The main belt as a source of near-Earth asteroids     

Mario Menichella  Paolo Paolicchi  Paolo Farinella 《Earth, Moon, and Planets》1995,71(3):133-149

We investigate the flux of main-belt asteroid fragments into resonant orbits converting them into near-Earth asteroids (NEAs), and the variability of this flux due to chance interasteroidal collisions. A numerical model is used, based on collisional physics consistent with the results of laboratory impact experiments. The assumed main-belt asteroid size distribution is derived from that of known asteroids extrapolated down to sizes of 40 cm, modified in such a way to yield a quasi-stationary fragment production rate over times 100 Myr. The results show that the asteroid belt can supply a few hundred km-sized NEAs per year, well enough to sustain the current population of such bodies. On the other hand, if our collisional physics is correct, the number of existing 10-km objects implies that these objects either have very long-lived orbits, or must come from a different source (i.e., comets). Our model predicts that the fragments supplied from the asteroid belt have initially a power-law size distribution somewhat steeper than the observed one, suggesting preferential removal of small objects. The component of the NEA population with dynamical lifetimes shorter than or of the order of 1 Myr can vary by a factor reaching up to a few tens, due to single large-scale collisions in the main belt; these fluctuations are enhanced for smaller bodies and faster evolutionary time scales. As a consequence, the Earth's cratering rate can also change by about an order of magnitude over the 0.1 to 1 Myr time scales. Despite these sporadic spikes, when averaged over times of 10 Myr or longer the fluctuations are unlikely to exceed a factor two.  相似文献   

The First Record of Long Headed Eagle Ray,Aetobatus flagellum (Pisces: Myliobatidae) from Korea     

Jina Oh  Sung Kim  Choong-Gon Kim  Ho Young Soh  Dawa Jeong  Youn-Ho Lee 《Ocean Science Journal》2006,41(1):53-57

A specimen ofAetobatus flagellum was collected at Uljin in June 2005 for the first time in Korea. This specimen is characterized by the cephalic fin, the long snout, the dorsal fin between pelvic fins, spiracles on the dorsal side of the disc, the deeply notched nasal curtain and the one row of the teeth in the lower and the upper jaws. And unlikeAetobatus narinari, it does not have any spots on the its dorsal side of the disc. We report this specimen as the first record from Korea and name it ‘Bak-jui-ga-o-ri’ in Korean.  相似文献   

A survey on Hydroidomedusae from the upwelling region of southern part of the Taiwan Strait of ChinaⅠ.On new species and records of Anthomedusae   总被引：1，自引：2，他引：1  

XU Zhenzu  HUANG Jiaqi  GUO Donghui 《海洋学报(英文版)》2007,26(5):66-75

Through the examination of 377 samples of the Anthomedusae, eight new species and one new record are described.  相似文献   

历史城区旅游化：时空过程、功能转换及关联协调——以甘肃省天水市历史城区为例     

田小波  胡静  张志斌  贾垚焱  吕丽  徐欣 《地理科学》2021,41(8):1371-1379

借助史料、历史地图及谷歌POI数据,运用空间句法和GIS空间分析探讨天水古城自明清以来空间和功能的演化过程及其内在关联。研究发现：① 明清以来,天水古城的旅游化经历了传统游憩的普及（明清）、衰落（民国）、停滞（1949—1987年）阶段,现代旅游兴起与传统游憩恢复（1988—2000年）及融合创新（2001年至今）阶段。古城传统格局逐渐消失,但是历史轴线的主轴线地位延续,可达性、穿越性最好,协同度最高,“空间结构惯性”规律作用明显。② 在不同施动主体的共同作用下,天水古城由安全防御、军事政治、集市商贸等传统功能向工业生产,进而向现代旅游、居住功能转变,游憩休闲功能显著增强。③ 居民和游客共享公共活动空间,但服务功能相互分离。游客活动空间与功能服务协调性较好,居民公共活动空间与服务设施一致性较差。  相似文献   

甘肃河西地区历史遗址分布及其自然环境背景     

朱陇强  王晓云  刘佳敏  颉耀文 《中国沙漠》2021,41(4):121-128

研究历史遗址的空间分布,有助于深入了解历史时期人类活动与自然地理环境的关系,从而更加科学有效地管理和保护历史文化遗址。以河西地区3?654处历史文化遗址为研究对象,结合当时的历史文化背景,在GIS技术的支持下,对河西地区历史文化遗址的空间分布特征及其自然影响因素进行了研究。结果显示：研究区历史文化遗址受自然环境的影响较大,但不同遗址类型受影响的程度不大相同,军事设施遗址和石窟寺及石刻遗址更反映地形和区位因素,居住址和城址则对水源和综合自然条件的要求更严格;除军事和宗教遗址外,生产和生活型遗址点多分布于海拔2?200?m以下、距河流不超过10 km、坡度小于15°的平地、微斜坡、缓斜坡和斜坡上;为获取更多的生存空间,人们生产生活一般选择在走廊绿洲平原区,对坡向的要求不显著;遗址呈现出以石羊河中游、黑河中游及北大河中下游3个高密集分布区,另外在疏勒河中游和党河中下游存在2个较高密集分布区。  相似文献   

Sustainable Development and Tourists’ Satisfaction in Historical Districts: Influencing Factors and Features     

YU Wenting  ZHU He  ZHANG Shuying 《资源与生态学报(英文版)》2021,12(5):669-681

Determining how to achieve sustainable development and protection of historical districts is a worthwhile research topic. As a vital way to update urban space, tourism development in historical districts is an effective tool for redistributing urban functions and increasing urban vitality. This paper takes the Insa-dong in South Korea as a case study for the evaluation of tourist satisfaction in historical districts, based on 550 questionnaires. The questionnaire contains 33 evaluation indicators, and the subjects' subjective judgment and satisfaction with Insa-dong are measured in the form of the Likert scale. Using SPSS to analyze tourists’ satisfaction with historical districts, this studyfinds that: (1) The tourist satisfaction evaluation of Insa-dong includes six dimensions in total: “Embodiment of historical elements”, “The blend of tradition and modernity”, “Industry distribution and type”, “Consumer demand”, “ Street layout and function” and “Landscaping”. (2) “The blend of tradition and modernity” has the highest weight while “Industry distribution and type” has the lowest one in the analysis of factors influencing overall satisfaction. (3) The analysis of the common factor weights and the common factor satisfaction levels shows that “The blend of tradition and modernity” and “Street layout and function” are the parts that need to be improved. “Consumer demand” also has a lot of room for promotion. The research results will be beneficial for enhancing the tourist experiences of historical districts and they provide a theoretical basis and practical experience reference for effective protection and sustainable development of historical districts.  相似文献   

1930s-2000年广西地区石漠化分布的变迁   总被引：4，自引：1，他引：3  

韩昭庆  冉有华  刘俊秀  李军 《地理学报》2016,71(3):390-399

根据民国时期《广西省各县石山林木保护办法》中对石山的定义,对应目前石漠化等级划分方法,把该时期的石山区解读为轻度及以上石漠化分布地区.通过对1930s国民政府参谋本部陆地测量总局编绘的1:10万广西省地形图上所绘石山范围进行数字化,并与2000年广西壮族自治区石漠化分布现状图进行比较,揭示了近70年来广西地区石漠化分布在空间上的变化.研究表明:① 1930s广西地区轻度及以上石漠化面积为31922.25 km²,比2000年的27123.21 km²多出4799 km²,说明民国时期广西地区轻度及以上石漠化土地分布范围比当代的大,但在空间变迁上表现出此增彼减的变化特点;② 2000年的数据中有47个县石漠化面积比1930s的少,减少的总面积为9045.5 km²,集中分布在广西西部和中部,都安瑶族自治县的变化最明显,近70年内减少了894.8 km²;③ 2000年有30个县的石漠化面积比1930s的多,共增加了4246 km²,集中分布在广西东北部,其中全州县增加了556.55 km².  相似文献   

基于分层分区法的中国历史耕地数据的网格化重建     

魏希文  缪丽娟  江源  崔雪锋 《地理学报》2016,71(7):1144-1156

网格化历史耕地数据集能为历史时期耕地变化研究提供更精确的支持,并且为全球环境气候变化研究模型模拟提供驱动数据。本文综合考虑了中国历代土地利用开发的特点及自然人文因子对耕地的影响,设计了一套对中国耕地先分区再分层分配的网格化方法。基于国内3个主流区域耕地数据研究成果,采用上述方法建立了1820年（清仁宗嘉庆二十五年）和1936年（民国二十五年）中国10 km×10 km分辨率的耕地数据集,并绘制了分布图。本文还利用国内具有代表性的区域数据集对重建结果进行对比验证。结果表明,该方法可以保证耕地数量的权威性,并且建立具有区域性的高精度历史耕地数据集。  相似文献   

上新世-第四纪亚洲内陆干旱化过程——柴达木中部鸭湖剖面孢粉和盐类化学指标证据   总被引：1，自引：0，他引：1       下载免费PDF全文

方小敏  吴福莉  韩文霞  王亚东  张玺正  张伟林 《第四纪研究》2008,28(5):874-882

亚洲内陆干旱区是与亚洲季风区密切关联的一个环境系统,其形成演化与青藏高原的隆起和全球变化等因素密切相关,揭示其干旱化过程和趋势具有重大的理论和现实意义。文章通过对亚洲内陆干旱区柴达木盆地中央鸭湖剖面孢粉和盐类化学指标的初步研究,表明5.3Ma时湖泊周围已分布着以蒿属、藜科、麻黄、禾本科等为主的草原,气候已经变干,由此期较高的氯离子、硫酸根离子及碳酸钙含量表明湖泊演化处于沉积碳酸盐和少量硫酸盐阶段;3.4Ma后草本植物含量略有增加,但离子含量普遍降低,推测气候季节性增强;2.6Ma后耐旱植物花粉含量与盐度指标均呈现明显长期增加趋势,气候快速向更干旱方向发展。  相似文献   

钻井工程实时多参数监测控制系统的研究   总被引：1，自引：0，他引：1  

蒋维平  孟宪民 《中国煤炭地质》2008,20(5):68-71

钻井实时多参数监测控制系统,是对钻压、泵量等十二个参数直接测量,并对其参数进行数学建模与计算,以提高目的层采心率．以及提前预警钻进事故,在此基础上开发出专家系统对钻机运行状态进行有效控制。该系统在完成实验室检测、调试及模拟式运行后,分别在XY-44和XY-5两种型号的液压钻机上做了现场时运行,运行结果表明．该系统运行稳定,适于钻井现场复杂的钻进过程,实用性强,实现了设计目标。  相似文献