A hybrid simulation of Mercury’s magnetosphere for the MESSENGER encounters in year 2008

The latest measurements from the two encounters of the MESSENGER spacecraft in year 2008 have discovered several interesting features of the magnetosphere of Mercury. We have performed high-resolution 3D hybrid model calculations to simulate the solar wind interaction with the Hermean magnetosphere during the first two Mercury encounters of the MESSENGER spacecraft in 2008. It is found that the global structure of the Hermean magnetosphere is significantly controlled by the direction of the interplanetary magnetic field. The bow shock size and shape and the magnetotail configuration have very large differences in these two encounters with northward-pointing and southward-pointing interplanetary magnetic field, respectively. Comparisons are also given with the observed magnetic field profiles and the computational results. In general, good agreement can be found including the interesting feature of the relatively thick magnetopause current layer at outbound measurements. Our work shows that 3D hybrid simulation is a promising method to study in detail the Hermean magnetosphere in parallel with the post-MOI observations of the MESSENGER spacecraft and the Bepi-Colombo mission in future.     

From planetesimals to terrestrial planets: N-body simulations including the effects of nebular gas and giant planets

We present results from a suite of N-body simulations that follow the formation and accretion history of the terrestrial planets using a new parallel treecode that we have developed. We initially place 2000 equal size planetesimals between 0.5 and 4.0 AU and the collisional growth is followed until the completion of planetary accretion (>100 Myr). A total of 64 simulations were carried out to explore sensitivity to the key parameters and initial conditions. All the important effect of gas in laminar disks are taken into account: the aerodynamic gas drag, the disk-planet interaction including Type I migration, and the global disk potential which causes inward migration of secular resonances as the gas dissipates. We vary the initial total mass and spatial distribution of the planetesimals, the time scale of dissipation of nebular gas (which dissipates uniformly in space and exponentially in time), and orbits of Jupiter and Saturn. We end up with 1-5 planets in the terrestrial region. In order to maintain sufficient mass in this region in the presence of Type I migration, the time scale of gas dissipation needs to be 1-2 Myr. The final configurations and collisional histories strongly depend on the orbital eccentricity of Jupiter. If today’s eccentricity of Jupiter is used, then most of bodies in the asteroidal region are swept up within the terrestrial region owing to the inward migration of the secular resonance, and giant impacts between protoplanets occur most commonly around 10 Myr. If the orbital eccentricity of Jupiter is close to zero, as suggested in the Nice model, the effect of the secular resonance is negligible and a large amount of mass stays for a long period of time in the asteroidal region. With a circular orbit for Jupiter, giant impacts usually occur around 100 Myr, consistent with the accretion time scale indicated from isotope records. However, we inevitably have an Earth size planet at around 2 AU in this case. It is very difficult to obtain spatially concentrated terrestrial planets together with very late giant impacts, as long as we include all the above effects of gas and assume initial disks similar to the minimum mass solar nebular.     

Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets

We used chemical equilibrium and chemical kinetic calculations to model chemistry of the volatiles released by heating different types of carbonaceous, ordinary and enstatite chondritic material as a function of temperature and pressure. Our results predict the composition of atmospheres formed by outgassing during accretion of the Earth and other terrestrial planets. Outgassing of CI and CM carbonaceous chondritic material produces H₂O-rich (steam) atmospheres in agreement with the results of impact experiments. However, outgassing of other types of chondritic material produces atmospheres dominated by other gases. Outgassing of ordinary (H, L, LL) and high iron enstatite (EH) chondritic material yields H₂-rich atmospheres with CO and H₂O being the second and third most abundant gases. Outgassing of low iron enstatite (EL) chondritic material gives a CO-rich atmosphere with H₂, CO₂, and H₂O being the next most abundant gases. Outgassing of CV carbonaceous chondritic material gives a CO₂-rich atmosphere with H₂O being the second most abundant gas. Our results predict that the atmospheres formed during accretion of the Earth and Mars were probably H₂-rich unless the accreted material was dominantly CI and CM carbonaceous chondritic material. We also predict significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at high temperatures (1500-2500 K). Finally, our results may be useful for interpreting spectroscopic observations of accreting extrasolar terrestrial planets.     

Abundances of Jupiter's trace hydrocarbons from Voyager and Cassini

The flybys of Jupiter by the Voyager spacecraft in 1979, and over two decades later by Cassini in 2000, have provided us with unique datasets from two different epochs, allowing the investigation of seasonal change in the atmosphere. In this paper we model zonal averages of thermal infrared spectra from the two instruments, Voyager 1 IRIS and Cassini CIRS, to retrieve the vertical and meridional profiles of temperature, and the abundances of the two minor hydrocarbons, acetylene (C₂H₂) and ethane (C₂H₆). The spatial variation of these gases is controlled by both chemistry and dynamics, and therefore their observed distribution gives us an insight into both processes. We find that the two gases paint quite different pictures of seasonal change. Whilst the 2-D cross-section of C₂H₆ abundance is slightly increased and more symmetric in 2000 (northern summer solstice) compared to 1979 (northern fall equinox), the major trend of equator to pole increase remains. For C₂H₂ on the other hand, the Voyager epoch exhibits almost no latitudinal variation, whilst the Cassini era shows a marked decrease polewards in both hemispheres. At the present time, these experimental findings are in advance of interpretation, as there are no published models of 2-D Jovian seasonal chemical variation available for comparison.     

Saturn eddy momentum fluxes and convection: First estimates from Cassini images

We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere (). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events.     

塔里木盆地英买力地区原油地球化学特征与族群划分

塔里木盆地英买力地区油气成因复杂,采用多馏分、多组分化学成分分析及单体烃碳同位素分析途径,对该区海相、陆相油进行精细分析。结果表明,英买力油田主要有两大类油组,分别是南部YM2井区的海相油(I类)和北部YM7井区的陆相油(Ⅱ类),各自具有典型的海相油与陆相油的特征。进一步分析表明,两大油组内部仍有分异。将第二油组进一步分为两亚类,以中、古生界产层为主的正常黑油和重质油为一类(Ⅱa),以古近系为主的凝析油为另一类(Ⅱb)。两亚类原油正构烷烃单体烃同位素稍有差异,芳烃组成与分布具有显著差异。较之于Ⅱa原油,Ⅱb原油富含联苯系列与氧芴系列、不同类型芳烃系列中以低分子量同系物占绝对优势,其正构烷烃单体烃碳同位素更重,与相邻的羊塔克地区原油表现出一定的相似性,表明两者之间有一定的成因联系,前者可能为混源油,由位于库车坳陷中拜城凹陷提供两种陆相烃源岩。英买力地区陆相油并非以往确认的一种成因类型,可能至少由两套性质迥异的中生界烃源岩供烃。英买力地区第一类海相油(I类)尽管生物标志物特征相近,但分析原油的单体烃碳同位素具有显著差异,同样存在油气混源的可能。英买力地区海、陆相原油特征与成因的深入剖析,对于该区精细油源确认、油气成藏机制乃至油气资源评价具有重要意义。     

地下水-陆生植被系统研究评述

依赖于地下水的生态系统广泛分布,潜水含水层本身是个生态系统,地下水不仅维持许多水生生态系统,而且是干旱-半干旱地区陆生植被生存的重要水源。研究地下水与植被的相互联系是生态水文地质学的主要组成部分,重点介绍地下水-植被系统的研究方法。植被吸收的地下水量占总蒸腾量的比例是评价植被依赖地下水程度的一个定量指标,蒸腾量比例呈季节性与区域性变化,取决于植被类型、地下水位埋深与动态变化、土壤岩性与气候条件。植被总蒸腾量可以用涡流相关系统测量。地下水的蒸腾量可以用植物茎流计测量,或者用地下水位昼夜变化估算。用同位素混合模型也能估算地下水占总蒸腾量的比例。植被对地下水位下降的响应可能有两种形式,一种是线性比例响应,另一种是临界突变响应,只有通过长期系统观测植被长势与地下水位的变化才能利用统计分析建立两者响应关系。目前,植被-水文模型只能模拟植被冠层截流与蒸腾对土壤水与地下水的影响,或者模拟土壤水-地下水对植被生长的影响,地下水与植被相互作用与反馈的耦合模型还是个空白。     

Cassini图像中受散射光影响的7颗土星近环内卫星的天体测量

卡西尼号探测器携带了光学成像科学子系统(ISS: Imaging Science Subsystem), 在2004--2017年间拍摄了一些土星内卫星的图像. 部分图像中土星内卫星非常靠近土星环, 观测对象受土星环散射光影响导致测量精度差, 甚至无法测量. 由此提出一种适用于这类图像的背景消除算法, 可对受土星光环散射光影响的土星内卫星进行测量. 归算了7颗近环内卫星(土卫十(Janus)、土卫十一(Epimetheus)、土卫十五(Atlas)、土卫十六(Prometheus)、土卫十七(Pandora)、土卫三十二(Methone)和土卫四十九(Anthe))的70张ISS图像, 并与不消除散射光的方法进行了比较. 结果显示, 该方法至少可以提高43%的精度. 和喷气推进实验室的土星卫星历表\lk SAT415相比, 测量得到的赤经和赤纬方向的残差均值分别为0.72km和2.26km, 标准差分别为10.99km和11.36km.     

Last glacial terrestrial vegetation record of leaf wax n-alkanols in the northern South China Sea: Contrast to scenarios from long-chain n-alkanes

Long-chain n-alkanols and n-alkanes in core sediments from the northern South China Sea (SCS) were measured to make a comparison during terrestrial vegetation reconstruction from ~42 ka to ~7 ka. The results showed that terrestrial vegetation record from long-chain n-alkanes matched well with previous studies in nearby cores, showing that more C₄ plants developed during the Last Glacial Maximum (LGM) and C₃ plants dominated in the interglacial period. However, these scenarios were not revealed by terrestrial vegetation reconstruction using long-chain n-alkanols, which showed C₃ plant expansion during the LGM. The discrepancy during the interglacial period could be attributed to the aerobic degradation of functionalized long-chain n-alkanols in the oxygen-rich bottom water, resulting in poor preservation of terrestrial vegetation signals. On the other hand, the different advantages of functionalized n-alkanols and non-functional n-alkanes to record local and distal vegetation signals, respectively, may offer a potential explanation for the contradiction during the LGM when the SCS was characterized by low-oxygen deep water. Nevertheless, large variations on n-alkyl lipid compositions in C₃/C₄ plants could play a part in modulating sedimentary long-chain n-alkanols and n-alkanes toward different vegetation signals, thereby suggesting that caution must be taken in respect to the terrestrial vegetation reconstruction using long-chain n-alkanes and long-chain n-alkanols.     

基于GPS垂向位移测定云南地区陆地水储量及其对累积降雨的响应机制

本文基于中国大陆构造环境监测网（陆态网）GPS台站垂向形变数据,通过主成分分析计算云南省6大流域陆地水负荷形变,并与降雨数据进行相关性分析和交叉小波分析。结果表明,云南省各流域陆地水储量变化相对于降雨在半年和年周期上分别存在1.5~2.25和3~6个月的滞后,不同周期滞后的叠加使陆地水储量与累积降雨量之间存在显著的负相关性,6大流域的降雨累积天数各不相同,均在47~109 d之间。本文由此提出一种云南地区陆地水储量对累积降雨的响应机制。