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901.
Water, vital for life, not only maintains the integrity of structural and metabolic biomolecules, it also transports them in solution or colloidal suspension. Any flow of water through a dormant or fossilized microbial community elutes molecules that are potentially recognizable as biomarkers. We hypothesize that the surface seepage channels emanating from crater walls and cliffs in Mars Orbiter Camera images results from fluvial erosion of the regolith as low-temperature hypersaline brines. We propose that, if such flows passed through extensive subsurface catchments containing buried and fossilized remains of microbial communities from the wet Hesperian period of early Mars (approximately 3.5 Ga ago), they would have eluted and concentrated relict biomolecules and delivered them to the surface. Life-supporting low-temperature hypersaline brines in Antarctic desert habitats provide a terrestrial analog for such a scenario. As in the Antarctic, salts would likely have accumulated in water-filled depressions on Mars by seasonal influx and evaporation. Liquid water in the Antarctic cold desert analogs occurs at -80 degrees C in the interstices of shallow hypersaline soils and at -50 degrees C in salt-saturated ponds. Similarly, hypersaline brines on Mars could have freezing points depressed below -50 degrees C. The presence of hypersaline brines on Mars would have extended the amount of time during which life might have evolved. Phototrophic communities are especially important for the search for life because the distinctive structures and longevity of their pigments make excellent biomarkers. The surface seepage channels are therefore not only of geomorphological significance, but also provide potential repositories for biomolecules that could be accessed by landers. 相似文献
902.
The dynamics of dust particles in the solar system is dominated by solar gravity, by solar radiation pressure, or by electromagnetic interaction of charged dust grains with the interplanetary magnetic field. For micron-sized or bigger dust particles solar gravity leads to speeds of about 30 to 40 km s–1 at the Earths distance. Smaller particles that are generated close to the Sun and for which radiation pressure is dominant (the ratio of radiation pressure force over gravity F
rad/F
grav is generally termed ) are driven out of the solar system on hyperbolic orbits. Such a flow of -meteoroids has been observed by the Pioneer 8, 9 and Ulysses spaceprobes. Dust particles in interplanetary space are electrically charged to typically +5 V by the photo effect from solar UV radiation. The dust detector on Cassini for the first time measured the dust charge directly. The dynamics of dust particles smaller than about 0.1 m is dominated by the electromagnetic interaction with the ambient magnetic field. Effects of the solar wind magnetic field on interstellar grains passing through the solar system have been observed. Nanometer sized dust stream particles have been found which were accelerated by Jupiters magnetic field to speeds of about 300 km s–1. 相似文献
903.
Cometary nuclei consist of ices intermixed with dust grains and are thought to be the least modified solar system bodies remaining
from the time of planetary formation. Flyby missions to Comet P/Halley in 1986 showed that cometary dust is extremely rich
in organics (∼50% by mass). However, this proportion appears to be variable among different comets. In comparison with the
CI-chondritic abundances, the volatile elements H, C, and N are enriched in cometary dust indicating that cometary solid material
is more primitive than CI-chondrites. Relative to dust in dense molecular clouds, bulk cometary dust preserves the abundances
of C and N, but exhibits depletions in O and H. In most cases, the carbonaceous component of cometary particles can be characterized
as a multi-component mixture of carbon phases and organic compounds. Cluster analysis identified a few basic types of compounds,
such as elemental carbon, hydrocarbons, polymers of carbon suboxide and of cyanopolyynes. In smaller amounts, polymers of
formaldehyde, of hydrogen cyanide and various unsaturated nitriles also are present. These compositionally simple types, probably,
are essential "building blocks", which in various combinations give rise to the variety of involatile cometary organics.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
904.
F Raulin P Coll N Smith Y Benilan P Bruston M C Gazeau 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(4):453-460
Titan, the largest satellite of Saturn, with a dense atmosphere very rich in organics, and many couplings in the various parts of its "geofluid", is a reference for studying prebiotic chemistry on a planetary scale. New data have been obtained from experiments simulating this organic chemistry (gas and aerosol phases), within the right ranges of temperature and a careful avoiding of any chemical contamination. They show a very good agreement with the observational data, demonstrating for the first time the formation of all the organic species already detected in Titan atmosphere including, at last, C4N2, together with many other species not yet detected in Titan. This strongly suggests the presence of more complex organics in Titan's atmosphere and surface, including high molecular weight polyynes and cyanopolyynes. The NASA-ESA Cassini-Huygens mission has been successfully launched in October 1997. The Cassini spacecraft will reach the Saturn system in 2004 and become an orbiter around Saturn, while the Huygens probe will penetrate into Titan's atmosphere. In situ measurements, in particular from Huygens GC-MS and ACP instruments, will provide a detailed analysis of the organics present in the air, aerosols, and surface. This very ambitious mission should yield much information of crucial importance for our knowledge of the complexity of Titan's chemistry, and, more generally for the field of exobiology. 相似文献
905.
Thomas W. Becker 《Space Policy》1993,9(1)
The subject of space education is attracting increasing attention, but there are diverging views as to how it should be approached, as can be seen from the following two reports of Education Remote Sensing '92, a conference held in Cardiff, Wales, 28–30 June 1992. 相似文献
906.
C. Done 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):211-219
The X-ray properties of Active Galactic Nuclei (AGN) support recent theories which unify the Seyfert 1 nuclei with the Seyfert 2's and radio-quiet quasars. In these objects the underlying spectrum is strongly distorted by the effects of reflection from the accretion flow and by absorption in partially ionised material. These obscure any intrinsic changes in the spectrum, making it difficult (though not impossible) to constrain the nature of the emission process. Conversely, there is no evidence for either of these spectral distortions in the radio-loud AGN, supporting the hypothesis that the X-rays are dominated by beamed emission from the relativistic jet. 相似文献
907.
T. J. -L. Courvoisier A. Orr P. Bü hler A. Zchnder R. Henneck F. Stauffacher J. Biakhowski N. Schlumpf W. Schoeps A. Mchedlishvili R. Sunyaev V. Arefev A. Yascovich G. Babalyan M. Pavlinsky J. P. Delaboudini re T. Carone O. Siegmund J. Warren D. Leahy N. Salaschenko J. Platonov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):299-302
EUVITA is a set of 8 extreme UV normal incidence imaging telescopes, each of them sensitive in a narrow band (λ/Δλ = 15 to 80), centered at wavelengths between 50 and 175 Å. Each telescope has an effective area of a few cm2; a field of view of 1.2° and a spatial resolution of 10 arcsec.
EUVITA will be flown on the Russian mission SPECTRUM X-G. This satellite will be launched in a highly eccentric orbit with a period of 4 days, allowing long, uninterrupted observations (e.g. 105 seconds). EUVITA's narrow spectral bands allow the measurement of source parameters such as temperature or power law index as well as interstellar absorption, and will resolve groups of strong lines emitted by optically thin hot plasmas. 相似文献
908.
C. De Jager 《Space Science Reviews》1993,66(1-4):7-19
The paper gives a summary of the situation mid-1993 of theory and observations regarding massive stars. I describe: stellar mass loss and its implications, pre-main-sequence evolution, the main sequence, problems of atmospheric instability, Luminous Blue Supergiants, Yellow Hypergiants, Wolf-Rayet stars and supernovae. 相似文献
909.
910.
A. Claret J. Ballet A. Goldwurm F. Lebrun J. P. Roques L. Bouchet M. C. Schmitz-Fraysse P. Mandrou R. Sunyaev E. Churazov M. Gilfanov N. Khavenson A. Dyachkov B. Novikov R. Kremnev V. Kovtunenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):735-738
We report the first detection of two soft gamma ray transients GX 354-0 and Nova Persei (GRO J0422+32) by the coded-mask telescope SIGMA. Only preliminary results are presented with regard to Nova Persei while special emphasis is given to the data on GX 354-0 which has been monitored during the 1992 February–April survey of the Galactic Center. GX 354-0 underwent two flares of about two week duration and its average spectrum is well described by a thermal Bremsstrahlung model or a broken power law whereas the Nova Persei spectrum seems to be similar to the one observed for GRS 1124-684 in Musca. 相似文献