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1.
Nature ; 526(7575): 678-81, 2015 Oct 29.
Artigo em Inglês | MEDLINE | ID: mdl-26511578

RESUMO

The composition of the neutral gas comas of most comets is dominated by H2O, CO and CO2, typically comprising as much as 95 per cent of the total gas density. In addition, cometary comas have been found to contain a rich array of other molecules, including sulfuric compounds and complex hydrocarbons. Molecular oxygen (O2), however, despite its detection on other icy bodies such as the moons of Jupiter and Saturn, has remained undetected in cometary comas. Here we report in situ measurement of O2 in the coma of comet 67P/Churyumov-Gerasimenko, with local abundances ranging from one per cent to ten per cent relative to H2O and with a mean value of 3.80 ± 0.85 per cent. Our observations indicate that the O2/H2O ratio is isotropic in the coma and does not change systematically with heliocentric distance. This suggests that primordial O2 was incorporated into the nucleus during the comet's formation, which is unexpected given the low upper limits from remote sensing observations. Current Solar System formation models do not predict conditions that would allow this to occur.


Assuntos
Meteoroides , Oxigênio/análise , Monóxido de Carbono/análise , Meio Ambiente Extraterreno/química , Gelo/análise , Nitrogênio/análise , Oxigênio/efeitos da radiação , Fotólise , Sistema Solar/química , Astronave , Água/análise
2.
Nature ; 438(7069): 785-91, 2005 Dec 08.
Artigo em Inglês | MEDLINE | ID: mdl-16319827

RESUMO

On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.

3.
Science ; 168(3930): 470-3, 1970 Apr 24.
Artigo em Inglês | MEDLINE | ID: mdl-5436082

RESUMO

A mixture of gases roughly simulating the primitive terrestrial atmosphere has been subjected to shock heating followed by a rapid thermal quench. Under strictly homogeneous conditions there is a very high efficiency of 5 x 10(10) molecules per erg of shock-injected energy for production of alpha-amino acids. Calculations suggest that rapid quenching bypasses the usual thermochemical barrier. The product of energy flux and efficiency implies the unexpected conclusion that shocks occurring on atmospheric entry of cometary meteors and micrometeorites and from thunder may have been the principal energy sources for pre-biological organic synthesis on the primitive earth.


Assuntos
Aminoácidos/síntese química , Amônia , Metano , Eletricidade , Gases , Pressão , Temperatura
4.
Science ; 356(6342): 1069-1072, 2017 06 09.
Artigo em Inglês | MEDLINE | ID: mdl-28596364

RESUMO

The origin of cometary matter and the potential contribution of comets to inner-planet atmospheres are long-standing problems. During a series of dedicated low-altitude orbits, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta spacecraft analyzed the isotopes of xenon in the coma of comet 67P/Churyumov-Gerasimenko. The xenon isotopic composition shows deficits in heavy xenon isotopes and matches that of a primordial atmospheric component. The present-day Earth atmosphere contains 22 ± 5% cometary xenon, in addition to chondritic (or solar) xenon.

5.
Science ; 347(6220): 1261952, 2015 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-25501976

RESUMO

The provenance of water and organic compounds on Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the deuterium-to-hydrogen (D/H) ratios in the reservoirs for comets and Earth's oceans. Here, we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the European Space Agency's Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10(-4)­that is, approximately three times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean-like water.

6.
Science ; 348(6231): 232-5, 2015 Apr 10.
Artigo em Inglês | MEDLINE | ID: mdl-25791084

RESUMO

Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta spacecraft. A N2/CO ratio of (5.70 ± 0.66) × 10(-3) (2σ standard deviation of the sampled mean) corresponds to depletion by a factor of ~25.4 ± 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ~30 kelvin.

7.
Science ; 347(6220): aaa0276, 2015 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-25613892

RESUMO

Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well-sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.

9.
Phys Rev B Condens Matter ; 36(17): 9219-27, 1987 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-9942788

RESUMO

Further insight into the structure and dynamics of amorphous water ice, at low temperatures, was obtained by trapping in it Ar, Ne, H2, and D2. Ballistic water-vapor deposition results in the growth of smooth, approximately 1 x 0.2 micrometer2, ice needles. The amorphous ice seems to exist in at least two separate forms, at T < 85 K and at 85 < T < 136.8 K, and transform irreversibly from one form to the other through a series of temperature-dependent metastable states. The channels formed by the water hexagons in the ice are wide enough to allow the free penetration of H2 and D2 into the ice matrix even in the relatively compact cubic ice, resulting in H2-(D2-) to-ice ratios (by number) as high as 0.63. The larger Ar atoms can penetrate only into the wider channels of amorphous ice, and Ne is an intermediate case. Dynamic percolation behavior explains the emergence of Ar and Ne (but not H2 and D2) for the ice, upon warming, in small and big gas jets. The big jets, each containing approximately 5 x 10(10) atoms, break and propel the ice needles. Dynamic percolation also explains the collapse of the ice matrix under bombardment by Ar , at a pressure exceeding 2.6 dyn cm-2, and the burial of huge amounts of gas inside the collapsed matrix, up to an Ar-to-ice of 3.3 (by number). The experimental results could be relevant to comets, icy satellites, and icy grain mantles in dense interstellar clouds.


Assuntos
Argônio/química , Deutério/química , Meio Ambiente Extraterreno , Hidrogênio/química , Gelo/análise , Neônio/química , Fenômenos Astronômicos , Astronomia , Monóxido de Carbono/química , Metano/química , Modelos Químicos , Nitrogênio/química , Temperatura
10.
Phys Rev B Condens Matter ; 38(11): 7749-54, 1988 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-9945502

RESUMO

Our studies on gas trapping in amorphous water ice at 24-100 K were extended, by using mixtures of CH4, CO, N2, and Ar, rather than single gases. In 1:1 gas:(water vapor) mixtures, the competition among these gases on the available sites in the ice showed that the trapping capacity for the various gases is determined not only by the structure and dynamics of the ice, but is also influenced by the gas itself. Whereas at 24-35 K all four gases are trapped in the ice indiscriminantly, at 50-75 K there is a clear enhancement, in the order of CH4 > CO > N2 > or approximately Ar. This order is influenced by the gas-water interaction energy, the size of the trapped gas atom or molecule, the type of clathrate-hydrate formed (I or II) and, possibly, other factors. It seems that the gas can be trapped in the amorphous ice in several different locations, each being affected in a different way by the deposition temperature and gas composition. Once a gas atom or molecule is trapped in a specific location, it is predestined to emerge in one of eight different temperature ranges, which are associated with changes in the ice. The experimentally observed enhancements, together with the findings on the gas composition of comet Halley, might enable an estimation of the gas composition in the region of comet formation.


Assuntos
Argônio/análise , Monóxido de Carbono/análise , Gelo/análise , Metano/análise , Nitrogênio/análise , Água/análise , Fenômenos Astronômicos , Astronomia , Meio Ambiente Extraterreno , Gases/análise , Meteoroides , Temperatura
11.
Adv Space Res ; 19(7): 1103-12, 1997.
Artigo em Inglês | MEDLINE | ID: mdl-11541339

RESUMO

The photochemistry of hydrocarbons in Titan's atmosphere is modeled by a comprehensive kinetic scheme, containing 732 elementary reactions and 147 species up to C60. Four groups of the hydrocarbons are considered: Polyacetylenes (PA), Polyvinyles (PV), Vinylacetylenes (VA) and Allenes (Polyenes).


Assuntos
Aerossóis/química , Atmosfera/química , Hidrocarbonetos/química , Modelos Químicos , Saturno , Meio Ambiente Extraterreno , Cinética , Fotoquímica
12.
Adv Space Res ; 7(5): 45-7, 1987.
Artigo em Inglês | MEDLINE | ID: mdl-11538219

RESUMO

The trapping of various gases by water ice at low temperatures (20-80K) and their release from the ice upon warming, was studied experimentally. The results of these experiments, together with a computation of the thermal evolution of a cometary nucleus, can explain the gas and dust jets which were observed to emanate from the nucleus of P/Halley. The experimental results are important also to the gas content of Titan.


Assuntos
Argônio , Gases , Gelo , Meteoroides , Saturno , Acetileno , Dióxido de Carbono , Monóxido de Carbono , Meio Ambiente Extraterreno , Cianeto de Hidrogênio , Metano , Nitrogênio , Temperatura , Água
13.
Phys Rev B Condens Matter ; 35(5): 2427-2435, 1987 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-9941692
14.
Orig Life ; 6(1-2): 109-15, 1975.
Artigo em Inglês | MEDLINE | ID: mdl-168532

RESUMO

The synthesis of amino acids behind shock waves in methane, ethane, ammonia, and water vapor was further investigated. Aldehydes and HCN are formed separately in the non-homogeneous gas during the high temperature period and recombine with ammonia during the thermal quench period, to form chi-amino nitriles. The chi-amino nitriles are either hydrolyzed by excess water vapor during the quench period or emerge as such after the reaction is completed. A combined gas chromatograph-mass spectrometer analysis of the reaction product showed identical amounts of D and L amino acids, thus confirming the absence of contaminants. Thunder shock waves were shown to be a suitable source of energy for the production of amino acids.


Assuntos
Aminoácidos/síntese química , Temperatura Alta , Aldeídos/síntese química , Amônia , Etano , Cianeto de Hidrogênio/síntese química , Metano , Nitrilas/síntese química , Fenômenos Físicos , Física , Água
15.
Astrophys J ; 313(2): 893-905, 1987 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-11542134

RESUMO

The thermal evolution of a spherical cometary nucleus (initial radius of 2.5 km), composed initially of very cold amorphous ice and moving in comet Halley's orbit, is simulated numerically for 280 revolutions. It is found that the phase transition from amorphous to crystalline ice constitutes a major internal heat source. The transition does not occur continuously, but in five distinct rounds, during the following revolutions: 1, 7, 40-41, 110-112, and 248-252. Due to the (slow) heating of the amorphous ice between crystallization rounds, the phase transition front advances into the nucleus to progressively greater depths: 36 m on the first round, and then 91 m, 193 m, 381 m, and 605 m respectively. Each round of crystallization starts when when the boundary between amorphous and crystalline ice is brought to approximately 15 m below the surface, as the nucleus radius decreases due to sublimation. At the time of crystallization, the temperature of the transformed ice rises to 180 K. According to experimental studies of gas-laden amorphous ice, a large fraction of the gas trapped in the ice at low temperatures is released. Whereas some of the released gas may find its way out through cracks in the crystalline ice layer, the rest is expected to accumulate in gas pockets that may eventually explode, forming "volcanic calderas." The gas-laden amorphous ice thus exposed may be a major source of gas and dust jets into the coma, such as those observed on comet Halley by the Giotto spacecraft. The activity of new comets and, possibly, cometary outbursts and splits may also be explained in terms of explosive gas release following the transition from amorphous to crystalline ice.


Assuntos
Gelo , Meteoroides , Modelos Químicos , Temperatura , Cristalização , Evolução Planetária , Gases , Matemática
16.
Icarus ; 74: 272-83, 1988.
Artigo em Inglês | MEDLINE | ID: mdl-11538225

RESUMO

The growth of a permanent, permeable, dust mantle on the surface of a comet nucleus, composed initially of dusty amorphous water ice, is investigated. Numerical simulations of the evolution of one-dimensional comet nucleus models, in Comet Halley's orbit, are carried out for various parameters, allowing for the crystallization of the amorphous ice. It is assumed that the mantle forms gradually, by the accumulation of a constant fraction (0.001-0.01) of the dust, which is not carried away with the sublimating ice. It is found that an approximately 1-cm-thick dust mantle diminishes the average sublimation rate by a factor of approximately 5, and a further growth of the dust mantle may decrease the surface activity of the nucleus by another factor of 10. Therefore, the activity of a dust-covered nucleus is expected to result mainly from exposed patches of ice and from craters, such as were observed on Comet Halley by Giotto. These are formed by explosions of gas-filled pockets in the crystalline outer layer of the nucleus. The insulating effect of the dust mantle causes the crystallization of the amorphous ice to proceed at a slower rate than in the case of a bare icy nucleus. Thus, the thickness of the outer crystalline shell, overlying the amorphous ice core, is always greater than 15 m, but does not exceed a few tens of meters. This size range is compatible with the amount of gas released in the numerous small explosions which were observed on Comet Halley.


Assuntos
Poeira Cósmica/análise , Meio Ambiente Extraterreno , Meteoroides , Cristalização , Temperatura Alta , Gelo/análise , Modelos Teóricos , Água/análise
17.
Astrophys J ; 324(1): L31-4, 1988 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-11538469

RESUMO

An observational test--the detection of a hydrogen coma around comets at large heliocentric distances--is proposed for determining whether comets were formed by the agglomeration of unaltered, ice-coated, interstellar grains. Laboratory experiments showed that amorphous water ice traps H2, D2, and Ne below 20 K and does not release them completely until the ice is heated to 150 K. Gas/ice ratios as high as 0.63 are obtainable. Thus, if the ice-coated interstellar grains were not heated above approximately 110 K, prior to their agglomeration into cometary nuclei, the inward propagating heat waves should release from the comets a continuous flux of molecular hydrogen. This flux would exceed that of water molecules at approximately 3 AU preperihelion and approximately 4 AU postperihelion.


Assuntos
Astronomia , Meio Ambiente Extraterreno , Hidrogênio/análise , Meteoroides , Fenômenos Astronômicos , Deutério/análise , Poeira/análise , Gelo/análise , Modelos Teóricos , Neônio/análise , Temperatura , Água/análise
18.
Astron Astrophys ; 258: L9-12, 1992.
Artigo em Inglês | MEDLINE | ID: mdl-11538062

RESUMO

The post-perihelion eruption of comet P/Halley, detected in Feb. 1991 and believed to have started 3 months earlier, can be explained by crystallization of amorphous ice taking place in the interior of the porous nucleus, at depths a few tens of meters, accompanied by the release of trapped gases. Numerical calculations show that for a bulk density of 0.5 g cm-3 and a pore size of 1 millimicron crystallization occurs on the outbound leg of comet P/Halley's orbit, at heliocentric distances between 5 AU and 17 AU. The trapped gas is released and flows to the surface through the porous medium. It may also open wider channels, as the internal pressures obtained surpass the tensile strength of cometary ice. The outflowing gas carries with it grains of ice and dust, and thus can explain the large amounts of dust observed in the coma at 14.3 AU and beyond. The typical decline time of the process is found to be on the order of months, in agreement with observations. The rate of outgassing is two or three orders of magnitude higher than in quiescence. In an asymmetric, non-uniform nucleus--in contrast to the one-dimensional spherical model--the process should occur intermittently, such as was observed for comet P/Halley beyond 5 AU.


Assuntos
Gelo , Sistema Solar , Fenômenos Astronômicos , Astronomia , Cristalização , Poeira , Gases , Modelos Teóricos
19.
Icarus ; 116: 215-26, 1995.
Artigo em Inglês | MEDLINE | ID: mdl-11539473

RESUMO

We are proposing a model for the delivery of volatiles to the inner planets by icy planetesimals (comets). Laboratory studies of the trapping of gases in ice forming at low temperatures simulate the formation of comet nuclei at various distances from the Sun in the solar nebula. The total gas content as well as the relative proportions of gases trapped in the ice are strong functions of temperature. As they trap N2 inefficiently, all planetesimals formed interior to Neptune are deficient in nitrogen, acquiring values of C/N resembling those found in the inner planet volatile inventories. A mixture of three basic types of comets appears capable of accounting for the observed volatile inventories on Venus, Earth, and Mars, with the caveat that impact erosion is necessary to explain the present condition of the martian atmosphere. The model includes the possibility of several epochs of clement conditions on early Mars. Some tests of these ideas are suggested, including measurements in Jupiter's atmosphere by the Galileo probe.


Assuntos
Atmosfera/análise , Monóxido de Carbono/análise , Evolução Planetária , Meteoroides , Nitrogênio/análise , Gases Nobres/análise , Monóxido de Carbono/química , Planeta Terra , Meio Ambiente Extraterreno , Gases/análise , Júpiter , Marte , Nitrogênio/química , Saturno , Vênus
20.
Earth Moon Planets ; 72: 425-32, 1996.
Artigo em Inglês | MEDLINE | ID: mdl-11539471

RESUMO

The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe.


Assuntos
Atmosfera/análise , Criptônio , Meteoroides , Gases Nobres/análise , Xenônio , Argônio , Planeta Terra , Meio Ambiente Extraterreno , Marte , Neônio , Vênus
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