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J. Teemu T. Mäkinen 《Icarus》2005,177(1):269-279
As a part of a project to study the cometary particle environment, a self-consistent ballistic rigid body particle colliding simulator, capable of producing fractal aggregates of icy particles modelled by hard or soft (metaball) spheres has been developed, to be used for creating test particles in future statistical studies of the cometary coma-particle interaction. Instead of a predefined sticking probability, actual surface interactions are modelled and a method for calculating internal stresses and fragmentation of an aggregate during the accretion phase is described. Simulations suggest that taking fragmentation into account has two major consequences, increase in the fractal dimension of resulting particles and a noticeable change in the mass spectrum of an ensemble. 相似文献
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Shock metamorphic features at the Saarijärvi (D > 2 km) and Söderfjärden (D = 6.5 km) structures in Finland have so far only been studied tentatively, although both are considered to be proven impact structures. This work presents the first detailed universal stage study of planar deformation features (PDFs), feather feature lamellae (FFL), and planar fractures (PFs) in quartz grains from a polymict impact breccia dike from Söderfjärden, and from sedimentary crater‐fill rocks from Saarijärvi. Planar microstructures, particularly PDFs, are very rare and poorly developed or preserved in Saarijärvi, whereas in Söderfjärden they are much more common and well defined. Miller–Bravais indices of the planar microstructures in both Saarijärvi and Söderfjärden are indicative of relatively low‐shock pressure but high shear conditions, only compatible with an impact origin for these structures. Although a Proterozoic age for Saarijärvi cannot be ruled out, the observations of shock features throughout the sedimentary crater‐fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a Lower Cambrian (or younger) impact age. 相似文献
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The Characteristics of Polygonal Impact Craters on Venus 总被引：1，自引：0，他引：1
Marko Aittola Teemu Öhman Johannes J. Leitner Jouko Raitala 《Earth, Moon, and Planets》2007,101(1-2):41-53
Polygonal impact craters (PICs) are craters whose shape in plan view is more or less angular instead of being circular or ellipsoidal. This type of craters are present and often common on the Moon, Mercury, Mars and several asteroids and icy moons and after the careful analysis we found on Venus 131 impact craters, which show at least two straight rim segments. This survey proves that there are polygonal impact craters on Venus and they may provide a good tool to analyse the properties of the planet’s surface/crust/lithosphere as well as the impact process itself. This study also collaborates our previous results, that PICs are not an anomaly among craters, but an integral part of all impact craters regardless of their size or environment. We compared the polygonal impact craters to “normal”-shaped craters by using different characteristics (diameter, altitude, geologic setting, morphologic class, floor reflectance, degradation stage, and wall terracing). It turned out that the smaller crater sizes favor the formation of straight rim segments, but otherwise these craters show similar characteristics to other craters. Our study also shows that there are regions where the straight segments of the crater rims most clearly follow the orientations of the dominant tectonic features of the area. Thus, the orientations of crater walls reflect–at least in some places–the local tectonics and zones of weakness also on Venus and could thus tell us about the directions and distributions of fractures or other zones of weakness in the crust. 相似文献
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Martin SCHMIEDER Winfried H. SCHWARZ Elmar BUCHNER Mario TRIELOFF Jarmo MOILANEN Teemu ÖHMAN 《Meteoritics & planetary science》2010,45(4):572-582
Abstract– 40Ar/39Ar dating of recrystallized feldspar glass particles separated from clast‐rich impact melt rocks from the approximately 10 km Paasselkä impact structure (SE Finland) yielded a Middle to Late Triassic (Ladinian‐Karnian) pseudo‐plateau age of 228.7 ± 3.0 (3.4) Ma (2σ). This new age makes Paasselkä the first known Triassic impact structure dated by isotopic methods on the Baltic Shield. The new Paasselkä impact age is, within uncertainty, coeval with isotopic ages recently obtained for the Lake Saint Martin impact structure in Canada, indicating a new Middle to Late Triassic impact crater population on Earth. The comparatively small crater size, however, suggests no relationship between the Paasselkä impact and a postulated extinction event at the Middle/Late Triassic boundary. 相似文献
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Anne?OjalaEmail author Satu?Kokkonen Timo?Kairesalo 《Aquatic Sciences - Research Across Boundaries》2003,65(3):287-296
The role of phosphorus in phytoplankton growth was studied in Lake Vesijärvi, a large previously eutrophic body of water with a history of flourishing fishery. The study combined different approaches: long-term algal enrichment experiments with natural phytoplankton assemblages were carried out together with observations on nutrient and chlorophyll a concentrations, elemental ratios (N:P, C:P, C:N) of particulate matter, and analysis of P uptake using [33P]. None of the approaches revealed periods of P limitation, but some growth experiments as well as elemental ratios indicated slight deficiency in early summer. Concentrations of total dissolved phosphorus (TDP), which were usually 20–30 g l–1, also indicated luxurious P resources. Thermal stratification was weak and the lake mixed twice during the study period; this was reflected in the phytoplankton biomass that increased up to 4-fold. Results of elemental ratios usually suggested the occurrence of nitrogen limitation, and in general these ratios were low for a lake. All size fractions >0.22 m in the experiments with [33P] showed P uptake. In August most of the P was taken up by picoplankton, but when the lake turned over in September, the uptake of P by this fraction was absent. Thus, there was always a plentiful supply of P for phytoplankton, but the shortage of inorganic N may have affected those algae not capable of fixing N2. These conditions should have favoured the growth of heterocystous cyanobacteria, but these prokaryotes never attained high abundances. This may have been due to the weak stability of the water column, or the growth of cyanobacteria may have been limited by trace elements such as molybdenum or iron. 相似文献
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The two Suvasvesi impact structures,Finland: Argon isotopic evidence for a “false” impact crater doublet 
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下载免费PDF全文 Martin Schmieder Winfried H. Schwarz Mario Trieloff Elmar Buchner Jens Hopp Eric Tohver Lauri J. Pesonen Martti Lehtinen Jarmo Moilanen Stephanie C. Werner Teemu Öhman 《Meteoritics & planetary science》2016,51(5):966-980
The two neighboring Suvasvesi North and South impact structures in central‐east Finland have been discussed as a possible impact crater doublet produced by the impact of a binary asteroid. This study presents 40Ar/39Ar geochronologic data for impact melt rocks recovered from the drilling into the center of the Suvasvesi North impact structure and melt rock from glacially transported boulders linked to Suvasvesi South. 40Ar/39Ar step‐heating analysis yielded two essentially flat age spectra indicating a Late Cretaceous age of ~85 Ma for the Suvasvesi North melt rock, whereas the Suvasvesi South melt sample gave a Neoproterozoic minimum (alteration) age of ~710 Ma. Although the statistical likelihood for two independent meteorite strikes in close proximity to each other is rather low, the remarkable difference in 40Ar/39Ar ages of >600 Myr for the two Suvasvesi impact melt samples is interpreted as evidence for two temporally separate, but geographically closely spaced, impacts into the Fennoscandian Shield. The Suvasvesi North and South impact structures are, thus, interpreted as a “false” crater doublet, similar to the larger East and West Clearwater Lake impact structures in Québec, Canada, recently shown to be unrelated. Our findings have implications for the reliable recognition of impact crater doublets and the apparent rate of binary asteroid impacts on Earth and other planetary bodies in the inner solar system. 相似文献
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A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure,Finland 下载免费PDF全文
下载免费PDF全文 Martin Schmieder Fred Jourdan Jarmo Moilanen Elmar Buchner Teemu Öhman 《Meteoritics & planetary science》2016,51(2):303-322
Field investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone‐bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8–10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40Ar/39Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new 40Ar/39Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä impact, followed by impact‐induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic 40Ar/39Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new 40Ar/39Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide. 相似文献
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The Huygens entry probe descended through the atmosphere of Titan and provided an excellent set of observations of the atmosphere and the surface of Titan. During the 150-min descent the Huygens Atmospheric Structure Instrument (HASI) observed a comprehensive set of variables, including pressure, temperature, density and atmospheric electricity. The atmospheric pressure profile was recorded by the Pressure Profile Instrument (PPI), provided by Finnish Meteorological Institute (FMI). The instrument started measurements at an altitude of 150 km, and produced about 28 bits of data per second. Data were also obtained through the time of 31 min beyond the time of surface impact. The first-order scientific analysis of the PPI results has been performed. The observations together with hydrostatic assumption and in combination with other measurements have provided the first atmospheric pressure profile and the surface pressure (of approximately ) for Titan's atmosphere. To carry out the pressure profile reconstruction we developed a real gas formulation, which is applicable also for other Titan atmospheric investigations. The altitude versus time speed of the descent was calculated and the results were compared with the direct altitude observations by the radar altimeter during the last 40 km of the descent. The fit was excellent demonstrating the high-quality level of the PPI observations as well as the utilized investigation methods. 相似文献
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