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Register a new userFUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov-Gerasimenko
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Paul D. Feldman
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The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H2O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H2O, CO2, and O2. CO emissions were found to be due to both electron and photoexcitation processes. Thus we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and CO2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.
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Comet 67P/Churyumov-Gerasimenko is the main target of ESAs Rosetta mission and will be encountered in May 2014. As the spacecraft shall be in orbit the comet nucleus before and after release of the lander {it Philae}, it is necessary necessary to know the conditions in the coma. Study the dust environment, including the dust production rate and its variations along its preperihelion orbit. The comet was observed during its approach to the Sun on four epochs between early-June 2008 and mid-January 2009, over a large range of heliocentric distances that will be covered by the mission in 2014. An anomalous enhancement of the coma dust density was measured towards the comet nucleus. The scalelength of this enhancement increased with decreasing heliocentric distance of the comet. This is interpreted as a result of an unusually slow expansion of the dust coma. Assuming a spherical symmetric coma, the average amount of dust as well as its ejection velocity have been derived. The latter increases exponentially with decreasing heliocentric distance (rh), ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these results we describe the dust environment at those nucleocentric distances at which the spacecraft will presumably be in orbit. Astronomy and Astrophysics, in press
The ratios of the three stable oxygen isotopes 16O, 17O and 18O on Earth and, as far as we know in the solar system, show variations on the order of a few percent at most, with a few outliers in meteorites. However, in the interstellar medium there are some highly fractionated oxygen isotopic ratios in some specific molecules. The goal of this work is to investigate the oxygen isotopic ratios in different volatile molecules found in the coma of comet 67P/Churyumov-Gerasimenko and compare them with findings from interstellar clouds in order to assess commonalities and differences. To accomplish this goal, we analyzed data from the ROSINA instrument on Rosetta during its mission around the comet. 16O/18O ratios could be determined for O2, methanol, formaldehyde, carbonyl sulfide and sulfur monoxide/dioxide. For O2 the 16O/17O ratio is also available. Some ratios are strongly enriched in the heavy isotopes, especially for sulfur bearing molecules and formaldehyde, whereas for methanol the ratios are compatible with the ones in the solar system. O2 falls in-between, but its oxygen isotopic ratios clearly differ from water, which likely rules out an origin of O2 from water, be it by radiolysis, dismutation during sublimation or the Eley-Rideal process from water ions hitting the nucleus as postulated in the literature.
The ROSINA mass spectrometer DFMS on board ESAs Rosetta spacecraft detected the major isotopes of the noble gases argon, krypton, and xenon in the coma of comet 67P/Churyumov-Gerasimenko. Earlier, it has been shown that xenon exhibits an isotopic composition distinct from anywhere else in the solar system. However, argon isotopes, within error, were shown to be consistent with solar isotope abundances. This discrepancy suggested an additional exotic component of xenon in comet 67P/Churyumov-Gerasimenko. Here we show that also krypton exhibits an isotopic composition close to solar. Furthermore, we found a depletion compared to solar of argon with respect to krypton and of krypton with respect to xenon, which is a necessity to postulate an addition of exotic xenon in the comet.
We analyze the physical properties and dynamical origin of a curved jet of comet 67P/Churyumov-Gerasimenko that was observed repeatedly in several nucleus rotations starting on May 30 and persisting until early August, 2015. We simulated the motion of dust grains ejected from the nucleus surface under the influence of the gravity and viscous drag effect of the expanding gas flow from the rotating nucleus. The formation of the curved jet is a combination of the size of the dust particles (~0.1-1 mm) and the location of the source region near the nucleus equator. This enhances the spiral feature of the collimated dust stream after the dust is accelerated to a terminal speed on the order of m/s.
Magnetohydrodynamics simulations have been carried out in studying the solar wind and cometary plasma interactions for decades. Various plasma boundaries have been simulated and compared well with observations for comet 1P/Halley. The Rosetta mission, which studies comet 67P/Churyumov-Gerasimenko, challenges our understanding of the solar wind and comet interactions. The Rosetta Plasma Consortium observed regions of very weak magnetic field outside the predicted diamagnetic cavity. In this paper, we simulate the inner coma with the Hall magnetohydrodynamics equations and show that the Hall effect is important in the inner coma environment. The magnetic field topology becomes complex and magnetic reconnection occurs on the dayside when the Hall effect is taken into account. The magnetic reconnection on the dayside can generate weak magnetic filed regions outside the global diamagnetic cavity, which may explain the Rosetta Plasma Consortium observations. We conclude that the substantial change in the inner coma environment is due to the fact that the ion inertial length (or gyro radius) is not much smaller than the size of the diamagnetic cavity.
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