On the Prediction of CO Outgassing from Comets Hale-Bopp and Wirtanen

The gas flux from a volatile icy-dust mixture is computed using a comet nucleus thermal model in order to study the evolution of CO outgassing during several apparitions from long-period Comet Hale-Bopp and short-period Comet Wirtanen. The comet model assumes a spherical, porous body containing a dust component, one major ice component (H₂O), and one minor ice component of higher volatility (CO). The initial chemical composition is assumed to be homogeneous. The following processes are taken into account: heat and gas diffusion inside the rotating nucleus; release of outward diffusing gas from the comet nucleus; chemical differentiation by sublimation of volatile ices in the surface layers and recondensation of gas in deeper, cooler layers. A 2-D time dependent solution is obtained through the dependence of the boundary conditions on the local solar illumination as the nucleus rotates. The model for Comet Hale-Bopp was compared with observational measurements (Biver et al., 1999). The best agreement was obtained for a model with amorphous water ice and CO, assuming that a part of the latter is trapped by the water ice, another part is condensed as an independent ice phase. The model confirms that sublimation of CO ice at large heliocentric distance produces a gradual increase in the comet's activity as it approaches the Sun. Crystallization of amorphous water ice begins at 7 AU from the Sun, but no outbursts were found. Seasonal effects and thermal inertia of the nucleus material lead to larger CO outgassing rates as the comet recedes from the Sun. In the second part of this work the model was run with the orbital parameters of Comet Wirtanen. Unlike Comet Hale-Bopp, the predicted CO outgassing from Comet Wirtanen is almost constant throughout its orbit. Such behavior can be explained by a thermally evolved and chemically differentiated comet nucleus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Loss of the Surface Layers of Comet Nuclei

The Deep Impact observations of low thermal inertia for comet 9P/Tempel 1 are of profound importance for the observations to be made by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. While sub-surface sublimation is necessary to explain the observations, the depth at which this occurs is no more than 2–3 cm and possibly less. The low thermal conductivity when combined with local surface roughness (also observed with Deep Impact) implies that local variations in outgassing rates can be substantial. These variations are likely to be on scales smaller than the resolution limits of all experiments on the Rosetta orbiter. The observed physico-chemical inhomogeneity further suggests that the Rosetta lander will only provide a local snapshot of conditions in the nucleus layer.     

The Rosetta Mission: Flying Towards the Origin of the Solar System

The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will rendezvous in 2014 with comet 67P/Churyumov-Gerasimenko close to its aphelion and will study the physical and chemical properties of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the interaction region of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus. On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia.     

Deep Impact and the Origin and Evolution of Cometary Nuclei

The Deep Impact mission revealed many properties of comet Tempel 1, a typical comet from the Jupiter family in so far as any comet can be considered typical. In addition to the properties revealed by the impact itself, numerous properties were also discovered from observations prior to the impact just because they were the types of observations that had never been made before. The impact showed that the cometary nucleus was very weak at scales from the impactor diameter (～1 m) to the crater diameter (～100 m) and suggested that the strength was low at much smaller scales as well. The impact also showed that the cometary nucleus is extremely porous and that the ice was close to the surface but below a devolatilized layer with thickness of order the impactor diameter. The ambient observations showed a huge range of topography, implying ubiquitous layering on many spatial scales, frequent (more than once a week) natural outbursts, many of them correlated with rotational phase, a nuclear surface with many features that are best interpreted as impact craters, and clear chemical heterogeneity in the outgassing from the nucleus.     

The Deep Impact Earth-Based Campaign

Prior to the selection of the comet 9P/Tempel 1 as the Deep Impact mission target, the comet was not well observed. From 1999 through the present there has been an intensive world-wide observing campaign designed to obtain mission critical information about the target nucleus, including the nucleus size, albedo, rotation rate, rotation state, phase function, and the development of the dust and gas coma. The specific observing schemes used to obtain this information and the resources needed are presented here. The Deep Impact mission is unique in that part of the mission observations will rely on an Earth-based (ground and orbital) suite of complementary observations of the comet just prior to impact and in the weeks following. While the impact should result in new cometary activity, the actual physical outcome is uncertain, and the Earth-based observations must allow for a wide range of post-impact phenomena. A world-wide coordinated effort for these observations is described.     

Alice: The rosetta Ultraviolet Imaging Spectrograph

We describe the design, performance and scientific objectives of the NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will be the first UV spectrograph to study a comet at close range. It is designed to obtain spatially-resolved spectra of Rosetta mission targets in the 700–2050 Å spectral band with a spectral resolution between 8 Å and 12 Å for extended sources that fill its ～0.05^ × 6.0^ field-of-view. ALICE employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a toroidal concave holographic reflection grating. The microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and employs a two-dimensional delay-line readout array. The instrument is controlled by an internal microprocessor. During the prime Rosetta mission, ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and nucleus/coma coupling; during cruise to the comet, ALICE will make observations of the mission's two asteroid flyby targets and of Mars, its moons, and of Earth's moon. ALICE has already successfully completed the in-flight commissioning phase and is operating well in flight. It has been characterized in flight with stellar flux calibrations, observations of the Moon during the first Earth fly-by, and observations of comet C/2002 T7 (LINEAR) in 2004 and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing campaign.     

From Coma Abundances to Nucleus Composition

A major goal of comet research is to determine conditions in the outer solar nebula based on the chemical composition and structure of comet nuclei. The old view was to use coma abundances directly for the chemical composition of the nucleus. However, since the composition of the coma changes with heliocentric distance, r, the new view is that the nucleus composition msut be determined from analysis of coma mixing ratios as a function of r. Taking advantage of new observing technology and the early detection of the very active Comet Hale-Bopp (C/1995 O1) allows us to determine the coma mixing ratios over a large range of heliocentric distances. In our analysis we assume three sources for the coma gas: (1) the surface of the nucleus (releasing water vapor), (2) the interior of the porous nucleus (releasing many species more volatile than water), and (3) the distributed source (releasing gases from ices and hydrocarbon polycondensates trapped and contained in coma dust). Molecules diffusing inside the nucleus are sublimated by heat transported into the interior. The mixing ratios in the coma are modeled assuming various chemical compositions and structural parameters of the spinning nucleus as it moves in its orbit from large heliocentric distance through perihelion. We have combined several sets of observational data of Comet Hale-Bopp for H₂O (from OH) and CO, covering the spectrum range from radio to UV. Many inconsistencies in the data were uncovered and reported to the observers for a reanalysis. Since post-perihelion data are still sparse, we have combined pre- and post-perihelion data. The resulting mixing ratio of CO relative to H₂O as a function of r is presented with a preliminary analysis that still needs to be expanded further. Our fit to the data indicates that the total CO release rate (from the nucleus and distributed sources) relative to that of H₂O is 30% near perihelion. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interrelation between KOSI experiments and comet nucleus sampling

In situ observations of comet Halley yielded information on the nucleus and its environment. These measurements are related to properties of and processes at the nucleus by theoretical modelling and by simulation experiments in the laboratory. The objective of the KOSI (Kometensimulation) experiments is to study in detail processes which occur near the surface of ice-dust mixtures under irradiation by light, like heat transport into the sample, chemical fractionation of sample material, emission of gases, and others. The KOSI experiments are carried out at the large space simulation chamber in Köln. By providing an in-depth understanding of potential cometary processes the results from the KOSI experiments are relevant to any comet nucleus sample return mission.     

A Portrait of the Nucleus of Comet 67P/Churyumov-Gerasimenko

In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two regions for the direction of the rotational axis of the nucleus: RA = 220°^+50° _−30° and Dec = −70^° ± 10^° (retrograde rotation) or RA = 40°^+50° _-30° and Dec = +70^°± 10^° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function, although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel 1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m⁻³, lead to a mass of ~ 8 × 10¹² kg which should ease the landing of Philae and insure the overall success of the Rosetta mission.     

Morphology–Composition–Isotopes: Recent Results from Observations

This article presents some recent imaging and spectroscopic observations that led to results which are significant for understanding the properties of comet nuclei. The coma morphology and/or composition were investigated for 12 comets belonging to different dynamical classes. The data analysis showed that the coma morphology of three non-periodic comets is not consistent with the general assumption that dynamically new comets still have a relatively uniform nucleus surface and therefore do not exhibit gas and/or dust jets in their coma. The determination of carbon and nitrogen isotopic ratios revealed the same values for all comets investigated at various heliocentric distances. However, the relative abundance of the rare nitrogen isotope ¹⁵N is about twice as high as in the Earth’s atmosphere. Observations of comets at splitting events and during outbursts led to indications for differences between material from the nucleus surface and the interior. The monitoring of the induced outburst of 9P/Temple revealed that under non-steady state conditions the fast disintegration of species is detectable.     

Mupus – A Thermal and Mechanical Properties Probe for the Rosetta Lander Philae

MUPUS, the multi purpose sensor package onboard the Rosetta lander Philae, will measure the energy balance and the physical parameters in the near-surface layers – up to about 30 cm depth- of the nucleus of Rosetta’s target comet Churyumov-Gerasimenko. Moreover it will monitor changes in these parameters over time as the comet approaches the sun. Among the parameters studied are the density, the porosity, cohesion, the thermal diffusivity and conductivity, and temperature. The data should increase our knowledge of how comets work, and how the coma gases form. The data may also be used to constrain the microstructure of the nucleus material. Changes with time of physical properties will reveal timescales and possibly the nature of processes that modify the material close to the surface. Thereby, the data will indicate how pristine cometary matter sampled and analysed by other experiments on Philae really is.     

Deep Impact: A Large-Scale Active Experiment on a Cometary Nucleus

The Deep Impact mission will provide the first data on the interior of a cometary nucleus and a comparison of those data with data on the surface. Two spacecraft, an impactor and a flyby spacecraft, will arrive at comet 9P/Tempel 1 on 4 July 2005 to create and observe the formation and final properties of a large crater that is predicted to be approximately 30-m deep with the dimensions of a football stadium. The flyby and impactor instruments will yield images and near infrared spectra (1–5 μm) of the surface at unprecedented spatial resolutions both before and after the impact of a 350-kg spacecraft at 10.2 km/s. These data will provide unique information on the structure of the nucleus near the surface and its chemical composition. They will also used to interpret the evolutionary effects on remote sensing data and will indicate how those data can be used to better constrain conditions in the early solar system.     

Comet Geology with Deep Impact Remote Sensing

The Deep Impact mission will provide the highest resolution images yet of a comet nucleus. Our knowledge of the makeup and structure of cometary nuclei, and the processes shaping their surfaces, is extremely limited, thus use of the Deep Impact data to show the geological context of the cratering experiment is crucial. This article briefly discusses some of the geological issues of cometary nuclei.     

Concepts for the curation,primary examination,and allocation of comet nucleus samples by a comet sample receiving laboratory

The Comet Nucleus Sample Return Mission ROSETTA, a cornerstone mission of ESA jointly planned with NASA, requires the implementation of a highly sophisticated curatorial facility for the returned samples. A concept for the instrumentation and the mode of operation of a Comet Sample Receiving Laboratory (CSRL) is proposed. The main elements of the facility are: (1) cryogenic evacuated cabinets with robotic manipulators, (2) devices for sample dissection, aliquotisation, phase separation, and thin section preparation, and (3) instrumentation for non-destructive chemical and physical analyses and facilities for destructive mineralogical, textural, and (micro)chemical analyses. It is recommended that a very detailed Primary Sample Examination and Analysis be performed on a small representative fraction of the samples at the P-T-conditions of the parent comet nucleus before sample aliquots are released to Principal Investigators. The CSRL should be staffed with top rank personnel and supervised by an international peer review panel which may also be responsible for the selection of investigators and the allocation of samples.     

On the Existence of Distributed Sources in Comet Comae

The main production process of species occurring in the coma of comets is the photodestruction of molecules initially present in the nucleus ices and non-refractory grains or trapped inside the nucleus "material". Grains can also be a source of molecules in the coma. Chemical reactions may occur between coma species. Consequently, although chances that an abundant coma species has not been detected are now small, the coma composition is certainly quite different from that of the nucleus. Except for the molecules released directly at the nucleus surface, all coma species are produced in an "extended region" or come from "a distributed source". Since the recent literature is rich in reports on observations of molecules and species possibly not initially present in the comet ices or not released at the nucleus, a general discussion of how coma species are stored, processed or produced is presented, based mostly on observational results. What is at stake is the proper modeling of the coma structure, hence an accurate derivation of the nucleus composition from coma observations. This revised version was published online in June 2006 with corrections to the Cover Date.     

某型增强型复合材料发射筒强度特性评估

为了评估复合材料发射筒的设计是否满足使用要求,建立了某型复合材料发射筒的三维有限元仿真计算模型,并对该型发射筒在吊装、运输、发射工作状态下的受力与变形情况进行了分析评估,得出该型复合材料发射筒在这些过程中的最大变形,采用最大应力准则推导出复合材料发射筒的最小安全系数.评估结果表明:3种工况下,该型复合材料发射筒的最大变...     

Near-surface stratigraphy in comets inferred from observation,laboratory studies,and modeling

A brief discussion is given of the physical processes that may lead to a differentiation of the nucleus of short period comets. It is concluded that samples from the near-surface layers of such comets may give us important information on the initial state of cometary organics and refractories. Cometary ices are more likely to be indicators of the recent evolution of the comet.