Compositional and physical results for Rosetta's new target Comet 67P/Churyumov-Gerasimenko from narrowband photometry and imaging

We present compositional and physical results of Comet 67P/Churyumov-Gerasimenko, the new target of ESA's Rosetta mission. A total of 16 nights of narrowband photometry were obtained at Lowell Observatory during the 1982/83 and 1995/96 apparitions, along with one night of imaging near perihelion in 1996. These data encompass an interval of −61 to +118 days from perihelion, corresponding to a range of heliocentric distances before perihelion from 1.48 to 1.34 AU, and an outbound range from 1.30 to 1.86 AU. Production rates were determined for OH, NH, CN, C₃, and C₂, along with A(θ)fρ, a proxy of the dust production. Water production, based on OH, has a steep () power-law rH-dependence post-perihelion and the minor species are somewhat less steep ( to −4), while the dust is quite shallow (), possibly due to a lingering population of large, slow-moving grains. All species exhibit larger production rates after perihelion, with water having a ∼2×pre/post-perihelion asymmetry, while minor species and dust have larger asymmetries. These asymmetries imply a strong seasonal effect and probable high obliquity of the rotational axis, along with one or more isolated source regions coming into sunlight near perihelion. Peak water production (which occurred about 1 month after perihelion) was and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ∼1.5-2.2 km² or an active fraction of only about 3-4%. Abundances of carbon-chain molecules yield a classification of slightly “depleted” in the A'Hearn et al. [A'Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V., 1995. Icarus 118, 223-270] database. The peak dust production (as measured by A(θ)fρ, and uncorrected for phase angle) was ∼450 cm, while the color of the dust is moderately reddened, and the mean radial profile has a power-law slope of −1.3. Large night-to-night variability is also present, presumably due to the source region(s) rotating in and out of sunlight along with effects due to the use of differently sized apertures. A strong sunward radial feature was detected in images obtained near perihelion, along with a significant asymmetry between the two perpendicular directions from the Sun/tail line. These features may be the result of a mid-latitude source region sweeping out a cone with each rotation, which we are viewing from the side and where the sunward radial feature is one edge of the cone seen in projection. When combined with other constraints on the pole orientation, a possible pole solution is found having an obliquity of about 134° at an RA of about 223° and a Dec of −65°, with a source region located near +50° and in overall agreement with the photometric results. In comparison to the original Rosetta target Comet 46P/Wirtanen, Comet Churyumov-Gerasimenko has essentially the same peak water production but a peak dust production about 3 times greater than does Wirtanen based on A(θ)fρ (i.e., if one assumes that the properties of the dust grains are similar) (cf. Farnham and Schleicher [1998. Astron. Astrophys. 335, L50-L55]).     

CN, C2 radicals and dust in Comets Shoemaker-Levy 1991 T2 and P/deVico 1995 S1

The results of the multiaperture photometry of Comet Shoemaker-Levy 1991 T2 in the pre-perihelion and P/deVico in the post-perihelion period with the narrowband CN, C₂ and Blue Continuum (BC) IHW filters are presented. A Haser model of the molecular coma was used for the determination of the parent and daughter scale-lengths and production rates of the radicals. The comets showed some substantial differences between their parent scale-lengths. The CN parent scale-length (at 1.0 AU) was 16×¹⁰³ km for Comet Shoemaker-Levy and 39×¹⁰³ for P/deVico, the C₂ parent scale-lengths were respectively 29×¹⁰³ and 54×¹⁰³ km. Such divergences could be interpreted in the frame of different scenarios of emission of cometary parents, either from a nucleus or from a volume source. The daughter scale-lengths for these comets were quite similar, namely: 306×¹⁰³ and 318×¹⁰³ km for CN and 69×¹⁰³ and 66×¹⁰³ km for C₂. We determined the Afρ parameter for apertures of different radii. A Monte Carlo model of the dust coma was used to obtain the dust ejection velocity. It was of the order of 0.1 km s⁻¹ for both comets. The power index of the distribution of the β-parameter of dust particles (ratio of light pressure to the solar gravitation) was of the order of 3 for C/Shoemaker-Levy and close to 2 for P/deVico. The dependence on heliocentric distance (rh) of the radical and dust production rates for P/deVico in the range of 0.7-1.0 AU was described by the power law function with a power index equal to: 5.55±0.14 for CN, 5.70±0.24 for C₂ and 5.22±0.19 for dust. Relative abundances of the dynamically new Comet Shoemaker-Levy and short-period P/deVico were quite similar with an enhancement of C₂ comparing with standard values taken from A'Hearn et al. (1995).     

Absolute spectrophotometry of Comet kohoutek 1973f during post-perihelion passage

Absolute spectrophotometry of the coma of Comet Kohoutek 1973f during post-perihelion period has been presented for seven nights in January 1984. Moderately wide range of heliocentric distance (0.436–0.799 AU) covered during observations allowed us to study the flux variation of emission bands with heliocentric distance. The emission features of CN, CH, C₂, C₃, and NaI have been identified in this comet. The abundances of CN and C₂ have been estimated and the production rates of CN, C₂ and C₃ have been derived. Production rates of CN and C₂ seem to vary as r ^–0.33 and r ^–3.50 respectively. The continuum of the comet became more and more redder as the heliocentric distance of the comet increased and phase angle decreased.     

Observations of the inner coma of C/1995O1 (Hale-Bopp)—gas and dust production

Hale-Bopp (C/1995 O1) was the most productive recent comet observed in terms of gas and dust output. Since its discovery in 1995 at a distance of 7.14 AU from the Sun, the comet has been well observed, revealing the dynamics of a rare and large comet. Hale-Bopp showed strong emissions of the principle cometary gases CN, C₃, and C₂, as well as an abundance of dust. The production rates of these gases were found to be 1.45×10²⁸, 1.71×10²⁸, and , respectively, with dust production, in terms of Afρ, , as measured in the green continuum (5260 Å). The observations for this paper are presented in two groups spanning 10 days each, one group centered near 32 days prior to and the other 21 days after perihelion. The averages of dust and gas production rates show a slightly higher value for each prior to perihelion than after perihelion, consistent with a possible peak in production a few weeks prior to perihelion passage.     

Evaluation of gas production rates in some comets

Emission fluxes of CN, C₂ and C₃ carbon-bearing molecular species observed in the coma of comets Bennett (1969i 1970II), West (1975n 1976VI), P Halley (1982i), Hartley-Good (19851) and Bradfield (1987s) are analysed in the framework of Haser model. CN, C₂ and C₃ production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C₂ and C₃ production rates on the heliocentric distance and the possible correlations among these radicals is studied and briefly discussed.     

Meridional variations of temperature, C2H2 and C2H6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C₂H₂ and C₂H₆ abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C₂H₆ in Saturn's middle and lower stratosphere (∼40-10,000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C₂H₂ (∼1-7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13-14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a LS≈270°, corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm⁻¹ with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C₂H₂ at −1° and −83° planetocentric latitude to be and , respectively. The C₂H₂ mixing ratio at 0.12 mbar is found to be at −1° planetocentric latitude and at −83° planetocentric latitude. The 2.3 mbar mixing ratio of C₂H₆ inferred from the data is and at −1° and −83° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C₂H₂ and C₂H₆ are affected by seasonal forcing and/or stratospheric circulation.     

Formation of radical species in photolyzed CH4:N2 ices

We report photochemical studies of thin cryogenic ice films composed of N₂, CH₄ and CO in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C₂H₆ and C₂H₂, and HCO are formed by the dissociation of CH₄ into H, CH₂ and CH₃ and the subsequent reaction of these radicals within the ice. Other radical species, such as C₂, , CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N₂ matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that , CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted.     

Vaporization in comets: The icy grain halo of comet west

The variation with heliocentric distance of the production rates of various species in Comet West (1975n=1976 VI) is explained with a cometary model consisting of a CO₂-dominated nucleus plus a halo of icy grains of H₂O or clathrate hydrate. We conclude that the parents of CN and C₃ are released primarily from the nucleus but that the parent of C₂ is released primarily from the halo of icy grains.     

Gas and dust release from comet West (1976VI) nuclear fragments

Emission fluxes of CN, G₂ and C₃ carbon-bearing molecular species observed in the spectra of the nuclear fragment A and D of comet West (1976VI) are analysed in the framework of Haser's two-component model with radial outflow symmetry. Gas production rates have been derived and their ratios obtained. We find these vary as Q r ⁿ (where 1.0r2.6 is the heliocentric distance in AU) with photometric parametern = -4.11 (CN), -2.68 (C₂), -1.89 (C₃) andQ _x /Q _y r ⁿ withn = 2.22 (C₃/CN), 1.42 (C₂/CN), -0.79 (C₂/C₃). The gas-to-dust ratio, dust production rates in arbitrary units, total number of dust particles in the field of view and an effective radius R_eff 2.0 km for the principal nuclear fragmentA is also estimated.     

On the monochromatic brightness variations of comets

A calculation of the non-steady development of a multi-species atmosphere of a comet moving in a near-parabolic heliocentric orbit is presented. The monochromatic brightness variations of the characteristic cometary emission bands due to OH, CN and C₂ are then evaluated assuming that the parent molecules of these chemically unstable species are respectively H₂O, HCN and H₂C₂ present in a homogeneous H₂O clathrate nucleus. For small heliocentric distances where a quasi-steady approximation is valid, the brightness variations follow Levin's (1943) law, provided all the destruction mechanisms of the cometary molecules vary as the inverse square of the heliocentric distance. On the other hand, at large heliocentric distances Levin's law breaks down, essentially due to the large time-scales of residence of the emitting species in the cometary atmosphere. This large residence time at large heliocentric distance also produces an asymmetry between the brightness profiles of the inbound and outbound passages, such that the brightness declines less steeply with distance on the outbound passage than on the inbound. Consequently, the monochromatic brightness of OH at 4 AU outbound is about twice as large as the corresponding value inbound. While some comets show such an effect, others show just the opposite effect. These deviations, which show the limitations of our simple homogeneous model, are discussed qualitatively in terms of the plausible time varying physical structure of the cometary nucleus. The variations of the relative monochromatic brightnesses of the various emissions are also discussed, and the need for extending monochromatic brightness measurements to larger heliocentric distances is stressed.     

Non-constant rotation period of Comet C/2001 K5 (LINEAR)

The article presents results of CCD photometry in R-band of a dynamically new Comet C/2001 K5 (LINEAR), obtained at a heliocentric distance of about 5.6 AU, after the perihelion passage. Being so distant from the Sun, this comet was extremely active (Afρ close to 2000 cm), exhibiting quite well developed dust coma and tail. During the observations, general photometric behavior of the comet with heliocentric distance r was well described by the 2.5nlog(r) function with coefficient n=5. The radial profiles of the coma were found to be undulated, with mean slope of the dependence between cometary magnitude and 2.5log of aperture radius (at comet distance) equal to . The light curve of Comet LINEAR exhibited short-term variability which we attributed to cyclic changes of dust emission, induced by nucleus rotation. Model computations by some authors have revealed that active comets can change their spin status quite substantially even during a single orbital revolution. Thus, attempting to search for a rotation frequency, we have modified the classical PDM approach by including the spin acceleration term. Such DynamicalPDM (DPDM) method revealed the most reliable solution for the frequency f₀=0.019048±0.000013 h⁻¹ and its first time-derivative (index “zero” denotes reference to the mid time of the whole observing run), indicating a rapid spin-down of the nucleus. These parameters are equivalent to the rotation period of 52.499±0.036 h and its relative increment of 0.02729±0.00013. We present the most probable evolution of the rotation frequency of Comet LINEAR, based on the results of periodicity analysis and a simple, almost parameter independent, dynamical model of nucleus rotation. It is also shown that the DPDM may be an effective tool for determination of a nucleus radius, which provided us with the value of 1.53±0.25 km for Comet LINEAR.     

Postperihelion interference filter photometry of the “annual” comet P/Encke

Interference filter photometry was taken of Comet Encke on June 14, 1974 (1.07 AU heliocentric distance, postperihelion) at the CTIO (Cerro Tololo Interamerican Observatory) 150-cm reflector. Production rates were calculated of 4.1 × 10²³ mol sec^?1 of CN, 5.3 × 10²³ mol sec^?1 of C₃, and 4.3 × 10²⁴ mol sec^?1 of C₂. These are about three times smaller than at comparable heliocentric distance preperihelion, assuming a value of 100 for the ratio H₂O/ (C₂ + C₃ + CN). An upper limit was placed on the production of nonvolatiles at about one-third that of volatiles in mass by assuming a bulk density of 1 g cm^?3, a particle geometric albedo of 0.1, and a phase function of 0.2.     

Observations of C2H6 and C2H2 in the stratosphere of Saturn

We have performed high-resolution spectral observations at mid-infrared wavelengths of C₂H₆ (12.16 μm), and C₂H₂ (13.45 μm) on Saturn. These emission features probe the stratosphere of the planet and provide information on the hydrocarbon photochemical processes taking place in that region of the atmosphere. The observations were performed using our cryogenic echelle spectrometer Celeste, in conjunction with the McMath-Pierce 1.5-m solar telescope in November and December 1994. We used Voyager IRIS CH₄ observations (7.67 μm) to derive a temperature profile on the saturnian atmosphere for the region of the stratosphere. This profile was then used in conjunction with height-dependent volume mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. Our ground-based measurements indicate abundances of for C₂H₆ (1.0 mbar pressure level), and for C₂H₂ (1.6 mbar pressure level). We also derived new mixing ratios from the Voyager mid-latitude IRIS observations; 8.6±0.9×¹⁰⁻⁶ for C₂H₆ (0.1-3.0 mbar pressure level), and 1.6±0.2×¹⁰⁻⁷ for C₂H₂ (2.0 mbar pressure level).     

Spectrophotometric study of periodic Comet Halley (1982i)

Spectral scans of the head of periodic Comet Halley (1982i) have been presented and analysed in detail in the optical region (3200–7000 Å); for ten nights during pre-perihelion period. Emission features due to NH, CN, CH, C₃, and C₂ molecules have been identified. The behaviour of the variation of different emission lines strength as a function of heliocentric distance has been investigated. It is found that the comet exhibits night-to-night variation of brightness. The abundances and production rates of CN and C₂ species have also been derived.     

Titan's corona: The contribution of exothermic chemistry

The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N₂, CH₄, H, H₂, ³CH₂, CH₃, C₂H₄, C₂H₅, C₂H₆, N(⁴S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N₂, CH₄, and H₂. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N₂ and CH₄ INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be and .     

The C/C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio ¹²C/¹³C in each of three chemical species in Titan's stratosphere: CH₄, C₂H₂ and C₂H₆. This is the first measurement of ¹²C/¹³C in any C₂ molecule on Titan, and the first measurement of ¹²CH₄/¹³CH₄ (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of ¹²C/¹³C within a given species, and also for differences between the ¹²C/¹³C in the three gases. For CH₄ alone, we find (1-σ), essentially in agreement with the ¹²CH₄/¹³CH₄ measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3±1.0, and also with measured values in H¹³CN and ¹³CH₃D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C₂ species, we find in C₂H₂ and 89.8±7.3 in C₂H₆, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of , also compatible with GCMS. Therefore, the evidence is mounting that ¹²C/¹³C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88±7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work.     

Spectrophotometry of comet Hartley-Good (1985l)

We present spectrophotometric studies of comet Hartley-Good (1985l) in the spectral region 3200–7000 Å. The emission features of molecules CN, CH, C₂, and C₃ are observed. The variation of the emission strength of different species has been studied as a function of heliocentric distance. The abundances (N) and production rates (Q) of the molecules are also estimated.