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1.
The Galileo spacecraft was launched in 1989, and—between 1995 and 2003—was the first spacecraft in orbit about Jupiter. The in-situ dust instrument on board was a highly sensitive impact-ionisation dust detector which measured the speed, mass and impact direction of dust particles hitting a metal target. It provided a unique 12-year record of cosmic dust in interplanetary and circumjovian space. Degradation of the instrument electronics caused by the harsh radiation environment in the inner jovian magnetosphere was recognised in various ways: the sensitivity for dust detection dropped by a factor of 7.5 between 1996 and 2003 while the noise sensitivity decreased by up to a factor of 100. Shifts in the parameters measured during dust impacts and noise events (charge amplitudes and signal rise times, etc.) required a time-dependent algorithm for noise identification. After noise removal a total of 21 224 complete data sets for dust impacts (i.e. impact charges, signal rise times, impact direction, etc.) is available from the entire Galileo mission between 1989 and 2003 (18 340 data sets from the Jupiter mission after 1996). This homogeneous data set has been used in many investigations of jovian dust published already or ongoing. Electronics degradation prevents the application of the mass and speed calibration to data obtained after 2000. Only in cases where the impact speed of grains is known by other means can grain masses be derived for later measurements. The drop of the detection sensitivity also required a time-dependent correction for fluxes of jovian dust streams, reaching a factor of 20 in 2002. We use the derived homogeneous noise-removed data set for long-term monitoring of the jovian dust streams with Galileo. The derived fluxes of dust stream particles were highly variable by about five orders of magnitude, between 3×10-3 and and exhibited strong orbit-to-orbit variability. This extensive and valuable data set is available for further detailed investigations.  相似文献   

2.
The paper presents simulations of the energetic neutral atom (ENA) production in the Mercury magnetosphere and the obtained ENA images for the equatorial and polar vantage points. The ENA fluxes are found to be 102–103 (cm2 srskeV)−1 and up to 104–105 (cm2 srskeV)−1 in the energy range 10–50 keV. Due to the small size of the magnetosphere, the particles injected in the tail can fill up the entire dayside magnetosphere making possible ENA imaging of the magnetospheric shape. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”) which can be used to study the global dynamics. The ENA instrument requirements, 10°×10° angular resolution and 20 s accumulation time, can be easily met by modern ENA instrumentation. Therefore, ENA imaging of the Mercury magnetosphere is feasible.  相似文献   

3.
During the last half of the 20th century, cumulative annual discharge from 137 representative rivers (watershed areas ranging from 0.3 to 6300 × 103 km2) to the global ocean remained constant, although annual discharge from about one-third of these rivers changed by more than 30%. Discharge trends for many rivers reflected mostly changes in precipitation, primarily in response to short- and longer-term atmospheric–oceanic signals; with the notable exception of the Parana, Mississippi, Niger and Cunene rivers, few of these “normal" rivers experienced significant changes in either discharge or precipitation. Cumulative discharge from many mid-latitude rivers, in contrast, decreased by 60%, reflecting in large part impacts due to damming, irrigation and interbasin water transfers. A number of high-latitude and high-altitude rivers experienced increased discharge despite generally declining precipitation. Poorly constrained meteorological and hydrological data do not seem to explain fully these “excess” rivers; changed seasonality in discharge, decreased storage and/or decreased evapotranspiration also may play important roles.  相似文献   

4.
《Planetary and Space Science》1999,47(3-4):363-383
The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79°). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraftreapproached the inner solar system, flew over the Suns south polar region in September 1994,crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Suns northpolar region in July 1995. We report on dust impact data obtained with the dust detector onboardUlysses between January 1993 and December 1995. We publish and analyse the complete dataset of 509 recorded impacts of dust particles with masses between 10−16 g–10−7 g. Together with 968 dust impacts from launch until the end of 1992 published earlier (Grün et al., 1995c), information about 1477 particles detected with theUlysses sensor between October 1990 and December 1995 is now available. The impact ratemeasured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day andvaried by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU arecompatible with particles of interstellar origin. Two populations of interplanetary particles havebeen recognized: big micrometer-sized particles close to the ecliptic plane and smallsub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is comparedwith a model for the flux of interstellar dust particles which gives relatively good agreement withthe observed impact rate. No change in the instruments noise characteristics or degradation of thechanneltron could be revealed during the three-year period.  相似文献   

5.
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6.
The possible effects of trace-gas induced climatic changes on Pyramid and Yellowstone Lakes are assessed using a model of lake temperature. The model is driven by years of hourly meteorological data obtained directly from the output of double-CO2 experiments (2 × CO2) conducted with a regional climate model nested in a general circulation model. The regional atmospheric model is the climate version of the National Center for Atmospheric Research/Pennsylvania State University mesoscale model, MM4.Average annual surface temperature of Pyramid Lake for the 2 × CO2 climate is 15.5 ± 5.4°C (±1 σ), 2.8°C higher than the control. Annual overturn of the lake ceases as a result of these higher temperatures for the 2 × CO2 climate. Evaporation increases from 1400 mm yr−1 in the control to 1595 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Pyramid Lake basin increases from −6 mm yr−1 in the control to +27 mm yr−1 in the 2 × CO2 simulation due to increased precipitation.For the open water periods, the average annual surface temperature of Yellowstone Lake is 13.2 ± 5.1°C for the 2 × CO2 climate, a temperature 1.6°C higher than the control. The annual duration of ice cover on the lake is 152 days in the 2 × CO2 simulation, a reduction of 44 days relative to the control. Warming of the lake for the 2 × CO2 climate is mostly confined to the near-surface. Simulated spring overturn for the 2 × CO2 climate occurs earlier in the year and fall overturn later than in the control. Evaporation increases from 544 mm yr−1 to 600 mm yr−1 in the 2 × CO2 simulation, but net water supplied to the Yellowstone Lake basin increases from +373 mm yr−1 in the control to +619 mm yr−1 due to increased precipitation. The effects of these climatic changes suggest possible deterioration of water quality and productivity in Pyramid Lake and possible enhancement of productivity in Yellowstone Lake.  相似文献   

7.
We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter. The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo's position relative to the Jovian magnetic field. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io's orbital period is evident in the dust impact rate. This feature indicates that Io most likely is the source of the dust streams. During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles. Within a distance of about 25 RJ(Jupiter radius, RJ= 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

8.
To explain the observed abundances of CO2 in Titan's atmosphere, a relatively high water deposition into the atmosphere needs to be invoked due to the importance of H2O photolysis in CO2 production. A likely source of H2O is icy dust particles from space. This paper considers the direct dust input to Titan's atmosphere from the interplanetary environment, and also ejecta particles from micrometeoroid impacts with the icy satellites Hyperion, Iapetus and Phoebe. It is found that the likely mass influx to Titan is 10–16 to 10–15 kg m–2 s–1. This mass influx is an order of magnitude too low to explain the observed levels of CO2 in Titan's atmosphere in the context of a recent photochemical model. This leads one to speculate as to the likelihood of one large impact to Titan in the recent past;i.e., that the atmosphere is not in equilibrium but is cnrrently losing CO2.  相似文献   

9.
The heat needed to melt snow over the Tien Shan mountains and Japanese Islands for 10-day period (TDP) was estimated. Melting curves and a map of snowmelt duration were obtained through the long-term data from 79 stations in the Tien Shan mountains and 20 stations in the Japanese Islands. At high elevations in the mountains, about 40% of the snow melts during penultimate 10 days of snow cover. In the Japanese Islands, about 80% of the snow melts during the last 20 days of snow cover. Over the mountains, 0.13×104 MJ m2 year−1 is needed to melt snow in the northern and western Tien Shan where maximum snow accumulation occurred. The volume of air cooled 10 °C by snowmelt amounted to 4.4×106 km3 year−1 over the Tien Shan mountains and 3×106 km3 year−1 over the Japanese Islands. The most significant impact of snowmelt on air temperature was observed at an elevation of 2500 m in the western and northern Tien Shan. Air that was cooled 10 °C could reach an elevation of 2.1 km day−1. Over the Japanese Islands, energy losses from snowmelt amounted to 0.26×1014 MJ year−1 and the maximum occurred over Honshu Island. The heat loss from snowmelt in the Tien Shan mountains and Japanese Islands amounted to about 2/3 of heat loss in the Eurasian continental plains.  相似文献   

10.
Galileo was the first artificial satellite to orbit Jupiter. During its late orbital mission the spacecraft made two passages through the giant planet’s gossamer ring system. The impact-ionization dust detector on board successfully recorded dust impacts during both ring passages and provided the first in-situ measurements from a dusty planetary ring. During the first passage—on 5 November 2002 while Galileo was approaching Jupiter—dust measurements were collected until a spacecraft anomaly at 2.33RJ (Jupiter radii) just 16 min after a close flyby of Amalthea put the spacecraft into a safing mode. The second ring passage on 21 September 2003 provided ring dust measurements down to about 2.5RJ and the Galileo spacecraft was destroyed shortly thereafter in a planned impact with Jupiter. In all, a few thousand dust impacts were counted with the instrument accumulators during both ring passages, but only a total of 110 complete data sets of dust impacts were transmitted to Earth. Detected particle sizes range from about 0.2 to 5 μm, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging [Showalter, M.R., de Pater, I., Verbanac, G., Hamilton, D.P., Burns, J.A., 2008. Icarus 195, 361-377; de Pater, I., Showalter, M.R., Macintosh, B., 2008. Icarus 195, 348-360]. The grain size distribution increases towards smaller particles and shows an excess of these tiny motes in the Amalthea gossamer ring compared to the Thebe ring. The size distribution for the Amalthea ring derived from our in-situ measurements for the small grains agrees very well with the one obtained from images for large grains. Our analysis shows that particles contributing most to the optical cross-section are about 5 μm in radius, in agreement with imaging results. The measurements indicate a large drop in particle flux immediately interior to Thebe’s orbit and some detected particles seem to be on highly-tilted orbits with inclinations up to 20°. Finally, the faint Thebe ring extension was detected out to at least 5RJ, indicating that grains attain higher eccentricities than previously thought. The drop interior to Thebe, the excess of submicron grains at Amalthea, and the faint ring extension indicate that grain dynamics is strongly influenced by electromagnetic forces. These findings can all be explained by a shadow resonance as detailed by Hamilton and Krüger [Hamilton, D.P., Krüger, H., 2008. Nature 453, 72-75].  相似文献   

11.
The atmosphere of Mars does little to attenuate incoming ultraviolet (UV) radiation. Large amounts of UV radiation sterilize the hardiest of terrestrial organisms within minutes, and chemically alter the soil such that organic molecules at or near the surface are rapidly destroyed. Thus the survival of any putative martian life near the surface depends to a large extent on how much UV radiation it receives. Variations in small-scale geometry of the surface such as pits, trenches, flat faces and overhangs can have a significant effect on the incident UV flux and may create “safe havens” for organisms and organic molecules. In order to examine this effect, a 1-D radiative transfer sky model with 836 meshed points (plus the Sun) was developed which includes both diffuse and direct components of the surface irradiance. This model derives the variation of UV flux with latitude and an object's Geometric Shielding Ratio (a ratio which describes the geometry of each situation). The best protection is offered by overhangs with flux reduced to a factor of 1.8±0.2×10−5 of the unprotected value, a reduction which does not vary significantly by latitude. Pits and cracks are less effective with a reduction in UV flux of only up to 4.5±0.5×10−3 for the modeled scenarios; however, they are more effective for the same geometric shielding ratio than overhangs at high latitudes due to the low height of the Sun in the sky. Lastly, polar faces of rocks have the least effective shielding geometry with at most a 1.1±0.1×10−1 reduction in UV flux. Polar faces of rocks are most effective at mid latitudes where the Sun is never directly overhead, as at tropical latitudes, and never exposes the back of the rock, as at polar latitudes. In the most favorable cases, UV flux is sufficiently reduced such that organic in-fall could accumulate beneath overhanging surfaces and in pits and cracks. As well, hardy terrestrial microorganisms such as Bacillus pumilus could persist for up to 100 sols on the outer surfaces of typical spacecraft or several tens of martian years in the most shielded surface niches.  相似文献   

12.
We quantify the level of polarization of the atmosphere due to Zeeman splitting of oxygen in the Earth’s magnetic field and compare it to the level of polarization expected from the polarization of the cosmic microwave background radiation. The analysis focuses on the effect at mid-latitudes and at large angular scales. We find that from stratospheric balloon borne platforms and for observations near 100 GHz the atmospheric linear and circular polarized intensities are about 10−12 and 100 × 10−9 K, respectively, making the atmosphere a negligible source of foreground. From the ground the linear and circular polarized intensities are about 10−9 and 100 × 10−6 K, making the atmosphere a potential source of foreground for the CMB E (B) mode signal if there is even a 1% (0.01%) conversion of circular to linear polarization in the instrument.  相似文献   

13.
We present the first diffraction-limited K-band image of the Red Rectangle with 76 mas resolution, an H-band image with 75 mas resolution, and an RG 715 filter image ( 800 nm wavelength) with 78 mas resolution (corresponding to 25 AU for a distance of 330 pc). The H and K images were reconstructed from 6 m telescope speckle data and the RG 715 image from 2.2 m telescope data using the speckle masking bispectrum method. At all wavelengths the images show a compact, highly symmetric bipolar nebula, suggesting a toroidal density distribution of the circumstellar material. No direct light from the central binary can be seen as it is obscured by a dust disk or circumbinary torus. Our first high-resolution H−K color image of the nebula shows a broad red plateau of H−K≈ 2m in the bright inner regions.The optical and near-infrared images and the available photometric continuum observations in a wide range of ultraviolet to centimeter wavelengths enabled us to model the Red Rectangle in detail using a two-dimensional radiative transfer code. Our model matches both the high-resolution images and the spectral energy distribution of this object very well, making the following picture much more certain. The central close binary system with a total luminosity of 3000 L is embedded in a very dense, compact circumbinary torus which has an average number density nH ≈5×1012 cm−3, an outer radius of the dense inner region of R≈30 AU (91 mas), and a ρ∝r−2 density distribution. The full opening angle of the bipolar outflow cavities in our model is 70°. By comparing the observed and theoretical images, we derived an inclination angle of the torus to the line of sight of 7°±1°.The radiative transfer calculations show that the dust properties in the Red Rectangle are spatially inhomogeneous. The modeling confirms that the idea of large grains in the long-lived disk around the Red Rectangle (Jura et al., 1997 [ApJ, 474, 741]) is quantitatively consistent with the observations. In our models, unusually large, approximately millimeter-sized grains dominate the emission of the compact, massive torus. Models with smaller average grain sizes can possibly be found in future studies, for instance, if it turns out that the radio spectrum is not mainly caused by continuum dust emission. Therefore, the large grains suggested by our models require further confirmation by both new observations and radiative transfer calculations. Assuming a dust-to-gas ratio ρd/ρg of 0.005, the dense torus mass is 0.25 M. The model gives a lower limit of 0.0018 M, for the mass of the large particles, which produce a gray extinction of A≈ 28m, towards the center. A much smaller mass of submicron-sized dust grains is presumably located in the polar outflow cavities, their conical surface layers, and in the outer low-density parts of the torus (where ρ∝r−4, in the region of 30 AUr 2000 AU corresponding to 0.′′09–6′′).  相似文献   

14.
High speed dust streams emanating from near Jupiter were first discovered by the Ulysses spacecraft in 1992. Since then the phenomenon has been re-observed by Galileo in 1995, Cassini in 2000, and Ulysses in 2004. The dust grains are expected to be charged to a potential of , which is sufficient to allow the planet's magnetic field to accelerate them away from the planet, where they are subsequently influenced by the interplanetary magnetic field (IMF). A similar phenomenon was observed near Saturn by Cassini. Here, we report and analyze simultaneous dust, IMF and solar wind data for all dust streams from the two Ulysses Jupiter flybys. We find that compression regions (CRs) in the IMF – regions of enhanced magnetic field – precede most dust streams. Furthermore, the duration of a dust stream is roughly comparable with that of the precedent CR, and the occurrence of a dust stream and the occurrence of the previous CR are separated by a time interval that depends on the distance to Jupiter. The intensity of the dust streams and their precedent CRs are also correlated, but this correlation is only evident at distances from the planet no greater than 2 AU. Combining these observations, we argue that CRs strongly affect dust streams, probably by deflecting dust grain trajectories, so that they can reach the spacecraft and be detected by its dust sensor.  相似文献   

15.
The Aeolian Dust Experiment on Climate Impact (ADEC) was initiated in April 2000 as a joint five-year Japan–China project. The goal was to understand the impact of aeolian dust on climate via radiative forcing (RF). Field experiments and numerical simulations were conducted from the source regions in northwestern China to the downwind region in Japan in order to understand wind erosion processes temporal and spatial distribution of dust during their long-range transportation chemical, physical, and optical properties of dust and the direct effect of radiative forcing due to dust. For this, three intensive observation periods (IOP) were conducted from April 2002 to April 2004.The in situ and network observation results are summarized as follows: (1) In situ observations of the wind erosion process revealed that the vertical profile of moving sand has a clear size dependency with height and saltation flux and that threshold wind velocity is dependent on soil moisture. Results also demonstrated that saltation flux is strongly dependent on the parent soil size distribution of the desert surface. (2) Both lidar observations and model simulations revealed a multiple dust layer in East Asia. A numerical simulation of a chemical transport model, CFORS, illustrated the elevated dust layer from the Taklimakan Desert and the lower dust layer from the Gobi Desert. The global-scale dust model, MASINGAR, also simulated the dust layer in the middle to upper free troposphere in East Asia, which originated from North Africa and the Middle East during a dust storm in March 2003. Raman lidar observations at Tsukuba, Japan, found the ice cloud associated with the dust layer at an altitude of 6 to 9 km. Analysis from lidar and the radio-sonde observation suggested that the Asian dust acted as ice nuclei at the ice-saturated region. These results suggest the importance of dust's climate impact via the indirect effect of radiative forcing due to the activation of dust into ice nuclei. (3) Studies on the aerosol concentration indicated that size distributions of aerosols in downwind regions have bimodal peaks. One peak was in the submicron range and the other in the supermicron range. The main soluble components of the supermicron peak were Na+, Ca2+, NO3−, and Cl−. In the downwind region in Japan, the dust, sea salt, and a mixture of the two were found to be dominant in coarse particles in the mixed boundary layer. (4) Observation of the optical properties of dust by sky-radiometer, particle shoot absorption photometer (PSAP), and Nephelometer indicated that unpolluted dust at source region has a weaker absorption than originally believed.A sensitivity experiment of direct RF by dust indicated that single scattering albedo is the most important of the optical properties of dust and that the sensitivity of instantaneous RF in the shortwave region at the top of the atmosphere to the refractive index strongly depends on surface albedo. A global scale dust model, MASINGAR, was used for evaluation of direct RF due to dust. The results indicated the global mean RF at the top and the bottom of the atmosphere were − 0.46 and − 2.13 W m− 2 with cloud and were almost half of the RF with cloud-free condition.  相似文献   

16.
Each year the Moon is bombarded by about 106 kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013.Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.  相似文献   

17.
《Planetary and Space Science》2006,54(9-10):911-918
As the data from space missions and laboratories improve, a research domain combining plasmas and charged dust is gaining in prominence. Our solar system provides many natural laboratories such as planetary rings, comet comae and tails, ejecta clouds around moons and asteroids, and Earth's noctilucent clouds for which to closely study plasma-embedded cosmic dust. One natural laboratory to study electromagnetically controlled cosmic dust has been provided by the Jovian dust streams and the data from the instruments which were on board the Galileo spacecraft. Given the prodigious quantity of dust poured into the Jovian magnetosphere by Io and its volcanoes resulting in the dust streams, the possibility of dusty plasma conditions exist. This paper characterizes the main parameters for those interested in studying dust embedded in a plasma with a focus on the Jupiter environment. I show how to distinguish between dust-in-plasma and dusty-plasma and how the Havnes parameter P can be used to support or negate the possibility of collective behavior of the dusty plasma. The result of applying these tools to the Jovian dust streams reveals mostly dust-in-plasma behavior. In the orbits displaying the highest dust stream fluxes, portions of orbits E4, G7, G8, C21 satisfy the minimum requirements for a dusty plasma. However, the P parameter demonstrates that these mild dusty plasma conditions do not lead to collective behavior of the dust stream particles.  相似文献   

18.
We present results of a study of the so-called “stickiness” regions where orbits in mappings and dynamical systems stay for very long times near an island and then escape to the surrounding chaotic region. First we investigated the standard map in the form xi+1 = xi+yi+1 and yi+1 = yi+K/2π · sin(2πxi) with a stochasticity parameter K = 5, where only two islands of regular motion survive. We checked now many consecutive points—for special initial conditions of the mapping—stay within a certain region around the island. For an orbit on an invariant curve all the points remain forever inside this region, but outside the “last invariant curve” this number changes significantly even for very small changes in the initial conditions. In our study we found out that there exist two regions of “sticky” orbits around the invariant curves: A small region I confined by Cantori with small holes and an extended region II is outside these cantori which has an interesting fractal character. Investigating also the Sitnikov-Problem where two equally massive primary bodies move on elliptical Keplerian orbits, and a third massless body oscillates through the barycentre of the two primaries perpendicularly to the plane of the primaries—a similar behaviour of the stickiness region was found. Although no clearly defined border between the two stickiness regions was found in the latter problem the fractal character of the outer region was confirmed.  相似文献   

19.
An analysis of the spectra from the PUMA dust-impact mass spectrometers onboard the Vega-1 and Vega-2 spacecraft shows that a large number of the observed, unidentified small-amplitude peaks are produced by impacts of very-low-mass (from 10?17 to 10?20 g) particles. The mass flux of very fine particles accounts for a few percent of the total dust mass flux from comet Halley. The elemental composition of the finest cometary particles is identical to the composition of large particles (10?12–10?16 g), in agreement with present views about the nucleus of comet Halley as an aggregate of interstellar dust.  相似文献   

20.
X-radiation may result from active plasma phenomena in the interactions of comets with the solar wind. We have carried out a limited but sensitive search for soft X-radiation from Comet Bradfield (1979 ), on 1980 Feb. 5. No X-radiation was detected at a level (3σ) of 1.7 × 10−13 erg(cm2sec keV)−1 in the 0.2 – 4.0 kev range. This corresponds to a limit on the power dissipated in the comet by non-thermal electrons of approximately 1019 ergs sec−1, averaged over the 2568-sec exposure to the comet. This energy deposition is near the magnitude suggested by simple theoretical ideas, and further searches of appropriate comets both in soft X-radiation and at radio wavelengths seem warranted.  相似文献   

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