%final version of CONTOUR coma paper \documentclass[namedreferences]{kluwer} \usepackage{graphicx} \bibliographystyle{klunamed} \newcommand{\iue}{{\em IUE}\/} \newcommand{\soho}{{\em SOHO}\/} \newcommand{\etal}{{et al.}\/} \newcommand{\Htwo}{\mbox{H$_{2}$}} \newcommand{\Htwoo}{\mbox{H$_{2}$O}} \newcommand{\Ntwo}{\mbox{N$_{2}$}} \newcommand{\ctwo}{\mbox{C$_{2}$}} \newcommand{\cthree}{\mbox{C$_{3}$}} \newcommand{\cotwo}{\mbox{CO$_{2}$}} \newcommand{\cotwop}{\mbox{CO$_{2}\!^{+}$}} \newcommand{\co}{\mbox{CO}} \newcommand{\cop}{\mbox{CO$^{+}$}} \newcommand{\ie}{{\it i.e.,}} \newcommand{\eg}{{\it e.g.,}} \newcommand{\cf}{cf.,} \newcommand{\asig}{$A\,^2\Sigma ^{+}$} \newcommand{\xpi}{$X\,^2\Pi$} \newcommand{\kms}{km~s$^{-1}$} \newcommand{\apj}{Astrophys. J.\/} \newcommand{\apjl}{Astrophys. J.\/} \newcommand{\aj}{Astron. J.\/} \newcommand{\aap}{Astron. Astrophys.\/} \newcommand{\mnras}{Mon. Not. Roy. Astron. Soc.\/} \newcommand{\jrasc}{J. Roy. Astron. Soc. Canada\/} \def\deg{\hbox{$^\circ$}} \def\mols{molecules s$^{-1}$} \def\Hone{H\,{\sc i}} \def\Oone{O\,{\sc i}} \begin{document} \begin{article} \begin{opening} \title{The Coma of Comet 2P/Encke} \author{Paul D. \surname{Feldman}\email{pdf@pha.jhu.edu}} \institute{Department of Physics and Astronomy, The Johns Hopkins University, Charles and 34th Streets, Baltimore, Maryland 21218} \author{Anita L. \surname{Cochran}\email{anita@barolo.as.utexas.edu}} \institute{The University of Texas McDonald Observatory, Austin, Texas 78712} \author{Andrew F. \surname{Cheng}\email{Andy.Cheng@jhuapl.edu}} \institute{The Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel Maryland 20723} \author{Michael J. S. \surname{Belton}\email{belton@azstarnet.com}} \institute{Belton Space Exploration Initiatives, LLC, 430 S. Randolph Way, Tucson, Arizona 85716} \runningtitle{The Coma of Comet 2P/Encke} \runningauthor{Feldman \etal} \begin{abstract} Space and ground-based observations of comet 2P/Encke made during the past thirty years together with recent modeling have provided a qualitative understanding of the nature and temporal evolution of the gaseous coma of the comet. Details of the interface between coma and nucleus remain obscure. In this context we outline the objectives and approach of the CONTOUR coma imaging investigation. \end{abstract} \end{opening} \newpage \section{Introduction} Periodic comet 2P/Encke, the first comet to be visited by CONTOUR, is perhaps the most studied comet of recent times (for an excellent historical review see \inlinecite{Sekanina:1991b} as well as the paper by Yeomans and Belton in this volume). Its short period (3.3 yr) and relatively stable orbit provided ample opportunity to measure the effects of ``non-gravitational'' forces on its orbit, the analysis of which in terms of asymmetric outgassing of the nucleus, played a pivotal role in the development of the dirty ice conglomerate model of the comet nucleus by \inlinecite{Whipple:1950}. The importance of water ice in the nucleus was subsequently confirmed by observations from above the terrestrial atmosphere of the dissociation products, H \cite{Bertaux:1973} and OH. Measurements of \Hone\ Lyman-$\alpha$ at 1216 \AA\ and OH fluorescence at 3085~\AA\ have continued to provide information about the water production rates of many comets, both short-period comets such as Encke, as well as for comets entering the inner solar system for the first time. These are supplemented by radio observations of OH at 18 cm, spectroscopy and narrow-band imaging of \Oone\ $^1D-^3P$ emission at 6300 and 6364 \AA, oxygen atoms in the $^1D$ state being another water dissociation product, and, more recently, direct observations of \Htwoo\ in the infrared, in many cases providing long term monitoring of the evolution of cometary activity over an entire apparition. However, at the spatial resolution of even the best current ground or space-based telescopes, the details of the mechanisms by which gas and dust are released from the nucleus remain obscure. Evidence from the spacecraft fly-bys of periodic comets Halley and Borrelly and from observations of other recent comets suggests that only a small fraction (10--20\%) of the sunlit surface of a comet nucleus is active, with periodic comets having much smaller active areas than long-period dynamically new comets \cite{Sekanina:1991a}. Imaging of the gas and dust close to the nucleus in order to elucidate these mechanisms is the principal objective of the CONTOUR coma imaging investigation. The CONTOUR Forward Imager (CFI) has the capability to obtain spectral images of gas emission using narrow band filters. Our focus will be on OH fluorescence in the \asig\ -- \xpi\ (0,0) band at 3085 \AA\ as a measure of \Htwoo\ outgassing and monitoring of CN (3883 \AA) and \ctwo\ (4850 \AA) emission associated with dust jets. Near the nucleus (within 50--100 km, depending on the heliocentric velocity of the comet), prompt emission of the OH (0,0) band from dissociative excitation of \Htwoo\ by solar extreme ultraviolet radiation dominates fluorescence and provides a direct tracer for \Htwoo\ escaping from the nucleus \cite{Bertaux:1986,Budzien:1991}. \section{The Gas Coma of Comet 2P/Encke} The visible coma of comet Encke (\eg\ \inlinecite{Djorgovski:1985}) shows a strong sunward fan that is attributed to localized outgassing from the nucleus \cite{Festou:2000}. Except very close to the nucleus, the images of the optical fan do not contain much reflected sunlight from micron-sized grains, but rather gas emissions at the wavelengths of the \ctwo, \cthree, and CN radicals. In this case, the visual light curve can be related directly to the production rates of these minor species. OH similarly displays an asymmetric coma \cite{Feldman:1984} which \citeauthor{Festou:2000} have modeled with the same geometry as for the optical fan. However, they caution that the \Htwoo\ production rate that they derive is $\sim30$\% lower than earlier published values based on spherically symmetric models. A visible image taken in November 1980 by S. M. Larson (from \inlinecite{Feldman:1984}) is shown in Figure~\ref{encke_image}. The aysmmetric distribution of the gas and dust in Encke's coma is strikingly illustrated by spatial profiles derived from long-slit spectroscopy shown in Figure~\ref{distribution}. \begin{figure}[t] \centerline{\includegraphics[width=3.5in]{encke_80.ps}} %\vspace*{-0.25in} \caption{Visible image of comet Encke taken by S. M. Larson on November 4, 1980. A star is located just northeast of the location of the comet's nucleus. From Feldman \etal\ (1984).} \label{encke_image} \end{figure} \begin{figure}[t] \centerline{\includegraphics[width=4.25in]{distribution.ps}} \vspace*{-0.25in} \caption{The band intensity of the gas and the average continuum flux were measured on 28 August 1990 with a long slit CCD spectrograph at McDonald Observatory. The mentioned asymmetries are quite apparent for the three gas species though the degree of asymmetry is different for different molecules. Note that the dust is confined to the very inner coma. Representative error bars are included on the left-most data point for each species except CN, where the error bars are the size of the points.} \label{distribution} \end{figure} \subsection{Water Production} In this section we shall review some of the indirect measurements of the water production rate of comet Encke made over the past thirty years. While the OH \asig\ -- \xpi\ (0,0) band at 3085 \AA\ had been observed in many comets, because its wavelength lies close to the atmospheric cut-off and comets are usually observed at large airmass, it was not until the first cometary observations by the {\it Orbiting Astronomical Observatory-2} in 1969 and 1970 that the strength of this emission was appreciated \cite{Code:1972}. At the same time, the very extended \Hone\ Lyman-$\alpha$ halo associated with fast hydrogen atoms produced by the photodissociation of \Htwoo\ and OH and interacting with the solar wind was also discovered. The launch of the {\it International Ultraviolet Explorer} (\iue) in January 1978 and its operation in geosynchronous orbit until late 1996 provided a stable, well-calibrated platform above the atmosphere for the observation of these emissions in nearly 50 comets, some, such as Encke, over multiple apparitions. Encke was observed by \iue\ in 1980, 1990, and 1993 pre-perihelion, and in 1984 and 1987 post-perihelion. The most extensive set of measurements was made during October--November 1980 when the viewing geometry was particularly favorable (the initial solar avoidance constraint of \iue\ was 45\deg), and this set included mapping of the OH emissions \cite{Feldman:1984,Festou:2000} and high-dispersion spectroscopy of the (0,0) band sunward and tailward in order to determine the non-gravitational parameters using the Swings effect \cite{A'Hearn:1988}. An attempt to ascertain the secular decrease of water production rate with time over the five apparitions was made by M.~Haken \etal\ (unpublished manuscript), but uncertainties in the pointing during the 1990 and 1993 apparitions made the results ambiguous. \begin{figure}[t] \centerline{\includegraphics[width=4.25in]{encke_comp.ps}} \caption{Composite \iue\ spectrum of comet 2P/Encke taken on November 4, 1980 from Feldman et al. (1984). Top: SWP10544 and SWP10548 (divided by 500; gray line). Bottom: LWR9232 and LWR9233 (divided by 20; gray line).} \label{fig_spec} \end{figure} A composite \iue\ spectrum taken November 4, 1980 is shown in Figure~\ref{fig_spec}. The data are the same as shown by \inlinecite{Feldman:1984} but have been reprocessed for the \iue\ archive using the {\it NEWSIPS} package. A long and short exposure is used for both wavelength ranges with the gray lines representing the brightness divided by 500 in the top spectrum and by 20 in the bottom panel. Note the dominance of the emissions of H and OH, but also the presence of resonance emissions of atomic carbon, oxygen, and sulfur, and the molecule CS, all routinely seen in comets in this spectral range. The continuum near 3000 \AA\ is barely detectable and is consistent with the very high gas/dust ratio deduced from optical spectroscopy and photometry. Water production rates are derived from these spectra by integrating the brightness in each OH band and by comparing this result to the predictions of a vectorial model \cite{Festou:1981} using photodissociation lifetimes and branching ratios appropriate to solar activity at the time \cite{Budzien:1994}. Velocity dependent fluorescence efficiencies for the OH bands are taken from \inlinecite{Schleicher:1988}. The results from the 1980 and 1984 apparitions are shown in Figure~\ref{fig_oh}. From the \iue\ data alone we find, for heliocentric distance, $r$, greater than 0.75 AU, a variation in water production rate of $r^{-3.4}$ and no difference between pre-perihelion and post-perihelion. Also included in the figure is the result of the model of \inlinecite{Festou:2000}, showing a 30\% decrease in water production rate relative to that derived from symmetric coma models. This result is in strong contrast with the production rates derived from narrow-band photometry of \ctwo\ and CN which show a post-perihelion decrease, at 0.8 AU, of a factor of $\sim$3.5 \cite{A'Hearn:1985}. The \iue\ spectra provide additional evidence of this asymmetry about perihelion for the minor coma species. \citeauthor{A'Hearn:1985} also show an \iue\ spectrum from 1984 in which the CS emission is sharply reduced relative to the OH seen in Fig.~\ref{fig_spec}. Using revised CS fluorescence efficiencies from R. Yelle (private communication), we find $Q_{\rm CS_2}/Q_{\rm H_2O} = 1.4 \times 10^{-3}$ at $r = 0.84$ AU pre-perihelion and $4.2 \times 10^{-4}$ at $r = 0.76$ AU post-perihelion, a decrease of a factor of 3.3, commensurate with that seen in \ctwo\ and CN. This result can be understood in terms of chemical heterogeneity between the two discrete outgassing vents identified by \inlinecite{Sekanina:1988a} from dynamical analysis of the sunward fan, the two vents being active during either the inbound or outbound legs of the orbit but not both simultaneously. Fortuitously, CONTOUR will fly by Encke at $r = 1.07$ AU, 47 days before perihelion, allowing the mass spectrometer (NGIMS) to sample a higher concentration of minor species. \begin{figure}[h] \centerline{\includegraphics[width=4.25in]{encke_water.ps}} \caption{Water production rate of comet 2P/Encke as derived from observations of OH (top) and \Hone\ Lyman-$\alpha$ (bottom). Error bars are shown when given with the original data.} \label{fig_oh} \end{figure} \inlinecite{A'Hearn:1985} also present ground-based OH observations made at four apparitions of Encke. In Fig.~\ref{fig_oh} their data from 1984, both pre- and post-perihelion, are shown, adjusted by a factor of two to convert from their use of a Haser model to the same parameters used above. In addition, they give $Q_{\rm OH}$ rather than $Q_{\rm H_2O}$, which we scale by a factor of 1.1. Their data suggest a lessening of the slope for $r < 0.8$ AU with a particularly severe downturn pre-perihelion, similar to what they reported from pre-perihelion observations in 1980 \cite{A'Hearn:1983}. The reality of the change in slope, though not relevant for the CONTOUR mission, can be addressed with recent \Hone\ Lyman-$\alpha$ observations of Encke made by two separate instruments on the {\it Solar and Heliospheric Observatory} (\soho). The lower panel of Figure~\ref{fig_oh} shows water production rates derived from 1997 \soho/SWAN data by \inlinecite{Makinen:2001} and from 2000 \soho/UVCS spectra taken very close to perihelion by \inlinecite{Raymond:2002}. For comparison with the upper panel, the dashed line with slope --3.4 is repeated and it is seen that the results of \citeauthor{Makinen:2001} for $r > 0.7$ AU give reasonably fair agreement to the post-perihelion OH data. Unfortunately there is a gap in the SWAN data between $\sim$0.4 and 0.7 AU caused by contamination from the very bright Lyman-$\alpha$ coma of comet C/1995 O1 (Hale-Bopp) which was present in the sky at the same time. Despite the unexplained discrepancy of a factor of $\sim$2.5 between the near-perihelion results of \citeauthor{Makinen:2001} and those of \citeauthor{Raymond:2002}, it is clear that there is a significant change in slope of $Q_{\rm H_2O}$ for $r < 0.75$ AU, and that at perihelion the water production rate of Encke reaches a maximum of $\sim 5 \times 10^{28}$ \mols. \citeauthor{Raymond:2002} also claim to see a 30\% increase in Lyman-$\alpha$ brightness two days after perihelion which they attribute to the activation of the second vent, in accordance with the predictions of the model of \inlinecite{Sekanina:1988a}. \inlinecite{Sanzovo:2001} have recently derived gas release rates for comet Encke and several other comets using an empirically derived relationship between $Q_{\rm H_2O}$ and visual magnitude in a fixed aperture. Their results are in complete discord with those presented above, as they find a strong asymmetry about perihelion and a peak water production rate at perihelion of $\sim 2 \times 10^{29}$ \mols. Their asymmetry is probably the result of the assumptions of their method which do not fit well with comet Encke since the visual magnitudes of Encke are so strongly influenced by the lack of dust in the coma. \subsection{CN and C$_{2}$} The optical spectrum of a comet is dominated by bands of molecular radicals, with CN, C$_{2}$ and C$_{3}$ being the strongest. Figure~\ref{optspectrum} shows the spectrum of comet Encke when the comet was at 1.3 AU, a distance slightly further from the Sun than the CONTOUR encounter distance. Shown in this figure are a spectrum from the optocenter and one from almost 16,000~km away. These spectra have had the background sky spectrum removed but no reflected solar spectrum has been removed. Despite this, there is almost no continuum observed. \begin{figure}[t] \centerline{\includegraphics[width=4.25in]{spectra.ps}} \vspace*{-0.25in} \caption{The visible spectrum of Encke's optocenter and a region of the coma 15,875~km away are shown. These spectra were obtained with a long-slit CCD spectrograph at McDonald Observatory. The sky spectrum has been subtracted but no solar spectrum has been removed. Note the lack of any continuum in these spectra, as well as the relative decrease of the various species.} \label{optspectrum} \end{figure} The lack of a continuum indicates that the coma of Encke is depleted in small dust particles. This is in accord with the findings of \inlinecite{Sekanina:1978} and \inlinecite{Reach:2000}. Indeed, \citeauthor{Reach:2000} found evidence for long-lived particles of size $\sim$5\,cm with data from the Infrared Space Observatory, but conclude there is no evidence of a ``classical'' cometary dust tail with small particles. Comparison of the upper and lower panels of Figure~\ref{optspectrum} shows that the molecular band strengths decrease with distance from the nucleus but the decrease is much stronger for C$_{3}$ than for C$_{2}$ or CN. \inlinecite{A'Hearn:1995} have shown that Encke produces CN, C$_{2}$ and C$_{3}$ in ``normal" proportions relative to OH. This is true despite the fact that the comet has been in the inner Solar System for an extremely long time and that it has one of the highest gas-to-dust ratios of any comet. \subsection{The Sunward Fan and its Source Regions} Encke's visible coma, as has been mentioned above, usually contains a diffuse structure referred to as a fan that emanates on the sunward side of the nucleus. Its dimensions are such that it stretches typically $\sim10^5$ km on the sunward side of the nucleus (but not necessarily directly towards the sun) with a included angle of 80 to 120\deg. There are no well-defined features in photographs of this structure, although early visual observers sometimes drew enhanced brightness along its outer edges. The projected geometry of this fan, which appears to be defined by molecular emissions, has been found to change in a systematic way, sometimes disappearing altogether to present an essentially circularly symmetric coma, depending on the position of the comet in orbit and the Earth viewing geometry. This phenomenon has been studied in detail by Sekanina~\shortcite{Sekanina:1988a,Sekanina:1988b} who provides an interpretation that is based on a rotating nucleus, with a fully relaxed spin state, with two well defined active areas, one in the north at $\sim$55\deg\ latitude and the other near --75\deg\ in the south. With the polar axis oriented towards $\alpha = 205$\deg, $\delta = +2$\deg\ (1950) at the present epoch, not far from the pole later deduced by \inlinecite{Festou:2000}, Sekanina invokes a narrowly collimated outflow (at least for dust entrained close to the nucleus) from the illuminated active area which forms, as a result of rotation, a diffuse conical sheet of material, with half angle of $\sim$35\deg, escaping from the nucleus that is roughly symmetric around the polar axis. With this geometry, and when viewed from the Earth with the line of sight outside of the cone, a fan shaped structure with the observed half angle and orientation should be seen. At those times when the line of sight from the earth is well within the conical surface the fan disappears to resolve itself into a more normal coma with roughly circular symmetry. The CONTOUR mission gives an excellent opportunity to test these ideas. The spacecraft will approach Encke's nucleus over its northern hemisphere at a time when Sekanina's rotation pole is $\sim$30\deg\ from the sunline and $\sim$40\deg\ from the trajectory approach vector (the angle between the sunline and the approach vector is small at 12\deg). This means that Sekanina's active area at 55\deg\ latitude will be illuminated at encounter and the line of sight (and trajectory) will traverse the coma just on the outside edge of the emission cone. If the fully relaxed spin assumed by Sekanina is borne out, the long axis of the elongated nucleus should be tilted by less than 40\deg\ from the image plane providing excellent viewing of the nucleus just before encounter. \section{Observations of Coma Emissions with CONTOUR} The CONTOUR encounter with comet Encke will occur at 1.07 AU pre-perihelion on November 12, 2003. The CONTOUR Forward Imager (CFI), described in detail in this volume by Warren \etal, has three filters designed for imaging in the coma emission bands of OH, CN and \ctwo\ as well as filters designed to sample nearby dust continuum emissions (see Table~\ref{filters}). Observations of coma emissions will begin about ten days before closest approach (CA) and continue until several seconds before CA. This will permit the sunward fan, as observed from Earth, to be mapped down to its source region on the nucleus. The coma images will also provide the context for the interpretation of the {\it in situ} mass spectrometer measurements. %Filter specs from A. Cheng, May 8, 2002 \begin{table}[h] \caption{CFI Coma Filters.} \label{filters} %\medskip \begin{tabular}{@{}lccc@{}} \hline Filter & Wavelength & FWHM & Transmission \\ & (\AA ) & (\AA ) & (\%) \\ \hline OH & 3084 & 53 & 53 \\ continuum & 3453 & 76 & 54 \\ CN & 3874 & 76 & 90 \\ \ctwo\ & 5124 & 110 & 87 \\ continuum & 5247 & 54 & 85 \\ \hline \end{tabular} \end{table} Additional evidence for asymmetrical outgassing from cometary nuclei has come from recent HST observations using the Space Telescope Imaging Spectrograph, with $0.\!''025$ resolution along the slit (18 km for a comet at a geocentric distance of 1 AU). Observations of comets C/1995 O1 (Hale-Bopp) \cite{Weaver:1999}, 103P/Hartley 2, and C/1999 H1 (Lee) all show that the OH emission is not centered at the same position as the optocenter of the dust coma (with the nucleus unresolved also at the optocenter). The magnitude of the offset is hundreds of km. While the interpretation that the observed asymmetry results from the superposition of OH derived from asymmetric outflow of water from the nucleus is in accord with the results from comet Encke described above, an icy grain halo, often postulated as a source of a large fraction of the water in the coma, cannot be ruled out with the data in hand. Such a halo would necessarily be composed of large grains so as to be able to survive at small heliocentric distances. CONTOUR imaging will be easily able to distinguish between these two mechanisms. As noted above, a primary goal of the coma imaging investigation is to study coma structures. %Belton text At the fast Encke approach speed of 28.2 \kms, the early coma images, at a range of $2.4 \times 10^7$ km, will have linear resolution in the coma similar to Earth-based observations and cover a rectangular area $\sim1 \times 10^6$ km on a side. These should reveal the fan structure with half angle near 90\deg\ and set the context for the rest of the observations. By 1 day before encounter the fan structure should fill the frame (now $1 \times 10^5$ km on a side) with a resolution of 104 km pixel$^{-1}$ at the nucleus. If Sekanina's model holds, a highly collimated structure should become apparent in the inner coma pointing to the source region on the still unresolved nucleus. At this time, the spacecraft will be entering the coma just on the outside of the emission cone and, as the encounter progresses and the viewing geometry is modified, we can expect dramatic changes in the apparent distribution of emissions in the coma. %Cheng text The CONTOUR pass through the central 50,000 km of the coma will occur in about 30 minutes, a time short compared with the approximate 15 hr rotation period of the nucleus \cite{Fernandez:2000}. At a range of 25,000 km, CFI will allow imaging at about 1 km~pixel$^{-1}$ (The nucleus will be about 4 pixels across) while the 2.5\deg\ FOV covers distances out to about 500 km from the nucleus. The spacecraft's rapid pass to within a closest approach distance of about 130 km will allow imaging of the coma at a predicted SNR of the order of 10 along many lines of sight passing completely through the coma as well as lines of sight originating from within it. If the coma does not vary significantly over the 30 minute interval, this data set may allow inference of near-nucleus coma structures such as jets or shocks by a tomographic process, although the interpretation of structures in coma images may be complex \cite{Kitamura:1990,Crifo:1995}. It may also be possible to associate coma structures with source regions on the nucleus. A primary consideration for the short wavelength filters is the rejection of out-of-band continuum (mainly red) from cometary dust. Simulations of cometary spectra through the measured filter response curves indicates that for Encke the amount of long wavelength contamination through the OH filter (3090 \AA) will be of the order of 1\% or less. Fairly long exposures ($\sim$10 seconds) and on-chip binning (at some loss of spatial resolution) will be required to achieve our goal of a signal-to-noise ratio of 10 in this band. \section{Summary} Our current knowledge of the coma of Comet 2P/Encke makes this comet an ideal first target for the CONTOUR mission. The small amount of micron-sized dust particles and the known gas production rate allow for planning optimum observing sequences for both the imagers and the {\it in situ} instruments. 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