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1.
Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10−9 g cm−2 for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm−3) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10−10 g cm−2), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm−3 vs. a few times 0.1 cm−3 for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).  相似文献   

2.
It is suspected that the lunar exosphere has a dusty component dispersed above the surface by various physical mechanisms. Most of the evidence for this phenomenon comes from observations of “lunar horizon glow” (LHG), which is thought to be produced by the scattering of sunlight by this exospheric dust. The characterization of exospheric dust populations at the Moon is key to furthering our understanding of fundamental surface processes, as well as a necessary requirement for the planning of future robotic and human exploration.We present a model to simulate the scattering of sunlight by complex lunar dust grains (i.e. grains that are non-spherical and can be inhomogeneous in composition) to be used in the interpretation of remote sensing data from current and future lunar missions. We numerically model lunar dust grains with several different morphologies and compositions and compute their individual scattering signatures using the Discrete Dipole Approximation (DDA). These scattering properties are then used in a radiative transfer code to simulate the light scattering due to a dust size distribution, as would likely be observed in the lunar exosphere at high altitudes 10's of km. We demonstrate the usefulness and relevance of our model by examining mode: irregular grains, aggregate of spherical monomers and spherical grains with nano-phase iron inclusions. We subsequently simulate the scattering by two grain size distributions (0.1 and radius), and show the results normalized per-grain. A similar methodology can also be applied to the analysis of the LHG observations, which are believed to be produced by scattering from larger dust grains within about a meter of the surface.As expected, significant differences in scattering properties are shown between the analyses employing the widely used Mie theory and our more realistic grain geometries. These differences include large variations in intensity as well as a positive polarization of scattered sunlight caused by non-spherical grains. Positive polarization occurs even when the grain size is small compared to the wavelength of incident sunlight, thus confirming that the interpretation of LHG based on Mie theory could lead to large errors in estimating the distribution and abundances of exospheric dust.  相似文献   

3.
We present the first in situ measurements of the secondary electron emission efficiency of lunar regolith, utilizing Lunar Prospector measurements of secondary electrons emitted from the negatively charged night side and accelerated upward by surface electric fields. By comparing measurements of secondary currents emitted from the surface and incident primary electron currents, we find that the secondary yield of lunar regolith is a factor of ∼3 lower than that measured for samples in the laboratory. This lower yield significantly affects current balance at the lunar surface and the resulting equilibrium surface potentials. This information must be folded into models of the near-surface plasma sheath, in order to predict the effects on dust and other components of the lunar environment, and ultimately determine the importance for surface exploration and scientific investigations on the Moon.  相似文献   

4.
Direct detection of water in its vapour phase in the tenuous lunar environment through in situ measurements carried out by the Chandra’s Altitudinal Composition Explorer (CHACE) payload, onboard the Moon Impact Probe (MIP) of Chandrayaan I mission vindicates the presence of water on the surface of the moon in form of ice at higher lunar latitudes inferred from IR absorption spectroscopy, (especially that of OH), by the Moon Mineralogy Mapper (M3) of Chandrayaan I. The quadrupole mass spectrometer based payload, CHACE, sampled the lunar neutral atmosphere every 4 s with a broad latitudinal (∼40°N to 90°S, with a resolution of ∼0.1°) and altitudinal (from 98 km up to impact on the lunar surface with a resolution of ∼0.25 km) coverage in the sunlit side of the moon for the first time. These two (CHACE and M3) complementary experiments are shown to collectively provide unambiguous signatures for the distribution of water in solid and gaseous phases in Earth’s moon.  相似文献   

5.
The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft will orbit the Moon at an altitude of ≈50 km with a payload that includes the Ultraviolet Spectrometer (UVS) instrument, which will obtain high spectral resolution measurements at near-ultraviolet and visible wavelengths (≈231-826 nm). When LADEE/UVS observes the lunar limb from within the shadow of the Moon it is anticipated that it will detect a lunar horizon glow (LHG) due to sunlight scattered from submicron exospheric dust, as well as emission lines from exospheric gases (particularly sodium), in the presence of the bright coronal and zodiacal light (CZL) background. A modularized code has been developed at NMSU for simulations of scattered light sources as observed by orbiting instruments in lunar shadow. Predictions for the LADEE UVS and star tracker cameras indicate that LHG, sodium (Na) emission lines, and CZL can be distinguished based on spatial morphology and spectral characteristics, with LHG dominant at blue wavelengths (∼250-450 nm) and small tangent heights. If present, LHG should be readily detected by LADEE/UVS and distinguishable from other sources of optical scattering. Observations from UVS and the other instruments aboard LADEE will significantly advance our understanding of how the Moon interacts with the surrounding space environment; these new insights will be applicable to the many other airless bodies in the solar system.  相似文献   

6.
Energetic solar electrons in the interplanetary medium   总被引：3，自引：0，他引：3  
R. P. Lin 《Solar physics》1985,100(1-2):537-561
ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with 40% detected only below 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events.In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above 10 keV with a break near 100 keV and flattening from 10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun.Solar type-III radio bursts are produced by the escaping 2–102 keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions.  相似文献   

7.
Meteorites ejected from the surface of the Moon as a result of impact events are an important source of lunar material in addition to Apollo and Luna samples. Here, we report bulk element composition, mineral chemistry, age, and petrography of Miller Range (MIL) 090036 and 090070 lunar meteorites. MIL 090036 and 090070 are both anorthositic regolith breccias consisting of mineral fragments and lithic clasts in a glassy matrix. They are not paired and represent sampling of two distinct regions of the lunar crust that have protoliths similar to ferroan anorthosites. 40Ar‐39Ar chronology performed on two subsplits of MIL 090070,33 (a pale clast impact melt and a dark glassy melt component) shows that the sample underwent two main degassing events, one at ~3.88 Ga and another at ~3.65 Ga. The cosmic ray exposure data obtained from MIL 090070 are consistent with a short (~8–9 Ma) exposure close to the lunar surface. Bulk‐rock FeO, TiO2, and Th concentrations in both samples were compared with 2‐degree Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data sets to determine areas of the lunar surface where the regolith matches the abundances observed on the sample. We find that MIL 090036 bulk rock is compositionally most similar to regolith surrounding the Procellarum KREEP Terrane, whereas MIL 090070 best matches regolith in the feldspathic highlands terrane on the lunar farside. Our results suggest that some areas of the lunar farside crust are composed of ferroan anorthosite, and that the samples shed light on the evolution and impact bombardment history of the ancient lunar highlands.  相似文献   

8.
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   

9.
We report new nitrogen and argon isotope and abundance results for single breccia clasts and agglutinates from four different sections of the Luna 24 drill core in order to re-evaluate the provenance of N trapped in lunar regolith, and to place limits on the flux of planetary material to the Moon’s surface. Single Luna 24 grains with 40Ar/36Ar ratios <1 show δ15N values between ?54.5‰ and +123.3‰ relative to terrestrial atmosphere. Thus, low-antiquity lunar soils record both positive and negative δ15N signatures, and the secular increase of the δ15N value previously postulated by Kerridge (Kerridge, J.F. [1975]. Science 188(4184), 162–164. doi:10.1126/science.188.4184.162) is no longer apparent when the Luna and Apollo data are combined. Instead, the N isotope signatures, corrected for cosmogenic 15N, are consistent with binary mixing between isotopically light solar wind (SW) N and a planetary N component with a δ15N value of +100‰ to +160‰. The lower δ15N values of Luna 24 grains compared to Apollo samples reflect a higher relative proportion of solar N, resulting from the higher SW fluence in the region of Mare Crisium compared to the central near side of the Moon. Carbonaceous chondrite-like micro-impactors match well the required isotope characteristics of the non-solar N component trapped in low-antiquity lunar regolith. In contrast, a possible cometary contribution to the non-solar N flux is constrained to be ?3–13%. Based on the mixing ratio of SW to planetary N obtained for recently exposed lunar soils, we estimate the flux of micro-impactors to be (2.2–5.7) × 103 tons yr?1 at the surface of the Moon. Our estimate for Luna 24 agrees well with that for young Apollo regolith, indicating that the supply of planetary material does not depend on lunar location. Thus, the continuous influx of water-bearing cosmic dust may have represented an important source of water for the lunar surface over the past ～1 Ga, provided that water removal rates (i.e., by meteorite impacts, photodissociation, and sputtering) do not exceed accumulation rates.  相似文献   

10.
P. Hedelt  Y. Ito  L. Esposito 《Icarus》2010,210(1):424-435
Based on measurements performed by the Hydrogen Deuterium Absorption Cell (HDAC) aboard the Cassini orbiter, Titan’s atomic hydrogen exosphere is investigated. Data obtained during the T9 encounter are used to infer the distribution of atomic hydrogen throughout Titan’s exosphere, as well as the exospheric temperature.The measurements performed during the flyby are modeled by performing Monte Carlo radiative transfer calculations of solar Lyman-α radiation, which is resonantly scattered on atomic hydrogen in Titan’s exosphere. Two different atomic hydrogen distribution models are applied to determine the best fitting density profile. One model is a static model that uses the Chamberlain formalism to calculate the distribution of atomic hydrogen throughout the exosphere, whereas the second model is a Particle model, which can also be applied to non-Maxwellian velocity distributions.The density distributions provided by both models are able to fit the measurements although both models differ at the exobase: best fitting exobase atomic hydrogen densities of nH = (1.5 ± 0.5) × 104 cm−3 and nH = (7 ± 1) × 104 cm−3 were found using the density distribution provided by both models, respectively. This is based on the fact that during the encounter, HDAC was sensitive to altitudes above about 3000 km, hence well above the exobase at about 1500 km. Above 3000 km, both models produce densities which are comparable, when taking into account the measurement uncertainty.The inferred exobase density using the Chamberlain profile is a factor of about 2.6 lower than the density obtained from Voyager 1 measurements and much lower than the values inferred from current photochemical models. However, when taking into account the higher solar activity during the Voyager flyby, this is consistent with the Voyager measurements. When using the density profile provided by the particle model, the best fitting exobase density is in perfect agreement with the densities inferred by current photochemical models.Furthermore, a best fitting exospheric temperature of atomic hydrogen in the range of TH = (150-175) ± 25 K was obtained when assuming an isothermal exosphere for the calculations. The required exospheric temperature depends on the density distribution chosen. This result is within the temperature range determined by different instruments aboard Cassini. The inferred temperature is close to the critical temperature for atomic hydrogen, above which it can escape hydrodynamically after it diffused through the heavier background gas.  相似文献   

11.
W.-H. Ip 《Icarus》1996,120(2):317-325
The newly detected oxygen atmosphere of Europa is modeled by invoking charged particle sputtering with H2O and O2molecules as the main ejecta. The magnetospheric corotating ions could provide the required source strength (∼3 × 1026sec−1) of O2molecules if a fraction (∼20%) of the exospheric ions were recycled to Europa's surface where they produce additional sputtering product. Two exospheric components are expected to form: an extended corona with a size of a few satellite radii which is composed of sputtered molecules in ballistic motion, and a thermal population with a surface density of 108–109cm−3and a scale height of about 20 km. The electron impact ionization of this exosphere would lead to an Io-like interaction with the jovian magnetosphere with a field-aligned Birkeland current of about 5 × 105A.  相似文献   

12.
Areas of lunar surface magnetic field are observed to ‘mirror’ low energy electrons present in the normal lunar space environment. The ambient electrons provide, in effect, a probe along the ambient magnetic field lines down to the lunar surface for remote sensing of the presence of surface fields. This probe, unlike direct measurement by the magnetometer, does not require low altitude or a very stable (magnetotail) ambient field to provide a mapping of regions of occurrence of such fields. Use of the on-board vector magnetometer measurements of the ambient magnetic field orientation allows accurate projection of such mapping onto the lunar surface. Preliminary maps of the lunar surface magnetic areas underlying the orbit of the ‘Particles and Fields Satellite deployed from Apollo-16’ have been generated, obtaining 40% coverage from partial data to demonstrate feasibility of the technique. As well as providing independent verification of areas such as Van de Graaff already discovered in the magnetometer data, these maps reveal many previously unreported areas of surface magnetism. The method is sensitive to fields of less than 0.1γ at the surface. Application to the full body of available PFS-1 & 2 electron data is expected to provide complete mapping of the lunar surface for areas of magnetization up to latitudes of 35–40 deg. The surface field regions observed are generally due to sources smaller than 10–50 km in size, although many individual regions are often so close together as to give much larger regions of effectively continuous mirroring. Absence of consistent mirroring by any global field places an upper limit on the size of any net lunar dipole moment of less than 1010 γ km3. Much additional information regarding the magnetic regions can be obtained by correlated analysis of both the electron return and vector magnetometer measurements at orbital altitude, the two techniques providing each other with directly complimentary measurements at the satellite and along the ambient field lines to the surface.  相似文献   

13.
Abstract— We report mass‐spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to >40x the solar‐wind value—‐and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high‐altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic‐ray‐induced spallation during estimated space residence times of IDPs, or by direct implantation of solar‐flare He. Minimum exposure ages inferred from the 3He excesses range from ～50 Ma to an impossible >10 Ga, compared to Poynting‐Robertson drag lifetimes for low‐density 20–30 μm particles on the order of ～0.1 Ma for an asteroidal source and ～10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar‐energetic‐particle (SEP) gases, to the virtual exclusion of solar‐wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ～1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low‐energy solar particle emission.  相似文献   

14.
We report the results of age determination of a lunar soil column, delivered by the Luna 16 mission in September 1970 from the Sea of Fertility. We elaborated and applied the soil age determination method using the kinetic parameter, the regolith accumulation rate. The age of the soil delivered by Luna 16 is about 90 Myr. The isotopic ratio of 3He/4He in the column is slightly higher than in the soil column delivered by the Luna 24 mission. The abundance of helium in the fine fraction of the soil (about 100 µm) is significantly higher and is close to the maximum abundance from the Luna 24 soil column. These differences are most likely associated with the variations of solar corpuscular fluxes. Based on the measurements of the helium isotope abundance in the samples of lunar soil columns, we have estimated the values of ancient solar fluxes of protons and helium and variations thereof in the time interval of up to 600 Myr. We demonstrate that during this epoch there were two strong bursts of the helium flux, about 80 and 470 Myr ago, respectively. The existence of the first peak was assumed earlier from the paleodendrochronological data.  相似文献   

15.
Measurements are reported of particle-track densities in 100–200µ crystalline grains taken from one level of the soil column returned from the lunar highlands between Mare Fecunditatis and Mare Crisium by Luna 20 and from two levels in that from Mare Fecunditatis by Luna 16. Ninety-three percent of the grains from Luna 16 have very high densities, > 108 cm–2 and the lower-track density grains are all in the deeper soil level. In contrast, most Luna 20 grains show densities < 108 cm–2. Track density gradients and exposure times have been measured for six Luna 16 grains with a wide spread in absolute track densities. The more extensive track counts in crystals strengthens our earlier conclusion that the Luna 16 soil has received long irradiations very close to the surface. Two possible histories are that the highly irradiated soil blanket at the Luna 16 site is either well mixed and thin, or else has accumulated by transport from surrounding higher regions. The single sample of doubtful depth from Luna 20 shows a much lesser near-surface irradiation, giving results similar to those on the Apollo 12 core and the 54–80 depth sample from the Apollo 15 deep core.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.  相似文献   

16.
Laboratory simulations using the Arizona State University Vortex Generator (ASUVG) were run to simulate sediment flux in dust devils in terrestrial ambient and Mars-analog conditions. The objective of this study was to measure vortex sediment flux in the laboratory to yield estimations of natural dust devils on Earth and Mars, where all parameters may not be measured. These tests used particles ranging from 2 to 2000 μm in diameter and 1300 to 4800 kg m−3 in density, and the results were compared with data from natural dust devils on Earth and Mars. Typically, the cores of dust devils (regardless of planetary environment) have a pressure decrease of ∼0.1-1.5% of ambient atmospheric pressure, which enhances the lifting of particles from the surface. Core pressure decreases in our experiments ranged from ∼0.01% to 5.00% of ambient pressure (10 mbar Mars cases and 1000 mbar for Earth cases) corresponding to a few tenths of a millibar for Mars cases and a few millibars for Earth cases. Sediment flux experiments were run at vortex tangential wind velocities of 1-45 m s−1, which typically correspond to ∼30-70% above vortex threshold values for the test particle sizes and densities. Sediment flux was determined by time-averaged measurements of mass loss for a given vortex size. Sediment fluxes of ∼10−6-100 kg m−2 s−1 were obtained, similar to estimates and measurements for fluxes in dust devils on Earth and Mars. Sediment flux is closely related to the vortex intensity, which depends on the strength of the pressure decrease in the core (ΔP). This study found vortex size is less important for lifting materials because many different diameters can have the same ΔP. This finding is critical in scaling the laboratory results to natural dust devils that can be several orders of magnitude larger than the laboratory counterparts.  相似文献   

17.
The NASA DS1 spacecraft encountered Comet P/Borrelly on September 22, 2001 at a distance of ∼2171 km on the sunward side of the comet. The flyby speed was ∼16.5 km s−1. Using high temporal resolution (50 μs) absolute electric field amplitude measurements from a ∼1 m dipole antenna, new features of plasma clouds created by cometary dust impacts have been detected. The pulses have 1/e exponential decays of ∼650 μs duration, exponentially shaped overshoots with rise times of ∼2 ms, and exponential-shaped overshoot decay times of ∼10 ms. Assuming a plasma temperature of 104 K, these pulse features have been explained as plasma cloud space charge effects from the electron, proton and heavy ion portions of the clouds passing the antenna. Complex pulse shapes were also detected. These are believed to be due to either plasma cloud scattering off of the spacecraft, or to secondary impacts. Small electric pulses of duration 10-15 ms of cometary origin were detected but are presently unexplained. The electric component of the plasma wave spectra at closest approach had an f−2.4 power law shape from 10 Hz to 1 kHz. The electron cyclotron frequency was approximately 1 kHz. One possible explanation of the wave spectrum is that whistler mode waves associated with phase steepened cometary plasma waves are dispersed, leading to the broad spectrum. Finally, based on the present results, a new type of low-cost, large-area dust detector is proposed.  相似文献   

18.
D. Reiss  M. Zanetti  G. Neukum 《Icarus》2011,215(1):358-369
Active dust devils were observed in Syria Planum in Mars Observer Camera - Wide Angle (MOC-WA) and High Resolution Stereo Camera (HRSC) imagery acquired on the same day with a time delay of ∼26 min. The unique operating technique of the HRSC allowed the measurement of the traverse velocities and directions of motion. Large dust devils observed in the HRSC image could be retraced to their counterparts in the earlier acquired MOC-WA image. Minimum lifetimes of three large (avg. ∼700 m in diameter) dust devils are ∼26 min, as inferred from retracing. For one of these large dust devil (∼820 m in diameter) it was possible to calculate a minimum lifetime of ∼74 min based on the measured horizontal speed and the length of its associated dust devil track. The comparison of our minimum lifetimes with previous published results of minimum and average lifetimes of small (∼19 m in diameter, avg. min. lifetime of ∼2.83 min) and medium (∼185 m in diameter, avg. min. lifetime of ∼13 min) dust devils imply that larger dust devils on Mars are active for much longer periods of time than smaller ones, as it is the case for terrestrial dust devils. Knowledge of martian dust devil lifetimes is an important parameter for the calculation of dust lifting rates. Estimates of the contribution of large dust devils (>300-1000 m in diameter) indicate that they may contribute, at least regionally, to ∼50% of dust entrainment by dust devils into the atmosphere compared to the dust devils <300 m in diameter given that the size-frequency distribution follows a power-law. Although large dust devils occur relatively rarely and the sediment fluxes are probably lower compared to smaller dust devils, their contribution to the background dust opacity by dust devils on Mars could be at least regionally large due to their longer lifetimes and ability of dust lifting into high atmospheric layers.  相似文献   

19.
Abstract— We studied 42 impact‐melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The 40Ar‐39Ar ages on 31 of the impact melts, the first ages on impact‐melt samples from outside the region of the Apollo and Luna sampling sites, range from ～4 to ～2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact‐melt rock age cluster at 3.9 Ga, but the meteorite impact‐melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact‐melt clast ages is a lunar‐wide phenomenon. No meteorite impact melts have ages more than 1s? older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.  相似文献   

20.
Observations of impulsive solar flare X-rays 10 keV by the OGO-5 satellite and the measurements of energetic solar electrons made with the Explorer-35 and Explorer-41 (IMP-5) satellites during the period March 1968–September 1969 have been analyzed in order to determine the ion density in the X-ray source region as well as the location of the electron acceleration region in the solar atmosphere. If we assume that the efficiency of escape of the accelerated electrons into interplanetary space is 10–3, the observations are found to be consistent with the following interpretation: (i) the ion density in the X-ray source region varies from event to event and lies between 109 and 1011 ions cm–3 for those events in which the impulsive X-ray emission could be detected; (ii) for those events in which no impulsive emission was detected above threshold, the ion density in the X-ray source was < 109 ions cm–3; (iii) at least in some small solar flares the region where the electrons are accelerated during the flash phase is located in the lower corona.  相似文献   

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