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.     

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.     

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.     

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.     

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.     

Comments on the analysis of returned comet samples

It is first argued that, when comet sampled are returned, they should be distributed to individual laboratories for analysis in the way that lunar samples, meteorites, and interplanetary dust particles have been studied in the past. The intellectual ferment engendered by recent discoveries should ensure the viability of groups working in extraterrestrial material research into the indefinite future. Many of the recent discoveries have resulted from application of increasingly sophisticated methods of microanalysis. #the interplay between technological developments and scientific work is underscored and it is argued that increased technical support for extraterrestrial material research should lead to instrumental developments that could have widespread practical applications.A brief review of certain potentially relevant technical developments in other fields is given and it is suggested that microanalytic measurements of extraterrestrial samples at the atom-counting limit appear promising for the future. The special problems raised by the necessity for cryogenic examination of comet samples are briefly discussed and it is concluded that the lack of expertise in this area is a current weakness in the ability of the extraterrestrial material community to handle comet samples. Computer tomography scan images of a dirty snowball are presented to illustrate the importance of developing new methods for comet sample analysis.     

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.     

The Coma of Comet 9P/Tempel 1

As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 10² km), it is in collisional equilibrium, at larger distances (≥10⁴ km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics of the coma, as well as with its variability with time, orbital position, and heliocentric distance. The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986. This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards that the coma presents for the spacecraft. An erratum to this article is available at .     

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.     

Distributed Sources in Comets

The distribution of some molecules and radicals (H₂CO, CO, HNC, CN,?…) in the atmosphere of several comets cannot be explained only by a direct sublimation from the nucleus, or by gas phase processes in the coma. Such molecules are in part the result of a distributed source in the coma, which could be the photo and thermal degradation of dust. We present a review of the degradation processes and discuss possible interpretations of the observations in which the degradation of solid complex organic material in dust particles seems to play a major role. The knowledge of such gas production mechanisms provides important clues on the chemical nature of the refractory organic material contained in comet nuclei.     

Virtis: An Imaging Spectrometer for the Rosetta Mission

The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison – using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied.     

Structural evolution of cometary surfaces

Comets with a high content of organics and light molecules are expected under cosmic radiation to gain a relatively unreactive crust and less volatile material to some ten metres deep. Interstellar dust impacts act to loosen and turn over 1 cm of the surface. We discuss how far this accords with observations of cometary dust halos and new versus old comets. Two key material properties have emerged from recent studies. Firstly, the source of cometary volatiles is not ice in the sense of material with a single sublimation energy. Secondly, the particulates are not simply mineral dust but include much organic material, some of which undergoes chemical processing and exchanges with the gaseous environment. Consistent with these properties, a coherent crust rather than a mantle of loose grains would build up to cover much of the nucleus of periodic comets. It would consolidate by cooking in the solar radiation, especially at peak temperatures around perihelion. There are two disjoint surface phases: one of volatile material, the other the refractory crust, the former deepening into crater-like hollows over successive apparitions. The transition to non-volatile crust is unstable, subject to competing consolidation and disruption processes, and sensitive to seasonal changes. A comet dims and becomes asteroidal as the inert crust extends over the erosion craters, and may only be rejuvenated via collision with a boulder-sized impactor or perturbation of the orbit to smaller perihelion distance.     

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.     

COTS based open systems for military avionics

The DoD has many acquisition programs that are aggressively implementing open architecture principles in new avionics systems. Since “open” is an unclear attribute, projects eventually give in to a point solution that has no flexibility to cost effectively keep up with rapid changes in technology. The Open Systems Development Initiative (OSDI) project utilized COTS products to study the feasibility of building an open system that has plug-and-play capabilities. Lessons learned from the AV-8B Open Systems Core Avionics Requirements (OSCAR) and the F/A-18 Advanced Mission Computers and Displays (AMC&D) programs clearly indicated that understanding the underlying interfaces is crucial to keeping the system as open as possible to take advantage of the rapid changes in technology. A matrix of Key Open Standard Interfaces (KOSI), called the KOSI matrix, was developed and an applicable standard was identified for each interface. A list of non-conforming interfaces was also identified and the use of extensions or wrappers was investigated in an attempt to comply with standards. Standardization, rather than optimization of such interfaces, was considered more beneficial. It became evident that, with the exception of ruggedization, there is no difference in the use of COTS products for either commercial or military systems. Performing a KOSI analysis helped identify the key interfaces and standards, thus enabling the OSDI system to be scalable, portable and interoperable. A good KOSI matrix provides a vehicle for clear communication and helps systems integration and technology insertion to be less painful than what it is today. It helps reduce time-to-market and provides guidance to systems engineers and vendors to keep the system open     

Expectations for Infrared Spectroscopy of 9P/Tempel 1 from Deep Impact

The science payload on the Deep Impact mission includes a 1.05–4.8 μm infrared spectrometer with a spectral resolution ranging from R∼200–900. The Deep Impact IR spectrometer was designed to optimize, within engineering and cost constraints, observations of the dust, gas, and nucleus of 9P/Tempel 1. The wavelength range includes absorption and emission features from ices, silicates, organics, and many gases that are known to be, or anticipated to be, present on comets. The expected data will provide measurements at previously unseen spatial resolution before, during, and after our cratering experiment at the comet 9P/Tempel 1. This article explores the unique aspects of the Deep Impact IR spectrometer experiment, presents a range of expectations for spectral data of 9P/Tempel 1, and summarizes the specific science objectives at each phase of the mission.     

Deep Impact: Working Properties for the Target Nucleus – Comet 9P/Tempel 1

In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus (summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter. Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological, geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values (circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25± 0.2 km, shape – irregular triaxial ellipsoid with a/b = 3.2± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85± 0.1 hr, pole directions (RA, Dec, J2000) = 46± 10, 73± 10 deg (Pole 1) or 287± 14, 16.5± 10 deg (Pole 2) (the two poles are photometrically, but not geometrically, equivalent), Kron-Cousins (V-R) color = 0.56± 0.02, V-band geometric albedo = 0.04± 0.01, R-band geometric albedo = 0.05± 0.01, R-band H(1,1,0) = 14.441± 0.067, and mass ∼7×10¹³ kg assuming a bulk density of 500 kg m⁻³. As these are working values, {i.e.}, based on preliminary analyses, it is expected that adjustments to their values may be made before encounter as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is estimated at 0.027–0.04 cm s⁻² and the escape velocity between 1.4 and 2 m s⁻¹. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7 deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain uncertain whether the impactor will collide with the broadside or the ends of the nucleus.     

Review of the CIV phenomenon

Alfvén's Critical Ionization Velocity (CIV) phenomenon is reviewed, with the main emphasis on comparisons between experimental and theoretical results. The review covers (1) the velocity measurements in laboratory experiments, (2) the effect of wall interaction, (3) the experimental and theoretical limits to the magnetic field strength and the neutral density, (4) ionospheric release experiments, (5) theoretical models for electron energization in comparison to experimental results, and (6) CIV models. All laboratory investigations of the CIV are found to obey the three following simple rules of thumb: (1) if the magnetic field is so strong that V _A > 3V ₀, and if there is enough neutral gas that the Townsend condition is fulfilled, then the CIV effect occurs, (2) when it occurs, the threshold velocity (or E/B value) is within ± 50% of Alfvén's proposed value V _c , and (3) for weaker magnetic fields, the effect gradually becomes irreproducible or weak and disappears altogether for V _A < V ₀. The theoretical understanding of the process has grown rapidly during the last decade, mainly due to the introduction of computer simulation models which have to a large degree confirmed and extended earlier analytical theories. The CIV mechanism is not due to one single plasma process, but to several different mechanisms which are applicable in different parameter regimes and geometries. The computer simulations have shown that in order to understand the mechanism properly it is necessary to consider a large number of interlocking collisional and plasma processes. The theoretical development has reached the stage where it should be possible to adapt computer simulation models to specific experiments and predict ionization rates, plasma flow velocities, E/B values, particle distributions, and wave spectra. Such models should for the first time provide a really firm basis for extrapolations of the CIV process to space applications.     

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.     

Origins of Cometary Materials

In this introductory presentation, material is categorized according to our state of knowledge: What do we know, what do we think we know but don’t know certainly, and what do we not know but often describe it as if it were a well-established fact about comets, their nuclei, their composition, and processes within comets and their nuclei. The material is presented not with the intend to criticize laboratory work simulating condition in comet nuclei, or observers analyzing their observations, nor modelers using data from both these sources to improve our understanding and make predictions. The intent is to provoke discussion and dialog between these groups to avoid overstating the results. What is a Comet? A comet is a diffuse appearing celestial phenomenon moving in an orbit about the Sun. The central body, the nucleus, is composed of ice and dust. It is the source of all cometary activity, including comae and tails. We distinguish between molecular (including atoms and ions) and dust comae. At heliocentric distances of about 1 AU and less, the hydrogen coma typically has dimensions larger than the Sun. The tails are composed of dust, neutral atoms and molecules, and plasma.