during the earliest times of the Solar System history, as it can give information both on the asteroids thermal evolution and on the localization of water sources in the asteroid belt. We present new spectral observations in the visible region of 80 asteroids belonging to the primitive classes C, G, F, B and P. We combine the present observations with the visible spectra of asteroids available in the literature for a total of 600 primitive main belt asteroids. Our analysis shows that the aqueous alteration sequence starts from the P-type objects, practically unaltered, and increases through the F, B, C, and G asteroids. Around 50% of the observed C-type asteroids show absorption features in the vis. range due to hydrated silicates, implying that more than 70% of them will have a 3 $μ$m absorption band and thus hydrated minerals on their surfaces. The process dominates in primitive asteroids located between 2.3 and 3.1 AU, that is at smaller heliocentric distances than previously suggested. The aqueous alteration process dominates in the 50--240 km sized primitive asteroids, while it is less effective for bodies smaller than 50 km. No correlation is found between the aqueous alteration process and the asteroids albedo or orbital elements. Comparing the $\sim$ 0.7 $μ$m band parameters of hydrated silicates and CM2 carbonaceous chondrites, we see that the band center of meteorites is at longer wavelengths than that of asteroids. This difference on center positions may be attributed to different minerals abundances, and to the fact that CM2 available on Earth might not be representative of the whole aqueous altered asteroids population.
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