中图分类号: P691
文献标识码: A
文章编号: 1001-8166(2018)05-0473-10
通讯作者:
收稿日期: 2018-01-1
修回日期: 2018-04-10
网络出版日期: 2018-05-20
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Zeng Xiandi(1992-), male, Guangzhou City, Guangdong Province, Master student. Research areas include lunar planetary science.E-mail:zengxiandi@mail.gyig.ac.cn
作者简介:曾献棣(1992-),男,广东广州人,硕士研究生,主要从事月球与行星科学研究.E-mail:zengxiandi@mail.gyig.ac.cn
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摘要
水是生命活动的基础,也是天体演化的重要部分。月球一直被认为是“无水”星体,但这一观点被最新的研究成果推翻。月球遥感红外光谱和Apollo样品分析结果均证实了月球表面能通过太阳风质子与月壤矿物相互作用来产生OH甚至是H2O。为探讨其反应过程,相关理论分析和离子注入模拟实验等研究已逐步开展。但是,目前对于太阳风成因水的成因机制,形成时的影响因素,产生后在月表的赋存、迁移和保留机制仍缺乏系统研究。针对这些问题,未来立足于嫦娥五号样品分析,建立月球表面太阳风成因水的形成和迁移运动的模型将会是推进月球水研究的重要部分。这不仅能为月球水资源开发利用提供线索,还可能为太阳系内其他无大气类地行星水来源和演化研究提供参考。
关键词：
Abstract
Water plays an important role in the evolution history of terrestrial planets and is also an indispensable resource for space exploration. The moon was used to be thought as “bone-dry”. However, this view was challenged by the latest achievements. Both the infrared remote sensing data and Apollo sample results have shown that some hydroxyl (and even H2O) can be produced by the reaction between the solar wind proton and regolith mineral on the Moon. A series of theoretical analysis and simulated ion implantation experiments have been carried out to discuss such processes. Many issues related to the solar wind-produced water have not been well understood yet, e.g., the formation mechanism, influencing factors, occurrence state, migration, and retention. To answer these questions, it is necessary to investigate the formation mechanism and migration of solar wind-produced water based on the Change’e-5 returned samples in the future. These studies can not only can provide clues for the exploitation and utilization of water on the Moon, but also help us to understand the origin and evolution of water on other airless terrestrial planets.
Keywords：
水是生命活动赖以维持的基础,是揭示太阳系天体形成演化过程的重要依据,同时也是重要的资源物质,因此,地外天体水一直是月球和深空探测的重要内容。但是月球水的探测却经历了漫长曲折的过程。月球水最早由Watson 等[1]提出,认为月球永久阴影极区的低温环境(约40 K)有助于保存水冰。但是Apollo和Luna时期的探测结果表明月球是一个极其干燥的天体,无论在月球表面还是月球样品中都没有发现水存在的明显证据[2]。
20世纪90年代的月球探测引起了人们对月球水的再次关注。1992年利用Arecibo天文台地基雷达对月球两极进行了探测,但没有发现面积大于1 km2的水冰区域[3]。1994年发射的Clementine通过雷达检测到月球极区永久阴影区中疑似水冰的信号,但是由于雷达数据的多解性,这一结论未获得广泛共识[4,5]。1998年发射的Lunar Prospector 搭载的中子光谱仪探测到月球两极有大量的氢(H)存在,却无法确定其存在形式(水冰、羟基或其他含氢化合物)[6,7,8,9]。更遗憾的是,1999年Lunar Prospector撞击到月球Shackleton撞击坑的试验中也没有获取到任何有关水冰的证据[10]。自此,月球水的探测活动又重新进入低迷时期。
21世纪是月球水探测和研究的新时期,无论是遥感探测还是月球样品的重新分析,都明确证实了月球水的存在。在遥感探测方面,Cassini,Chandrayaan-1和Deep Impact 这3次探测任务上搭载的红外光谱仪均发现几乎整个月球表面都出现2.8和3 μm的水吸收信号,证实了月表羟基和水的存在[11,12,13]。2008年发射的Chandrayaan-1搭载的微型雷达对月球北极撞击坑进行了探测,通过排除月表粗糙度的影响,初步确定了30个撞击坑含有水冰[14]。2009年发射的月球勘测轨道器(Lunar Reconnaissance Orbiter,LRO)搭载的中子探测仪在永久阴影区中也发现了丰富的H[15]。2009年月球陨坑观测与传感卫星(Lunar Crater Observation and Sensing Satellite,LCROSS)撞击月球Cabeus撞击坑,通过检测撞击溅射物质成分最终确认该地区有水的存在,并推测其水含量为(5.6±2.9)wt.%(wt.%为重量百分含量)[16]。在月球样品分析方面,随着实验室精细分析技术的发展和提高,对Apollo时期返回月球样品的重新检测,相继发现月球中的火山玻璃、磷灰石、钙长石、胶结质玻璃、熔融包裹体中有微量水的存在,而且这些水并非来自地球污染[17,18,19,20,21,22]。
目前认为月球水的来源主要有3种[23,24,25,26,27]:①月球内部岩浆洋的水;②彗星或陨石撞击携带的水;③太阳风质子与月表含氧物质相互作用产生的水。其中,产生于太阳风质子与月表含氧矿物之间相互作用的太阳风成因水,会受月表温度发生迁移运动,部分最终在月球永久阴影区中保存下来[28,29]。因此开展月球太阳风成因水的研究,是认识月球水的来源、成因及其演化的重要基础,有助于评估太阳风成因水对月球极区水冰来源和储存的贡献,对月表水开发利用具有重要意义。
太阳风成因水不仅在月球出现,太阳系其他无大气天体如水星、小行星以及星际尘埃物质也一直接受太阳辐射,很有可能在其表面形成水。Starukhina[9]总结了雷达探测水星极区水冰、中子谱仪探测月表H分布以及红外光谱仪探测M和E群小行星3 μm 附近水吸收峰的结果,提出月球、水星和小行星表面探测到的水主要形成于太阳风质子注入作用;轨道观测器Dawn探测到在小行星带的Vesta和Ceres上有疑似水的存在[30,31],Farrell等[32]认为其部分水也可能产生于太阳风作用;Delitsky等[33]探讨水星极地阴影区的挥发分时认为其中部分水由太阳风注入产生;Bradley等[34]在行星际尘埃硅酸盐颗粒表面的非晶质环带中也检测到太阳风成因水;Djouadi等[35]通过模拟实验得出星际尘埃的颗粒表面能产生太阳风成因的OH基团,并指出太阳成因的OH可能在类地行星吸积过程中提供一个重要的水源。太阳系形成距今已约有46亿年,从太阳开始燃烧并产生辐射,太阳风就会持续不断作用于行星吸积形成前的太阳星云尘埃颗粒并形成OH,最终部分水可能保留于行星内部。Vattuone等[36]指出在地球吸积时,太阳风注入产生的羟基在高温条件下仍能保留于矿物结构中,这部分水可能是早期地球水的重要来源之一。所以,深入探讨月球太阳风成因水,不仅能对比太阳系中其他无大气天体表面太阳风成因水的形成和演化过程,还有助于揭示内太阳系行星内部水的来源和演化。
虽然月球太阳风成因水的假设很早就被提出,但因为缺少实证,长期以来没有被学术界重视,直到Cassini,Chandrayaan-1和Deep Impact 3次绕月遥感探测搭载的红外光谱仪获得了月球表面存在水的重要证据[11,12,13],月球太阳风成因水的研究才不断深入。
Cassini搭载的可见光—红外成像光谱仪(Visual and Infrared Mapping Spectrometer,VIMS)由2个光学系统组成,光谱范围为0.3~5.1 μm,能同时产生352个二维图像,空间分辨率为每像素0.5 mrad×0.5 mrad,红外波段的光谱分辨率为16 nm[37]。Chandrayaan-1搭载的红外光谱仪是Moon Mineralogy Mapper(M3),光谱范围为0.43~3.00 μm,0.7 mrad的空间分辨率对应的光谱分辨率为10 nm[38]。Deep Impact上搭载的是HRI-IR(High Resolution Instrument-Infrared),测量范围为1.05~4.80 μm[39]。从光谱的测量范围看,这3个红外遥感光谱仪均包含OH和H2O基频的吸收范围[40,41],且这3种红外遥感光谱仪都是探测月表水有效的仪器。
通过对这3次红外遥感探测数据的分析,获得了以下结论[11~13,42~46]:①3个红外光谱均发现月球表面广泛存在2.8和3.0 μm的信号,代表了OH/H2O,其中2.8 μm的特征峰在整个月球表面都有检测到,而3 μm主要分布在中高纬度地区(图1);②OH和H2O的吸收强度与纬度存在正相关性,即在赤道的吸收强度最弱,随着纬度增加其吸收强度逐渐增强,极区最强,这反映了水从低纬度到高纬度存在丰度变化;③OH和H2O的吸收强度与温度(光照条件)具有负相关性(图2),Deep Impact检测的光谱显示,相同或相近区域在早晨其OH和H2O的吸收强度最强,中午表现出最弱,到傍晚基本恢复到早上的吸收强度,说明日照面的月表能持续产生OH/H2O,并且其水含量是瞬时变化的,随着光照和温度的增强,OH和H2O不断减少;④月表水含量与物质成分也存在一定的相关性,月表高地OH和H2O的吸收强度均明显强于月海,特别是几个年轻的富斜长石撞击坑表现出非常强的OH/H2O吸收,并且高地和月海OH/H2O受温度和太阳辐射的影响也有所不同,表现为高地长石保存水的能力要高于月海玄武岩;⑤OH/H2O的红外光谱信号强度与月壤成熟度成正相关,随着月表矿物颗粒受太空风化越强烈,其成熟度越高,而光谱数据显示该区域的水含量要更高;⑥紫外辐射对月表OH也具有一定的影响,通过分析Boguslawsky撞击坑的M3数据,发现在清晨含OH的逃逸主要由热效应作用控制,而午后则主要受紫外辐射造成的光解作用影响;⑦这3次红外光谱均分别估算出了月表水含量,VIMS检测到水含量范围为10×10-6~1 000×10-6,M3数据反映的水含量最高可达770×10-6,HRI-IR估算出水含量小于5 000×10-6。
这3次红外光谱探测不仅证实了月表全球范围内OH和H2O的存在,并且还发现OH和H2O的吸收强度与纬度、光照条件(温度)和矿物成分具有一定的相关性。由于红外光谱仅代表月表最上层毫米级范围内物质的吸收特征,且月表OH/H2O信号表现具有全球性分布和随光照条件变化的特点,结合Lunar Prospector搭载的中子光谱探测到月表数厘米内H的信号分布[6],说明红外光谱仪探测到的OH和H2O形成于月球最表层的水合作用,并且这种动态的水合作用明显受太阳辐射的驱动,这意味着探测到的羟基和水主要来源于太阳风的注入作用。
图1 月表上遥感红外水吸收峰分布
(a)VIMS检测在月表不同纬度范围的水吸收峰变化[
Fig.1 Infrared remote sensing of water absorption peaks on lunar surface
(a) Varying water absorbance distribute on lunar surface in different latitudes from VIMS[
图2 对比1/4天内月海和高地区域在不同纬度和不同时间的水吸收峰变化[
(a)不同纬度月海区域的水吸收峰变化;(b)不同纬度高地区域的水吸收峰变化;(c)月海区域在不同时间的水吸收峰变化; (d)高地区域在不同时间的水吸收峰变化;(e)1,2,3,9,M为月海区域,5,6,7,8,H为高地区域
Fig.2 Comparisons of mare and highland terrains as a function of latitude and time of day as observed over a quarter of a lunar day[
(a)Variation of water absorption in mare region at different latitude;(b)Variation of water absorption in highland region at different latitude; (c)Variation of water absorption in mare region at different time;(d)Variation of water absorption in highland region at different time;(e)1, 2, 3, 9, M for mare region, 5,6,7,8, H for highland region
除了遥感探测,月壤样品分析结果也是太阳风成因水的重要证据之一。最早由Housley等[47,48]提出太阳风质子进入月表矿物颗粒中能使部分矿物发生还原反应,产生亚微米级铁颗粒以及OH或H2O。此外,他们还提出在月壤胶结质玻璃中出现的小囊泡结构是由太阳风气体(主要为H+)及其产物(OH或者H2O)饱和充填所形成的。尽管如此,这仍不是月表太阳风成因水存在的直接证据。
近年,Apollo计划时期返回月壤的分析结果确认了部分样品中含有微量水,并根据氢同位素特征推断了这些水的可能来源,其中就包括太阳风来源。Liu等[20]利用傅里叶红外光谱(Fourier Transform Infrared Spectroscopy,FTIR)和二次离子探针(Secondary Ion Mass Spectroscopy,SIMS)分析了Apollo 11和17月海月壤、Apollo 16高地月壤样品。FTIR在Apollo 11和17的胶结质玻璃中发现了OH的特征峰,其水含量为70×10-6~170×10-6,同时,SIMS检测出样品水含量为160×10-6~200×10-6,氢同位素比值为δD<-500‰(图3),与太阳风的δD≈-1 000‰相近(其他来源如地球、陨石和彗星等的δD远高于胶结质玻璃中所测的δD值),这意味着太阳风对月壤胶结质玻璃中水的贡献,证实了太阳风成因水的存在。此外,Izawa等[49]用反射光谱分析从月球返回后一直保存在真空中且未受地球大气污染的月球样品,样品类型包括Apollo11,12,15,16和17。光谱分析发现所有样品都出现了3 μm附近水的吸收峰,含水量为800×10-6~1 600×10-6,与VIMS,HRI-IR和M3的光谱数据结果相一致。由于反射光谱检测到的3 μm附近的信号反映了样品极表面的水合作用,且其吸收峰深度和样品的成熟度具有正相关性,因此得出了分析月壤样品中的水主要来源于太阳风质子注入作用的结论。
Fig.3 FTIR spectra and SIMS data of two agglutinates[
δD(‰)=[(D/H)measured/(D/H)standard-1]×1 000,(D/H)standard=1.5576×10-4; ppm为×10-6
针对月表太阳风质子与月壤含氧物质之间的相互作用形成水这一问题,国内外部分学者从理论上分析了该过程的成因机制,论证反应形成条件与合理性,并开展了相关的模拟实验,验证了太阳风成因水的反应过程,并对一些影响因素进行了分析。
太阳风是指从太阳日冕向行星际空间辐射的连续的等离子体粒子流,包含约95%的质子,约4%的氦核和极少量的其他元素,其到达地月系统的通量约为4×108/(cm2·s),平均能量约为1 keV(范围为0.5~3 keV)[50,51,52]。而月壤是受太阳风作用的直接物质,主要矿物有斜长石、辉石、橄榄石及大量的胶结质玻璃等[53]。在一系列太空风化的长期作用下,月壤矿物颗粒表面会产生大量晶格缺陷,这为太阳风质子在晶格结构中保留并成键提供了条件[54]。通常注入的太阳风质子能进入到矿物颗粒表面0.5×10-5~1×10-5cm以下的位置[8],其中部分以弱键形式,如氢键或范德华力等停留在月壤颗粒表层结构中,但这些键在月表环境中不稳定,容易受到温度等影响发生逃逸。另外,部分质子能进入矿物颗粒内部的缺陷中,并与O离子相结合形成-OH甚至是H2O[42,55],这部分OH/H2O相对稳定。Starukhina[9]结合太阳风质子注入量以及月表矿物对H的储集能力,通过数学模型论证太阳风作用生成OH的饱和浓度以及温度和月壤物性对OH形成的影响。Farrell等[32]在Starukhina[9]的基础上,细化了理论计算公式,指出颗粒表面活化能较大时(>1 eV),注入的H就能长时间保留在月壤中并可能形成OH,活化能较小时(<0.2 eV)则很快逃逸,当活化能能量范围在0.3~0.9 eV时,注入的H极易受环境温度影响,其中月壤中的晶格缺陷起关键作用。
为验证太阳风质子与含氧物质相互作用生成水这一过程,国内外学者开展了模拟太阳风质子注入实验。最早Zeller等[56]通过质子注入实验来证明月表羟基的形成,并为太阳风成因水假设提供实验证据。该实验使用的注入能量远高于太阳风注入月表矿物的能量,所用样品是化学合成的模拟玻璃质物质。数据结果显示H生成OH的转化率能达到5%~100%,虽然实验条件未达到真实的月表环境,但为太阳风作用成因水提供了很好的研究思路和方法。
近年来,随着太阳风作用成因水的研究热潮重新掀起,并且实验设备和检测技术不断完善,太阳风模拟研究实验也取得了新的进展。
Zent等[57]使用Apollo 17月壤样品开展了太阳风生成OH的模拟实验,利用红外光谱仪检测经H等离子体辐射前后样品中水的吸收峰变化,确认有OH/H2O产生。Managadze 等[58]对橄榄石和SiO2的粉末样品注入D+来模拟太阳风质子注入生成水实验,SIMS检测发现注入后不仅有OD产生,而且还有D2O,由于H+和D+性质相近,由此推断太阳风质子注入在月表矿物中会产生OH/H2O。Ichimura等[59]使用了月壤颗粒样品进行模拟实验,分别使用1.1 keV的H+和D+开展了注入实验,实验结果也都显示注入后的样品有OH和OD生成,同样验证了太阳风成因水的存在。Bradley等[34]用5 keV 的H+对标准无水硅酸盐矿物橄榄石、单斜辉石和钙长石样品进行了离子注入模拟实验,并通过透射电镜和价电子损失能谱仪(Valence Electron Eenergy-Loss Spectrometer,VEELS)检测出离子注入后样品表层非晶质环带中的小气泡有新形成的水存在,其中钙长石中的水信号最强。目前所有模拟实验中仅有Burke等[60]的实验没有羟基或水生成,Managadze等[58]推测其可能原因是实验使用的样品为抛光薄片样品,其表面积要比粉末样品小得多,从而导致实验效果不理想。Farrell等[32]则认为样品的晶格缺陷起关键作用,Burke等[60]使用的样品是抛光后完整矿物,内部缺陷极少,不利于H保留。
以上模拟实验主要是验证太阳风质子与含氧物质相互作用是否能够产生水,实验流程和条件相对单一。为了更深入地认识太阳风作用生成水过程的影响因素,Djouadi等[35]选用橄榄石作为模拟实验的样品,Schaible等[61]则用非晶质化SiO2和橄榄石作为模拟实验的样品,实验设计不同的能量、通量及惰性离子预处理(提高样品成熟度)等条件,探究这些因素对太阳风成因水的具体影响。实验结果表明:这些H+注入实验均产生了OH,并且注入H+通量越高,样品生成OH的量越多,但是会达到一个饱和值,其中橄榄石通量的饱和值要高于非晶质SiO2;注入能量越高能OH的饱和值越高;此外提高样品成熟度同样也能提高OH的生成量。
总的来说,目前太阳风注入的模拟实验已经获得了新的进展,不仅证实了月表条件下太阳风成因OH/H2O的可能性,并且还揭示了太阳风质子与月壤物质相互作用产生水这一过程会受到含氧物质类型、太阳风通量(暴露时间)和月壤成熟度等因素影响。
迄今为止,在月球表面太阳风质子轰击到月壤颗粒上并与矿物中的氧发生反应生成OH或者H2O已经得到了很多重要的证据,包括一系列的红外遥感数据、月壤返回样品的分析结果以及相关的理论分析和模拟实验。但太阳风成因水的具体形成机制、特征和影响因素等重要问题尚不明确,还需要进行系统深入的研究。
(1)太阳风成因水的成因机制。①关于质子与颗粒物质是如何相互作用的,目前成因机制仅是假说,具体反应过程尚不清楚。②虽然遥感红外数据中出现2.8与3 μm的红外特征峰,但两者的分布并不完全统一,可能有不同生成条件。这个问题即使是在模拟实验中也未得到很好的验证和解释,虽然目前认为在质子注入过程中形成OH达到饱和后,在H继续补充的情况下,OH会进一步形成H2O[42,59],但仅有Bradley等[34]的实验有H2O产生,具体原因仍不清楚。所以关于太阳风成因水的形成过程,是一步生成还是分步进行,羟基和水生成对应的具体反应所需条件和反应过程是什么,都需要更深入的分析。
(2)太阳风成因水的特征。目前学术界对太阳风生成的OH或者H2O的化学状态、赋存位置、含量以及稳定性等特征都还未有明确的认识。Mccord等[42]初步认为太阳风成因的OH和H2O赋存形态主要有2种形式:一是H与颗粒表层悬挂的O结合,以范德华力或弱键形式存在于极表层区域,容易受外界影响而分解;二是扩散到内部的H与矿物内部的晶格缺陷中的O结合形成结构水,其状态相对更稳定。Bradley 等[34]用透射电镜和VEELS检测到实验后矿物样品非晶质环带与矿物界面上的气泡有水的存在,认为OH/H2O可能存在于气泡之中。因此太阳风成因水的化学状态、赋存位置、含量变化和稳定性仍需要进一步研究。
(3)太阳风成因水生成的影响因素。太阳风成因水会受内外两大因素影响,内在因素包括月壤物质类型和成熟度,外在因素包括光照、温度、紫外辐射、真空等。但目前仍未对这些因素做全面系统的分析和探讨。遥感红外光谱结果分析和模拟实验中均反映了不同类型区域(样品)中生成水和保存水的能力存在差异[12,13,34,61]。这是由于不同硅酸盐矿物的化学成分和结构具有明显差异,注入质子与氧结合的形态和化学位置也会不同,进而影响不同矿物中保留H和形成水的能力。但是目前仅对比分析了部分矿物在形成OH之间的差异,尚未形成系统全面的认识。其次,Izawa等[49]的研究揭示了月壤成熟度与太阳风成因水存在一定正相关性,成熟度越高意味着对矿物晶格的破坏越严重,更有利于质子的进入和扩散,从而促进OH和H2O的生成,但对该影响仅进行了简单分析,其具体影响程度尚不清楚。外在因素包括温度、紫外辐射和真空,对于太阳风成因水的保存具有重要影响,但是目前尚没有开展定量、全面的分析探讨,仅进行了单一因素的定性对比。
(4)太阳风成因水的迁移运动。在月表真空环境中受温度和辐射的驱动下,太阳风成因水由于其特殊成因表现出不稳定性,极易发生分解、逃逸和迁移。部分学者在理论上探讨了月表水的迁移运动过程。Butler[28]用数值模拟简单计算出月表及水星表面的水分子迁移率分别为20%~50%和5%~15%,随后Crider等[62,63]用Monte-Carlo模型结合月表温度、大气情况和月壤矿物表面的解吸作用,计算出太阳风质子进入到月表有60%转化为H2,27%仍以H原子形式存在,而形成OH的仅占10%,迁移到极区的H主要以OH为主,约占66.7%。但是,由于月表环境的复杂性,包括质子进入矿物后反应生成水的含量和位置、不同区域太空风化作用的程度、月表不同纬度和时间对水的影响差异等,这些影响因素对太阳风成因水的生成、迁移、逃逸和保存都没有得到清楚的认识,限制了关于太阳风成因水迁移运动的研究,还需要更为系统全面的理论分析和实验验证。
总体来说,太阳风成因水作为月表水的重要来源之一已经普遍被学术界所认可,但还需要开展系统的研究,才能深入理解太阳风成因水的形成和特征。结合太阳风成因水的研究进展,立足于理论分析的基础上,根据月壤中不同矿物的晶格结构,计算不同阳离子和氧的结合能,探讨在太阳风质子注入下矿物结构损坏程度以及与氧反应生成OH甚至H2O的能力和稳定性,从而推断月表太阳风成因水在月壤不同载体矿物中的生成机制。目前仅在胶结质玻璃中发现太阳风成因水[20],所以未来需要结合CE-5返回样品来开展太阳风成因水的深入研究,分析不同类型月壤颗粒的水含量及氢同位素,结合采集点环境和月壤矿物性质来分析其赋存状态和稳定性,探讨其形成机理、保存机制和迁移过程。同时,以模拟实验的方式根据月表环境设计不同的条件,包括辐照的能量、通量、速流以及真空、温差、紫外辐射等,综合探讨月表太阳风成因水的形成和保存的影响因素。最后结合遥感数据、月球样品分析、理论计算和模拟实验的结果,建立月球表面太阳风成因水的形成和迁移运动的模型,全面认识月表太阳风成因水生成—分解—逃逸—迁移—储存这一系列复杂的过程,并以此评估太阳风成因水对月球极区水冰的贡献。总之,深入认识太阳风成因水不仅能更好地认识月球水的来源、形成和演化过程,更能进一步延伸到太阳系其他无大气天体表面太阳风成因水的特征对比,并且有助于探讨内太阳系行星内部水的来源。
The authors have declared that no competing interests exist.
| [1] |
The behavior of volatiles on the lunar surface [J].
Volatiles, and water in particular, have been thought to be unstable on the lunar surface because of the rapid removal of constituents of the lunar atmosphere by solar radiation, solar wind, and gravitational escape. The limiting factor in removal of a volatile from the moon, however, is actually the evaporation rate of the solid phase, which will be collected at the coldest points on the lunar surface. We present a detailed theory of the behavior of volatiles on the lunar surface based on solid-vapor kinetic relationships, and show that water is far more stable there than the noble gases or other possible constituents of the lunar atmosphere. Numerical calculations indicate the amount of water lost from the moon since the present surface conditions were initiated is only a few grams per square centimeter of the lunar surface. The amount of ice eventually detected in lunar 090004cold traps090005 thus will provide a sensitive indication of the degree of chemical differentiation of the moon.
|
| [2] |
et al. Review in Mineralogy and Geochemistry Vol.60: New Views of the Moon [M]. |
| [3] |
Arecibo radar mapping of the Lunar Poles: A search for ice deposits [J].
The Arecibo 12.6-centimeter wavelength radar system was used to image the polar regions of the moon at a resolution of 125 meters in a search for ice deposits in areas of possible permanent shadow from the sun. No areas greater than 1 square kilometer were found with high radar backscatter cross sections and high circular polarization ratios, properties suggestive of the presence of ice. A number of areas smaller than 1 square kilometer were found with these properties, but optical images from spacecraft missions have shown some of these features to be in sunlight. Arecibo radar images of Sinus Iridum at latitude 47掳$N also showed a number of small features with similar properties. The coincidence of some of these features with the radar-facing slopes of craters and their presence in sunlit areas suggests that very rough surfaces rather than ice deposits are responsible for their unusual radar properties.
|
| [4] |
The clementine bistatic radar experiment [J].
Abstract During the Clementine 1 mission, a bistatic radar experiment measured the magnitude and polarization of the radar echo versus bistatic angle, beta, for selected lunar areas. Observations of the lunar south pole yield a same-sense polarization enhancement around beta = 0. Analysis shows that the observed enhancement is localized to the permanently shadowed regions of the lunar south pole. Radar observations of periodically solar-illuminated lunar surfaces, including the north pole, yielded no such enhancement. A probable explanation for these differences is the presence of low-loss volume scatterers, such as water ice, in the permanently shadowed region at the south pole.
|
| [5] |
The possibility of ice on the moon [J].
) have dealt a blow to the hypothesis that ice deposits may exist in permanently shadowed regions at the lunar poles. Their ground-based radar observations detected several areas with high backscatter cross sections and circular polarization ratios consistent with ice, but in locations that are at least occasionally illuminated by sunlight. These features are associated with walls and rims of small craters; the most likely explanation for their occurrence is high surface roughness at the scale of the radar wavelength. Mercury has regions with similarly anomalous radar properties located near its poles, in permanently shadowed floors of large craters (
|
| [6] |
Fluxes of fast and epithermal neutrons from Lunar Prospector: Evidence for water ice at the lunar poles [J].
Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles. Depressions in epithermal fluxes were observed close to permanently shaded areas at both poles. The peak depression at the North Pole is 4.6 percent below the average epithermal flux intensity at lower latitudes, and that at the South Pole is 3.0 percent below the low-latitude average. No measurable depression in fast neutrons is seen at either pole. These data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.
|
| [7] |
Polar hydrogen deposits on the Moon [J].
Neutron and gamma-ray data measured using the Lunar Prospector spectrometers were analyzed to define the enhanced hydrogen deposits near both poles of the Moon. Combining the new low-altitude neutron data (3000±15 km) with previous high-altitude (10000±20 km) neutron data and the results of several recent radar investigations sharply constrains the characteristics of each of the polar deposits. The deposits at the north appear to be in the form of many small pockets or of generally distributed hydrogen that average to a 100 ppm weight fraction enhancement over that which exists in regolith at more equatorial latitudes. Those deposits in the permanently shaded craters near the south pole are consistent with a thick ferroan anorthosite regolith containing an enhancement of 167000±890 ppm hydrogen, which, if in the form of water ice, amounts to 1.500±0.8% weight fraction of H2O. Neutron data alone cannot discriminate between hydrogen implanted in lunar soil from the solar wind, hydrated minerals, or H2O. These craters appear to be surrounded by regolith that either contains small pockets of enhanced hydrogen or is soil that is uniformly impregnated with hydrogen enhanced on average by about 100 ppm above that contained in soils at more equatorial latitudes.
|
| [8] |
The lunar poles: Water ice or chemically trapped hydrogen? [J].
An alternative explanation is proposed for the hydrogen excess observed by the Lunar Prospector neutron spectrometer: solar wind protons trapped on radiation defects in regolith particles and effectively retained at the temperature of the lunar poles can be misinterpreted as water. Protons from the Earth's magnetotail plasma can be a source of hydrogen atoms in the regolith of permanently shadowed areas of the lunar surface.
|
| [9] |
Water detection on atmosphereless celestial bodies: Alternative explanations of the observations [J].
Alternative explanations are proposed for the results of the three types of remote sensing experiments in which water ice is supposed to be found on the surfaces of atmosphereless celestial bodies: (1) measurements of hydrogen content by the neutron spectrometer on Lunar Prospector, (2) observations of the absorption bands near 3 脦录m in reflectance spectra of asteroids, and (3) radar observations of polar regions of Mercury. Calculations have shown that in the first two types of observations solar wind protons chemically trapped by oxygen atoms in hydroxyl groups or other radiation defects in oxygen-bearing particles on the surfaces of atmosphereless bodies can be mistaken for water. Higher hydrogen content in the lunar polar regions, especially in polar craters, as compared to equatorial zones can be due to sharp decrease of escape probability with temperature. Spots of high hydrogen content in equatorial zones of the Moon can be explained by variations of degassing rates in different materials. Solar wind origin of OH groups may account for 3-脦录m absorption by asteroids of M and E classes thought to be differentiated. Strong, highly depolarized radar echoes from polar and lower latitude craters of Mercury can be explained by decrease of the dielectric loss of silicate material with temperature, which solves the problems of delivery and thermal stability of low-loss material on Mercury surface, being consistent with the observed regional variations of radar brightness. The possibilities considered in the paper should be taken into account in interpretation of the observations aimed on search of water.
|
| [10] |
Impacting lunar prospector in a cold trap to detect water ice [J].
Abstract Top of page Abstract References Lunar Prospector data support the contention that water ice reservoirs exist in the permanently shaded craters near the lunar poles. Yet the question remains whether the detected hydrogen abundance is actually water ice or is hydrogen in some other form. Present plans call for a controlled impact of Lunar Prospector into a polar crater at the end of July, 1999, in an attempt to liberate a small amount of water vapor that may be detected by ground- and space-based observatories. A positive spectral detection of water vapor or its photo-dissociated byproduct, OH , would be definite proof of the presence of water ice in the regolith. The following represents both an analysis of this method of searching for water ice as well as an announcement to the observing community of the event.
|
| [11] |
Detection of adsorbed water and hydroxyl on the Moon [J].
Data from the Visual and Infrared Mapping Spectrometer (VIMS) on Cassini during its flyby of the Moon in 1999 show a broad absorption at 3 micrometers due to adsorbed water and near 2.8 micrometers attributed to hydroxyl in the sunlit surface on the Moon. The amounts of water indicated in the spectra depend on the type of mixing and the grain sizes in the rocks and soils but could be 10 to 1000 parts per million and locally higher. Water in the polar regions may be water that has migrated to the colder environments there. Trace hydroxyl is observed in the anorthositic highlands at lower latitudes.
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| [12] |
Character and spatial distribution of OH/H2O on the surface of the Moon seen by M3 on Chandrayaan-1 [J].
The search for water on the surface of the anhydrous Moon had remained an unfulfilled quest for 40 years. However, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 has recently detected absorption features near 2.8 to 3.0 micrometers on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer hydrogen abundance data suggests that the formation and retention of hydroxyl and water are ongoing surficial processes. Hydroxyl/water production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.
|
| [13] |
Temporal and spatial variability of lunar hydration as observed by the deep impact spacecraft [J].
The Moon is generally anhydrous, yet the Deep Impact spacecraft found the entire surface to be hydrated during some portions of the day. Hydroxyl (OH) and water (H60O) absorptions in the near infrared were strongest near the North Pole and are consistent with <0.5 weight percent H60O. Hydration varied with temperature, rather than cumulative solar radiation, but no inherent absorptivity differences with composition were observed. However, comparisons between data collected 1 week (a quarter lunar day) apart show a dynamic process with diurnal changes in hydration that were greater for mare basalts (~70%) than for highlands (~50%). This hydration loss and return to a steady state occurred entirely between local morning and evening, requiring a ready daytime source of water-group ions, which is consistent with a solar wind origin.
|
| [14] |
Bussey D B J, Baloga S M, et al. Initial results for the north pole of the Moon from Mini-SAR, Chandrayaan-1 mission [J]. |
| [15] |
Hydrogen mapping of the lunar south pole using the LRO neutron detector experiment LEND [J]. |
| [16] |
Detection of water in the LCROSS ejecta plume [J].
Several remote observations have indicated that water ice may be presented in permanently shadowed craters of the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to provide direct evidence (1). On 9 October 2009, a spent Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris, dust, and vapor. This material was observed by a second "shepherding" spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Near-infrared absorbance attributed to water vapor and ice and ultraviolet emissions attributable to hydroxyl radicals support the presence of water in the debris. The maximum total water vapor and water ice within the instrument field of view was 155 卤 12 kilograms. Given the estimated total excavated mass of regolith that reached sunlight, and hence was observable, the concentration of water ice in the regolith at the LCROSS impact site is estimated to be 5.6 卤 2.9% by mass. In addition to water, spectral bands of a number of other volatile compounds were observed, including light hydrocarbons, sulfur-bearing species, and carbon dioxide.
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| [17] |
Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior [J].
The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.
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| [18] |
Lunar apatite with terrestrial volatile abundances [J].
Abstract The Moon is thought to be depleted relative to the Earth in volatile elements such as H, Cl and the alkalis. Nevertheless, evidence for lunar explosive volcanism has been used to infer that some lunar magmas exsolved a CO-rich and CO(2)-rich vapour phase before or during eruption. Although there is also evidence for other volatile species on glass spherules, until recently there had been no unambiguous reports of indigenous H in lunar rocks. Here we report quantitative ion microprobe measurements of late-stage apatite from lunar basalt 14053 that document concentrations of H, Cl and S that are indistinguishable from apatites in common terrestrial igneous rocks. These volatile contents could reflect post-magmatic metamorphic volatile addition or growth from a late-stage, interstitial, sulphide-saturated melt that contained approximately 1,600 parts per million H(2)O and approximately 3,500 parts per million Cl. Both metamorphic and igneous models of apatite formation suggest a volatile inventory for at least some lunar materials that is similar to comparable terrestrial materials. One possible implication is that portions of the lunar mantle or crust are more volatile-rich than previously thought.
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| [19] |
Water in lunar anorthosites and evidence for a wet early Moon [J].
The Moon was thought to be anhydrous since the Apollo era(1), but this view has been challenged by detections of water on the lunar surface(2-4) and in volcanic rocks(5-9) and regolith(10). Part of this water is thought to have been brought through solar-wind implantation(2-4,7,10) and meteorite impacts(2,3,7,11), long after the primary lunar crust formed from the cooling magma ocean(12,13). Here we show that this primary crust of the Moon contains significant amounts of water. We analysed plagioclase grains in lunar anorthosites thought to sample the primary crust, obtained in the Apollo missions, using Fourier-transform infrared spectroscopy, and detected approximately 6 ppm water. We also detected up to 2.7 ppm water in plagioclase grains in troctolites also from the lunar highland upper crust. From these measurements, we estimate that the initial water content of the lunar magma ocean was approximately 320 ppm; water accumulating in the final residuum of the lunar magma ocean could have reached 1.4 wt%, an amount sufficient to explain water contents measured in lunar volcanic rocks. The presence of water in the primary crust implies a more prolonged crystallization of the lunar magma ocean than a dry moon scenario and suggests that water may have played a key role in the genesis of lunar basalts.
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| [20] |
Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water [J].
Remote sensing discoveries of hydroxyl and water on the lunar surface have reshaped our view of the distribution of water and related compounds on airless bodies such as the Moon. The origin of this surface water is unclear, but it has been suggested that hydroxyl in the lunar regolith can result from the implantation of hydrogen ions by the solar wind. Here we present Fourier transform infrared spectroscopy and secondary ion mass spectrometry analyses of Apollo samples that reveal the presence of significant amounts of hydroxyl in glasses formed in the lunar regolith by micrometeorite impacts. Hydrogen isotope compositions of these glasses suggest that some of the observed hydroxyl is derived from solar wind sources. Our findings imply that ice in polar cold traps could contain hydrogen atoms ultimately derived from the solar wind, as predicted by early theoretical models of water stability on the lunar surface. We suggest that a similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface, such as Mercury and the asteroid 4-Vesta.
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| [21] |
High pre-eruptive water contents preserved in lunar melt inclusions [J].
The Moon has long been thought to be highly depleted in volatiles such as water, and indeed published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these samples of primitive lunar magma, by virtue of being trapped within olivine crystals before volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain 615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as much water as Earth鈥檚 upper mantle.
|
| [22] |
Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon [J].
Water plays a critical role in the evolution of planetary bodies, and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth-Moon system. During the Apollo programme, the lunar samples were found to be devoid of indigenous water. The severe depletion of lunar volatiles, including water, has long been seen as strong support for the giant-impact origin of the Moon. Recent studies have found water in lunar volcanic glasses and in lunar apatite, but the sources of lunar water have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, found in crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon's magmatic history. Variations of hydrogen isotope ratios in apatite suggest the lunar mantle, solar wind protons, and comets as possible sources for water in lunar rocks and imply a significant delivery of cometary water to the Earth-Moon system shortly after the Moon-forming impact.
|
| [23] |
Ice in the lunar polar regions [J].
The idea that ice and other trapped volatiles exist in permanently shadowed regions near the lunar poles was proposed by Watson, Murray, and Brown [1961]. It is reexamined in the present paper, in the light of the vast increase of our lunar knowledge. The stability of the traps and the trapping mechanism are verified. Four potential sources of lunar H2O, (1) solar wind reduction of Fe in the regolith, (2) H2O-containing meteoroids, (3) cometary impact, and (4) (the least certain) degassing of the interior, can supply amounts of trapped H2O estimated in the range of 1016芒聙聯1017 g. Two important destructive mechanisms have been identified: photodissociation of H2O molecules adsorbed on the sunlit surface and sputtering or decomposition of trapped H2O by solar wind particles. The effect of impact gardening is mainly protective. The question of the presence of H2O in the traps remains open; it can be settled by experiment.
|
| [24] |
A lunar ater world [J]. |
| [25] |
Lunar water: A brief review [J].
One of the most exciting recent developments in the field of lunar science has been the unambiguous detection of water (either as OH or H 2 O) or water ice on the Moon through instruments flown on a number of orbiting spacecraft missions. At the same time, continued laboratory-based investigations of returned lunar samples by Apollo missions using high-precision, low-detection, analytical instruments have for the first time, provided the absolute abundance of water (present mostly as structurally bound OH 鈭 in mineral phases) in lunar samples. These new results suggest that the Moon is not an anhydrous body, questioning conventional wisdom, and indicating the possibility of a wet lunar interior and the presence of distinct reservoirs of water on the lunar surface. However, not all recent results point to a wet Moon and it appears that the distribution of water on the Moon may be highly heterogeneous. Additionally, a number of sources are likely to have contributed to the water inventory of the Moon ranging from primordial water to meteorite-derived water ice through to the water formed during the reaction of solar-wind hydrogen with the lunar soil. Water on the Moon has implications for future astrobiological investigations as well as for generating resources in situ during future exploration of the Moon and other airless bodies in the Solar System.
|
| [26] |
An asteroidal origin for water in the Moon [J].
The Apollo-derived tenet of an anhydrous Moon has been contested following measurement of water in several lunar samples that require water to be present in the lunar interior. However, significant uncertainties exist regarding the flux, sources and timing of water delivery to the Moon. Here we address those fundamental issues by constraining the mass of water accreted to the Moon and modelling the relative proportions of asteroidal and cometary sources for water that are consistent with measured isotopic compositions of lunar samples. We determine that a combination of carbonaceous chondrite-type materials were responsible for the majority of water (and nitrogen) delivered to the Earth–Moon system. Crucially, we conclude that comets containing water enriched in deuterium contributed significantly <20% of the water in the Moon. Therefore, our work places important constraints on the types of objects impacting the Moon 654.5–4.3 billion years ago and on the origin of water in the inner Solar System. Recent samples have shown that the Moon's interior, previously thought to be anhydrous, contains water, yet how this water was delivered is unclear. Here, using isotopic analyses and modelling, Barneset al. show that carbonaceous chondrite-type objects delivered >80% of the Moon's bulk water.
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| [27] |
Simulations of a comet impact on the Moon and associated ice deposition in polar cold traps [J].
Modeling results of the water vapor plume produced by a comet impact on the Moon and of the resulting water ice deposits in the lunar cold traps are presented. The water vapor plume is simulated near the point of impact by the SOVA hydrocode and in the far field by the Direct Simulation Monte Carlo (DSMC) method using as input the SOVA hydrocode solution at a fixed hemispherical interface. The SOVA hydrocode models the physics of the impact event such as the surface deformation and material phase changes during the impact. The further transport and retention processes, including gravity, photodestruction processes, and variable surface temperature with local polar cold traps, are modeled by the DSMC method for months after impact. In order to follow the water from the near field of the impact to the full planetary induced atmosphere, the 3D parallel DSMC code used a collision limiting scheme and an unsteady multi-domain approach. 3D results for the 45° oblique impact of a 2 km in diameter comet on the surface of the Moon at 30 km/s are presented. Most of the cometary water is lost due to escape just after impact and only 653% of the cometary water is initially retained on the Moon. Early downrange focusing of the water vapor plume is observed but the later material that is moving more slowly takes on a more symmetric shape with time. Several locations for the point of impact were investigated and final retention rates of 650.1% of the comet mass were observed. Based on the surface area of the cold traps used in the present simulations, 651 mm of ice would have accumulated in the cold traps after such an impact. Estimates for the total mass of water accumulated in the polar cold traps over 1 byr are consistent with recent observations.
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| [28] |
The migration of volatiles on the surfaces of Mercury and the Moon [J].
Radar observations of Mercury have provided the startling discovery of the probable existence of substantial deposits of ices in permanently shaded polar regions [Butler et al., 1993; Harmon et al., 1994]. This has renewed the old argument about the existence of such deposits in the polar regions of the Moon. It is most likely necessary for volatiles to be able to migrate to the polar regions of these bodies in order to build up such deposits. This paper presents the results of the extension of a Monte Carlo simulation of the migration of molecules on the surface of Mercury by Butler et al. [1993]. The results of the simulations show that for typical conditions on Mercury, 090804509000915% of all H2O which is placed randomly on the surface migrates to stable (permanently shaded) polar regions. For the Moon the numbers are 0908042009000950%. The numbers for the migration of CO2 are 0908042% for Mercury and 09080413% for the Moon. These percentages are similar to those previously calculated and support the idea of ice deposits in the polar regions of both Mercury and the Moon.
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| [29] |
Subsurface migration of H2O at lunar cold traps [J].
Permanently shaded areas near the poles of the Moon and Mercury may harbor water ice. We develop a physical model for migration of water molecules in the regolith and discover two pathways that can lead to accumulation of HO in the subsurface. A small fraction of water molecules delivered, either continuously or abruptly, to permanently cold areas diffuses into the regolith and can remain there longer than on the surface. Higher temperatures lead to deeper burial. At constant temperature, this diffusive migration produces less than one molecular layer of volatile HO on grains, because it is driven by differences in surface concentrations. The water is therefore expected to be in adsorbed form, and the amount stored in this fashion could be at most a few hundred ppm of HO. A second pathway is pumping by diurnal temperature oscillations from a transient ice cover that may have formed during a large comet impact. It can lead to high ground ice densities, but the ground ice layer lasts not long beyond the disappearance of the ice cover. Both types of subsurface charging mechanism work best for temperatures typical of permanently shaded areas with sunlit surfaces in their field of view.
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| [30] |
Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres [J].
Dawn is in development for a mission to explore main belt asteroids in order to yield insights into important questions about the formation and evolution of the solar system. Its objective is to acquire data from orbit around two complementary bodies, (4) Vesta and (1) Ceres, the two most massive asteroids. The project relies on extensive heritage from other deep-space and Earth-orbiting missions, thus permitting the ambitious objectives to be accomplished with an affordable budget.
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| [31] |
Detection of widespread hydrated materials on vesta by the VIR imaging spectrometer on board the dawn mission [J].
Water plays a key role in the evolution of terrestrial planets, and notably in the occurrence of Earth's oceans. However, the mechanism by which water has been incorporated into these bodies—including Earth—is still extensively debated. Here we report the detection of widespread 2.8 μm OH absorption bands on the surface of the asteroid Vesta by the VIR imaging spectrometer on board Dawn. These observations are surprising as Vesta is fully differentiated with a basaltic surface. The 2.8 μm OH absorption is distributed across Vesta's surface and shows areas enriched and depleted in hydrated materials. The uneven distribution of hydrated mineral phases is unexpected and indicates ancient processes that differ from those believed to be responsible for OH on other airless bodies, like the Moon. The origin of Vestan OH provides new insight into the delivery of hydrous materials in the main belt and may offer new scenarios on the delivery of hydrous minerals in the inner solar system, suggesting processes that may have played a role in the formation of terrestrial planets.
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| [32] |
Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects [J].
Solar wind protons are implanted directly into the top 100nm of the lunar near-surface region, but can either quickly diffuse out of the surface or be retained, depending upon surface temperature and the activation energy, U, associated with the implantation site. In this work, we explore the distribution of activation energies upon implantation and the associated hydrogen-retention times; this for comparison with recent observation of OH on the lunar surface. We apply a Monte Carlo approach: for simulated solar wind protons at a given local time, we assume a distribution of U values with a central peak, Uc and width, Uw, and derive the fraction retained for long periods in the near-surface. We find that surfaces characterized by a distribution with predominantly large values of U (>1eV) like that expected at defect sites will retain implanted H (to likely form OH). Surfaces with the distribution predominantly at small values of U (<0.2eV) will quickly diffuse away implanted H. However, surfaces with a large portion of activation energies between 0.3eV<U<0.9eV will tend to be H-retentive in cool conditions but transform into H-emissive surfaces when warmed (as when the surface rotates into local noon). These mid-range activation energies give rise to a diurnal effect with diffusive loss of H at noontime.
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| [33] |
Ices on mercury: Chemistry of volatiles in permanently cold areas of Mercury’s north polar region [J].
Observations by the MESSENGER spacecraft during its flyby and orbital observations of Mercury in 2008鈥2015 indicated the presence of cold icy materials hiding in permanently-shadowed craters in Mercury's north polar region. These icy condensed volatiles are thought to be composed of water ice and frozen organics that can persist over long geologic timescales and evolve under the influence of the Mercury space environment. Polar ices never see solar photons because at such high latitudes, sunlight cannot reach over the crater rims. The craters maintain a permanently cold environment for the ices to persist. However, the magnetosphere will supply a beam of ions and electrons that can reach the frozen volatiles and induce ice chemistry. Mercury's magnetic field contains magneticcusps, areas of focused field lines containing trapped magnetospheric charged particles that will be funneled onto the Mercury surface at very high latitudes. This magnetic highway will act to direct energetic protons, ions and electrons directly onto the polar ices. The radiation processing of the ices could convert them into higher-order organics and dark refractory materials whose spectral characteristics are consistent with low-albedo materials observed by MESSENGER Laser Altimeter (MLA) and RADAR instruments. Galactic cosmic rays (GCR), scattered UV light and solar energetic particles (SEP) also supply energy for ice processing. Cometary impacts will deposit H2O,CH4, CO2and NH3raw materials onto Mercury's surface which will migrate to the poles and be converted to more complex CHNOS-containing molecules such as aldehydes, amines, alcohols, cyanates, ketones, hydroxides, carbon oxides and suboxides, organic acids and others. Based on lab experiments in the literature, specific compounds produced may be: H2CO, HCOOH, CH3OH, HCO, H2CO3, CH3C(O)CH3, C2O, CxO, C3O2, CxOy, CH3CHO, CH3OCH2CH2OCH3, C2H6, CxHy,NO2,HNO2, HNO3, NH2OH, HNO, N2H2, N3, HCN, Na2O, NaOH, CH3NH2,SO, SO2, SO3, OCS, H2S, CH3SH, even BxHy. Three types of radiation processing mechanisms may be at work in the ices: (1) Impact/dissociation, (2) Ion implantation and (3) Nuclear recoil (hot atom chemistry). Magnetospheric energy sources dominate the radiation effects. Total energy fluxes of photons, SEPs and GCRs are all around two or more orders of magnitudelessthan the fluxes from magnetospheric energy sources (in the focused cusp particles). However, SEPs and GCRs cause chemical processing at greater depths than other particles leading to thicker organic layers. Processing of polar volatiles on Mercury would be somewhat different from that on the Moon because Mercury has a magnetic field while the Moon does not. The channeled flux of charged particles through these magnetospheric cusps is a chemical processing mechanism unique to Mercury as compared to other airless bodies.
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| [34] |
Detection of solar wind-produced water in irradiated rims on silicate minerals [J].
The solar wind (SW), composed of predominantly 鈭1-keV H(+) ions, produces amorphous rims up to 鈭150 nm thick on the surfaces of minerals exposed in space. Silicates with amorphous rims are observed on interplanetary dust particles and on lunar and asteroid soil regolith grains. Implanted H(+) may react with oxygen in the minerals to form trace amounts of hydroxyl (-OH) and/or water (H2O). Previous studies have detected hydroxyl in lunar soils, but its chemical state, physical location in the soils, and source(s) are debated. If -OH or H2O is generated in rims on silicate grains, there are important implications for the origins of water in the solar system and other astrophysical environments. By exploiting the high spatial resolution of transmission electron microscopy and valence electron energy-loss spectroscopy, we detect water sealed in vesicles within amorphous rims produced by SW irradiation of silicate mineral grains on the exterior surfaces of interplanetary dust particles. Our findings establish that water is a byproduct of SW space weathering. We conclude, on the basis of the pervasiveness of the SW and silicate materials, that the production of radiolytic SW water on airless bodies is a ubiquitous process throughout the solar system.
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| [35] |
Hydroxyl radical production and storage in analogues of amorphous interstellar silicates: A possible “wet” accretion phase for inner telluric planets [J]. |
| [36] |
Accretion disc origin of the Earth’s water [J].
Abstract Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10090103090100 bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.
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| [37] |
The cassini visual and infrared mapping spectrometer (Vims) investigation [J].
The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth鈥揗oon system.
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| [38] |
The Moon Mineralogy Mapper (M3) imaging spectrometer for lunar science: Instrument description, calibration, on-orbit measurements, science data calibration and on-orbit validation [J].
The NASA Discovery Moon Mineralogy Mapper imaging spectrometer was selected to pursue a wide range of science objectives requiring measurement of composition at fine spatial scales over the full lunar surface. To pursue these objectives, a broad spectral range imaging spectrometer with high uniformity and high signal-to-noise ratio capable of measuring compositionally diagnostic spectral absorption features from a wide variety of known and possible lunar materials was required. For this purpose the Moon Mineralogy Mapper imaging spectrometer was designed and developed that measures the spectral range from 430 to 3000 nm with 10 nm spectral sampling through a 24 degree field of view with 0.7 milliradian spatial sampling. The instrument has a signal-to-noise ratio of greater than 400 for the specified equatorial reference radiance and greater than 100 for the polar reference radiance. The spectral cross-track uniformity is >90% and spectral instantaneous field-of-view uniformity is >90%. The Moon Mineralogy Mapper was launched on Chandrayaan-1 on the 22nd of October. On the 18th of November 2008 the Moon Mineralogy Mapper was turned on and collected a first light data set within 24 h. During this early checkout period and throughout the mission the spacecraft thermal environment and orbital parameters varied more than expected and placed operational and data quality constraints on the measurements. On the 29th of August 2009, spacecraft communication was lost. Over the course of the flight mission 1542 downlinked data sets were acquired that provide coverage of more than 95% of the lunar surface. An end-to-end science data calibration system was developed and all measurements have been passed through this system and delivered to the Planetary Data System (PDS.NASA.GOV). An extensive effort has been undertaken by the science team to validate the Moon Mineralogy Mapper science measurements in the context of the mission objectives. A focused spectral, radiometric, spatial, and uniformity validation effort has been pursued with selected data sets including an Earth-view data set. With this effort an initial validation of the on-orbit performance of the imaging spectrometer has been achieved, including validation of the cross-track spectral uniformity and spectral instantaneous field of view uniformity. The Moon Mineralogy Mapper is the first imaging spectrometer to measure a data set of this kind at the Moon. These calibrated science measurements are being used to address the full set of science goals and objectives for this mission. Copyright 2011 by the American Geophysical Union.
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| [39] |
An overview of the instrument suite for the deep impact mission [J]. |
| [40] |
The quantitative IR spectroscopic determination of OH in Apatite based on 1.4 μm [J].基于1.4 μm红外光谱测量磷灰石结构水的定量方法探讨 [J].
水在地球科学各个领域以及行星演化和太空探索中都是非常重要的物质。我国嫦娥-3号的近红外光谱数据在1.4 μm附近出现了微弱的峰,可能代表了水的存在。为了定量计算其水含量,以磷灰石为研究对象,通过对磷灰石结构水在1.4 μm和2.8 μm红外光谱相关性的分析和验证,获得了其在1.4 μm红外光谱的摩尔吸收系数。磷灰石在1.4 μm和2.8 μm处的吸收峰与其晶体定向有关,当光矢量E平行于磷灰石晶体c轴时,根据Beer-Lambert定律,可用公式<em>C=ωA/ερd</em>定量计算磷灰石中水的含量。这一结果可为解译嫦娥-3号近红外光谱数据中水的信号提供借鉴。该方法能为月球其他矿物在近红外光谱中结构水的定量计算提供依据。
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| [41] |
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| [42] |
Sources and physical processes responsible for OH/H2O in the lunar soil as revealed by the Moon Mineralogy Mapper (M3) [J].
[1] Analysis of two absorption features near 3 脦录m in the lunar reflectance spectrum, observed by the orbiting M3 spectrometer and interpreted as being due to OH and H2O, is presented, and the results are used to discuss the processes producing these molecules. This analysis focuses on the dependence of the absorptions on lunar physical properties, including composition, illumination, latitude, and temperature. Solar wind proton-induced hydroxylation is proposed as the creation process, and its products could be a source for other reported types of hydrogen-rich material and water. The irregular and damaged fine-grained lunar soil seems especially adapted for trapping solar wind protons and forming OH owing to abundant dangling oxygen bonds. The M3 data reveal that the strengths of the two absorptions are correlated and widespread, and both are correlated with lunar composition but in different ways. Feldspathic material seems richer in OH. These results seem to rule out water from the lunar interior and cometary infall as major sources. There appear to be correlations of apparent band strengths with time of day and lighting conditions. However, thermal emission from the Moon reduces the apparent strengths of the M3 absorptions, and its removal is not yet completely successful. Further, many of the lunar physical properties are themselves intercorrelated, and so separating these dependencies on the absorptions is difficult, due to the incomplete M3 data set. This process should also operate on other airless silicate surfaces, such as Mercury and Vesta, which will be visited by the Dawn spacecraft in mid-2011.
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| [43] |
Temperature regime and water/hydroxyl behavior in the crater Boguslawsky on the Moon [J].
In this work we examine the lunar crater Boguslawsky as a typical region of the illuminated southern lunar highlands with regard to its temperature regime and the behavior of the depth of the water/hydroxyl-related spectral absorption band near 3聽碌m wavelength. For estimating the surface temperature, we compare two different methods, the first of which is based on raytracing and the simulation of heat diffusion in the upper regolith layer, while the second relies on the thermal equilibrium assumption and uses Moon Mineralogy Mapper (M鲁) spectral reflectance data for estimating the wavelength-dependent thermal emissivity. A method for taking into account the surface roughness in the estimation of the surface temperature is proposed. Both methods yield consistent results that coincide within a few K. By constructing a map of the maximal surface temperatures and comparing with the volatility temperatures of Hg, S, Na, Mg, and Ca, we determine regions in which these volatile species might form stable deposits. Based on M鲁 data of the crater Boguslawsky acquired at different times of the lunar day, it is found that the average OH absorption depth is higher in the morning than at midday. In the morning a dependence of the OH absorption depth on the local surface temperature is observed, which is no more apparent at midday. This suggests that water/OH accumulates on the surface during the lunar night and largely disappears during the first half of the lunar day. We furthermore model the time dependence of the OH fraction remaining on the surface after having been exposed to the temporally integrated solar flux. In the morning, the OH absorption depth is not correlated with the remaining fraction of OH-containing species, indicating that the removal of water and/or OH-bearing species is mainly due to thermal evaporation after sunrise. In contrast, at midday the OH absorption depth increases with increasing remaining fraction of OH-containing species, suggesting photolysis by solar photons as the main mechanism for removal of the remaining OH-containing species later in the lunar day.
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| [44] |
Widespread distribution of OH/H2O on the lunar surface inferred from spectral data [J].
A new set of time-of-day–dependent global maps of the lunar near-infrared water/hydroxyl (H2O/OH) absorption band strength near 2.8 to 3.0 μm constructed on the basis of Moon Mineralogy Mapper (M06) data is presented. The analyzed absorption band near 2.8 to 3.0 μm indicates the presence of surficial H2O/OH. To remove the thermal emission component from the M06 reflectance spectra, a reliable... [Show full abstract]
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| [45] |
Time-of-day-dependent global distribution of lunar surficial water/hydroxyl [J].
Abstract A new set of time-of-day-dependent global maps of the lunar near-infrared water/hydroxyl (H 2 O/OH) absorption band strength near 2.8 to 3.0 0204m constructed on the basis of Moon Mineralogy Mapper (M 3 ) data is presented. The analyzed absorption band near 2.8 to 3.0 0204m indicates the presence of surficial H 2 O/OH. To remove the thermal emission component from the M 3 reflectance spectra, a reliable and physically realistic mapping method has been developed. Our maps show that lunar highlands at high latitudes show a stronger H 2 O/OH absorption band in the lunar morning and evening than at midday. The amplitude of these time-of-day-dependent variations decreases with decreasing latitude of the highland regions, where below about 3000°, absorption strength becomes nearly constant during the lunar day at a similar level as in the high-latitude highlands at midday. The lunar maria exhibit weaker H 2 O/OH absorption than the highlands at all, but showing a smaller difference from highlands absorption levels in the morning and evening than at midday. The level around midday is generally higher for low-Ti than for high-Ti mare surfaces, where it reaches near-zero values. Our observations contrast with previous studies that indicate a significant concentration of surficial H 2 O/OH at high latitudes only. Furthermore, although our results generally support the commonly accepted mechanism of H 2 O/OH formation by adsorption of solar wind protons, they suggest the presence of a more strongly bounded surficial H 2 O/OH component in the lunar highlands and parts of the mare regions, which is not removed by processes such as diffusion/thermal evaporation and photolysis in the course of the lunar day.
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| [46] |
Water on the surface of the Moon as seen by the Moon Mineralogy Mapper: Distribution, abundance, and origins [J].
Abstract A new thermal correction model and experimentally validated relationships between absorption strength and water content have been used to construct the first global quantitative maps of lunar surface water derived from the Moon Mineralogy Mapper near-infrared reflectance data. We find that OH abundance increases as a function of latitude, approaching values of ~500 to 750 parts per million (ppm). Water content also increases with the degree of space weathering, consistent with the preferential retention of water originating from solar wind implantation during agglutinate formation. Anomalously high water contents indicative of interior magmatic sources are observed in several locations, but there is no global correlation between surface composition and water content. Surface water abundance can vary by ~200 ppm over a lunar day, and the upper meter of regolith may contain a total of ~1.2 脙聴 10 14 g of water averaged over the globe. Formation and migration of water toward cold traps may thus be a continuous process on the Moon and other airless bodies.
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| [47] |
Origin and characteristics of excess Fe metal in lunar glass welded aggregates [C]// |
| [48] |
Solar wind and micrometeorite alteration of the lunar regolith [C]// |
| [49] |
Laboratory spectroscopic detection of hydration in pristine lunar regolith [J].
Reflectance spectroscopy of Apollo lunar soil samples curated in an air- and water-free, sealed environment since recovery and return to Earth has been carried out under water-, oxygen-, CO 2 - and organic-controlled conditions. Spectra of these pristine samples contain features near 3 μm 3 μm mathContainer Loading Mathjax wavelength similar to those observed from the lunar surface by the Chandrayaan-1 Moon Mineralogy Mapper (M 3 ), Cassini Visual and Infrared Mapping Spectrometer (VIMS), and Deep Impact Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) High-Resolution Instrument (HRI) instruments. Spectral feature characteristics and inferred OH/H 2 O concentrations are within the range of those observed by spacecraft instruments. These findings confirm that the 3 μm 3 μm mathContainer Loading Mathjax feature from the lunar surface results from the presence of hydration in the form of bound OH and H 2 O. Implantation of solar wind H + appears to be the most plausible formation mechanism for most of the observed lunar OH and H 2 O.
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| [50] |
Introduction to the Solar Wind [M]. |
| [51] |
Computer simulation of sputtering of lunar regolith by solar wind protons: Contribution to change of surface composition and to hydrogen flux at the lunar poles [J].
A computer simulation of the sputtering of lunar soil by solar wind protons was performed with the TRIM program. The rate of the sputtering-induced erosion of regolith particles was shown to be less than 0.2 脜 per year. A preferential sputtering of Ca, Mg, and O was found along with a less intense sputtering of Fe, Si, and Ti. However, with no other selection mechanisms, surface concentrations of the atoms would differ from the volume ones by no more than 6 %. The enrichment of rims of regolith particles with iron occurs as a result of selective removal of lighter atoms from the lunar surface because of different energies of escape from the Moon's gravity. The energy distributions proved to be the same for all sorts of the sputtered atoms, except for implanted hydrogen; thus, a greater fraction of the atoms left on the lunar surface corresponds to heavier elements. According to simulation results, the concentration of reduced iron observed in the mature regolith could be attained during the time of regolith particle exposure to the present flux of solar wind (10 5 years). Thus, sputtering can provide the concentration of Fe 0 observed in regolith. On periphery of a cloud of impact vapor the temperature is too low for an irreversible selective removal of evaporation products; thus, a meteoritic bombardment contributes to the formation of composition of the rims of regolith particles mainly through enrichment of the rims with elements from the bulk of the particles. The estimates of fluxes of backscattered solar wind protons and of sputtered protons, earlier implanted to the regolith, demonstrated that their contribution to the proton flux near the poles is only 10 4 cm 鈥2 s 鈥1 . This is by two orders of magnitude smaller than the proton flux from the Earth's magnetosphere which is, therefore, the main source of protons for permanently shaded polar craters of the Moon.
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| [52] |
Investigation of the solar wind-Moon interaction onboard Chandrayaan-1 mission with the SARA experiment [J].
The SARA instrument (Sub-keV Atom Reflecting Analyser) comprises a low energy neutral atom (LENA) sensor for the energy range 10 eV-3.3 keV and an ion mass spectrometer (10 eV-15 keV). It is the first ever experiment to study the solar wind-planetary surface interaction via measurements of the sputtered atoms and neutralized back-scattered solar wind hydrogen. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analysed via surface interaction technique. The ion mass spectrometer is based on the same principle. SARA performs LENA imaging of the Moon's elemental surface composition including that of permanently shadowed areas, and imaging of the lunar surface magnetic anomalies. It will also investigate processes of space weathering and sputtered sources of the exospheric gases.
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| [53] |
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| [54] |
The nature and origin of rims on lunar soil grains [J].
The formation of rims on lunar soils is complex and involves several processes whose effects may be superimposed. From this study, it is shown that one process does not dominate and that the relative importance of vapor-deposition is comparable to radiation-damage in the formation of rims on lunar silicate grains. The presence of rims on lunar soil grains, particularly those with nanometer-sized Fe metal inclusions, may have a major influence on the optical and magnetic properties of lunar soils.
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| [55] |
Energetic Charged-Particle Interactions with Atmospheres and Surfaces [M]. |
| [56] |
Proton-induced hydroxyl formation on the lunar surface [J].
The interaction of both the particle and photon component of the solar wind with the lunar surface material is expected to produce diverse chemical reactions. Experimental evidence for proton-induced OH formation was obtained by bombarding a glass, chemically similar in composition to common silicate minerals, with high-energy protons. The concentration of OH, before and after irradiation, was determined by infrared absorption measurements. The OH formation rate was greatest at the start of the bombardment and decreased with increasing dose. The maximum proton to OH conversion rate, at the start of the irradiation, is at least 5 or 10% and may be as high as 100%. Using this result, together with estimates of the lunar age and recent solar proton flux data, we were able to make very rough calculations of the minimum proton-induced OH content in the lunar surface. If mixing or churning is not important, the upper centimeter could contain 4脙聴1016 OH per cm3. When protons below 40 Mev and the higher conversion rate are included in the computation, the estimated OH concentrations could increase by a factor of 10 or more. If surface mixing or churning has occurred, they should be divided by an average churning depth.
|
| [57] |
Production of OH/H2O in lunar samples via proton bombardment [C]// |
| [58] |
Simulating OH/H2O formation by solar wind at the lunar surface [J].
We simulate the OH/H 2O production from the action of keV protons on the lunar regolith using a vacuum chamber and a mass analyzer to examine the molecular products released from olivine and SiO 2 powders during their irradiation by deuterium ions. The measured mass spectra, showing the OD/D 2O signature, confirm the possibility of OH/H 2O formation on the lunar surface by solar-wind hydrogen.
|
| [59] |
Hydroxyl (
78 H+ and D+ implantation experiments on lunar soils support the solar-wind hypothesis. 78 IR spectra of highland and mare soils are consistent with remote sensing and RELAB data. 78 Proton bombardment creates and depletes OH in lunar soil surfaces.
|
| [60] |
Solar wind contribution to surficial lunar water: Laboratory investigations [J].
Remote infrared spectroscopic measurements have recently re-opened the possibility that water is present on the surface of the Moon. Analyses of infrared absorption spectra obtained by three independent space instruments have identified water and hydroxyl (–OH) absorption bands at 653 μm within the lunar surface. These reports are surprising since there are many mechanisms that can remove water but no clear mechanism for replenishment. One hypothesis, based on the spatial distribution of the –OH signal, is that water is formed by the interaction of the solar wind with silicates and other oxides in the lunar basalt. To test this hypothesis, we have performed a series of laboratory simulations that examine the effect of proton irradiation on two minerals: anorthite and ilmenite. Bi-directional infrared reflection absorption spectra do not show any discernable enhancement of infrared absorption in the 3 μm spectral region following 1 or 100 keV proton irradiation at fluences between 10 16 and 10 18 ions cm 612. In fact, the post-irradiation spectra are characterized by a decrease in the residual O–H band within both minerals. Similarly, secondary ion mass spectrometry shows a decrease rather than an increase of the water group ions following proton bombardment of ilmenite. The absence of significant formation of either –OH or H 2O is ascribed to the preferential depletion of oxygen by sputtering during proton irradiation, which is confirmed by post-irradiation surface analysis using X-ray photoelectron spectroscopy measurements. Our results provide no evidence to support the formation of H 2O in the lunar regolith via implantation of solar wind protons as a mechanism responsible for the significant O–H absorption in recent spacecraft data. We determine an upper limit for the production of surficial –OH on the lunar surface by solar wind irradiation to be 0.5% (absorption depth).
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| [61] |
Hydrogen implantation in silicates: The role of solar wind in Si-OH bond formation on the surfaces of airless bodies in space [J].
Abstract Hydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H+ implantation of amorphous SiO2 and olivine in ultrahigh vacuum. The samples were implanted with 2–1065keV65H+, in the range of solar wind energies, and the OH absorption band at ~2.86508m measured by transmission Fourier transform infrared spectroscopy. For 265keV protons in SiO2, the OH band depth saturated at fluences F ~565×65101665H+/cm2 to a maximum 0.0032 absorption band depth, corresponding to a column density ηs 65=651.165×651016 OH/cm2. The corresponding values for 565keV protons in olivine are >265×651017/cm2, 0.0067, and 4.065×651016 OH/cm2. The initial conversion rate of implanted H+ into hydroxyl species was found to be ~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.
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| [62] |
The solar wind as a possible source of lunar polar hydrogen deposits [J]. |
| [63] |
Hydrogen migration to the lunar poles by solar wind bombardment of the moon [J].
We investigate the deposition of hydrogen at the lunar poles from the incident solar wind proton flux. We follow an incident solar wind proton as it interacts with the lunar surface, is emitted into the atmosphere, and migrate through a series of ballistic hops across the surface of the Moon. We trace the path of the particle until it is removed from the system by photo-processes such as ionization or dissociation, by thermal escape, by surface chemistry, or by reaching a cold trap. Iterating with various compounds formed through surface chemistry, we follow all hydrogen atoms input into the system, regardless of their molecular composition. Accumulating statistics on the outcomes for various input particles, we determine that .6 of the hydrogen forms H 2 through chemical sputtering. Most of the remaining H thermally desorbs as atomic in a fraction of .27. Although the amount converted to OH is small (10%), most hydrogen reaches the pole through OH migration, .008 OH/H + . Finally, we calculate the amount of time required for the solar wind to supply the amount of hydrogen detected at the poles by the Lunar Prospector Neutron Spectrometer to be 7 Myr. If the poles only retain water due to the low freezing points of the other molecules, the time required is 100 Myr.
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