Estimation of the escape of photoelectrons from Mars in 2004 liberated by the ionization of carbon dioxide and atomic oxygen

Photoelectron peaks in the atmosphere of Mars caused by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons have been observed by the Electron Spectrometer (ELS), a component of the Mars Express (MEx) Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment. Ionization mostly occurs at the Mars exobase with the majority of the photoionized electron flux trapped in the remanent and induced magnetic field, with a portion of that flux escaping the planet down its tail. Since Mars is overall charge neutral, the number of electrons must be identical to the number of ion charges which escape the planet. An estimate of the fraction of the total number of escaping electrons is obtained for the year 2004, specifically those produced by the ionization of carbon dioxide and atomic oxygen by solar 30.4 nm photons. In achieving this process, an illustrative example pass is used to show how the electron spectrum is adjusted for the potential on the spacecraft; then the region of the electron spectrum which shows photoelectron peaks is integrated over energy, yielding a flux of 5.74 × 10⁶ electrons/(cm² s sr). This technique is then applied to a subset of 22 sample averaged spectra from the 2004 data (5 January 2004 through 25 January 2005), yielding an average result of 4.15 × 10⁶ electrons/(cm² s sr) for the 22 cases. The observation cone of 33.75° is used to integrate over solid angle (assuming the flux is constant), giving 4.39 × 10⁶ electrons/(cm² s). This average value was taken as representative of the full data interval. Frequency of occurrence statistics showing about a 6.2% occurrence rate for the 2004 data is applied to give an average escape flux from Mars of 2.72 × 10⁵ electrons/(cm² s) during 2004. By estimating the outflow area as 1.16 × 10¹⁸ cm² at X = −1.5 R_Mars the electron escape rate of 3.14 × 10²³ electrons/s is obtained. Thus about 9.92 × 10³⁰ electrons or 16.5 Mmole of electrons escaped Mars during 2004 due to the ionization of carbon dioxide and atomic oxygen by the He 30.4 nm line. Due to the caveats of the analysis, these derived escape rates should be considered lower limits on the total electron escape rate from Mars.     

A comparison of global models for the solar wind interaction with Mars

We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion ‘plume’ in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars’ past using models.     

Isotope fractionation in the photochemical escape of O from Mars

The production of energetic and escaping neutral O atoms at the current epoch in the martian thermosphere is thought to be dominated by the dissociative recombination process:

     

Impact-induced N2 production from ammonium sulfate: Implications for the origin and evolution of N2 in Titan’s atmosphere

Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N₂ production from reduced nitrogen-bearing solids proposed to be contained in Titan’s crust, ammonium sulfate ((NH₄)₂SO₄), for the replenishment of N₂ to the atmosphere in Titan’s history. To investigate the conversion of (NH₄)₂SO₄ into N₂ by hypervelocity impacts, we measured gases released from (NH₄)₂SO₄ that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N₂ production from (NH₄)₂SO₄ as a function of peak shock pressure ranging from ∼8 to ∼45 GPa. Our results indicate that the initial and complete shock pressures for N₂ degassing from (NH₄)₂SO₄ are ∼10 and ∼25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity v_i > ∼8 km/s) produce N₂ efficiently; whereas satellitesimal impacts during the accretion (i.e., v_i < 4 km/s) produce N₂ only inefficiently. Even when using the proposed small amount of (NH₄)₂SO₄ content in the crust (∼4 wt.%) (Fortes, A.D. et al., 2007. Icarus 188, 139-153), the total amount of N₂ provided through cometary impacts over 4.5 Ga reaches ∼2-6 times the present atmospheric N₂ (i.e., ∼7 × 10²⁰-2 × 10²¹ [mol]) based on the measured production efficiency and results of a hydrodynamic simulation of cometary impacts onto Titan. This implies that cometary impacts onto Titan’s crust have the potential to account for a large part of the present N₂ through the atmospheric replenishment after the accretion.     

The KC method: Numerical investigation of a new analysis method for reinforced soil walls

Reinforced soil walls commonly include facing elements which affect the mechanical behavior of the system. However, the design procedures involved in the existing codes and manuals (e.g. FHWA, BS8006, AASHTO, etc.) do not consider the structural contribution of the facing to the wall stability. Recently, a new computer based method for the analysis of reinforced soil walls which takes into account the interaction between the facing and the soil reinforcement layers was presented [Klar A and Sas T. Rational approach for the analysis of segmental reinforced soil walls based on kinematic constraints. Geotextiles and Geomembranes 2009;27:332-340]. This method demands full compatibility between the reinforcement layers and the deforming wall, and is solved as an optimization problem on this constraint. This kinematic compatibility (KC) method entails several assumptions regarding the interaction between the three components of the system (soil, wall, and reinforcement). This paper compares the KC method to a more rigorous continuum analysis. Results show that the KC method is capable of replicating the behavior of the more rigorous system, with a good agreement on both the value of maximum tensile forces in the reinforcement and shear and bending moment distributions along the wall. The KC method has a certain advantage over continuum methods, such as finite element or finite difference, since it requires limited input data that can easily be obtained from field tests.     

Post-glacial rock weathering processes on a roche moutonn@e in the Riksgränsen area (68°N), northern Norway

Weathering processes responsible for landscape denudation in Arctic environments are poorly understood. Traditionally, gelifraction has been widely invoked as the dominant weathering process, but empirical support for this is frequently lacking. In Norwegian Lapland, post-glacial weathering has been recently attributed to predominantly biophysical processes, based on field measurements of landscape lowering undertaken by André (1995). The study reported here examined bedrock samples from one of André's sites for mineralogical and chemical transformations since post-glacial exposure. Backscatter scanning electron microscopy reveals extensive mineral grain dissolution and accompanying development of rock porosity. Wavelength dispersive spectroscopy shows these mineral alterations to be the result of the loss of major chemical constituents including Ca, Mg, K, Si, and Al. Chemical weathering is clearly a component of post-glacial landscape denudation in Norwegian Lapland, in addition to the important role of biophysical processes identified by André (1995). Cosmogenic dating of the sampled bedrock surface confirms that much of this weathering has occurred in the last 10,000 years.     

Source dependency of exospheric sodium on Mercury

Due to a large solar radiation effect, the sodium exosphere exhibits many interesting effects, including the formation of an extended corona and a tail-like structure. The current suite of observations allows us to study some physical properties of the sodium exosphere, such as the source rates and the interaction with the surface, both experimentally and theoretically. In order to quantify the complex variations in the sodium exosphere in more detail, we use an exospheric model with the Monte-Carlo method to examine the surface interactions of a sodium atom, including the surface thermal accommodation rate and the sticking coefficient. The source rates from different components, such as the photon stimulated desorption (PSD), the meteoroid impact vaporization (MIV), and the solar wind ion sputtering (IS), can be constrained by comparing our exospheric model calculations with the published observational data. The detected terminator to limb (TL) ratio on the disk and the tail production rate can be explained with no sticking effect and small thermal accommodation rates. We also examine the best fit of the MIV source evolution, through comparison with the disk-averaged emission. The resultant discrepancy between the observations and the model fit may reflect the surface variation in the sodium abundance. A comprehensive mapping of the surface geochemical composition of the surface by the MESSENGER and Bepi-Colombo missions should give us more information about the nature of this surface-bound exosphere.     

基于STK的寿命末期GEO卫星轨道演化分析

地球同步轨道上的GEO卫星接近寿命末期时,星上的有效载荷和电子元器件等仍能正常应用,此时如果只做东西方向的位置保持,可以大幅度延长卫星的在轨工作寿命。简要介绍了引起GEO卫星轨道变化的主要摄动力,利用STK软件分析寿命末期GEO卫星的运动规律,并给出这类GEO卫星轨道演化的仿真分析流程,用以指导寿命末期GEO卫星的开发应用工作。     

一次雨雪的回波演变及大气结构特征

利用多普勒天气雷达和探空资料,对2009年3月2日南京的雨雪过程进行了研究,分析了雨雪发展的不同阶段,基本反射率场、基本速度场、基本谱宽场、VWP廓线以及大气要素垂直剖面图的演变特征,分析显示:(1)降雪过程的回波分布连续、具有丝缕状纹理,强度比降雨弱。(2)速度场西北方向有逆风区出现,逆风区持续期间降雪稳定、持续。(3)利用探空资料绘制的大气风矢与位温剖面图可较细致地分析雨雪天气的大气垂直结构特征。(4)雨雪天气过程和大气层结密切相关,K指数、沙氏指数及sθe850-500与雨雪天气的演变有着较好的对应关系。     

非活动论与地球脉动式演化学说的再崛起──纪念张伯声院士逝世二周年

以强调岩石圈作大规模水平移动为特色的活动论在兴旺了２０余年之后，由于首先在大陆上，继之在海洋上碰上了麻烦；在研究地球演化的过程中，又在古地磁学与古气候学上遇到了不解之谜；地球的分层结构及其各种物理、化学参数对于经典的乃至修正了的板块学说的支持都显得软弱无力。人们不得不回过头来冷静思考其反对者们的意见。其实，持否定态度的学者一直没有停止过他们的研究，尤其脉动论者。如今，脉动学说有了新的发展，其中“地球多级驻波式脉动理论模型”的提出，使活动论的各种麻烦都成了它的根据。尽管以再崛起的脉动论为代表的非活动论尚未形成一股强大的科学势力，但其前景是不可低估的。