An overview of the descent and landing of the Huygens probe on Titan

Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. Here we report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity--probably due to methane-and a darker area that looked like a river- or lake-bed. Post-landing images showed centimetre-sized surface details.     

The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe

Saturn's largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellite's surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.     

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturn's largest satellite--Titan--is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.     

D to H ratio and the origin and evolution of Titan's atmosphere

A value of 1.7 x 10(-3) has been reported for the ratio of CH3D to CH4 in the stratosphere of the saturnian moon Titan. A lower value of 6 x 10(-4) for this ratio in the deeper part of Titan's atmosphere was reported by de Bergh et al. For comparison we note that the CH3D to CH4 ratio on Saturn and Jupiter is 8.7 x 10(-5) and 6.7 x 10(-5), respectively. We estimate the uncertainties in all these observations and data reduction to be about a factor of 2. Despite these uncertainties it appears that Titan's atmosphere is enriched in deuterium by a factor of > or = 3 relative to Jupiter and Saturn. Potential causative factors examined here for this enrichment are condensation to form tropospheric methane clouds, fractionation occurring over a hypothetical CH4-C2H6 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan and fractionation occurring as a result of the evolution of Titan's atmosphere. We conclude that the greater part of the observed fractionation is probably derived from the formation of Titan and the subsequent evolution of Titan's atmosphere driven by photochemistry.     

一次霾过程的气溶胶特征分析

利用2008年5月26日河北省机载粒子探测系统PMS获得的飞行探测资料,分析了大气气溶胶包括粗细粒子的浓度、尺度谱分布及其时空变化特征,并对不同高度的谱分布进行了拟合.分析表明:霾天气下,气溶胶在近地面到1km高度层内有所积累,随后数浓度迅速减少.1-4km高度段中,气溶胶数浓度减少趋势变缓,4km以上气溶胶数浓度保持在30-40cm-3.低空气溶胶数浓度和平均直径水平变化不大;中高空由于云区的出现变化幅度相对较明显.霾天气下的同尺度气溶胶数密度谱值一般要高出晴天下一个量级以上.负幂函数对积聚态粒子有较好的拟合效果;修正后的伽马谱分布函数对粗粒子谱分布平均曲线的拟合较好.     

Aerosol oxalate and its implication to haze pollution in Shanghai, China

A total of 238 samples of PM2.5and TSP were analyzed to study the characteristics,sources,and formation pathways of aerosol oxalate in Shanghai in four seasons of 2007.The concentrations of oxalate were0.07–0.41 lg/m3in PM2.5and 0.10–0.48 lg/m3in TSP,respectively.Oxalate displayed a seasonal variation of autumn[summer[winter[spring in both PM2.5and TSP and was dominantly present in PM2.5in all samples.Correlation between oxalate and K?and high ratio of oxalate/K?suggested that biomass burning was a secondary source of aerosol oxalate in Shanghai,in addition to urban VOCs sources(vehicular and industrial emissions),especially in autumn.Secondary formation accounted for the majority of aerosol oxalate in Shanghai,which was supported by the high correlation of oxalate with nssSO42-,K?and NO3-,proceeding from different mechanisms.Relatively high ambient RH together with high cloud cover was found benefiting the secondary formation of aerosol oxalate.The in-cloud process(aqueous-phase oxidation)was proposed to be likely the major formation pathway of aerosol oxalate in Shanghai,which was supported by the high correlation of oxalate with nss-SO42-and K?,dominant residence of oxalate in droplet mode and result of favorable meteorological condition analysis.High correlation of oxalate and NO3-reflected the OH radical involved oxidation chemistry of the two species in the atmosphere and also suggested that gas-particle surface reactions and the evaporation–condensation process were both possible secondary formation pathways of aerosol oxalate in coarser particle mode([1.0 lm).As a major water-soluble organic compound in aerosols,concentration of oxalate showed a distinct negative correlation to the atmospheric visibility,which implied that aerosol organic compounds could play an important role in haze pollution as well as in air quality in Shanghai.     

2005～2010年河池市酸雨变化趋势和特征分析

根据河池市环境保护监测站2005～2010年的大气降水监测资料,分析河池市近年来酸雨变化趋势和特征.结果表明,河池市酸雨的变化趋势总体下降明显,降雨的pH值3.28～8.60,酸雨发生频率为50.1%.河池市的酸雨是典型的硫酸型酸雨,SO42-离子占总量的大部分,其次NO3-离子含量也较高.河池市工业和能源消耗大量煤炭...     

灰霾天气下北京海淀城区空气中溴代二恶英在不同粒径颗粒物上的分布特征研究

采用大气颗粒物分级采样器在同一地点连续采集灰霾天北京海淀城区空气样品,调研了空气中溴代二恶英（PBDD/Fs）的污染水平和在不同粒径颗粒物上的分布特征。研究结果表明：灰霾天城区空气样品中PBDD/Fs的质量浓度为1 535~6 053 fg/m³;PBDD/Fs的浓度随着颗粒物粒径（aerodynamic diameter,d_ae）的减小而增加,在d_ae<1.0 μm的颗粒物上浓度最高,80%以上的PBDD/Fs分布在d_ae<2.5 μm的颗粒物上;PBDD/Fs在不同粒径颗粒物上的含量不等,但均以高溴代二苯并呋喃（PBDFs）为主,且其所占比例随颗粒物粒径的增加而减小,然而多溴代二苯并-对-二恶英（PBDDs）在不同粒径颗粒物上的分布差异性却不显著。     

1981-2013年宝鸡市雾霾日数变化趋势及突变分析

目的分析1981-2013年33a宝鸡市雾霾日数变化趋势及突变。方法收集宝鸡市1981-2013年气象站观测的雾霾日数数据,采用趋势系数、倾向率、一元线性回归、距平百分率、MannKendall突变检验和滑动t检验方法分析研究。结果与结论①在33a间,宝鸡市雾霾日数以16.253d/10a的趋势下降,且存在15a和24a的周期动荡;②冬季雾霾发生频率最高,春季略低,其次秋季,夏季最低;③1981-2013年7月份雾霾日数距平百分率波动范围最大,表明在33a内雾霾日数同期波动较为剧烈,且出现极端雾霾现象,9和10月略低,4、5、6和8月次之,2和3月较1月波动范围大,1月波动范围最小;④1981-2013雾霾日数在年、季都有明显的突变趋势,其突变类型表现不同。     

混凝土的抗酸雨腐蚀性及其机理研究

为了考察混凝土建筑在酸雨环境腐蚀下的耐久性问题,研究不同配合比的混凝土材料在酸雨侵蚀作用下的抗压强度、抗折强度和抗冲击性等性能,通过X射线衍射(XRD)、压汞测孔法(MIP)、扫描电镜和能谱仪(SEM-EDS)等测试方法,从粉煤灰、膨胀剂、聚丙烯纤维的作用机理入手,研究了侵蚀前后混凝土试块中水化产物微观结构及内部孔结构特征的变化.结果表明,聚丙烯纤维粉煤灰膨胀混凝土的抗酸雨腐蚀性效果最佳.基准混凝土水化产物在酸雨介质侵蚀下易转换为石膏和SiO2·nH2O,而聚丙烯纤维粉煤灰膨胀混凝土结构致密,有害孔显著减少,可有效阻碍酸雨介质侵蚀作用.     

Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.     

拓扑一致降标与单值延拓性质

设H为Hilbert空间.算子T∈B(H)称作有单值延拓性质,若对任意一个开集U(∈)C,满足方程(T-λI)f(λ)=0((A)λ∈U)的惟一的解析函数为零函数.若存在整数d∈N使得当n≥d时,N(Tn)+R(T)=N(Td)+R(T)并且R(Tn)在R(Td)的算子值域拓扑中闭,称T当n≥d时有拓扑一致降标.本文给出了拓扑一致降标与单值延拓性质之间的关系,并利用算子的拓扑一致降标性质研究了单值延拓性质的稳定性.     

酸雨对环境的危害及防治对策

简述了酸雨特性及酸雨成因，描述了酸雨对自然生态环境和人类造成的种种危害，指出迄今我国酸雨发生和发展的严峻状况，针对性的提出了依靠政策措施、经济手段和先进环保技术防治酸雨的对策建议。     

应用机器学习技术预测强雨雪天气过程中的积雪

2017年12月～2018年2月冬季,在中国长江中下游流域发生了两次强度强、范围广的强雨雪冰冻天气。在第一次强降雪天气中,由于2018年1月3～4日和5～8日两阶段降雪在中国东部落区高度重叠,导致了较为严重的灾害。为了预测日积雪深度,利用2017年12月～2018年2月和2007年12月～2008年2月这两个时间段上的国家测站日值数据,利用CART决策树算法根据各气象要素生成一个预测当天是否有积雪的二元判别决策树模型。从决策树结构中可以看出,前一日的积雪深度、日最高气温、日平均气温、日最低相对湿度等要素对预测结果的影响重大。且两决策树的结构相似度极高,故该模型对是否有积雪的预测存在普适性。随后利用深度学习方法训练两个时间段上所有预测为有积雪的个例,建立预测积雪深度的回归模型,结果表明,利用该模型训练得到的误差较小,但不足之处在于,预测极端降雪个例的误差大于普通降雪个例。将决策树模型与深度学习模型串接,便能得到预测当天是否有积雪,及积雪深度的模型。相比于前人的研究,该模型能拟合更复杂的特征,得到更精确的预测,使用2018年的数据也能更好地模拟当前的气候背景。     

算子代数上保持升降标的线性映射

设B（ H）为无限维可分的复Hilbert空间H上的有界线性算子的全体,矱为B（ H）上满的线性映射。若矱保持上半Browder谱或降标谱并且保持孤立点集,则矱为B（ H）上的自同构。当矱保持Drazin谱并且保持孤立点集时,刻画了线性映射矱的两种可能结构。     

行星着陆——推进下降过程的建模与控制(英文)

登陆火星或月球时,释放降落伞后进入推进阶段,通过轴向推力倾斜实现水平方向运动,能够与速度矢量(重力)的相反方向一致或与需要的加速度方向保持一致.后一种策略能实现精确着陆,在该策略下,倾斜角(俯仰角和偏转角)与水平加速度成正比.这种策略并未采用以水平加速度为姿态控制目标的递阶导航与控制方案,而是基于角加速度对位置作用的四阶动力学特性设计了一个独特的控制系统.除去倾斜角的非线性因素(垂直制动规定的轴向推力)后,系统综合动力学方程能够实现(准)输入-状态线性化.论文同时表明:一方面,基于参考轨迹(倾斜角和位置)的控制器设计仅能实现部分输入-状态线性化;另一方面,系统的内部稳定性可以证明.即使存在不可镇定的外部干扰动力学模态时,仍能确保系统稳定性.蒙特卡洛仿真验证了闭环控制策略的有效性.     

一次冻雨过程的云微物理过程分析、模拟和预测

冻雨是一种对交通、电网以及人民生活带来严重影响的小概率灾害性天气.通过对2018年1月25日—26日中国南方大面积冻雨的天气形势、云微物理量变化的分析,探讨了本次大范围冻雨天气形成的气象条件,进而利用耦合了改进Ramer参数化方案的天气预报模式(weather research and forecasting model,WRF),对本次冻雨过程进行预报并评估了其预报能力,得到了如下结论:①冷干与暖湿气流在贵州-湖南-江西一带形成来回摆动的高空切变线,出现逆温层,地面温度小于0℃,天气形势有利于冰冻天气出现.②冻雨形成必须同时满足三个气象条件:一定厚度的暖层、较高的液水含量和地面温度低于0℃.在一次完整的冻雨过程中,气象要素并不能总是满足上述三个条件,因而一次冻雨过程并不一定是连续的,期间往往夹杂着降雨或降雪事件的出现.③耦合改进Ramer参数化方案的WRF模式对冻雨有很好的预报能力,平均威胁分数(threat score,TS)评分达到了0.52.该研究对冻雨预报有较大的参考价值,并为交通、电网的防灾减灾提供了坚实的理论支撑.     

Extreme winds and waves in the aftermath of a Neoproterozoic glaciation

The most severe excursions in the Earth's climatic history are thought to be associated with Proterozoic glaciations. According to the 'Snowball Earth' hypothesis, the Marinoan glaciation, which ended about 635 million years ago, involved global or nearly global ice cover. At the termination of this glacial period, rapid melting of continental ice sheets must have caused a large rise in sea level. Here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples. These structures occur at homologous stratigraphic levels in Australia, Brazil, Canada, Namibia and Svalbard. Our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds, significantly longer than those typical for today's oceans. The reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch-unlimited ocean basins. We propose that these extraordinary wind and wave conditions were characteristic of the climatic transit, and provide observational targets for atmospheric circulation models.     

关于雾霾天气对太阳辐射量的影响研究实验

本实验主要通过对不同天内太阳辐射量和PM2.5的数值关系的定性分析来研究雾霾天气对太阳辐射量的影响.就上海市松江地区而言,我们对日照计测得的太阳辐射量以及上海松江气象局采集到的PM2.5数据进行分析,为了排除其他因素的影响,选取相同温度和天气条件下的两天的数据对比,最后得出结论:雾霾天气对太阳能辐射量有负面影响.     

Ku波段卫星通信雨衰分析及对抗措施

雨衰是影响Ku波段卫星通信系统传输质量及系统性能的主要因素之一。介绍了Ku波段卫星通信中雨衰产生的机理及其对信道的影响,并提出了有效减少雨衰对Ku波段卫星通信不利影响的措施。