-
在历次月球探测任务中,对月球周边等离子体的探测主要依赖两类技术:无线电掩星观测和就位等离子体探测。首次对月球周围等离子体的观测由“先驱者7号”(Pioneer 7)于1966年实施,采用了无线电掩星观测方法,当时获得的电子密度约为4 × 107el/m3[30]。首次就位观测实施于1971年2月,由“阿波罗14号”(Apollo 14)搭载的月球环境带电粒子探测仪(the Charged-Particle Lunar Environment,CPLEE)在距离月表26 cm高处对40~200 eV的带电粒子进行了观测。根据CPLEE观测数据推断,在月面日间,距离月表几百米左右高度处的电子密度为104el/cm3[31]。
前苏联的两个探测器“月球19号”(Luna 19)和“月球22号”(Luna 22)均携带了同源的双频无线电信标,其星上晶振稳定度为10–7(约600 s),相位采样率为0.2 sps,采用三轴稳定状态。两者分别于1972年、1974年进行了月球等离子体的无线电掩星观测试验。根据观测数据反演结果(图3)可知,月球等离子体电子密度自高度30~40 km开始随高度的降低快速增加;电子密度峰值出现在2~10 km处;在日间峰值密度范围为400~2 000 cm–3,标高约为10~30 km;在几乎所有的廓线中,电子密度在接近月球表面时随着高度的降低而减少[32]。
2003年,ESA的首个探月任务“智慧1号”(Smart 1)发射升空。该卫星具备S、X及Ka波段的下行能力。Smart 1探测器于2006年开始进行多次的双频月球无线电掩星观测试验,并获得了大量数据,但是迄今为止尚未有详细的试验结果发布。Pluchino等[30]给出了该探测器获得的等离子体总电子含量,约为1013el/m2。
2007年9月,JAXA发射了SELENE探测器,该探测器携带的两个子卫星Rstar和Vstar。两个子卫星均具备甚长基线干涉测量(Very Long Baseline Interferometry,VLBI)无线电信标信号下行能力。其中子卫星Vstar与主卫星于2007年11月—2009年6月开展了S和X波段双频的月球临近空间环境的无线电掩星观测,获得了378个观测序列。观测期间,中国科学院国家天文台和新疆天文台参加了该实验,并获得了掩星观测数据。由于Vstar采用约0.18 Hz的稳定自旋控制卫星姿态,导致0.18 Hz基频和高倍频谐波项、缓慢进动的低频周期项以及长期项给卫星发出的电磁波带来偏差,特别是其中的低频项和长期项,在SELENE掩星观测中无法扣除,给结果带来了较大影响[33]。根据上述观测序列进行反演获得的电子数密度较小,且变化无明显规律。对SELENE观测结果进行特定的筛选后,挑选出太阳天顶角小于60°的16个序列,获得了其平均电子密度廓线,如图4所示[34]。
印度月球探测任务“月船1号”(Chandrayaan 1)发射于2008年10月。Chandrayaan 1具备S波段下行通信能力,于2009年开展了该波段的双程单频掩星试验,其公布的一例观测结果(也是唯一一例公开结果)发生于太阳天顶角101.72°时,掩星点坐标为77.2°N和90.2°W。Chandrayaan 1这例结果与SELENE观测结果的电子密度较为接近[35]。
2015—2016年,利用中国再入返回飞行器服务舱,中国科学院国家天文台和北京航天飞行控制中心联合开展了S/X相干双频掩星观测的试观测试验。根据试验数据成功获取了月表以上至50 km左右的电子柱浓度信息(如图5所示),其中电子柱浓度最大处约为0.4×1016~0.5 × 1016el/m2,且浓度随高度的降低而增加[36]。
上述观测中,Apollo 14 CPLEE的观测结果、Luna 19和Luna 22、Smart 1和再入返回飞行器服务舱的观测结果支持月球表面存在较强的等离子体层,其中,再入返回飞行器服务舱获得的电子密度峰值与CPLEE接近;而SELENE和Chandrayaan 1获得的电子密度较前述小1.5~2个量级。
历次月球无线电掩星观测结果的不一致与其试验中的关键参数(表1)也密切相关。由表1可见,由于观测目标较为稀薄,除Chandrayaan 1以外的掩星试验均采用了具有同源频率参考的相干双频掩星方法;SELENE任务利用其小卫星开展了星星双频掩星观测,这种设计能够极大地提高观测数据的时空覆盖,但是由于SELENE采用的S波段双频信号频点相近,同时其子卫星特有的自旋稳定带来的信号误差无法消除,直接影响了其观测结果的有效性[33];再入返回飞行器服务舱和Luna19/Luna22在观测频点选择和观测模式上都更为可信,两者的观测结果也相近。再入返回飞行器服务舱掩星试验具有同类试验中最高的星上晶振稳定度和最高的地面采样率,提供了更精细的电子浓度随高度的变化特征。
表 1历次月球掩星观测试验情况对比
Table 1.The key parameters in previous successful lunar occultation experiments
航天器 工作频率/MHz 地面观测站 观测模式 星上晶振稳定度 采样率/sps 卫星状态 Luna 19/Luna 22 936.85/3 747.4 相干双频星地掩星 10–7(约600 s) 0.2 三轴稳定 Smart 1 S波段:2 235.1
X波段:8 453.02
Ka波段:32 121.5Medicina, Noto 相干双频星地掩星 三轴稳定 SELENE S波段 2 218.0, 2 287.3125 Usuda 相干双频星星掩星 短稳10–7 40 自旋,频率为0.18 Hz Chandrayaan 1 S波段:2 230.8 Bylallu 单频双程星地掩星 1 三轴稳定 再入返回飞行器服务舱 S波段 X波段 喀什,佳木斯 相干双频星地掩星 短稳10–9 100 三轴稳定 由于月球外逸层大气稀薄,由氦、氩等组成的气体浓度在夜半球仅约为2 × 105cm–3,在日半球约为104cm–3。因此月球外逸层光致电离所能产生的电子远小于前述的较强电子密度(Apollo 14等5个探测器结果)。而月球不仅处在太阳辐射的影响下,太阳风等离子体、地球磁尾和行星际空间环境也会对其产生影响。太阳辐射作用于月球表面所发生的光化学反应使得日间的月表附近存在一层光电子;月表尘埃云在太阳辐射作用下,也会发生光化学反应;同时,来自太阳风的等离子体,月球位于磁尾时来自地球磁层的等离子体,均为月球临近空间等离子体的可能来源。月球尘埃云的涨落也必然影响着月球临近空间的等离子体。
前述观测结果均为来自于月球的日半球。在月球的夜半球,光化学反应不存在,此时月球表面处在太阳风和地球磁尾等离子体的影响下。位于月球表面或近地表层的尘埃粒子吸收周围等离子体中的电子和离子,从而使尘埃粒子带电,与带电的月球表面相互作用,并导致尘埃悬浮和运动。对于直径约100 nm的尘埃粒子,带电尘埃粒子的数密度约为10–2~10–1cm–3[37]。月球日半球和夜半球带电粒子数密度的巨大差异必将导致月球晨昏线附近离子和尘埃粒子的复杂变化。
A Review of Lunar Space Environment Study
-
摘要:虽然月球是除地球以外人类探测最为频繁的目标天体之一,但是月球空间环境仍然是未解之谜。美国国家航空航天局(National Aeronautics and Space Administration,NASA)的“阿波罗”(Apollo)系列、“勘测者”(Surveyor)系列、“阿尔忒弥斯”(Artemis)任务、月球大气和粉尘环境探测器(Lunar Atmosphere and Dust Environment Explorer,LADEE),前苏联的“月球”(Luna)系列探测任务均进行了(或计划进行)月面就位探测,提供了一些月球电磁环境和月尘的信息;历次月球探测任务中的无线电观测提供了部分关于月球等离子体环境的信息。本文首先对当前月球空间环境研究进展和存在的问题进行介绍;其次,探讨了日、地对月球空间环境的可能影响;最后,基于“嫦娥四号”中继星及着陆器搭载的甚低频射电探测载荷,对未来月球空间环境研究及其探测进行了展望。Abstract:After more than fifty years lunar exploration, our understanding of the lunar space environment is still superficial. The lunar dusty exosphere research was based on ARTEMIS mission and the lunar atmosphere and dust environment detector which were developed by NASA. Based on the radio experiments of several lunar missions, the existence of lunar ionosphere is determined. The current status and observation of lunar exosphere and ionosphere are introduced in this paper. With the help of the low frequency radio astronomical payloads carried by Chang'E-4 relay satellite and the lander, more of lunar space environment will be uncovered.Highlights
● The space-time characteristics and generation mechanism of the lunar ionosphere is a key scientific issue in the exploration of the lunar space environment. ● In future lunar exploration missions, it is necessary to explore the lunar ionosphere and its interaction with the moon's permanent asymmetric dust cloud. ● The low-frequency radio detection payloads onboard Chang'E-4 will study the radio environment on the farside of the moon. -
表 1历次月球掩星观测试验情况对比
Table 1The key parameters in previous successful lunar occultation experiments
航天器 工作频率/MHz 地面观测站 观测模式 星上晶振稳定度 采样率/sps 卫星状态 Luna 19/Luna 22 936.85/3 747.4 相干双频星地掩星 10–7(约600 s) 0.2 三轴稳定 Smart 1 S波段:2 235.1
X波段:8 453.02
Ka波段:32 121.5Medicina, Noto 相干双频星地掩星 三轴稳定 SELENE S波段 2 218.0, 2 287.3125 Usuda 相干双频星星掩星 短稳10–7 40 自旋,频率为0.18 Hz Chandrayaan 1 S波段:2 230.8 Bylallu 单频双程星地掩星 1 三轴稳定 再入返回飞行器服务舱 S波段 X波段 喀什,佳木斯 相干双频星地掩星 短稳10–9 100 三轴稳定 -
[1] HOFFMAN J H, HODGES J R R, EVANS D E. Lunar atmospheric composition results from Apollo 17[C]//Proceedings of the Fourth Lunar Science Conference. Houson, Texas: [s.n.], 1973. [2] POTTER A E,MORGAN T H. Discovery of sodium and potassium vapor in the atmosphere of the Moon[J]. Science,1988,241(4866):675-680.doi:10.1126/science.241.4866.675 [3] BENNA M, MAHAFFY P R, HALEKAS J S, et al. Variability of Helium, Neon, and Argon in the lunar exosphere as observed by the LADEE NMS instrument[C]//2014 NASA Exploration Science Forum. Moffett Field, California: NASA, 2014. [4] COLAPRETE A, WOODEN D, COOK A, et al. Observations of titanium, aluminum, and magnesium in the lunar exosphere by LADEE UVS[C]//Lunar and Planetary Science Conference. Woodlands, Texas: [s. n.], 2016. [5] SARANTOS M,KILLEN R M,SHARMA A S,et al. Sources of sodium in the lunar exosphere:modeling using ground-based observations of sodium emission and spacecraft data of the plasma[J]. Icarus,2010,205(2):364-374.doi:10.1016/j.icarus.2009.07.039 [6] SARANTOS M,KILLEN R M,GLENAR D A,et al. Metallic species,oxygen and silicon in the lunar exosphere:upper limits and prospects for LADEE measurements[J]. Journal of Geophysical Research Space Physics,2012,117(A3):3103-3119. [7] HODGES R R. Helium and hydrogen in the lunar atmosphere[J]. Journal of Geophysical Research,1973,78(34):8055-8064.doi:10.1029/JA078i034p08055 [8] HODGES R R. Methods for Monte Carlo simulation of the exospheres of the Moon and Mercury[J]. Journal of Geophysical Research,1980,85(A1):164-170.doi:10.1029/JA085iA01p00164 [9] WURZ P,ROHNER U,WHITBY J A,et al. The lunar exosphere:the sputtering contribution[J]. Icarus,2007,191(2):486-496.doi:10.1016/j.icarus.2007.04.034 [10] HILCHENBACH M,HOVESTADT D,KLECKER B,et al. Detection of singly ionized energetic lunar pick-up ions upstream of Earth's bow shock[J]. Solar Wind Seven,1992,A93-33554:349-355. [11] MALL U,KIRSCH E,CIERPKA K,et al. Direct observation of lunar pick-up ions near the Moon[J]. Geophysical Research Letters,1998,25(20):3799-3802.doi:10.1029/1998GL900003 [12] YOKOTA S,SAITO Y,ASAMURA K,et al. First direct detection of ions originating from the Moon by MAP-PACE IMA onboard SELENE (KAGUYA)[J]. Geophysical Research Letters,2009,36(11):L11201.doi:10.1029/2009GL038185 [13] TANAKA T,SAITO Y,YOKOTA S,et al. First in situ observation of the Moon-originating ions in the Earth's Magnetosphere by MAP-PACE on SELENE (KAGUYA)[J]. Geophysical Research Letters,2009,36(22):L22106.doi:10.1029/2009GL040682 [14] GRÜN E,HORANYI M,STERNOVSKY Z. The lunar dust environment[J]. Planetary & Space Science,2011,59(14):1672-1680. [15] SEVERNY A B,TEREZ E I,ZVEREVA A M. The measurements of sky brightness on lunokhod-2[J]. Moon,1975,14(1):123-128.doi:10.1007/BF00562978 [16] CRISWELL D R. Horizon-glow and the motion of lunar dust[J]. Photon and Particle In-teraction in Space,1973,37:545-556. [17] RENNILSON J J,CRISWELL D R. Surveyor observations of lunar horizon-glow[J]. Moon,1974,10(2):121-142.doi:10.1007/BF00655715 [18] COLWELL J E,BATISTE S,HORÁNYI M,et al. Lunar surface:dust dynamics and regolith mechanics[J]. Reviews of Geophysics,2007,45(2):RG2006. [19] AUER A,SITTE K. Detection technique for micrometeoroids using impact ionization[J]. Earth and Planetary Science Letters,1968,4(2):178-183.doi:10.1016/0012-821X(68)90013-7 [20] COLLETTE A,STERNOVSKY Z,HORANYI M. Production of neutral gas by micrometeoroid impacts[J]. Icarus,2014,227:89-93.doi:10.1016/j.icarus.2013.09.009 [21] HARTMANN W K. Impact experiments:1. ejecta velocity distributions and related results from regolith targets[J]. Icarus,1985,63(1):69-98.doi:10.1016/0019-1035(85)90021-1 [22] MARSHALL J,RICHARD D,DAVIS S. Electrical stress and strain in lunar regolith simulants[J]. Planetary & Space Science,2011,59(14):1744-1748. [23] STUBBS T J,VONDRAK R R,FARRELL W M. A dynamic fountain model for lunar dust[J]. Advances in Space Research,2006,37(1):59-66.doi:10.1016/j.asr.2005.04.048 [24] GLENAR D A,STUBBS T J,HAHN J M,et al. Search for a high-altitude lunar dust exosphere using Clementine navigational star tracker measurements[J]. Journal of Geophysical Research:Planets,2014,119(12):2548-2567. [25] FELDMAN P D,GLENAR D A,STUBBS T J,et al. Upper limits for a lunar dust exosphere from far-ultraviolet spectroscopy by LRO/LAMP[J]. Icarus,2014,233:106-113.doi:10.1016/j.icarus.2014.01.039 [26] GLENAR D A,STUBBS T J,MCCOY J E,et al. A reanalysis of the Apollo light scattering observations,and implications for lunar exospheric dust[J]. Planetary & Space Science,2011,59(14):1695-1707. [27] HORANYI M,SZALAY J R,KEMPF S,et al. A permanent,asymmetric dust cloud around the Moon[J]. Nature,2015,522(7556):324-326.doi:10.1038/nature14479 [28] WOODEN D H,COOK A M,COLAPRETE A,et al. Evidence for a dynamic nanodust cloud enveloping the Moon[J]. Nature Geoscience,2016,9:665-668.doi:10.1038/ngeo2779 [29] SZALAY J R,HORÁNYI M. Annual variation and synodic modulation of the sporadic meteoroid flux to the Moon[J]. Geophysical Research Letters,2015,42:10580-10584.doi:10.1002/2015GL066908 [30] PLUCHINO S,SCHILLIRÒ F,SALERNO E,et al. Radio occultation measurements of the lunar ionosphere[J]. Memorie Della Società Astronomica Italiana Supplement,2008,12(12):53-59. [31] REASONER D L,BURKE W J. Direct observation of the lunar photoelectron layer[J]. Proceedings of the Third Lunar Science Conference,1972,3:2639-2654. [32] VYSHLOV A S. Preliminary results of circumlunar plasma research by the Luna 22 spacecraft[C]//Proceedings of the Open Meetings of Working Groups on Physical Sciences. Varna, Bulgaria: 1975. [33] 王震. 月球电离层探测研究[D]. 北京: 中国科学院大学, 2015.WANG Z. Study of lunar ionosphere detection[D]. Beijing: University of Chinese Academy Sciences, 2015. [34] IMAMURA T,NABATOV A,MOCHIZUKI N,et al. Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission[J]. Journal of Geophysical Research Space Physics,2012,117:A06303. [35] CHOUDHARY R K,AMBILI K M,CHOUDHURY S,et al. On the origin of the ionosphere at the Moon using results from Chandrayaan-1 S-band radio occultation experiment and a photochemical model[J]. Geophysical Research Letters,2016,3:10,025-10,033. [36] 韩松涛,王明远,平劲松,等. 应用我国再入返回飞行器服务舱探测到较强月球电离层信号[J]. 科学通报,2016,61(32):3476-3481.HAN S T,WANG M Y,PING J S,et al. Exploring strong lunar ionosphere successfully with the service module of Chinese circumlunar return and reentry spacecraft[J]. Chinese Science Bulletin,2016,61(32):3476-3481. [37] POPEL S I,ZELENYI L M,GOLUB' A P,et al. Lunar dust and dusty plasmas:recent developments,advances,and unsolved problems[J]. Planetary and Space Science,2018,156:71-84.doi:10.1016/j.pss.2018.02.010 [38] LUO Q Y,YANG L,JI J H. Global distribution of the kinetic scale magnetic turbulence around the Moon[J]. The Astrophysical Journal Letters,2016,816(1):L3. [39] KAISER M L,ALEXANDER J K,Riddle A C,et al. Direct measurements by Voyagers 1 and 2 of the polarization of terrestrial kilometric radiation[J]. Geophysical Research Letters,1978,10:857-860. [40] BENSON R F,CALVERT W. ISIS 1 observations at the source of auroral kilometric radiation[J]. Geophysical Research Letters,1979,6:479-482.doi:10.1029/GL006i006p00479 [41] LEE L C,WU C S. Amplification of radiation near cyclotron frequency due to electron population inversion[J]. Physics of Fluids,1980,23(7):1348.doi:10.1063/1.863148 [42] GOTO Y,FUJIMOTO T,KASAHARA Y,et al. Lunar ionosphere exploration method using auroral kilometric radiation[J]. Earth,Planets and Space,2011,63(1):47-56. [43] 纪奕才,赵博,方广有,等. 在月球背面进行低频射电天文观测的关键技术研究[J]. 深空探测学报(中英文),2017,4(2):150-157.JI Y C,ZHAO B,FANG G Y,et al. Key technologies of very low frequency radio observations on the lunar far-side[J]. Journal of Deep Space Exploration,2017,4(2):150-157. [44] JIA Y, ZOU Y, PING J, et al. The scientific objectives and payloads of Chang’E-4 mission[J]. Planetary & Space Science, 2018: S0032063317300211, 162: 207-215.