Akiva Bar-Nun

Trapping and release of gases by water ice and implications for icy bodies

Abstract The trapping and release of H 2 , CO, CO 2 , CH 4 , Ar, Ne, and N 2 by amorphous water ice was studied experimentally under dynamic conditions, at low temperatures starting at 16°K, with gas pressure of 5 × 10 −8 −10 −6 Torr. CO, CH 4 , Ar, and N 2 were found to be released in three or four distinct temperature ranges, each resulting from a different trapping mechanism: (a) 30–55°K, where the gas frozen on the water ice evaporates; (b) 135–155°K, where gas is squeezed out of the water ice during the transformation of amorphous ice to cubic ice; (c) 165–190°K, where gas and water are released simultaneously, probably by the evaporation of a clathrate hydrate, and, occasionally (d) 160–175°K, where deeply buried gas is released during the transformation of cubic ice to hexagonal ice. If the third range is indeed due to clathrate formation, CO was found to form this compound. CO 2 did not form a clathrate under the experimental conditions. Excess hydrogen did not affect the occlusion of other gases. Hydrogen itself was trapped only at 16°K. Neon was not trapped at 25°K. With cubic ice, the only trapping mechanism is freezing of gas on the ice surface. No fractionation between the gas phase and the ice was observed with a mixture of CO and Ar. Massive ejection of ice grains was observed during the evaporation of the gas in three (a,c,d) out of the four ranges. The experimental results are used to explain several cometary phenomena, especially those occurring at large heliocentric distances, and are applied also to Titans atmospheric composition and to the possible ejection of ice grains from Enceladus.

67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio

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.

Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko

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.

Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature

Making comets in the cold The speciation of nitrogen compounds in comets can tell us about their history. Comets are some of the most ancient bodies in the solar system and should contain the nitrogen compounds that were abundant when they formed. Using the ROSINA mass spectrometer aboard the Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko, Rubin et al. found molecular nitrogen at levels that are depleted compared to those in the primordial solar system. Depletion of such a magnitude suggests that the comet formed either from the low-temperature agglomeration of pristine amorphous water ice grains or from clathrates. Science, this issue p. 232 Direct measurements of N2 by instruments aboard the Rosetta spacecraft provide clues about the comet’s long history. 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.

Abundant molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko

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.

DEVELOPMENT OF A DUST MANTLE ON THE SURFACE OF AN INSOLATED ICE-DUST MIXTURE : RESULTS FROM THE KOSI-9 EXPERIMENT

Astronomical observations indicate that formation and destruction of dust mantles on cometary nuclei may be the cause for erratic and systematic variations of cometary activity, i.e. emission of dust. A laboratory experiment (KOSI-9) has been performed to study the evolution of a dust mantle on top of a sublimating ice-dust mixture in vacuum. A sample consisting of water ice with a 10% (by weight) admixture of olivine grains has been insolated in three periods at variable intensities from 200 to 1900 W/m2. Both increasing surface temperature of the sample and decreasing gas and particle emissions indicated the formation of a dust mantle during the first period. During the second insolation period after the gas flux had reached a critical value of a few 1021 water molecules m−2 s−1, avalanches of mantle material occurred on the inclined sample surface, broke up the mantle locally, and opened up a fresh icy surface. Enhanced ice and dust particle emission resumed for some time from these spots. A large number of the emitted dust particles were of a fluffy aggregate structure, i.e., they had large cross section to mass ratios compared to compact particles. During the third period the critical gas flux was not reached and no enhanced dust and ice emission was observed. A dry dust mantle of a few millimeters thickness developed during the course of the experiment. Consequences of these findings for cometary scenarios are discussed.

Prebiotic chemicals—amino acid and phosphorus—in the coma of comet 67P/Churyumov-Gerasimenko

The detection of glycine and phosphorus in the coma of 67P shows that comets contain all ingredients to help spark life on Earth. The importance of comets for the origin of life on Earth has been advocated for many decades. Amino acids are key ingredients in chemistry, leading to life as we know it. Many primitive meteorites contain amino acids, and it is generally believed that these are formed by aqueous alterations. In the collector aerogel and foil samples of the Stardust mission after the flyby at comet Wild 2, the simplest form of amino acids, glycine, has been found together with precursor molecules methylamine and ethylamine. Because of contamination issues of the samples, a cometary origin was deduced from the 13C isotopic signature. We report the presence of volatile glycine accompanied by methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko measured by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) mass spectrometer, confirming the Stardust results. Together with the detection of phosphorus and a multitude of organic molecules, this result demonstrates that comets could have played a crucial role in the emergence of life on Earth.

Ejection of H2O, O2, H2 and H from water ice by 0.5–6 keV H+ and Ne+ ion bombardment

Abstract The ejection of H2O, O2, H2 and H from water ice at 30–140 K, bombarded by 0.5–6 keV H+ and Ne+ was studied experimentally. Neon ions in this energy range deposit their energy in the ice by nuclear collisions, whereas with protons of 0.5 to 6 keV the energy deposition mechanism shifts gradually from predominantly nuclear collisions to predominantly electronic processes. The existing theory of nuclear sputtering predicts very well the yield of ejected water molecules and the experimental results in the region of electronic processes agree well with the experimental results of Lanzerotti, Brown and Johnson. However, the major mass loss from water by ion bombardment is via the ejection of O2, H2 and H atoms, which exceed the ejection of water molecules. O2 and H2 production is markedly enhanced at temperatures exceeding ~100 K, whereas H2O and H production are temperature independent, suggesting that O2 and H2 are produced in the bulk of the ice whereas H2O and H atoms are ejected from the surface or near surface layers.

Gas trapping in water ice at very low deposition rates and implications for comets

Abstract The effect of water ice formation temperature and rate of ice deposition on a cold plate on the amount of trapped argon (equivalent to CO), and the ratios of Ar/Kr/Xe trapped in the water ice were studied at 50, 27 and 22 K and at ice formation rates ranging over four orders of magnitude, from 10−1 to 10−5 μm min−1. Contrary to our previous conclusions that cometary ices were formed at 50–60 K, we now conclude that these ices were formed at about 25 K. At 25 K the enrichment ratios for Ar, Kr, and Xe remained the same as those at 50 K, reinforcing our suggestion of cometary contribution of these noble gases to the atmospheres of Earth and Mars.

THE CHARACTERISATION OF TITAN'S ATMOSPHERIC PHYSICAL PROPERTIES BY THE HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT (HASI)

The Huygens Atmospheric Structure Instrument (HASI) is a multi-sensor package which has been designed to measure the physical quantities characterising the atmosphere of Titan during the Huygens probe descent on Titan and at the surface. HASI sensors are devoted to the study of Titans atmospheric structure and electric properties, and to provide information on its surface, whether solid or liquid.