| 1. |
- Kueppers, Michael, et al.
(författare)
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Triple F-a comet nucleus sample return mission
- 2009
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 23:3, s. 809-847
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Tidskriftsartikel (refereegranskat)abstract
- The Triple F (Fresh From the Fridge) mission, a Comet Nucleus Sample Return, has been proposed to ESA's Cosmic Vision program. A sample return from a comet enables us to reach the ultimate goal of cometary research. Since comets are the least processed bodies in the solar system, the proposal goes far beyond cometary science topics (like the explanation of cometary activity) and delivers invaluable information about the formation of the solar system and the interstellar molecular cloud from which it formed. The proposed mission would extract three sample cores of the upper 50 cm from three locations on a cometary nucleus and return them cooled to Earth for analysis in the laboratory. The simple mission concept with a touch-and-go sampling by a single spacecraft was proposed as an M-class mission in collaboration with the Russian space agency ROSCOSMOS.
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| 2. |
- Kumar, Nitin, et al.
(författare)
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Adaptation of host transmission cycle during Clostridium difficile speciation
- 2019
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Ingår i: Nature Genetics. - : Nature Publishing Group. - 1061-4036 .- 1546-1718. ; 51:9, s. 1315-1320
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Tidskriftsartikel (refereegranskat)abstract
- Bacterial speciation is a fundamental evolutionary process characterized by diverging genotypic and phenotypic properties. However, the selective forces that affect genetic adaptations and how they relate to the biological changes that underpin the formation of a new bacterial species remain poorly understood. Here, we show that the spore-forming, healthcare-associated enteropathogen Clostridium difficile is actively undergoing speciation. Through large-scale genomic analysis of 906 strains, we demonstrate that the ongoing speciation process is linked to positive selection on core genes in the newly forming species that are involved in sporulation and the metabolism of simple dietary sugars. Functional validation shows that the new C. difficile produces spores that are more resistant and have increased sporulation and host colonization capacity when glucose or fructose is available for metabolism. Thus, we report the formation of an emerging C. difficile species, selected for metabolizing simple dietary sugars and producing high levels of resistant spores, that is adapted for healthcare-mediated transmission.
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| 3. |
- Meier, Matthias, et al.
(författare)
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Graphite grains in supernova ejecta - Insights from a noble gas study of 91 individual KFC1 presolar graphite grains from the Murchison meteorite
- 2012
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 76, s. 147-160
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Tidskriftsartikel (refereegranskat)abstract
- We have measured helium and neon concentrations, elemental and isotopic ratios of 91 individual presolar graphite grains from the KFC1 density separate of the Murchison meteorite. Eleven grains contain measurable amounts of either (4)He, (20)Ne, (21)Ne or (22)Ne, or a combination thereof. We report the first detection of (21)Ne from an individual presolar graphite grain and the first detection of (4)He and (20)Ne in individual KFC1 graphite grains. Six of the gas-rich grains originate from asymptotic giant branch (AGB) stars, while another five are likely derived from core-collapse supernovae. The mono-isotopic (22)Ne detected in one supernova grain is either radiogenic and compatible with condensation in the O/Ne zone, or nucleosynthetic and derived from the He/C zone. Two grains with (20)Ne and (12)C/(13)C < 10 are consistent with condensation and Ne acquisition in a similar to 80:20 mixture of material from the H envelope and He/N zone. The isotopic ratios of a single grain with (21)Ne and (22)Ne, and a further grain with (20)Ne and (22)Ne are compatible with condensation and Ne acquisition in the C/O zone. We discuss the implications of our study on the understanding of processes in supernova ejecta. (C) 2011 Elsevier Ltd. All rights reserved.
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| 4. |
- von Mentzer, Astrid, 1983, et al.
(författare)
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Identification of enterotoxigenic Escherichia coli (ETEC) clades with long-term global distribution
- 2014
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Ingår i: Nature Genetics. - : Springer Science and Business Media LLC. - 1061-4036 .- 1546-1718. ; 46:12, s. 1321-1326
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Tidskriftsartikel (refereegranskat)abstract
- Enterotoxigenic Escherichia coil (ETEC), a major cause of infectious diarrhea, produce heat-stable and/or heat-labile enterotoxins and at least 25 different colonization factors that target the intestinal mucosa. The genes encoding the enterotoxins and most of the colonization factors are located on plasmids found across diverse E. coli serogroups. Whole-genome sequencing of a representative collection of ETEC isolated between 1980 and 2011 identified globally distributed lineages characterized by distinct colonization factor and enterotoxin profiles. Contrary to current notions, these relatively recently emerged lineages might harbor chromosome and plasmid combinations that optimize fitness and transmissibility. These data have implications for understanding, tracking and possibly preventing ETEC disease.
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| 5. |
- Wieler, R., et al.
(författare)
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Comment on "Radiation History of Fossil Meteorites from Sweden" by V.A. Alexeev (2010), Solar System Research, 44, 311-319
- 2011
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Ingår i: Solar System Research. - 1608-3423. ; 45:5, s. 459-461
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We dispute the claim by Alexeev (2010) that the numerous fossil meteorites found in mid-Ordovician sediments in southern Sweden can be explained by a single large meteorite shower some 470 million years ago, and-in particular-that the reported variable cosmic ray exposure ages derived from cosmogenic Ne-21 in chromite grains extracted from the fossil meteorites actually reflect variable contributions of nucleogenic Ne-21 instead.
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| 6. |
- Alwmark, Carl, et al.
(författare)
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A global rain of micrometeorites following breakup of the L-chondrite parent body-Evidence from solar wind-implanted Ne in fossil extraterrestrial chromite grains from China
- 2012
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 47:8, s. 1297-1304
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies of limestone beds of mid-Ordovician age from both Sweden and China show that the Earth saw an at least two orders of magnitude increase in the influx of extraterrestrial material approximately 470 Ma, following the disruption of an L-chondrite parent body in the asteroid belt. Recovered extraterrestrial material consists of fossil meteorites and sediment-dispersed extraterrestrial chromite (SEC) grains, both with L-chondritic origin. Ne isotope analysis of SEC grains from one of the Swedish limestone sections revealed that the vast majority of the grains were delivered to Earth as micrometeorites. In this study, we extend the previous work, both in time and geographically, by measuring concentrations and isotopic ratios of Ne in individual SEC grains (60120 mu m in diameter) from three different beds from a contemporary Middle Ordovician limestone section in China. All of the Chinese SEC grains, 44 in total, contain surface-implanted Ne of fractionated solar wind composition, implying that these grains were, as in the case of the Swedish SEC grains, delivered to Earth as micrometeorites. This gives further compelling evidence that the two to three orders of magnitude increase in the influx of micrometeoritic material following the breakup of the L-chondrite parent body was indeed a global event. The rain of micrometeorites prevailed for at least 2 Myr (the estimated time of the deposition of the topmost Chinese bed) after the breakup event.
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| 7. |
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| 8. |
- Heck, PR, et al.
(författare)
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Fast delivery of meteorites to Earth after a major asteroid collision
- 2004
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 430:6997, s. 323-325
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Tidskriftsartikel (refereegranskat)abstract
- Very large collisions in the asteroid belt could lead temporarily to a substantial increase in the rate of impacts of meteorites on Earth. Orbital simulations predict that fragments from such events may arrive considerably faster than the typical transit times of meteorites falling today, because in some large impacts part of the debris is transferred directly into a resonant orbit with Jupiter(1,2). Such an efficient meteorite delivery track, however, has not been verified. Here we report high-sensitivity measurements of noble gases produced by cosmic rays in chromite grains from a unique suite of fossil meteorites(3) preserved in similar to480 million year old sediments. The transfer times deduced from the noble gases are as short as similar to10(5) years, and they increase with stratigraphic height in agreement with the estimated duration of sedimentation. These data provide powerful evidence that this unusual meteorite occurrence was the result of a long-lasting rain of meteorites following the destruction of an asteroid, and show that at least one strong resonance in the main asteroid belt can deliver material into the inner Solar System within the short timescales suggested by dynamical models.
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| 9. |
- Heck, Philipp R., et al.
(författare)
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Noble gases in fossil micrometeorites and meteorites from 470 Myr old sediments from southern Sweden, and new evidence for the L-chondrite parent body breakup event
- 2008
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Ingår i: Meteoritics and Planetary Science. - 1086-9379. ; 43:3, s. 517-528
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Tidskriftsartikel (refereegranskat)abstract
- We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from similar to 470 Myr old sediments from two quarries (Hallekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhogen quarry, all located ill Southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhogen 001 were generated in the L-chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during similar to 470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment-dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into all orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhogen 001 has a low cosmic-ray exposure age of similar to 0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.
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| 10. |
- Kita, Noriko T., et al.
(författare)
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Fall, classification, and exposure history of the Mifflin L5 chondrite
- 2013
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 48:4, s. 641-655
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Tidskriftsartikel (refereegranskat)abstract
- The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock-melted veins are present. The U,Th-He age is 0.7Ga, and the K-Ar age is 1.8Ga, indicating that Mifflin might have been heated at the time of the 470Ma L-chondrite parent body breakup and that U, Th-He, and K-Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 3065cm. Assuming this exposure geometry, a cosmic-ray exposure age of 25 +/- 3Ma is calculated from cosmogenic noble gas concentrations. The low 22Ne/21Ne ratio may, however, indicate a two-stage exposure with a longer first-stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.
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