Martian meteorite launch: high-speed ejecta from small craters

We performed high-resolution computer simulations of impacts into homogeneous and layered martian terrain analogs to try to account for the ages and characteristics of the martian meteorite collection found on Earth. We found that craters as small as approximately 3 kilometers can eject approximately 10(7) decimeter-sized fragments from Mars, which is enough to expect those fragments to appear in the terrestrial collection. This minimum crater diameter is at least four times smaller than previous estimates and depends on the physical composition of the target material. Terrain covered by a weak layer such as an impact-generated regolith requires larger, therefore rarer, impacts to eject meteorites. Because older terrain is more likely to be mantled with such material, we estimate that the martian meteorites will be biased toward younger ages, which is consistent with the meteorite collection.     

Radar glory from buried craters on icy moons

Three ice-covered moons of Jupiter, in comparison with rocky planets and Earth's moon, produce radar echoes of astounding strengths and bizarre polarizations. Scattering from buried craters can explain these and other anomalous properties of the echoes. The role of such craters is analogous to that of the water droplets that create the apparition known as "the glory," the optically bright region surrounding an observer's shadow on a cloud. Both situations involve the electromagnetic phenomenon of total internal reflection at a dielectric interface, operating in a geometry that strongly favors exact backscattering. Dim surface craters are transformed into bright glory holes by being buried under somewhat denser material, thereby increasing the intensity of their echoes by factors of hundreds. The dielectric interface thus formed at the crater walls nicely accounts for the unusual polarizations of the echoes.     

Impact craters on venus: initial analysis from magellan

Magellan radar images of 15 percent of the planet show 135 craters of probable impact origin. Craters more than 15 km across tend to contain central peaks, multiple central peaks, and peak rings. Many craters smaller than 15 km exhibit multiple floors or appear in clusters; these phenomena are attributed to atmospheric breakup of incoming meteoroids. Additionally, the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across. Ejecta is found at greater distances than that predicted by simple ballistic emplacement, and the distal ends of some ejecta deposits are lobate. These characteristics may represent surface flows of material initially entrained in the atmosphere. Many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid. Craters are absent from several large areas such as a 5 million square kilometer region around Sappho Patera, where the most likely explanation for the dearth of craters is volcanic resurfacing. There is apparently a spectrum of surface ages on Venus ranging approximately from 0 to 800 million years, and therefore Venus must be a geologically active planet.     

A sharper view of impact craters from clementine data

The ultraviolet-visible camera on the Clementine spacecraft obtained high-spatial resolution images of the moon in five spectral channels. Impact craters mapped with these multispectral images show a scale of lithologic diversity that varies with crater size and target stratigraphy. Prominent lithologic variations (feldspathic versus basaltic) occur within the south wall of Copernicus (93 kilometers in diameter) on the scale of 1 to 2 kilometers. Lithologic diversity at Tycho (85 kilometers in diameter) is less apparent at this scale, although the impact melt of these two large craters is remarkably similar in this spectral range. The lunar surface within and around the smaller crater Giordano Bruno (22 kilometers in diameter) is largely dominated by the mixing of freshly excavated material with surrounding older soils derived from a generally similar feldspathic lithology.     

Endogenetic craters interpreted from crater counts on the inner wall of copernicus

Collapse craters formed in terrestrial basalt flows exhibit size-frequency distributions that are similar to distributions for craters located in terraces in the inner wall of the lunar crater Copernicus. These distributions and surface morphology suggest that the interior terraces are basalt lava flows containing collapse craters as well as impact craters.     

Filamentary crystal growth associated with impact craters from hypervelocity microparticles

A filamentary crystal growth is associated with impacts of hypervelocity microparticles upon copper foil. They are copper whiskers formed by the condensation of the target material vaporzied by the impact.     

广州莲藕斑点病病原线虫鉴定

报道了在广州市番禺区万顷沙莲藕生产基地发现的莲藕斑点病,病原鉴定为分离潜根线虫Hirschmanniella diversa。记述了病原线虫的检验方法、形态测量数据和分类鉴定特征。讨论了病害的症状和病原线虫与近似种在形态特征方面的差异,并对不同地区的病原线虫标本的测量数据和形态特征进行了比较。     

济南市发现的青铜犁铧再探

许久以来，专家学者对青铜农具的起源及商周是否大量使用青铜农具等问题，多有见解。犁　铧，即木犁前端所套金属刃，也就是耕地用的犁头。它到底始于何时？也是众说纷纭，并因　之滋生许多异论。大多资料认为：铜犁在春秋以前尚未使用，战国时期虽有发现但数量甚少　。随着考古发掘的重大收获，尤其是江西新干大洋洲商代大墓出土的４８０多件青铜器中，　有农具多件，其中包括两件青铜犁铧。由此，便使得在我国商周青铜文化的繁荣时代是否已　有铜犁的使用，这一悬而未决的课题终于明朗化了。“新干商墓青铜犁的出土，将青铜犁的　出现提早…     

巨大节毛菌在水杉上的发现

以采自四川省成都市金堂县赵镇的水杉树上的巨大节毛菌标本为材料,对其子实体形态,菌丝和孢子等进行了形态学上的描述,初步判定其属于巨大节毛菌。对其核糖体内转录间隔区(ITS)序列进行分析,建立同源进化树,与其它种类的真菌的亲缘关系进行比较,发现试验的标本材料与GenBank中基因号为GQ355959的巨大节毛菌最相近并被聚类到一起,同源相似性为87%。     

Radionuclide composition of the allende meteorite from nondestructive gamma-ray spectrometric analysis

The concentrations of beryllium-7, sodium-22, aluminum-26, potassium-40, scandium-46, vanadium-48, chromium-51, manganese-54, cobalt-57, cobalt-60, and thorium-232 (thallium-208) have been measured in the Allende meteorite by nondestructive gamma-ray spectrometry. The high cobalt-60 content of the meteorite is indicative of a preatmospheric body with a minimum effective radius of 50 centimeters and a weight of 1650 kilograms; the aluminum-26 activity indicates a minimum exposure age of 3 million years.     

The frequency of meteorite falls on the Earth

Photographic observations from a network of 60 cameras in western Canada are used to derive the influx rate of meteorites on the earth's surface, the first time instrumental data have been used for this purpose. Forty-three observed events are believed to have dropped between 0.1 and 12 kilograms of meteorites each. The flux values are corrected for a minor latitude effect and agree with earlier estimates near 10 kilograms but vary more slowly with mass. Eight events per year drop at least 1 kilogram of meteorites in an area of 10(6) square kilometers.     

Iodine-129/xenon-129 age of magnetite from the orgueil meteorite

Magnetite from the Orgueil C1 chondrite is only 2.0 +/- 2.4 million years older by the iodine-xenon method than the next oldest meteorite, the Karoonda C4 chondrite. This age ties the primitive C1 chondrites to the extensive iodine-xenon chronology of normal chrondrites. If Karoonda and Orgueil magnetite formed from similar material, then the age difference is an upper limit to the formation time of these meteorites-and by customary extension, the solar system. Condensation, chondrule formation, accretion, and metamorphism of the Karoonda parent body all seem to have been completed within a few million years.     

Oblique impact: a process for obtaining meteorite samples from other planets

Cratering flow calculations for a series of oblique to normal (10 degrees to 90 degrees ) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25 degrees to 60 degrees and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth.     

Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages

Lunar meteorites represent a more random sampling of lunar material than the Apollo or Luna collections and, as such, lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis. Within the lunar meteorite breccias MAC 88105, QUE 93069, DaG 262, and DaG 400, seven to nine different impact events are represented with 40Ar-39Ar ages between 2.76 and 3.92 billion years ago (Ga). The lack of impact melt older than 3.92 Ga supports the concept of a short, intense period of bombardment in the Earth-moon system at approximately 3.9 Ga. This was an anomalous spike of impact activity on the otherwise declining impact- frequency curve.     

The Tagish Lake meteorite: a possible sample from a D-type asteroid

A new type of carbonaceous chondrite, the Tagish Lake meteorite, exhibits a reflectance spectrum similar to spectra observed from the D-type asteroids, which are relatively abundant in the outer solar system beyond the main asteroid belt and have been inferred to be more primitive than any known meteorite. Until the Tagish Lake fall, these asteroids had no analog in the meteorite collections. The Tagish Lake meteorite is a carbon-rich (4 to 5 weight %), aqueously altered carbonaceous chondrite and contains high concentrations of presolar grains and carbonate minerals, which is consistent with the expectation that the D-type asteroids were originally made of primitive materials and did not experience any extensive heating.     

Rapid growth of magnesium-carbonate weathering products in a stony meteorite from antarctica

Nesquehonite, a hydrous magnesium carbonate, occurs as a weathering product on the surface of the Antarctic meteorite LEW 85320(H5 chondrite). Antarctic meteorites have resided on the earth for periods of 10(4) to 10(6) years, but the time needed for weathering products to form has been uncertain. Isotopic measurements of delta(13)C and delta(18)O indicate that the nesquehonite formed at near freezing temperatures by reaction of meteoritic minerals with terrestrial water and carbon dioxide. Results from carbon-14 dating suggest that, although the meteorite has been in Antarctica for at least 3.2 x 10(4) to 3.3 x 10(4) years, the nesquehonite formed after A.D.1950.     

Origin and evolution of prebiotic organic matter as inferred from the Tagish Lake meteorite

The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites' asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration.     

Deepest known plant life discovered on an uncharted seamount

The discovery of abundant autotrophic macrophytes living below 200 meters indicates their importance to primary productivity, food webs, sedimentary processes, and as reef builders in clear oceanic waters. Estimates concerning minimum light levels for macroalgal photosynthesis and macrophytic contributions to the biology and geology of tropical insular and continental borderlands must now be revised.