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.

In situ measurements of the physical characteristics of Titan's environment.

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.

Shock synthesis of amino acids in simulated primitive environments.

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.

Xenon isotopes in 67P/Churyumov-Gerasimenko show that comets contributed to Earth's atmosphere.

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.

Cometary science. 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.

Cometary science. 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.

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.

Trapping of gas mixtures by amorphous water ice.

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.

Structure and dynamics of amorphous water ice.

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.

An adequate kinetic model of photochemical aerosol formation in Titan's atmosphere.

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).

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

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.

Shock synthesis of amino acids II.

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.

On the temperature and gas composition in the region of comet formation.

The findings of the Giotto and Vega spacecrafts on the gas composition of comet Halley, together with an experimental study on the trapping of gas mixtures in amorphous water ice, enable estimation of the gas composition and temperature in the region of comet Halley's formation: If Halley was formed in the solar nebula by condensation of water vapor in the presence of gas, in the region of its formation the CO/CH4 ratio had to be at least 100 and the temperature about 48 K. The ice particles that formed the comet could not have condensed at a higher temperature and subsequently cool down because then the 7% CO found as a parent molecule could not have been trapped in the ice. A approximately 48 K formation implies that the ice was in amorphous form. This temperature is surprisingly close to the temperatures observed by IRAS for the circumstellar dust shells around alpha PsA (55 K) and epsilon Eri (45 K) and supports the suggestion that short-period comets were formed outside the region of planet formation. The CO content of comet Halley and sensitivity to explosion of irradiated, ice-coated, interstellar grains seem to exclude the possibility of their direct incorporation into comets. Yet, they might have provided the condensed organics--the "CHON" materials.

The formation of a permanent dust mantle and its effect on cometary activity.

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.

The possible formation of a hydrogen coma around comets at large heliocentric distances.

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.

On the evolution and activity of cometary nuclei.

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.

The mechanisms of amino acids synthesis by high temperature shock-waves.

The mechanisms of amino acids synthesis behind high temperature shock-waves were elucidated and distinction was made between the steps occurring in the gas phase and those occurring in solution. In the presence of water vapor, aldehydes and HCN are formed separately in regions of different temperatures along the reacting gas. The aldehydes and ammonia condense to aldimines which add HCN to form alpha-amino nitriles, all in the gas phase. The hydrolysis to amino acids takes place in solution. In the absence of water vapor, aldimines and HCN are formed in the gas phase but condense to alpha-amino nitriles only in solution. A fair amount of oxygen only lowers the production of amino acids, which consequently could be still produced in the presence of oxygen in the Earth's primitive atmoshere. The waterless mechanism can operate in the Jovian atmosphere and supply it with ample amounts of amino acids, especially aspartic.

Contributions of icy planetesimals to the Earth's early atmosphere.

Laboratory experiments on the trapping of gases by ice forming at low temperatures implicate comets as major carriers of the heavy noble gases to the inner planets. These icy planetesimals may also have brought the nitrogen compounds that ultimately produced atmospheric N2. However, if the sample of three comets analyzed so far is typical, the Earth's oceans cannot have been produced by comets alone, they require an additional source of water with low D/H. The highly fractionated neon in the Earth's atmosphere may also indicate the importance of non-icy carriers of volatiles. The most important additional carrier is probably the rocky material comprising the bulk of the mass of these planets. Venus may require a contribution from icy planetesimals formed at the low temperatures characteristic of the Kuiper Belt.