Remote sensing evidence for an ancient carbon-bearing crust on Mercury
Remote sensing evidence for an ancient carbon-bearing crust on Mercury
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DOI：
10.1038/ngeo2669
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发表时间：
2016-04
影响因子：
18.3
通讯作者：
P. Peplowski;R. Klima;D. Lawrence;C. Ernst;B. Denevi;E. Frank;J. Goldsten;S. Murchie;L. Nittler;S. Solomon
中科院分区：
文献类型：
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作者：
P. Peplowski;R. Klima;D. Lawrence;C. Ernst;B. Denevi;E. Frank;J. Goldsten;S. Murchie;L. Nittler;S. Solomon
水星的全球表面比从其测量的元素组成预测的要暗得多。这种以前没有被发现的变暗剂主要集中在水星反射率最低的光谱单位，即低反射率物质中。这种低反射率的物质通常存在于大型撞击坑及其喷出物中，这表明其起源于中下地壳。在这里，我们提出了从信使号航天器的水星表面的中子光谱测量，揭示了热中子计数率的增加，在空间上与低反射率材料的沉积物。与中子测量和低反射率材料的可见光到近红外光谱两者一致的唯一元素是碳，其丰度比周围的高反射率材料高1- 3wt%。我们推断，碳是水星上主要的暗化剂，低反射率材料采样了行星地壳内的含碳沉积物。我们的研究结果是一致的形成石墨漂浮地壳从早期的岩浆海洋，我们建议，这个古老的层的严重破坏的残余物坚持在现在的上地壳。在这种情况下，水星的全球低反射率是由于富含石墨的古老地壳与上覆火山物质的混合，通过撞击过程或在次级地壳形成期间将碳同化到上升的岩浆中。
Mercury’s global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury’s lowest-reflectance spectral unit, the low-reflectance material. This low-reflectance material is generally found in large impact craters and their ejecta,, which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury’s surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra of low-reflectance material is carbon, at an abundance that is 1–3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet’s crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean, and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury’s globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.