Preservation of layered paleodeposits in high-latitude pedestal craters on Mars     

Seth J. Kadish  James W. Head 《Icarus》2011,213(2):443-450

An outstanding question in Mars’ climate history is whether or not pedestal craters represent the armored remnants of ice-rich paleodeposits. We address this question using new high-resolution images; in a survey of several hundred high-latitude pedestal craters, we have identified 12 examples in which visible and/or topographically expressed layers are exposed on the marginal scarp of the pedestal. One example, located on the south polar layered deposits, preserves ice-rich layers that have otherwise been completely removed from the polar cap. These observations provide empirical evidence that the pedestal crater formation mechanism is capable of armoring and preserving ice-rich layered paleodeposits. Although layered exposures have not yet been observed in mid-latitude pedestal craters, high-latitude instances of discontinuous, partially covered layers suggest that layers can be readily concealed, likely through mantling and/or mass wasting processes along the marginal scarp. This interpretation is supported by the observation that high-latitude pedestals with exposed layers along their margins are, on average, taller than mid-latitude examples, and have larger, steeper marginal scarps, which may help to maintain layer exposures. These observations favor the interpretation that mid- to high-latitude pedestal craters represent the armored remnants of ice- and dust-rich paleodeposits, which occurred transiently due to changes in the climate regime. Preservation of fine-scale layering of ice and dust at these latitudes implies that the climate change did not involve regional melting conditions.  相似文献   

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies     

Joseph S. Levy  David R. Marchant 《Icarus》2010,206(1):229-216

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.  相似文献   

Viscous liquid film flow on dune slopes of Mars     

Diedrich Möhlmann  Akos Kereszturi 《Icarus》2010,207(2):654-658

It is shown that viscous liquid film flow (VLF-flow) on the surfaces of slopes of martian dunes can be a low-temperature rheological phenomenon active today on high latitudes. A quantitative model indicates that the VLF-flows are consistent with the flow of liquid brines similar to that observed by imaging at the Phoenix landing site. VLF-flows depend on the viscosity, dynamics, and energetics of temporary darkened liquid brines. The darkening of the flowing brine is possibly, at least partially, attributed to non-volatile ingredients of the liquid brines. Evidence of previous VLF-flows can also be seen on the dunes, suggesting that it is an ongoing process that also occurred in the recent past.  相似文献   

西风角动量输送的气候特征及其与急流关系研究     

杨玮  王盘兴  何金海  李丽平  成丽萍 《大气科学》2014,38(2):363-372

利用1958～1997年NCEP/NCAR一日四次的风场再分析资料,系统地分析了季节平均西风角动量（即u角动量）经向、垂直输送通量及其三个分量（平均经圈环流、定常波、瞬变涡输送通量）的气候特征,特别是讨论了12～2月、6～8月它们与东、西风带、副热带西风急流、极夜急流之间的联系。结果表明:（1）包含纬度因子的角动量通量与动量通量在高纬地区存在显著差别,高纬对流层上部的强动量输送中心在角动量通量中不明显。而u角动量强经向输送主要在中低纬对流层顶附近和冬半球高纬平流层顶附近,副热带西风急流和极夜西风急流均位于u角动量强向极输送中心及其高纬一侧的辐合区中。（2）发现三个输送分量对急流维持的作用随纬度、季节不同。北半球冬季（夏季）的副热带西风急流主要由平均经圈环流（强度相当的定常波和瞬变涡）强经向输送及辐合维持;南半球西风急流全年均由平均经圈环流和瞬变涡旋输送及辐合维持;冬半球中平流层极夜急流主要由定常波、瞬变涡旋输送及其辐合共同维持。（3）热带东风区是牵连角动量（即Ω角动量）的高值区,它主要由平均经圈环流向对流层上部输送;冬半球副热带及中纬西风区存在u角动量垂直输送的切变区,它主要由平均经圈环流和瞬变涡旋完成;热带对流层顶附近有u角动量的定常波弱向下输送。  相似文献   

Performance Evaluation of a Self-Developed Ozonesonde and Its Application in an Intensive Observational Campaign   总被引：1，自引：0，他引：1       下载免费PDF全文

ZHANG Jin-Qiang  XUAN Yue-Jian  XIA Xiang-Ao  LIU Ming-Yuan  YAN Xiao-Lu  PANG Li  BAI Zhi-Xuan  WAN Xiao-Wei 《大气和海洋科学快报》2014,(3):175-179

To make a detailed test on the reliability and detection performance of the electrochemical concentra- tion cell （ECC） type ozonesonde which had been devel- oped and preliminarily evaluated by the authors, an inten- sive ozonesonde release experiment was held at two sites in Beijing and Changchun in June 2013. The results showed that the mean background current and its standard deviation were 0.03 （0.04） pA and 0.02 （0.03） pA in Bei- jing （Changchun）. The average response time and its standard deviation were 27.8 s （30.4 s） and 4.0 s （3.7 s） in Beijing （Changchun）. The ozone partial pressure profiles at both sites showed a central peak in the stratosphere and a side peak in the boundary layer. Large variation in ozone partial pressure was observed at the middle levels of the atmosphere （10-17 kin）. A more marked gradient of ozone change was observed in Beijing （3.4 mPa km-1） at the lower atmosphere level, as compared to that in Changchun （0.4 mPa km-l）. The results presented in this paper indicate that this self-developed ozonesonde shows a high level of reliability and good performance. The ozonsonde is expected to play an important role in opera- tional observations of ozone profiles.  相似文献   

Quantifying the Response Strength of the Southern Stratospheric Polar Vortex to Indian Ocean Warming in Austral Summer简     

LI Shuanglin  CHEN Xiaoting 《大气科学进展》2014,31(2):492-503

A previous multiple-AGCM study suggested that Indian Ocean Warming （IOW） tends to warm and weaken the southern polar vortex.Such an impact is robust because of a qualitative consistency among the five AGCMs used.However,a significant difference exists in the modeled strengths,particularly in the stratosphere,with those in three of the AGCMs （CCM3,CAM3,and GFS） being four to five times as strong as those in the two other models （GFDL AM2,ECHAM5）.As to which case reflects reality is an important issue not only for quantifying the role of tropical ocean warming in the recent modest recovery of the ozone hole over the Antarctic,but also for projecting its future trend.This issue is addressed in the present study through comparing the models＇ climatological mean states and intrinsic variability,particularly those influencing tropospheric signals to propagate upward and reach the stratosphere.The results suggest that differences in intrinsic variability of model atmospheres provide implications for the difference.Based on a comparison with observations,it is speculated that the impact in the real world may be closer to the modest one simulated by GFDL AM2 and ECHAM5,rather than the strong one simulated by the three other models （CCM3,CAM3 and GFS）.In particular,IOW during the past 50 years may have dynamically induced a 1.0℃ warming in the polar lower stratosphere （～100 hPa）,which canceled a fraction of radiative cooling due to ozone depletion.  相似文献   

Geochronology and paleoenvironment of pluvial Harper Lake,Mojave Desert,California, USA     

Anna L. Garcia  Jeffrey R. Knott  Shannon A. Mahan  Jordon Bright 《Quaternary Research》2014

Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500 ka. Previous studies inferred a polar jet stream south of 35°N at 18 ka and at ~ 40°N at 17–14 ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to > 30,000 ¹⁴C yr BP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~ 45–40 ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40 ka, but shifted to 40°N by ~ 35 ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~ 45–40 ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.  相似文献   

Evidence for an Hesperian-aged South Circum-Polar Lake Margin Environment on Mars     

James Dickson 《Planetary and Space Science》2006,54(3):251-272

A broad pitted plain and an elongated low rise occur near the south pole of Mars between a region of major cavi (Cavi Angusti) and a regionally smooth and broad valley (Argentea Planum). Viking, Mars Global Surveyor (MGS), and Odyssey data reveal a densely pitted plain covering ∼6750 km², and containing >300 irregularly shaped, steep-walled and flat-floored depressions with a mean diameter of ∼3.5 km. At the southernmost (poleward) extent of this plain are 12 north/south trending linear valleys that are characterized by theater-shaped heads abutting a major cavi within Cavi Angusti. The pitted plain, which abuts Cavi Angusti to the southwest, is separated from the floor of Argentea Planum by a smooth, elongated low rise that extends parallel to the plain for ∼200 km. These unusual features are all found within the Hesperian-aged circumpolar Dorsa Argentea Formation, which has been interpreted by some workers to be an ice-rich glacier-related deposit. We interpret the pitted plain to represent the maximum northern extent of the Angusti lobe ice deposit. The pits are analogous in morphology and distribution to terrestrial kettle holes, which form from the melting of isolated ice-blocks surrounded and partly buried by sediment, to leave hollows. The linear valleys are consistent with sapping valleys formed from the release of an elevated groundwater table, fed by meltwater lakes. On the basis of these characteristics, relationships and analogs, we interpret the marginal facies to represent an ice-sheet/lake contact environment that existed during Hesperian time.  相似文献   

北半球平流层臭氧的时空分布特征   总被引：3，自引：0，他引：3  

黄仪方  郭世昌  陈辉  秦瑜  吴涧 《成都信息工程学院学报》2001,16(1):8-11

利用1963－1985年北半球的平流层臭氧观测资料，采用自然平交分解方法，初步分析了北半球、特别是东亚地区大气臭氧层的时空分布特征。结果表明：（1）北半球大气臭氧的空分间分布可能与下垫面的性能有关；（2）东亚地区大气臭氧的时间分布主要具有2年和11年变化的周期特征，这与太阳黑子的准2年和准11年变化有一定的对应关系；（3）北美地区的臭氧变化则主要包含4年和7年的周期变经成分。  相似文献   

武汉市2018年4—9月臭氧及其前体物非线性关系研究     

陈安雄  胡柯  沈龙娇  钟章雄  段佳鹏  汪羽商  陈文泰 《南京气象学院学报》2020,12(6):729-738

近年来武汉市臭氧污染日益严峻,成为影响空气质量达标的瓶颈,弄清臭氧及其前体物非线性关系是臭氧防控的关键和基础.本研究基于武汉中心城区2018年4—9月臭氧及其前体物在线观测数据,分析出武汉市臭氧浓度受前体物和气象条件等因素的共同影响,呈较为明显的季节变化和日变化特征.观测期间武汉市大气挥发性有机物（VOCs）平均体积分数为32.5×10^-9,烷烃是武汉市VOCs的主要组分,其次是含氧VOCs （OVOCs）和卤代烃.利用基于观测的模型定量分析臭氧与前体物之间的关系,发现削减VOCs会引起臭氧生成潜势的显著下降,而削减氮氧化物则会使臭氧生成潜势升高,说明武汉市臭氧生成处于VOCs控制区.在人为源VOCs中,间/对二甲苯和邻二甲苯的相对增量反应活性（RIR）最高,是影响臭氧生成的关键组分.  相似文献