Neutral upper atmospheres of Venus and Mars

Numerous measurements of the neutral upper atmosphere above 100 km have been made from spacecraft over Venus and over Mars. The Venus exospheric temperatures are unexpectedly low (less than 300°K near noon and less than 130°K near midnight). These very low temperatures may be partially caused by collisional excitation of CO₂ vibrational states by atomic oxygen and partially by eddy cooling. The Venus atmosphere is unexpectedly insensitive to solar EUV variability. On the other hand, the Martian dayside exospheric temperature varies from 150°K to 400°K over the 11-year solar cycle, where CO₂ 15-μm cooling may be less effective because of lower atomic oxygen mixing ratios. On Venus, temperature increases with altitude on the dayside (thermosphere), but decreases with altitude from 100 to 150 km on the nightside (cryosphere). However, dayside Martian temperatures near solar minimum for maximum planet-sun distance and low solar activity are essentially isothermal from 40 km to 200 km. During high solar activity, the thermospheric temperatures of Mars sharply increase. The Venus neutral upper atmosphere contains CO₂, O, CO, C, N₂, N, He, H, D and hot nonthermal H, O, C, and N, while the dayside Mars neutral upper atmosphere contains CO₂, O, O₂, CO, C, N₂, He, H, and Ar. There is evidence on Venus for inhibited day-to-night transport as well as superrotation of the upper atmosphere. Both atmospheres have substantial wave activity. Various theoretical models used to interpret the planetary atmospheric data are discussed.     

电离对高超声速热化学非平衡气动热环境的影响

高超声速飞行,激波后高温气体会发生电离,飞行器气动热环境复杂。5组元（N₂,O₂,NO,O,N）、7组元（N₂,O₂,NO,O,N,NO+,e-）和11组元（N₂,O₂,NO,O,N,N₂+,O₂+,NO+,O+,N+,e-）热化学反应采用Gupta化学反应模型,分别数值研究电离作用对高超声速热化学非平衡气动热环境影响。本文分析了不同催化壁面条件下,高超声速热化学非平衡电离流场气动热环境特性。电离作用对激波离体距离和气动力载荷的影响很小。5组元热化学非平衡不考虑电离作用,流场温度和壁面热流密度偏大。11组元热化学平衡强电离流场温度最低;7组元热化学非平衡弱电离流场NO+和e-生成量过低;11组元热化学反应能对热化学非平衡电离流场气动力和热流密度载荷可靠预测。壁面催化作用会增大壁面热流密度,但它对高超声速热化学非平衡电离流场温度和气动力载荷的影响很小。      

Energy deposition in the upper atmosphere in the EXCEDE III experiment

The EXCEDE III sounding rocket flight of April 27, 1990 used a 18 Ampere 2.5 keV electron beam to produce an artificial aurora in the region 90 to 115 km. A “daughter” sensor payload remotely monitored the low-energy X-ray spectrum while scanning photometers measured the spatial profile of prompt emissions of N₂⁺ (1N) and N₂ (2P) transitions (3914Å and 3805Å, respectively). Two Ebert-Fastie spectrometers measured the spectral region from 1800 to 8000Å. On the “mother” accelerator payload, the return current electron differential energy spectra were monitored by an electrostatic analyzer (up to 10 keV) and by a retarding potential analyzer (0 eV to 100 eV). We present an overview of the results from this experiment.     

太赫兹火星大气临边探测仿真研究

对火星大气进行连续高分辨率观测是研究火星大气物理和化学过程的重要手段.太赫兹临边探测技术通过测量火星大气中的风和光化学循环中的重要气体（CO,O₃,H₂O,H₂O₂等）提高对火星的认知.针对火星大气遥感的探测需求,分析了300～1000GHz频段的频谱特征.针对探测卫星对于载荷质量、功耗等参数的要求,提出一个560GHz频段的火星大气太赫兹临边探测仪设计方案,并利用辐射传输模型ARTS中的行星工具箱进行仿真.仿真结果显示:火星大气温度的反演精度优于4K,其中45km高度以下优于2K;H₂O丰度的反演精度在90km以下优于50%,30km以下优于2%;H₂O₂的反演精度在40km以下优于50%;O₃的反演精度在50km以下优于60%;大气风速度的反演精度在65km以上优于5m·s-1,最高可以达到2m·s-1.研究结果表明,利用太赫兹波段的吸收谱线可以很好地探测火星大气中各成分的丰度、变化趋势以及中高层大气的风,可为后续火星表面及大气探测提供参考.      

火星大气模型参数对MSL气动特性的影响

火星大气与地球大气截然不同,飞行器在进入火星时气动特性不同于地球再入. 大气模型的差异主要表现为气体组份、密度和温度等物理参数. 针对火星进入器MSL在进入-下降-着陆过程中的高超声速进入段,利用三维并行程序求解耦合真实气体模型的流体动力学Navier-Stokes方程,分析MSL进入火星大气时大气模型参数对进入器气动特性的影响. 结果表明,通过与海盗号飞行数据的对比,验证了所采用的火星气体模型和计算方法,且其与NASA的 LAURA代码气动特性计算结果也较为一致;大气模型气体性质,即CO₂环境对进入器阻力系数和俯仰力矩系数影响较大,利用空气得到的计算和实验数据必须考虑CO₂效应;密度增大促进了化学非平衡效应,但对进入器气动特性基本没有影响;温度升高大大增强了化学非平衡效应,而对进入器气动特性影响较小.      

激发跃迁速率对热力学非平衡氮气紫外辐射的影响

基于氮气的碰撞-辐射（CR）模型,计算了速度为6.2 km/s、初始压力为133 Pa的高超声速流动激波中N₂和N₂+分子电子能级的分布情况,分析了不同激发跃迁速率模型对电子能级分布及辐射光谱模拟的影响。针对流动中热力学非平衡区域和平衡区域,在300~440 nm的辐射光谱开展了逐线法的数值模拟,并与激波管实验测量光谱进行了对比。结果表明,目前的激发跃迁速率均存在偏差,综合Park模型的爱因斯坦系数和Johnston模型的碰撞激发速率可以得到与实验结果最为接近的辐射光谱。      

Polar/TIMAS statistical results on the outflow of molecular ions from earth at solar minimum

Ion composition data from the first 22 months of operation of the Polar/TIMAS instrument, covering the 15-eV/e to 33-keV/e energy range, have been surveyed to determine the typical abundance, at solar minimum, of N₂⁺, NO⁺ and O₂⁺ ions in the auroral ion outflow, as compared to that of the better known O⁺ ions. The results indicate that molecular ions have roughly the same energy distribution as the O⁺ ions, with maximum differential flux occurring below 400 eV, but are far less abundant, by two orders of magnitude. The molecular ions also differ from the O⁺ ions in that they seem more specifically associated with enhanced geomagnetic activity.     

Biological nitrogen fixation under primordial Martian partial pressures of dinitrogen.

Early Earth and early Mars were similar enough such that past geochemical and climatic conditions on Mars may have also been favorable for the origin of life. However, one of the most striking differences between the two planets was the low partial pressure of dinitrogen (pN2) on early Mars (18 mb). On Earth, nitrogen is a key biological element and in many ecosystems the low availability of fixed nitrogen compounds is the main factor limiting growth. Biological fixation of dinitrogen on Earth is a crucial source of fixed nitrogen. Could the low availability of dinitrogen in the primordial Martian atmosphere have prevented the existence, or evolution of Martian microbiota? Azotobacter vinelandii and Azomonas agilis were grown in nitrogen free synthetic medium under various partial pressures of dinitrogen ranging from 780-0 mb (total atmosphere=1 bar). Below 400 mb the biomass, cell number, and growth rate decreased with decreasing pN2. Both microorganisms were capable of growth at a pN2 as low as 5 mb, but no growth was observed at a pN2 < or = 1 mb. The data appear to indicate that biological nitrogen fixation could have occurred on primordial Mars (pN2=18 mb) making it possible for a biotic system to have played a role in the Martian nitrogen cycle. It is possible that nitrogen may have played a key role in the early evolution of life on Mars, and that later a lack of available nitrogen on that planet (currently, pN2=0.2 mb) may have been involved in its subsequent extinction.     

Bio-markers and the search for extinct life on Mars.

Geologic and climatologic studies suggest that conditions on early Mars were similar to early Earth. Because life on Earth is believed to have originated during this early period (3.5 billion years ago), the Martian environment could have also been conducive to the origin of life. To investigate this possibility we must first define the attributes of an early Martian biota. Then, specific geographic locations on Mars must be chosen where life may have occurred (i.e. areas which had long standing water), and within these distinct locations search for key signatures or bio-markers of a possible extinct Martian biota. Some of the key signatures or bio-markers indicative of past biological activity on Earth may be applicable to Mars including: reduced carbon and nitrogen compounds, CO3(2-), SO4(2-), NO3-, NO2- [correction of NO2(2)], Mg, Mn, Fe, and certain other metals, and the isotopic ratios of C, N and S. However, we must also be able to distinguish abiotic from biologic origins for these bio-markers. For example, abiotically fixed N2 would form deposits of NO3- and NO2-, whereas biological processes would have reduced these to ammonium containing compounds, N2O, or N2, which would then be released to the atmosphere. A fully equipped Mars Rover might be able to perform analyses to measure most of these biomarkers while on the Martian surface.     

VEGA three-channel spectrometer experiment: Spectroscopy of comet Halley in the visible and near-ultraviolet ranges

A total of 3600 spectra of Comet Halley in the 275–710 nm were obtained on March, 8, 9, 10 and 11, 1986, from the VEGA 2 spacecraft. The emissions of OH, NH, CN, C₃, CH, C₂, NH₂ and H₂O⁺ are identified. From the OH intensity in the (0,0) band: 1.1 Megarayleigh at 5400 km from the nucleus, it can be inferred that the OH production rate was (1.4 ± 0.5)×10³⁰ molecules s⁻¹. The NH, C₃, CH and NH₂ bands became comparatively more intense at distances from the nucleus shorter than 3000km. At 06:40 U.T. when the instrument field of view was 6000×4500 km, two jets were observed. Spectra from the jets show significant differences with other spectra. Inside a jet NH, C₃ and NH₂ are comparatively more intense and the rotational distributions of OH, CN and C₂ are strongly distorted. This shows that part of the observed emissions probably comes from radicals directly produced in the excited state during the initial process of photolysis of the parent molecules.     

Geminga pulsar observations with gamma-telescope Gamma-1

The Geminga light curve obtained with the “Gamma-1” telescope features two peaks separated by 0.5 ± 0.03 period. The light curve is pronounced for γ-quanta energies higher than 400 MeV. The pulsed flux upper limit (1σ) in the energy interval 50 – 300 MeV is 6·10⁻⁷ cm⁻²sec⁻¹. For energies >300 MeV the pulsed component power law spectrum has an exponent 1.1 _−0.3^+1.1 and an integral flux (1.1±0.3)·10⁻⁶ cm⁻²sec⁻¹.     

火星表面热环境对航天器热控影响分析

火星大气对太阳辐射产生吸收和散射作用，同时还将与火星表面航天器发生对流换热。热设计时难以直接评估对流、辐射和导热三种换热对航天器的影响，从而确定主要的控温途径。在调研火星表面辐射、大气等热环境的基础上，从线性化传热系数和对流辐射比的角度对比分析了辐射、对流和导热对航天器的影响。器表辐射传热系数随光学属性和温度的变化范围为0.3～1.4W/(m2·℃)，对流传热系数随风速变化为0.2～1.5W/(m2·℃)，器内导热传热系数可控制在0.25W/(m2·℃)以下。结果表明，太阳辐射较火星表面和天空辐射而言是主要外热源，航天器表面的辐射和对流换热为两条并联换热途径，两者均可成为主要换热途径，器内导热传热是控制航天器内外隔热的主要可控因素。     

Hot atoms in cosmic chemistry

High energy chemical reactions and atom molecule interactions might be important for cosmic chemistry with respect to the accelerated species in solar wind, cosmic rays, colliding gas and dust clouds and secondary knock-on particles in solids. “Hot” atoms with energies ranging from a few eV to some MeV can be generated via nuclear reactions and consequent recoil processes. The chemical fate of the radioactive atoms can be followed by radiochemical methods (radio GC or HPLC). Hot atom chemistry may serve for laboratory simulation of the reactions of energetic species with gaseous or solid interstellar matter. Due to the effective measurement of 10⁸–10¹⁰ atoms only it covers a low to medium dose regime and may add to the studies of ion implantation which due to the optical methods applied are necessarily in the high dose regime.

Experimental results are given for the systems: C/H₂O (gas), C/H₂O (solid, 77 K), N/CH₄ (solid, 77K) and C/NH₃ (solid, 77 K). Nuclear reactions used for the generation of 2 to 3 MeV atoms are: ¹⁴N(p, ) ¹¹C, ¹⁶O(p, pn) ¹¹C and ¹²C(d,n) ¹³N with 8 to 45 MeV protons or deuterons from a cyclotron. Typical reactions products are: CO, CO₂, CH₄, CH₂O, CH₃OH, HCOOH, NH₃, CH₃NH₂, cyanamide, formamidine, guanidine etc. Products of hot reactions in solids are more complex than in corresponding gaseous systems, which underlines the importance of solid state reactions for the build-up of precursors for biomolecules in space. As one of the major mechanisms for product formation, the simultaneous or fast consecutive reactions of a hot carbon with two target molecules (reaction complex) is discussed.     

Atmospheric photochemistry above possible martian hot spots

Considering the possibility of outgassing from some localized sources on Mars, we have developed a one-dimensional photochemical model that includes methane (CH₄), sulfur dioxide (SO₂) and hydrogen sulfide (H₂S). Halogens were considered but were found to have no significant impact on the martian atmospheric chemistry. We find that the introduction of methane into the martian atmosphere results in the formation of mainly formaldehyde (CH₂O), methyl alcohol (CH₃OH) and ethane (C₂H₆), whereas the introduction of the sulfur species produces mainly sulfur monoxide (SO) and sulfuric acid (H₂SO₄). Depending upon the flux of the outgassed molecules from possible hot spots, some of these species and the resulting new molecules may be detectable locally, either by remote sensing (e.g., with the Planetary Fourier Spectrometer on Mars Express) or in situ measurements.     

S3 and S4 absorption cross sections in the range of 340 to 600 nm and evaluation of the S3 abundance in the lower atmosphere of Venus

S₃ absorption cross section equals 6×10⁻¹⁷ cm² at 400 nm, 6 × 10⁻¹⁹ cm² at 500 nm (less by a factor of 4 than that given by Sanko), 4×10⁻²⁰ cm² at 600 nm. That of S₄ equals 1.5 × 10⁻¹⁷ cm² at 450 nm, 8 × 10⁻¹⁷ cm² at 500 nm, and 4.7 × 10⁻¹⁷ cm² at 600 nm. Preliminary evaluation of the S₃ mixing ratio in the lower atmosphere of Venus is (8±3)×10⁻¹¹ at 5 to 25km according to the Venera 14 measurements and several times lower at the locations of the Veneras-11 and -13.     

Electric discharge in the Martian atmosphere,Paschen curves and implications for future missions

Electric discharge between two electrically charged surfaces occurs at a well-defined, gas-dependent combination of atmospheric pressure and the distance between those two surfaces, as described by Paschen’s law. The understanding of when the discharge will occur in the conditions present on Mars is essential for designing space-flight hardware that will operate on the Martian surface as well as understanding electrical discharge processes occurring in the Martian atmosphere. Here, we present experimentally measured Paschen curves for a gas mixture representative of the Martian atmosphere and compare our results to breakdown voltages of carbon dioxide, nitrogen, and helium as measured with our system and from the literature. We will discuss possible implications for instrument development as well as implications for processes in the Martian atmosphere. The DC voltage at which electric discharge occurred between two stainless steel spheres was measured at pressures from 10⁻² to 100 torr in all gases. We measured a minimum voltage for discharge in the Mars ambient atmosphere of 410 ± 10 V at 0.3 torr cm. As an application, the breakdown properties of space-qualified, electrical wires to be used in the Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) were studied.     

Radiation stability of organic matter in liquid and frozen H2O, NH3 and water-ammonia mixtures

The redox properties of irradiated liquid and frozen H₂O, NH₃ and H₂O/NH₃ mixtures at 298 K and 77 K, resp., towards some simple organic molecules have been checked by injecting carrierfree ¹¹C atoms and analyzing their chemical state by means of radiochromatography. The reactions and the stability of organic products versus radiation dose (in this study by MeV protons) depend on temperature, phase state, mobility of radicals, their concentration and reactivity. Especially dangerous are the reactive OH and O₂H radicals which oxidize organic material to inorganic CO₂. Highest stability has been found at low temperatures (solid state, reduced mobility of radicals) and for systems containing H-donors (H₂O/NH₃ mixtures), which reduce the concentration of oxidizing radicals. The fact that many bodies in space consist of H₂O-ice with NH₃ and CH₄ additives at temperatures between 10 and 150 K is promising in view of the survival of organic matter under high doses of radiation.     

Japanese research project on Arctic and Antarctic observations of the middle atmosphere

An all-sky optical imager is in routine observation at the South Pole. Monochromatic images of aurora and air glow at N₂⁺ 427.8nm, OI 557.7nm, OI 630nm and OH 730nm are supplying significant information on the magnetospheric process in the polar cap and cusp/cleft region along with atmospheric wave signature at this particular point. SuperDARN radars in Antarctica make observations over the South Pole.

At Syowa Station, Antarctica, a multi-instrumental observation project is now being implemented for the study of the polar upper atmosphere from the mesosphere to the thermosphere, where complex physical and chemical processes take place making the region very attractive for scientific research. Two HF radars, which are part of SuperDARN radars, have been already installed and started observations. By the end of 1999, all-sky imagers, photo meters, a Na temperature Lidar, an MF radar and a Fabry-Perot interferometer will be introduced and start collecting various physical parameters on a routine basis.

In the Arctic region, we are planning to deploy coordinated ground-based observations with optical, radio and radar sensing of the polar middle and upper atmosphere in conjunction with EISCAT radars. Scientific goals are versatile to shed light on the tangled coupling processes in response to magnetospheric disturbances from above and bi-lateral interactions with high-density lower atmospheric layers. These are outlined in this paper.     

飞行器进入火星大气的流场预测

针对火星着陆探测器进入-下降-着陆过程的高超声速进入阶段, 利用三维并 行程序求解流体动力学Navier-Stokes方程与化学反应动力学模型, 分析火星 科学实验室进入火星大气时探测器周围的流场结构、化学非平衡效应影响和气 动特性变化规律. 结果表明, 对于完全气体模型, 来流的热力学性质参数选 取影响激波位置和强度. 在化学非平衡效应影响下, 探测器头部激波脱体距离 大幅减小, 驻点压力变化不大, 波后温度显著降低. CO₂在激波后大量分解, 消耗相当能量. 流线结构显示, 探测器尾迹流动中存在复杂的旋涡运动等流动 分离现象.      

Photochemistry of planetary ionospheres

The basic photochemical processes in the upper atmospheres and ionospheres of the various bodies in our solar system (planets, moons and comets) are similar. However, there are many different factors (e.g. gas composition, energy input, gravity) which control/change the relative importance of these controlling processes. The photo-chemistry of the inner planets is reasonably well understood at this time, thus there is good agreement between model calculations and most of the observational data base. The extremely limited information that we have available on the ionospheres of the outer planets leads to significant uncertainties about some of the controlling processes. Some important questions (e.g. Is the charge exchange process H⁺ + H₂(v≥4) → H₂⁺ + H important? Is water vapor influx from the rings important?) remain unanswered at this time. In cometary atmospheres the freshly evaporated parent molecules are rapidly photodissociated and photoionized, therefore most of the chemical kinetics of cometary ionospheres involve these rapidly moving and highly reactive ions and radicals.