On the effect of the martian crustal magnetic field on atmospheric erosion

Without the shielding of a strong intrinsic magnetic field, the martian atmosphere directly interacts with the impacting solar wind. The neutral constituents of the atmospheric corona can be ionized, and then picked up and accelerated by the magnetic field and convection electric field in the solar wind. A significant fraction of pickup ions escape Mars’ gravitational pull and are lost to space. This non-thermal escape process of heavy species is an important mechanism responsible for atmospheric erosion. While there is a perception that the martian magnetic anomalies are significant for the ionospheric density distribution and the bow shock standoff location, little is known about the quantitative influence of the martian crustal magnetic field on the global distribution of escaping pickup ions. In this paper, we apply a newly developed Monte Carlo ion transport model to resolve the crustal field effect on the pickup oxygen ion distribution around Mars. The background magnetic and electric fields, in which test particles are followed, are calculated using an independent three-dimensional multispecies MHD model. The effects of the crustal magnetic field on particle escape are quantified by varying the crustal field orientation in the model setup and comparing the corresponding test particle simulation results. The comparison is made by turning on or off the crustal field or changing the local time of the strongest field from the dayside to the dawnside. It is found that without the protection of the crustal magnetic field, the total amount of atmospheric escape through the tail region would be enhanced by more than a factor of two. It is shown that the crustal magnetic field not only regionally deflects the solar wind around the martian atmosphere, but also has an important global effect on atmospheric erosion and thus on long-term atmospheric evolution.     

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.     

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.     

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.     

浅析末次间冰期以来柴达木盆地东南部古气候环境演化

在前人研究的基础上,以柴达木盆地察尔汗地区ZK2钻孔沉积物为研究对象。选取古气候替代指标——色度,分析了察尔汗地区近130kaB．P．以来气候的变化．揭示了该区对全球气候变化的响应模式以及古气候变化在色度指标上的表现。结果表明：末次间冰期以来,察尔汗地区环境由温暖湿润冷暖频繁波动干热环境变化,色度L＊,a＊,b＊具体表现为在不同水平的波动。其中b＊较L＊,a＊能更敏感地反映古环境的变化,在温暖湿润的间冰期,b＊值趋向于高值区间;而在寒冷干燥的冰期,b＊值趋向于低值区间波动。这种整体波动的趋势与氧同位素的5个阶段基本吻合。     

济阳坳陷石炭系—二叠系储层成岩作用及孔隙演化

石炭纪—二叠纪海陆交互沉积背景下发育的碎屑岩是济阳坳陷主要的储集岩。薄片、扫描电镜等多种微观分析研究表明,储层成岩作用主要有压实作用、胶结作用、交代作用、溶蚀作用及裂缝形成作用等。依据残留地层叠置样式及埋藏史,结合古地温、有机质热演化参数、黏土矿物转化序列、自生矿物以及成岩事件等多方面研究成果,将储层划分为古最大埋深等于现今埋深、古最大埋深大于现今埋深两种成岩阶段类型,前者处于中成岩B阶段和晚成岩阶段,后者处于中成岩A阶段和B阶段。多期复杂的构造运动导致储层具有多变的成岩环境及复杂的成岩演化序列,并控制了储层的成岩演化史、孔隙演化史及储集性能。     

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。     

一次暴雨过程天气尺度云系演变特征分析

通过对2005年6月18日至23日造成广西大范围持续性暴雨过程天气尺度云系特征分析,发现暴雨落区与阶段平均云图的TBB≤-60℃区域有很好的对应关系,表明暴雨主要是由中尺度天气系统造成的,利用这些关系可以很有助于做好强降雨的精细化预报。     

基于火箭探空资料的冰雹云内部结构个例分析

利用探空火箭、新一代天气雷达和气象探测资料对2015年7月17日延安宝塔区冰雹云进行了综合探测,结果表明:（1）当日08:00（北京时,下同）500 hPa河套低涡分裂东移,有较强冷平流且移动速度较快,地面14:00升温明显造成了这次降雹。（2）偏后位置的冰雹云内部温、湿条件以及对流指数（T_g）、整层比湿积分（IQ）、总指数（TT）均小于外部的自然大气;层结稳定度指数（K）、抬升指数（LI）、沙氏指数（SI）冰雹云内部比外部自然大气偏小;热力参数风暴强度指数（SSI）冰雹云内部低于外部自然大气;冰雹云内部能量参数（CAPE）明显低于自然大气;冰雹云内部0°C层高度低于冰雹云外部自然大气。（3）火箭探测的位置偏冰雹云后部,冰雹云由低层到高层风向呈逆时针变化,探空仪摆动明显,?20°C温度层偏高,气流较强,整层偏下沉气流。（4）冰雹云0°C附近,在温度区间?1.8～5.0°C、厚度1.0 km范围内有最大湿度区,湿度达80%以上,最大湿度87.1%,为冰雹的形成提供了水汽条件。（5）紧贴0°C下正温区,有最大水平风速为19 m s?1急流,厚度为0.022 km。在温度区间?4.8～5.0°C、厚度1.6 km范围内维持13 m s?1以上水平风速,为冰雹的形成提供了动力场条件。（6）在温度区间?8.7～?9.2°C、厚度0.2 km,有小于或等于2 m s?1弱风区;弱风区下方,在温度区间?4.6～?8.8°C、厚度0.889 km有上升气流,平均上升速度1.79 m s?1,最大上升速度4 m s?1,这种配置为冰雹的生长提供了环境场。