A radar survey of M- and X-class asteroids. III. Insights into their composition, hydration state, & structure
A radar survey of M- and X-class asteroids. III. Insights into their composition, hydration state, & structure
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M 级和 X 级小行星的雷达勘测。
DOI：
10.1016/j.icarus.2014.09.016
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发表时间：
2015
期刊：
影响因子：
3.2
通讯作者：
C. Magri
中科院分区：
文献类型：
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作者：
M. Shepard;P. Taylor;M. Nolan;E. Howell;A. Springmann;J. Giorgini;Brian D. Warner;A. Harris;R. Stephens;W. Merline;A. Rivkin;L. Benner;Daniel R. Coley;B. E. Clark;M. Ockert;C. Magri
利用阿雷西博天文台的S波段雷达，我们观测到了13颗X/M级小行星;其中9颗以前未被探测到，4颗被重新观测到，使雷达观测到的托伦X/M级小行星总数达到29颗。在这29颗M级小行星中，13颗也是W级，定义为M级天体，也显示出3 μ m的吸收特征，这通常被解释为水合矿物的特征（Jones，T.D.，Lebofsky，洛杉矶，刘易斯，J.S.，Marley，M.S. [1990年]。Icarus 88，172 - 192; Rivkin，A.S.，豪厄尔，E.S.，布里特，D. T.，Lebofsky，洛杉矶，M.C.诺兰D.D. [1995年]。Icarus 117，90 - 100; Rivkin，A.S.，豪厄尔，E.S.，Lebofsky，洛杉矶，克拉克，B.E.，布里特，DT [2000年]。Icarus 145，351 - 368）.与我们以前的工作一致（谢泼德，M.K.等人[2008]。Icarus 195，184 - 205;谢泼德，M.K.，哈里斯，A.W.，泰勒，私人助理，克拉克，B.E.，奥克-贝尔，M.，M.C.诺兰豪厄尔，E.S.，马格里角，Giorgini，J.D.，Benner，L.A.M.[2011年]。Icarus 215，547 - 551），我们发现38%的样品（29个中的11个）具有与金属主导成分一致的雷达散射。除了83 Beatrix和572 Rebekka之外，其余天体的雷达反射率明显高于平均S级或C级小行星（Magri，C.，M.C.诺兰Ostro，S.J.，乔治尼，JD [2007]。11颗高雷达散射小行星中有7颗（64%）也显示出3微米的吸收特征（W级），这被认为与金属为主的小行星的形成不一致。我们认为，水合吸收可能是一个次要的功能所造成的低速碰撞与水合小行星，如CI或CM类似物，和随后的植入的水合矿物到上层风化层。最近有证据表明灶神星上存在这一过程（Reddy，V.等人[2012]。Icarus 221，544 - 559; McCord，T.B.等人[2012]。Nature 491，83 - 86; Prettyman，T.H.等人[2012]。Science 338，242 - 246; Denevi，B.W.等人[2012]。我们的样本中有11个成员在某些旋转阶段显示出分叉的雷达回波，其中8个是高雷达回波目标。分叉回波的一种解释是接触双星，如216 Kleopatra，我们的几个样本是接触双星候选者。然而，其他目标的证据表明它们不是接触双星。相反，我们假设这些小行星的表面体积密度可能会有大规模的变化，即在近表面存在富含金属和富含硅酸盐的孤立区域，这可能是金属和富含硅酸盐的小行星之间碰撞的结果。
Using the S-band radar at Arecibo Observatory, we observed thirteen X/M-class asteroids; nine were previously undetected and four were re-observed, bringing the total number of Tholen X/M-class asteroids observed with radar to 29. Of these 29M-class asteroids, 13 are also W-class, defined as M-class objects that also display a 3-μm absorption feature which is often interpreted as the signature of hydrated minerals (Jones, T.D., Lebofsky, L.A., Lewis, J.S., Marley, M.S. [1990]. Icarus 88, 172–192; Rivkin, A.S., Howell, E.S., Britt, D.T., Lebofsky, L.A., Nolan, M.C., Branston, D.D. [1995]. Icarus 117, 90–100; Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E., Britt, D.T. [2000]. Icarus 145, 351–368).Consistent with our previous work (Shepard, M.K. et al. [2008]. Icarus 195, 184–205; Shepard, M.K., Harris, A.W., Taylor, P.A., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M. [2011]. Icarus 215, 547–551), we find that 38% of our sample (11 of 29) have radar albedos consistent with metal-dominated compositions. With the exception of 83 Beatrix and 572 Rebekka, the remaining objects have radar albedos significantly higher than the mean S- or C-class asteroid (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. [2007]. Icarus 186, 126–151).Seven of the eleven high-radar-albedo asteroids, or 64%, also display a 3-μm absorption feature (W-class) which is thought to be inconsistent with the formation of a metal dominated asteroid. We suggest that the hydration absorption could be a secondary feature caused by low-velocity collisions with hydrated asteroids, such as CI or CM analogs, and subsequent implantation of the hydrated minerals into the upper regolith. There is recent evidence for this process on Vesta (Reddy, V. et al. [2012]. Icarus 221, 544–559; McCord, T.B. et al. [2012]. Nature 491, 83–86; Prettyman, T.H. et al. [2012]. Science 338, 242–246; Denevi, B.W. et al. [2012]. Science 338, 246–249).Eleven members of our sample show bifurcated radar echoes at some rotation phases; eight of these are high radar albedo targets. One interpretation of a bifurcated echo is a contact binary, like 216 Kleopatra, and several of our sample are contact binary candidates. However, evidence for other targets indicates they are not contact binaries. Instead, we hypothesize that these asteroids may have large-scale variations in surface bulk density, i.e. isolated patches of metal-rich and silicate-rich regions at the near-surface, possibly the result of collisions between metal and silicate-rich asteroids.