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1.
Because of the ubiquity of subsurface microbial life on Earth, examination of the subsurface of Mars could provide an answer to the question of whether microorganisms exist or ever existed on that planet. Impact craters provide a natural mechanism for accessing the deep substrate of Mars and exploring its exobiological potential. Based on equations that relate impact crater diameters to excavation depth we estimate the observed crater diameters that are required to prospect to given depths in the martian subsurface and we relate these depths to observed microbiological phenomena in the terrestrial subsurface. Simple craters can be used to examine material to a depth of ∼270 m. Complex craters can be used to reach greater depths, with craters of diameters ≥300 km required to reach depths of 6 km or greater, which represent the limit of the terrestrial deep subsurface biosphere. Examination of the ejecta blankets of craters between 17.5 and 260 km in diameter would provide insights into whether there is an extant, or whether there is evidence of an extinct, deep subsurface microbiota between 500 and 5000 m prior to committing to large-scale drilling efforts. At depths <500 m some crater excavations are likely to be more important than others from an exobiological point of view. We discuss examples of impacts into putative intracrater paleolacustrine sediments and regions associated with hydrothermal activity. We compare these depths to the characteristics of subsurface life on Earth and the fossil microbiological record in terrestrial impact craters.  相似文献   

2.
Null tests are described for the strongly curved and highly aspheric mirrors of a set of 'exact optics' spectrograph cameras based on the design of Schwarzschild. For f /1.0 and f /1.333 cameras having a focal length of 100 mm the tests of each mirror are well within the Rayleigh limit and would permit construction on a larger scale. The ray-theoretical image diameters at the edge of the field of view of these two cameras are under 10 μm unless they are scaled up to an aperture greater than 450 mm.  相似文献   

3.
Abstract— We investigate the action of the martian atmosphere on entering meteoroids for present and past atmospheres with various surface pressures to predict the smallest observable craters, and to understand the implications for the size distributions of craters on Mars and meteoroids in space. We assume different strengths appropriate to icy, stone, and iron bodies and test the results against available data on terrestrial bolides. Deceleration, ablation, and fragmentation effects are included. We find that the smallest icy, stone, and iron meteoroids to hit the martian ground at crater forming speeds of ≥500 m/s have diameters of about 2 m, 0.03‐0.9 m (depending on strength), and 0.01 m, respectively, in the current atmosphere. For hypothetical denser past atmospheres, the cutoff diameters rise. At a surface pressure of 100 mb, the cutoff diameters are about 24 m, 5–12 m, and 0.14 m for the 3 classes. The weaker stony bodies in the size range of about 1–30 m may explode at altitudes of about 10–20 km above the ground. These figures imply that under the present atmosphere, the smallest craters made by these objects would be as follows: by ice bodies, craters of diameter (D) ?8 m, by stones about 0.5–6 m, and by irons, about 0.3 m. A strong depletion of craters should, thus, occur at diameters below about 0.3 m to 5 m. Predicted fragmentation and ablation effects on weak meteoroids in the present atmosphere may also produce a milder depletion below D ?500 m, relative to the lunar population. But, this effect may be difficult to detect in present data because of additional losses of small craters due to sedimentation, dunes, and other obliteration effects. Craters in strewn fields, caused by meteoroid fragmentation, will be near or below present‐day resolution limits, but examples have been found. These phenomena have significant consequences. Under the present atmosphere, the smallest (decimeter‐scale) craters in sands and soils could be quickly obliterated but might still be preserved on rock surfaces, as noted by Hörz et al. (1999). Ancient crater populations, if preserved, could yield diagnostic signatures of earlier atmospheric conditions. Surfaces formed under past denser atmospheres (few hundred mbar), if preserved by burial and later exposed by exhumation, could show: a) striking depletions of small craters (few meter sizes up to as much as 200 m), relative to modern surfaces; b) more clustered craters due to atmospheric breakup; and c) different distributions of meteorite types, with 4 m to 200 m craters formed primarily by irons instead of by stones as on present‐day Mars. Megaregolith gardening of the early crust would be significant but coarser than the gardening of the ancient lunar uplands.  相似文献   

4.
From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii ∼ 0.02 μmmaking up ∼ ;10% of the total grain mass, small dielectric spheres of radius about 0.04 μm making up ∼ 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of ∼ 2/3 μmmaking up ∼ 45% of the mass. The remaining ∼ 20%consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

5.
The surface of Enceladus consists almost completely of water ice. As the band depths of water ice absorptions are sensitive to the size of particles, absorptions can be used to map variations of icy particles across the surface. The Visual and Infrared Mapping Spectrometer (VIMS) observed Enceladus with a high spatial resolution during three Cassini flybys in 2005 (orbits EN 003, EN 004 and EN 011). Based on these data we measured the band depths of water ice absorptions at 1.04, 1.25, 1.5, and 2 μm. These band depths were compared to water ice models that represent theoretically calculated reflectance spectra for a range of particle diameters between 2 μm and 1 mm. The agreement between the experimental (VIMS) and model values supports the assumption that pure water ice characterizes the surface of Enceladus and therefore that variations in band depth correspond to variations in water ice particle diameters. Our measurements show that the particle diameter of water ice increases toward younger tectonically altered surface units with the largest particles exposed in relatively “fresh” surface material. The smallest particles were generally found in old densely cratered terrains. The largest particles (∼0.2 mm) are concentrated in the so called “tiger stripes” at the south pole. In general, the particle diameters are strongly correlated with geologic features and surface ages, indicating a stratigraphic evolution of the surface that is caused by cryovolcanic resurfacing and impact gardening.  相似文献   

6.
The orbital evolution of asteroidal fragments with diameters ranging from 10 cm to 20 km, injected into the 3:1 Kirkwood gap at 2.50 A.U., has been investigated using Monte Carlo techniques. It is assumed that this material can become Earth-crossing on a time scale of 106 years, as a result of a chaotic zone discovered by Wisdom, associated with the 3:1 resonance. This phenomenon, as well as close encounter planetary perturbations, the v6 secular resonance, and the ablative effects of the Earth's atmosphere are included in the determination of the orbital characteristics of meteorites impacting the Earth derived by fragmentation of this asteroidal material. It is found that the predicted meteorite orbits closely match those found for observed ordinary chondrites, and the total flux is in approximate agreement with the observed fall rate of ordinary chondrites. About 10% of the predicted impacting bodies are meteorite-size bodies originating directly from the asteroid belt. The remainder are obtained by subsequent fragmentation of larger (～1 m to 20 km diameter) Earth-crossing asteroidal fragments. The largest of these fragments are observable as Apollo-Amor objects. Thus the apparent paradox between the orbital characteristics of observed ordinary chondrites and those predicted from Apollo object sources is reconciled. Both appear to be complementary aspects of the same phenomena. No other asteroidal resonance is found to be satisfactory as a source of ordinary chondrites. These meteorites are therefore most likely to be derived from S asteroids in this limited region of the asteroidal belt, the largest of which are 11 Parthenope, 17 Thetis, and 29 Amphitrite.  相似文献   

7.
本文介绍了云南天文台一米望远镜附属的一台低色散光谱仪实验装置,介绍了该装置的设计原理、系统结构参数及试观测情况,并讨论了存在的问题和解决问题的设想。我们已用该装置观测到19m左右的具有大红移的类星体光谱。该类低色散光谱仪,将为我国星系研究工作者提供实测条件。  相似文献   

8.
For about 20 years SCHOTT has been supplying the glass ceramic ZERODUR, a material with very low thermal expansion. Besides many other applications, ZERODUR is excellently suited for the manufacture of mirror substrates for telescopes. About 80% of all telescopes in the western world with mirror diameters >1.8 m have been equipped with ZERODUR during the last 10 years. The development of modern astronomical telescopes is aimed at larger primary mirrors and lighter secondary mirrors.New techniques have been developed by SCHOTT for manufacture of thin monolithic mirror blanks of more than 8 m in diameter. The development of thin meniscus shaped shells using the spin-casting technique was successfully completed last year. During a test production several mirror substrates up to 4.1 m in diameter and down to 57 mm in thickness could be produced. The know-how has been acquired for the fabrication of mirror substrates of more than 8 m in diameter by the spin-casting technique.SCHOTT has also performed considerable developmental work in the field of lightweighted ZERODUR mirror substrates which can be generated using different techniques: forming of the lightweighted structure during casting, fusion of individual components to a total structure and lightweighting of a massive block by various mechanical machining methods.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.Registered trademark of SCHOTT Glaswerke, Mainz.  相似文献   

9.
From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii 0.02 m making up 10% of the total grain mass, small dielectric spheres of radius about 0.04 m making up 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of 2/3 m making up 45% of the mass. The remaining 20% consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material.  相似文献   

10.
Abstract— Microchondrules with apparent diameters of 2 to 150 μm are found in a black carbon—bearing inclusion in Mezö-Madaras. Some are homogeneous (glassy or micro-crystalline) others show two phases: mainly silica and a pyroxene-rich glass. The bulk chemical composition of the inclusion is related to the host-chondrite in which silica-pyroxene chondrules are ubiquitous. Small black lumps of the same kind are dispersed in bulk Mezö-Madaras. This L-related carbon-bearing material may represent a new specimen of C-rich ordinary chondrite.  相似文献   

11.
At present, there are few laboratory spectra of analogs of astrophysical interest in the far-infrared range (FIR). Laboratory infrared (IR) spectra of simple ices and its mixtures obtained at low temperature and pressure are found mainly up to 25 μm, and few up to 200 μm. On the other hand, there are some spectra for carbonaceous material and silicates up to 2000 μm. Our laboratory is equipped with an IR spectrometer that integrates a Michelson interferometer with a resolution better than 0.25 cm−1 and that operates under vacuum conditions of 10−1 mbar. There is also a silicon bolometer, a very high-sensitivity detector in comparison with the standard deuterated triglycine sulfate (DTGS) detectors. The use of the bolometer and the possibility of working under vacuum conditions inside the optics and the sample compartment of the spectrometer allow obtaining high-sensitivity spectra free from H2O vapor and CO2 gas bands. Those conditions are necessary to obtain high-quality spectra in the FIR where absorption bands are much less intense than those in the mid-IR region. In our laboratory there is also a high-vacuum chamber that allows different studies on ices deposited onto a cold finger. We have already carried out experiments on the study of ice density as a function of temperature, UV irradiation of ices, temperature-programmed desorption (TPD) and UV-vis reflectance. In this work, we present the design of the experimental setup we are building to carry out different experiments simultaneously on the same ice sample, including spectra measurements in the mid-IR range (MIR) and the FIR. This design integrates jointly the IR spectrometer, the high-vacuum chamber and the silicon bolometer. Lastly, we show a spectrum we have obtained of a solid of astrophysical interest such as crystalline forsterite grains by using the polyethylene pellet technique.  相似文献   

12.
The Haig (IIIA) iron meteorite material (BM 1968,280) in the Collection of the British Museum (Natural History) displays pre-terrestrial shear deformation which transects a small troilite-daubréelite-cohenite nodule. Five globular areas (< 1 mm) with dendritic structures indicating rapid solidification from melts occur within and around the larger part of the transected nodule. All shock deformation structures and shock-heating effects exhibited by Haig. are of pre-terrestrial origin and at least four successive alterations to the original structure are evident. Partial bulk analyses of the melt globules indicate complex mixtures of approximately 55 wt % troilite, 24 wt % metal, 14 wt % daubréelite and 7 wt % cohenite. It is suggested that a compressive environment maintained the melts as discrete pools of liquid. The diameters of the globules (< 0.5 mm) and spacing of the dendrite arms (< 1 μm) indicate solidification at > 106 °C sec?1 which is the fastest cooling rate yet recorded in meteorites.  相似文献   

13.
Groundbased radio observations indicate that Jupiter's ammonia is globally depleted from 0.6 bars to at least 4-6 bars relative to the deep abundance of ∼3 times solar, a fact that has so far defied explanation. The observations also indicate that (i) the depletion is greater in belts than zones, and (ii) the greatest depletion occurs within Jupiter's local 5-μm hot spots, which have recently been detected at radio wavelengths. Here, we first show that both the global depletion and its belt-zone variation can be explained by a simple model for the interaction of moist convection with Jupiter's cloud-layer circulation. If the global depletion is dynamical in origin, then important endmember models for the belt-zone circulation can be ruled out. Next, we show that the radio observations of Jupiter's 5-μm hot spots imply that the equatorial wave inferred to cause hot spots induces vertical parcel oscillation of a factor of ∼2 in pressure near the 2-bar level, which places important constraints on hot-spot dynamics. Finally, using spatially resolved radio maps, we demonstrate that low-latitude features exceeding ∼4000 km diameter, such as the equatorial plumes and large vortices, are also depleted in ammonia from 0.6 bars to at least 2 bars relative to the deep abundance of 3 times solar. If any low-latitude features exist that contain 3-times-solar ammonia up to the 0.6-bar ammonia condensation level, they must have diameters less than ∼4000 km.  相似文献   

14.
Abstract— Spectra of asteroid 4 Vesta and 21 small (estimated diameters less than 10 km) asteroids with Vesta‐like spectral properties (Vestoids) were measured at visible and near‐infrared wavelengths (～0.44 to ～1.65 μm). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled “non‐family” members that reside just outside the semi‐major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a‐e‐i) match of these “non‐family” objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut‐off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine‐rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best‐known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine‐grained eucritic material on their surfaces.  相似文献   

15.
S. Alan Stern 《Icarus》2009,199(2):571-573
In this Note, I present first-order scaling calculations to examine the efficacy of impacts by Kuiper Belt debris in causing regolith exchange between objects in the Pluto system. It is found that ejecta can escape Nix and Hydra with sufficient velocity to reach one another, as well as Charon, and even Pluto. The degree of ejecta exchanged between Nix and Hydra is sufficient to cover these bodies with much more material than is required for photometrically change. In specific, Nix and Hydra may have exchanged as up to 10s of meters of regolith, and may have covered Charon to depths up to 14 cm with their ejecta. Pluto is likely unaffected by most Nix and Hydra ejecta by virtue of a combination of dynamical shielding from Charon and Pluto's own annual atmospheric frost deposition cycle. As a result of ejecta exchange between Nix, Hydra, and Charon, these bodies are expected to evolve their colors, albedos, and other photometric properties to be self similar. These are testable predictions of this model, as is the prediction that Nix and Hydra will have diameters near 50 km, owing to having a Charon-like albedo induced by ejecta exchange. As I discuss, this ejecta exchange process can also be effective in many KBOs and asteroids with satellites, and may be the reason that very many KBO and asteroid satellite systems have like colors.  相似文献   

16.
Edward P. Ney 《Icarus》1974,23(4):551-560
Observations of Comets Kohoutek (1973f), Bradfield (1974b), and P/Encke have been made at a number of wavelengths between 0.55 and 18 μm. The silicate feature first observed in Comet Bennett (1969i) seems to be a common characteristic of cometary material. The comas of these comets radiate infrared with an effective temperature higher than the black-body temperature at the given distance from the Sun. The albedo of the dust particles is between 0.10 and 0.20. The particles in the coma and tail are small (diameter less than 2 μm), but the particles in the anti-tail of Comet Kohoutek must be larger than about 10 μm diameter. The observations give an absolute upper limit to the diameter of Comet Kohoutek of 30 km. A consistent interpretation would indicate that Comets Kohoutek and Bradfield have nuclear diameters of 5 to 10km, that Bennett was several times larger, and that P/Encke is 10 times smaller. The peculiar behavior of Bradfield showed that the coma of a single comet can abruptly change its dust composition.  相似文献   

17.
The purpose of this work is to study some characteristics of roundish features which are observed on Hα filtergrams, above the solar chromosphere, at the limb. The ‘Bulles’ - chromospheric bubbles - so called for their apparent shape, move upwards into the low corona and fade out when their material is completely ionised (Figures 1 to 3). The physical data for nine observed bubbles listed in Table I are: lifetime (2), average height (3), ascending velocity (4), lateral velocity (5), angle of the movement with the normal to the solar limb (6), intensity in Hα (7) and diameters (8). The time variations of some of these properties are pictured in Figure 4. It is noted that all measured velocities are super-sonic. We discuss the relation between the ascending velocity and the maximum height reached by the bubbles (Figure 5) and compare it with the one obtained by Rush and Roberts for spicules. The study of their diameters shows that the vertical diameter is generally greater than the horizontal one. Their ratio is a function of speed and becomes equal to unity when the bubble velocity reaches 70 km s?1 (Figure 6). The bubbles seem to diffuse in the corona as spicules do. We also compare the properties of bubbles with some already known chromospheric features (Table II) and there seems to be a link between bubbles and Becker's grains. Finally, we, however, propose two hypothesis regarding the origin of bubbles: (i) they are related to spicules, or (ii) they are accelerated in the low chromosphere by the divergence of the magnetic field.  相似文献   

18.
Near‐Earth objects (NEOs) with diameters of <300 m are difficult to detect from the Earth with radar or optical telescopes unless and until they approach closely. If they are on collisional courses with the Earth, there is little that can be done to mitigate the considerable damage. Although destructive collisions in space are rare for 1 km diameter bodies and above, once hit by a sizeable impactor, such a NEO can develop a relatively dense cloud of co‐orbiting material in which destructive collisions are relatively frequent. The gas and nanoscale dust released in the destructive collisions can be detected remotely by downstream spacecraft equipped with magnetometers. In this paper, we use such magnetic disturbances to identify regions of near‐Earth space in which high densities of small objects are present. We find that asteroid (138175) 2000EE104 currently may have a cloud of potentially threatening co‐orbiting material. Due to the scattered co‐orbitals, there can be a finite impact probability whenever the Earth approaches the orbit of asteroid 2000EE104, regardless of the position of the asteroid itself.  相似文献   

19.
The impacts of Comet Shoemaker-Levy 9 left spots on Jupiter with diameters on the order of tens of thousands of kilometers, which have the appearance of debris fields strewn upon the Jovian cloud tops. In this note we employ a measurement of the optical depth of the debris at the impact site of fragment G to estimate mass in the debris field and lower limits to the G fragment mass of 4×1012 – 4×1013 g and diameter of 0.1 – 0.3 km.The masses and sizes of the fragments of Comet Shoemaker-Levy 9 are still uncertain, with estimated sizes ranging from 0.1 to 4 km. The size of the cometary body before breakup is believed to have been between 1 and 10 km. (Asphaug & Benz 1994; Solen 1994; Weaver et al. 1994, Scott & Melosh 1993). These estimates were based on pre-impact images of the cometary fragments. A complimentary technique is to use post-impact images of the spots left on Jupiter to infer the sizes and masses of the fragments.Structure in the underlying clouds is clearly visible through spots imaged by the Hubble Space Telescope, implying that the debris fields are relatively thin. Shortly after the G impact, A'Hearn and collaborators (paper in preparation) used the University of Maryland CCD System at the Perth Observatory to image Jupiter in a variety of bandpasses. While a complete reduction is still underway, a preliminary examination of the raw data shows that the spot at the impact site of fragment G, when near the central meridian roughly three hours after impact, had an average optical depth of roughly 0.05 in several bandpasses between 0.62 and 0.73µm. The measured diameter of the spot was approximately D = 15,000 km.In this note we do not present the data for optical depth, but rather we show that measurements of this type can be used to determine the mass of the solid particles in the clouds and thus to set limits on the mass of the impactor. We assume that the spot consisted of a thin layer of dust in the upper atmosphere. Assuming a one-particle layer covering a fraction of 0.05 of the spot area (a valid assumption for an optically thin cloud), the mass of matter in the spot is M = (0.05/4) dD2, where and d are the particle density and diameter. Particle sizes are not directly measured. However, the particle diameters cannot be much less than 1 µm because the CCD observations when compared with HST ultraviolet images show that extinction is not strongly wavelength dependent at optical and near-uv wavelengths. Typical grain sizes in comets and in the zodiacal dust range from 1 to 10 µm. For particle densities of 0.5 g cm–3 and assumed particle diameters in the range 1 – 10 µm, we find masses, M = 4×1012 – 4×1013 g. Assuming an impactor density of 0.5 g cm–3 (Asphaug & Benz 1994), the corresponding fragment diameters are 0.1 – 0.3 km. Larger sizes for the grains would increase the estimated mass.The observed debris may not be actual comet dust. Since temperatures in the fireball are estimated to be several thousand degrees, all the material in the fragment should have been vaporized (Sekanina et al 1995; Takata et al 1994; Zahnle & MacLow 1994). Therefore the debris material could consist of recondensed matter, perhaps organics, from the fireball. An impactor collides with roughly its own mass of atmospheric material before disruption, so the estimates for the impactor mass hold to order of magnitude even if the debris contains matter with contributions from originally atmospheric gases.The estimate of 0.1 – 0.3 km diameter for the G fragment is a lower limit because the object would also contain material, for example ices, that would not appear in the debris field. Furthermore, since the HST images show structure in the spots that is unresolved in the observations used here, the spot may not be optically thin at all points, but only on average, and this leads to our estimate being a lower limit for the mass of particles. As noted above, the particles are unlikely to be much less than 1 µm in size; particles much larger than 10µm would also imply a larger mass of particles. The derived fragment size is comparable to those estimated from pre-impact observations.  相似文献   

20.
叙述了测定大气折射对工作星表的要求和星表系统误差的影响,并通过FK5星表相对于依巴谷星表的系统误差,来说明以往的各种基本星表都不适用的原因;文章说明了依巴谷星表所列星位置不含随天区而异的系统误差的特性,分析了该星表问世时的精度和十多年后的今天仍能达到的精度,说明它可以作为这种测定的工作星表,从而为直接测定天文大气折射值和折射率差提供了一个重要的条件;文章还介绍了依巴谷星表的星等分布,认为只需选用亮于6mag的依巴谷星,这一星等范围,为制定专用测量仪器终端的设计方案提供了依据。  相似文献   

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