科学家发现水星表面阴暗的原因
文章来源:未知 文章作者:enread 发布时间:2016-03-08 06:19 字体: [ ]  进入论坛
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Scientists have long been puzzled about what makes Mercury's surface so dark. The innermost planet reflects much less sunlight than the Moon, a body on which surface darkness is controlled by the abundance of iron-rich minerals. These are known to be rare at Mercury's surface, so what is the "darkening agent" there? About a year ago, scientists proposed that Mercury's darkness was due to carbon that gradually accumulated from the impact of comets that traveled into the inner Solar System. Now scientists, led by Patrick Peplowski of the Johns Hopkins University Applied1 Physics Laboratory, have used data from the MESSENGER mission* to confirm that a high abundance of carbon is present at Mercury's surface. However, they also have also found that, rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust, some of which was later brought to the surface by impact processes after most of Mercury's current crust had formed. The results are published in the March 7, 2016, Advanced Online Publication of Nature Geoscience.
 
Co-author and Deputy Principal Investigator2 of the MESSENGER mission, Carnegie's Larry Nittler, explained: "The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence. We used MESSENGER's Neutron3 Spectrometer to spatially4 resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust. Moreover, we used both neutrons5 and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface." 
 
MESSENGER obtained its statistically6 robust7 data via many orbits on which the spacecraft passed lower than 60 miles (100 km) above the surface of the planet during its last year of operation. The data used to identify carbon included measurements taken just days before MESSENGER impacted Mercury in April 2015. Repeated Neutron Spectrometer measurements showed higher amounts of low-energy neutrons, a signature consistent with the presence of elevated carbon, coming from the surface when the spacecraft passed over concentrations of the darkest material. Estimating the amount of carbon present required combining the neutron measurements with other MESSENGER datasets, including X-ray measurements and reflectance spectra8. Together, the data indicate that Mercury's surface rocks are made up of as much as a few weight percent graphitic carbon, much higher than on other planets. Graphite has the best fit to the reflectance spectra, at visible wavelengths9, and the likely conditions that produced the material. 
 
When Mercury was very young, much of the planet was likely so hot that there was a global "ocean" of molten magma. From laboratory experiments and modeling, scientists have suggested that as this magma ocean cooled, most minerals that solidified10 would sink. A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury.


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1 applied Tz2zXA     
adj.应用的;v.应用,适用
参考例句:
  • She plans to take a course in applied linguistics.她打算学习应用语言学课程。
  • This cream is best applied to the face at night.这种乳霜最好晚上擦脸用。
2 investigator zRQzo     
n.研究者,调查者,审查者
参考例句:
  • He was a special investigator for the FBI.他是联邦调查局的特别调查员。
  • The investigator was able to deduce the crime and find the criminal.调查者能够推出犯罪过程并锁定罪犯。
3 neutron neutron     
n.中子
参考例句:
  • Neutron is neutral and slightly heavier than the proton.中子是中性的,比质子略重。
  • Based on the neutron energy,the value of weighting factor was given.根据中子能量给出了相应的辐射权重因子的数值。
4 spatially ca2278711fa6d1d851ea605dea2e8224     
空间地,存在于空间地
参考例句:
  • A well-defined array of stable fringes results and the field is spatially coherent. 结果得到一组完全确定的稳定条纹,而光场是空间相干的。
  • The units are collaged together by a serial tectonic spatially and temporally. 这些构造单元是由构造原因,依一定的时空序列拼贴在一起的。
5 neutrons 8247a394cf7f4566ae93232e91c291b9     
n.中子( neutron的名词复数 )
参考例句:
  • The neutrons and protons form the core of the atom. 中子和质子构成了原子核。 来自《简明英汉词典》
  • When an atom of U235 is split,several neutrons are set free. 一个铀235原子分裂时,释放出几个中子。 来自《简明英汉词典》
6 statistically Yuxwa     
ad.根据统计数据来看,从统计学的观点来看
参考例句:
  • The sample of building permits is larger and therefore, statistically satisfying. 建筑许可数的样本比较大,所以统计数据更令人满意。
  • The results of each test would have to be statistically independent. 每次试验的结果在统计上必须是独立的。
7 robust FXvx7     
adj.强壮的,强健的,粗野的,需要体力的,浓的
参考例句:
  • She is too tall and robust.她个子太高,身体太壮。
  • China wants to keep growth robust to reduce poverty and avoid job losses,AP commented.美联社评论道,中国希望保持经济强势增长,以减少贫困和失业状况。
8 spectra RvCwh     
n.光谱
参考例句:
  • The infra-red spectra of quinones present a number of interesting features. 醌类的红外光谱具有一些有趣的性质。
  • This relation between the frequency and the field spectra was noted experimentally. 实验上已经发现频率和场频谱之间的这种关系。
9 wavelengths 55c7c1db2849f4af018e7824d42c3ff2     
n.波长( wavelength的名词复数 );具有相同的/不同的思路;合拍;不合拍
参考例句:
  • I find him difficult to talk to—we're on completely different wavelengths. 我没法和他谈话,因为我们俩完全不对路。 来自《简明英汉词典》
  • Sunlight consists of different wavelengths of radiation. 阳光由几种不同波长的射线组成。 来自辞典例句
10 solidified ec92c58adafe8f3291136b615a7bae5b     
(使)成为固体,(使)变硬,(使)变得坚固( solidify的过去式和过去分词 ); 使团结一致; 充实,巩固; 具体化
参考例句:
  • Her attitudes solidified through privilege and habit. 由于特权和习惯使然,她的看法变得越来越难以改变。
  • When threatened, he fires spheres of solidified air from his launcher! 当危险来临,他就会发射它的弹药!
TAG标签: surface mercury darkness
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