Comets in other planetary systems?

Comets in our solar system appear to have provided a bridge between the cold, volatile-rich outer solar system, and the warm, but volatile-poor inner solar system. Excluding tidal and possible extinct radionuclide heating sources, only in the inner solar system are temperatures high enough for liquid water, and therefore life as we know it, to exist for times comparable to the age of the solar system. Comets may have been crucial for providing biogenic volatiles and perhaps organic molecules to this warm environment. It is therefore interesting from an exobiological point of view to ask if comets exist in other planetary systems. Most attempts to detect comets around other stars or in interstellar space have failed. However, there is growing spectroscopic evidence for comet-like bodies orbiting the star Beta Pictoris.     

陨石、流星体与小行星及彗星的演化关系

小行星、彗星和流星体(meteoroid)都是绕太阳公转的小天体,它们只是在轨道特性和物理-化学性质方面有所不同,流星体泛指在行星际空间运行的、质量从10~(-16)克微流星体或微尘到10~8克的所有小天体,当它们闯入地球大气时与大气剧烈碰撞而产生发光的流星(meteor)现象,落到地面的流星体残余则称为陨石或陨星(meteorite)。     

论彗星的起源

彗星富含挥发物,这表明它们形成并长期保存于太阳系外部低温区。本文分析了星云盘外缘区的结构,论证了那里不可能存在彗星形成带。作者认为,彗星是木星到海王星这个区域中的残存星子演变而成的。     

Lunar apex–antapex cratering asymmetry as an impactor recorder in the Earth–Moon system

The degree of apex–antapex cratering asymmetry of a synchronously rotating satellite primarily depends on the mean encounter velocity of impactors with respect to the planetary system and the orbital velocity of the satellite. This means that we can estimate the mean encounter velocity of impactors by observing the apex–antapex cratering asymmetry, if the relationship between these is known. To apply this technique to the Moon, we attempt to derive the relationship between the mean encounter velocity of impactors and the degree of the lunar cratering asymmetry as a function of time, considering the temporal variation in the lunar orbital velocity during the last 4.0 Gyr. We used the cratering asymmetry of Zahnle et al. [Zahnle, K., Schenk, P., Sobieszczyk, S. et al. Differential cratering of synchronously rotating satellites by ecliptic comets. Icarus 153, 111–129, 2001] to obtain the relationship. Applying this relationship enables us to estimate the impactor’s velocity of the Earth–Moon system from an investigation of the spatial distribution of lunar craters. Furthermore, we re-evaluate the cratering asymmetry’s influence on lunar cratering chronology.     

Cratering records of the satellites of Jupiter and Saturn

The surfaces of most of the satellites of Jupiter and Saturn show marks of large-meteoroid impacts produced over much of solar system history and partly stemming from the early post-accretional heavy bombardment. An analysis of the crater size distributions shows that (i) crater densities on the most ancient terrains are comparable to those found on the lunar highlands, (ii) crater size distributions differ somewhat from those in the inner solar system but exhibit striking similarities in shape, (iii) the crater size distributions on all terrain types from oldest to youngest are very similar, i. e. the underlying impactor size distribution does not seem to have changed over time, (iv) there is no obvious difference in crater densities between apex and antapex parts of the satellites, (v) the cratering record can better be explained by impacts from bodies in planetocentric orbits rather than by bodies in heliocentric orbits.     

Aeronomical processes in cometary atmospheres: The carbon compounds' puzzle

The deduction of coma abundances, ultimately leading to the abundance of parent molecules escaping the nucleus of a comet, requires to study a very large number of physical phenomena. Those include excitation processes of the emission of the light, production and loss processes of coma species, interaction with the solar wind… etc. In this paper, as an example, we shall review our knowledge on carbon bearing species and attempt to estimate the C/OH abundance ratio in various comets. We shall reach the interesting conclusion that this parameter greatly varies from comet to comet. It will then be appropriate to discuss if this is the effect of an “aging” process or if this is due to a “primordial” difference among the objects under study.     

Meteorites,asteroids and comets — A study of their interrelationship

The origins of the three kinds of extra-terrestrial bodies, meteorites, asteroids and comets can be traced to the origin of solar system. From a study of their properties, a generic relatinship of the form interstellar gas and dust condensing in the primitive solar nebula leading to the formation of comets, the comets getting degassed and ablated to become asteroids and the asteroids on fragmentation due to collisions becoming meteorites, is advocated here. The isotopic abundances of carbon are suggested as the key to unlock the mystery of this relationship.     

On the evolution of dust in the solar vicinity.

The analysis of interplanetary dust shows that the majority of particles in out-of-ecliptic regions comes from comets and also that near solar dust, in the ecliptic regions, results most probably largely from comets. The intense radiation flux in the solar vicinity is expected to cause strong modifications in the material composition and surface structure of interplanetary dust particles and hence the analysis of near solar dust provides interesting insights into the evolution of meteoritic, especially cometary materials. Because of the lack of in-situ measurements our present knowledge concerning these processes derives from remote sensing, i.e. observations of the solar F-corona. In particular these are observations of albedo, polarization and colour temperature given in terms of average particle properties. For example the analysis of near infra-red F-corona data points to the existence of a strong component of irregularly structured silicate particles, most probably of cometary origin. The data may indicate a subsequent sublimation of different particles or different constituents of the particles. Here we compare particle properties derived from F-corona observations with model calculations of single particle properties and discuss perspectives of future analysis of cometary dust in the interplanetary cloud.     

Dark matter in the solar system

Many small bodies in the solar system, including planetary satellites, comets, and asteroids, have a surface component consisting at least in part of a very low albedo (0.06 or less) solid substance of neutral or red color in the wavelength region 0.3–2.5 μm. Laboratory spectra of organic residues from meteorites and mixtures with hydrous silicates suggest that complex mixtures of complex organic molecules (kerogens) together with aqueous alteration products of igneous minerals may be the source of the dark matter that is distributed so widely throughout the solor system.     

Large-scale impact cratering on the terrestrial planets

Impact cratering as a geologic process on the terrestrial planets is addressed. The crater densities on the Earth and Moon form the basis for a standard flux-time curve, which can be used to date unsampled planetary surfaces and constrain the temporal history of endogenic geologic processes. The attached uncertainties and the shape of the flux curve (a rapid exponential decay for the period 4.6 – 4.0 by, followed by the establishment of a constant fluid by 3.5 – 3.0 by which continues more or less to the present) are such that only very old (3.8 by) and very young ( 1.0 by) surfaces can be dated with some confidence. Dating of intermediate-aged surfaces is more imprecise; a problem which is most significant for the geologic history of Mars.

The cratering mechanics of simple craters are fairly well understood. A transient cavity of roughly parabolic cross-section results from the combined excavation and displacement of the target rocks by the cratering flow-field, which can be approximated by the Z-model derived from shallow-buried explosive events. The walls of the transient crater are unstable and slump inwards, resulting in a final bowl-shaped crater partially filled by breccia. The formation process of larger, shallow complex structures is less well understood. Recent models favor the complete collapse of the initial cavity, with the dynamic uplift of the excavated cavity floor. Regardless of the driving force for uplift, yield strength of the target rocks must be drastically reduced during cavity modification by an, as yet, imprecisely known process.

The formation of large impact basins had a profound effect on planetary evolution. They define the basic tectonic and stratigraphic framework of the Moon and their secondary effects lasted for 10⁸ y. The evidence is less compelling from other planets, but a general feature appears to be the concentration of later endogenic activity in and around basins. On Earth, it is possible that basin-formation contributed to the establishment of the dichotomy between proto-continental and proto-oceanic crusts. The effects of impact continue into recent geologic history and may be linked to major biological changes on Earth, such as at the Cretaceous-Tertiary boundary.     

等离子体彗星的某些相关分析

本文进行了彗星的分类统计,给出了"太刚活动"和"等离子体彗星出现频数"的统计,表示出它们之间无相关性。作了"太阳活动"与"彗星事件"的相关分析,表明其相关性是弱的;但是,"太阳风风速"与"彗星事件"的相关性较强。最后,用我国自己的资料作为一个实例,讨论了等离子体彗星的象差角及其扭折骚扰。     

Sharp boundaries of solar wind plasma structures and their relationship to solar wind turbulence

Sharp (<10 min) and large (>20%) solar wind ion flux changes are common phenomena in turbulent solar wind plasma. These changes are the boundaries of small- and middle-scale solar wind plasma structures which can have a significant influence on Earth’s magnetosphere. These solar wind ion flux changes are typically accompanied by only a small change in the bulk solar wind velocity, hence, the flux changes are driven mainly by plasma density variations. We show that these events occur more frequently in high-density solar wind. A characteristic of solar wind turbulence, intermittency, is determined for time periods with and without these flux changes. The probability distribution functions (PDF) of solar wind ion flux variations for different time scales are calculated for each of these periods and compared. For large time scales, the PDFs are Gaussian for both data sets. For small time scales, the PDFs from both data set are more flat than Gaussian, but the degree of flatness is much larger for the data near the sharp flux change boundaries.     

Large scale MHD properties of interplanetary magnetic clouds

Magnetic Clouds (MCs) are the interplanetary manifestation of Coronal Mass Ejections. These huge astrophysical objects travel from the Sun toward the external heliosphere and can reach the Earth environment. Depending on their magnetic field orientation, they can trigger intense geomagnetic storms. The details of the magnetic configuration of clouds and the typical values of their magnetohydrodynamic magnitudes are not yet well known. One of the most important magnetohydrodynamic quantities in MCs is the magnetic helicity. The helicity quantifies several aspects of a given magnetic structure, such as the twist, kink, number of knots between magnetic field lines, linking between magnetic flux tubes, etc. The helicity is approximately conserved in the solar atmosphere and the heliosphere, and it is very useful to link solar phenomena with their interplanetary counterpart. Since a magnetic cloud carries an important amount of helicity when it is ejected from the solar corona, estimations of the helicity content in clouds can help us to understand its evolution and its coronal origin. In situ observations of magnetic clouds at one astronomical unit are in agreement with a local helical magnetic structure. However, since spacecrafts only register data along a unique direction, several aspects of the global configuration of clouds cannot be observed. In this paper, we review the general properties of magnetic clouds and different models for their magnetic structure at one astronomical unit. We describe the corresponding techniques to analyze in situ measurements. We also quantify their magnetic helicity and compare it with the release of helicity in their solar source for some of the analyzed cases.     

Cometary origin of the biosphere: a progress report.

Empirical evidence of the accretion temperature for undifferentiated meteorites coming from the asteroid belt, combined with any reasonable temperature gradient extending from the asteroid belt to the Earth's zone, suggests that the Earth accreted from very hot dust grains that were degassed from all volatile elements and depleted in labile compounds. Isotopic evidence from the atmospheric noble gases also shows that no primary atmosphere has survived on the Earth. The only possible source for the atmosphere and the oceans is therefore the cometary bombardment that is predicted as the inescapable consequence of the formation of the giant planets. This implies that comets are the only source of organic carbon, nitrogen and water, hence of the total biosphere of the Earth.     

Modelling and observations of photospheric magnetic helicity

Mounting observational evidence of the emergence of twisted magnetic flux tubes through the photosphere have now been published. Such flux tubes, formed by the solar dynamo and transported through the convection zone, eventually reach the solar atmosphere. Their accumulation in the solar corona leads to flares and coronal mass ejections. Since reconnections occur during the evolution of the flux tubes, the concepts of twist and magnetic stress become inappropriate. Magnetic helicity, as a well preserved quantity, in particular in plasma with high magnetic Reynolds number, is a more suitable physical quantity to use, even if reconnection is involved.     

Solar UV radiation variations and their stratospheric and climatic effects

NIMBUS-7 SBUV measurements of the short-term solar UV variations caused by solar rotation and active-region evolution have determined the amplitude and wavelength dependence for the active-region component of solar UV variations. Intermediate-term variations lasting several months are associated with rounds of major new active regions. The UV flux stays near the peak value during the current solar cycle variation for more than two years and peaks about two years later than the sunspot number. NIMBUS-7 measurements have observed the concurrent stratospheric ozone variations caused by solar UV variations. There is now no doubt that solar UV variations are an important cause of short- and long-term stratospheric variations, but the strength of the coupling to the troposphere and to climate has not yet been proven.     

基于PCA方法对太阳活动区主要参量的分析

太阳耀斑与太阳质子事件的发生通常与太阳活动区存在非常密切的关系, 对这种关系的深入分析有助于太阳耀斑和太阳质子事件预报模型的建立. 本文利用主成分分析(Principal Component Analysis, PCA)方法对1997-2010年太阳质子事件所在活动区的主要参量进行分析, 选取的参量包括黑子磁分类、 McIntosh分类、太阳黑子群面积、10.7 cm射电流量、耀斑指数、质子耀斑位置和软X射线耀斑强度. 结果得到81个太阳活动主成分得分值排序(得分值代表每个事件的强弱), 与太阳质子事件峰值流量、太阳黑子年均值以及10.7 cm射电流量年均值的对比显示相似度非常高, 表明主成分得分值一定程度上可以反映太阳活动的强弱规律.     

The seeding of life by comets.

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.     

1997-2003年太阳质子事件源区的卡林顿经度分布

分析了1997-2003年期间的59个太阳质子事件,太阳质子事件源区的卡林顿经度带.分析结果表明,质子事件源区主要集中在10°-45°,135°-155°,180°-215°,230°-260°,265°-310°,345°-360°,还有4个质子耀斑分散在72°,74°,93°和107°上.其中最强的活动卡林顿经度带的经度范围是265°-310°,次之为卡林顿经度带135°-155°,最弱的是卡林顿经度带180°-215°最强的活动经度是卡林顿经度272°.1997-2003年期间峰值通量大于100 pfu,质子耀斑南半球发生次数为18,北半球发生次数为10;同一活动经度上的质子事件具有重现的规律,重复出现的时间间隔短的为27天,长的超过4年.     

Variations in polar rain flux according to IMF geometries observed by STSAT-1

We examined polar rain flux observed by STSAT-1 in the northern polar cap and compared it with solar wind parameters. We found that the differential energy spectrum of polar rain was similar to that of the solar wind for the energy range 100 eV – 1 keV, although we cannot rule out the possibility of a small amount of acceleration. On the other hand, the low-energy component of the solar wind showed no correlation and, naturally, the solar wind density had only a weak correlation with the polar rain flux. Polar rain flux in the northern hemisphere is most significant for the condition of the interplanetary magnetic field components Bz < 0, Bx < 0, and By > 0, and in this case it correlated well with the magnitude of By and Bz. For other interplanetary magnetic field conditions, the correlation was insignificant. The results are consistent with those reported previously.