North-south geological differences between the residual polar caps on Mars

Polar processes can be sensitive indicators of global climate, and the geological features associated with polar ice caps can therefore indicate evolution of climate with time. The polar regions on Mars have distinctive morphologic and climatologic features: thick layered deposits, seasonal CO2 frost caps extending to mid latitudes, and near-polar residual frost deposits that survive the summer. The relationship of the seasonal and residual frost caps to the layered deposits has been poorly constrained, mainly by the limited spatial resolution of the available data. In particular, it has not been known if the residual caps represent simple thin frost cover or substantial geologic features. Here we show that the residual cap on the south pole is a distinct geologic unit with striking collapse and erosional topography; this is very different from the residual cap on the north pole, which grades into the underlying layered materials. These findings indicate that the differences between the caps are substantial (rather than reflecting short-lived differences in frost cover), and so support the idea of long-term asymmetry in the polar climates of Mars.     

披碱草属植物的分类现状及主要存在的问题

披碱草属是禾本科、小麦族的一个有重要经济价值的属,其属内绝大多数种类是畜牧业上的优良牧草,饲用价值极高,有些种类具有耐寒、耐旱、耐碱等特性,是农牧业上良种繁育、牧草利用的重要基因资源;但该属同时又是分类学上的一个疑难属,其属内系统分类各学者意见不一、争议颇大,尤其在属的界限、属下类群划分、以及类群演化关系上问题较多,至今尚未解决.本研究查阅了国内外分类学文献,探讨其分类差异以及存在问题,为充分开发利用我国丰富的披碱草属植物资源提供理论依据.     

基于XGBoost的新零售优惠券使用行为预测

为实现新零售优惠券的定向投放,提出了对用户优惠券使用行为预测的模型.该文采用XGBoost算法,突破了以TAM模型(技术接受模型)为基础解释个人优惠券使用意愿的传统方法,并基于口碑网的真实交易数据进行了特征提取和用户使用行为建模.在K折交叉验证之后通过变量重要性评分,确定了对消费者使用决策贡献度较高的特征,并与随机森林和GBDT(梯度提升决策树)算法进行了AUC(Area under curve)准确率的对比.该研究证明了基于XGBoost的集成学习算法在优惠券使用行为预测中的有效性,对新零售精准营销有重要的现实意义.     

底材厚度对铝热反应熔化制备的块体纳米晶Fe3Al材料的组织和性能的影响

通过铝热反应熔化方法在厚度为5~15 mm的铜底材上制备块体纳米晶Fe3Al材料.通过X射线衍射(XRD)研究材料的晶粒尺寸,研究材料室温压缩下的力学性能和硬度.结果表明,所制备材料的晶粒尺寸均约为20nm,且晶粒尺寸随底材厚度的增加而增加.底材厚度为5 mm时纳米晶Fe3Al材料的屈服强度和硬度具有最大值,随着底材厚度的增加,屈服强度和硬度急剧减小,最后趋于稳定.5 mm时纳米晶Fe3Al材料的屈服强度约是10mm时的1.5倍.     

Aeolian processes at the Mars Exploration Rover Meridiani Planum landing site

The martian surface is a natural laboratory for testing our understanding of the physics of aeolian (wind-related) processes in an environment different from that of Earth. Martian surface markings and atmospheric opacity are time-variable, indicating that fine particles at the surface are mobilized regularly by wind. Regolith (unconsolidated surface material) at the Mars Exploration Rover Opportunity's landing site has been affected greatly by wind, which has created and reoriented bedforms, sorted grains, and eroded bedrock. Aeolian features here preserve a unique record of changing wind direction and wind strength. Here we present an in situ examination of a martian bright wind streak, which provides evidence consistent with a previously proposed formational model for such features. We also show that a widely used criterion for distinguishing between aeolian saltation- and suspension-dominated grain behaviour is different on Mars, and that estimated wind friction speeds between 2 and 3 m s(-1), most recently from the northwest, are associated with recent global dust storms, providing ground truth for climate model predictions.     

Titan Radar Mapper observations from Cassini's T3 fly-by

Cassini's Titan Radar Mapper imaged the surface of Saturn's moon Titan on its February 2005 fly-by (denoted T3), collecting high-resolution synthetic-aperture radar and larger-scale radiometry and scatterometry data. These data provide the first definitive identification of impact craters on the surface of Titan, networks of fluvial channels and surficial dark streaks that may be longitudinal dunes. Here we describe this great diversity of landforms. We conclude that much of the surface thus far imaged by radar of the haze-shrouded Titan is very young, with persistent geologic activity.     

不同Al含量的310S耐热钢热轧加工后的组织和力学性能

通过压力机和热轧机对WS-4非自耗真空电弧炉熔炼制备的Al质量分数为0、2％、4％、6％的310S耐热钢进行开坯轧制,用光学金相显微镜观察金相组织,用EPMA-1600电子探针分析组织中各元素分布,结合EPMA能谱分析确定合金中的相组成,并测定室温拉伸性能.结果表明:随着Al含量的增加,钢中碳化物的数量明显减少,且形态逐渐由团聚状向颗粒状转变;部分Al元素以Al4 C3相的形式出现富集,且Al元素含量越高富集现象越严重;合金的屈服强度略有降低,Al质量分数为2％的合金具有最高的抗拉强度及延伸率.各成分合金的综合力学性能明显高于国家标准对热轧310S板材的要求.     

等温处理对块体纳米晶Fe3Al材料磁学性能的影响

对铝热反应熔化法制备的含Ni质量分数10％的块体纳米晶Fe3Al材料进行1 000℃,8h的等温处理.通过在振动样品磁强计(VSM)上测得的等温处理前后合金的磁滞回线,分析其磁性能;采用X射线衍射仪进行结构分析和平均晶粒尺寸计算.结果表明:材料的磁滞回线呈倾斜状且狭长,磁滞损耗很小;等温处理后材料的晶粒尺寸没有明显差别,其矫顽力Hc和剩余磁化强度Mr变大,饱和磁化强度Ms没有明显变化,等温处理使材料综合磁学性能变差.