Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta

The OSIRIS cameras on the Rosetta spacecraft observed Comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impact projectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed through filters sensitive to emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showed regular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness of Tempel 1 decreased by about 10% during the OSIRIS observations. The analysis of the impact ejecta shows that no new permanent coma structures were created by the impact. Most of the material moved with . Much of it left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet ( of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations of the density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain.     

Preface and Dedication

Earth, Moon, and Planets -     

The Nucleus of Comet 22P/Kopff and Its Inner Coma

We report the detection of the nucleus of Comet 22P/Kopff with the Planetary Camera of the Hubble Space Telescope (HST) and with the Infrared Camera of the Infrared Space Observatory (ISOCAM). The HST observations were performed on 18 July 1996, 16 days after its perihelion passage of 2 July 1996, when it was at R_h=1.59 AU from the Sun and Δ=0.57 AU from the Earth. A sequence of images taken with four broad-band filters was repeated eight times over a 12-h time interval. The ISOCAM observations were performed on 15 October 1996, 106 days after the perihelion passage, when the comet was at R_h=1.89 AU from the Sun and Δ=1.32 AU from the Earth. Seven images were obtained with a broad-band filter centered at 11.5 μm. In both instances, the spatial resolution was appropriate to separate the signal of the nucleus from that of the coma. We determine the Johnson-Kron-Cousins BVRI magnitudes of the nucleus. The visible lightcurves constrain neither the rotation period nor the ratio of semiaxes. We favor the solution of a rather spherical nucleus, although the situation of a pole-on view of an irregular body cannot be excluded. The systematic decreasing trend of the lightcurves could suggest a period of several days. Combining the visible and infrared observations, we find that an ice-dust mixed model is ruled out, while the standard thermal model leads to a nuclear radius of R_n=1.67±0.18 km of albedo p_v=0.042±0.006. The red color of the nucleus is characterized by a nearly constant gradient of S′=14±5% per kÅ from 400 to 800 nm. We estimate a fractional active area of 0.35 which places 22P/Kopff in the class of highly active short-period comets. At R_h=1.59 AU, the dust coma is characterized by a red color with a reflectivity gradient S′=17±3% per kÅ, compatible with that of the nucleus, and Afρ=545 cm, yielding a dust production rate of Q_d=130 kg sec⁻¹.     

The rotational structure of the B-X system of sulfur dimers in the spectra of Comet Hyakutake (C/1996 B2)

We have constructed line-by-line fluorescence models for the ro-vibrational bands of the B-X system of sulfur dimers (S₂). For the first time the detailed rotational lines of the bands are clearly resolved in the high-resolution echelle spectra of Comet Hyakutake (C/1996 B2), which were obtained at the Kitt Peak 4-m telescope on 26.4 March 1996 (UT). In order to construct satisfactory band models of the B-X system, we first compared the models to laboratory spectra available in literature, and then compared the models including Swings effects to the high-resolution spectra of Hyakutake. From the model fits, we derived a rotational temperature of 70±10 K, which should represent a temperature of a coma region close to the nucleus, where most of the S₂ emissions originate. We present previously proposed scenarios for the origin of S₂ in cometary comae, and compare them with our spectral analysis.     

The nucleus of Deep Impact target Comet 9P/Tempel 1

On UT 2000 August 21 we obtained simultaneous visible and mid-infrared observations of Comet 9P/Tempel 1, the target of the upcoming NASA Discovery Program mission Deep Impact. The comet was still quite active while 2.55 AU from the Sun (post-perihelion). Two independent analyses of our data, one parameterizing the coma morphology and the other modeling infrared spectrophotometry, show that the nucleus's cross section at the time the data were taken corresponds to an effective radius of 3.0±0.2 km. Based on visible-wavelength photometry of the comet taken during this observing run and others in the summer of 2000, all of which show the rotational modulation of the nucleus's brightness, we find that the infrared data were obtained near the maximum of the light curve. If we assume that the nucleus's light curve had a peak-to-valley range of 0.6±0.2 mag, then the mean effective radius is 2.6±0.2 km. Visible-wavelength photometry of the nucleus, including data published by other groups, lets us constrain the nucleus's R-band geometric albedo: 0.072±0.016. The nucleus's flux contributed about 85% of the light in the mid-infrared images.     

Sulfur Chemistry at Millimeter Wavelengths in C/Hale-Bopp

The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years. Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H₂S, CS, and OCS, as well as observing H₂CS and SO. This was the first observation of H₂CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H₂S and CS. An example for H₂S is shown in Figure 2. This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H₂S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time. C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO₂ (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rotational state of the nucleus of Comet 9P/Tempel 1: Results from Hubble Space Telescope observations in 2004

The nucleus of Comet 9P/Tempel 1 was first observed with the Hubble Space Telescope (HST) in December 1997 [Lamy, P., Toth, I., A'Hearn, M.F., Weaver, H., Weissman, P.R., 2001. Icarus 154, 337-344], but the temporal coverage was insufficient to determine its rotational period. Because the success of the Deep Impact mission was critically dependent on understanding the rotational state and approximate shape and size of the nucleus, we extensively re-observed 9P/Tempel 1, this time with the Advanced Camera for Surveys (HST/ACS), from May 7.9 to 9.5, 2004 (UT). At the mid-point of the observing window, the comet was 3.52 AU from the Sun, 4.03 AU from the Earth, and at a solar phase angle of 13.3°. The program was comprised of 18 separate visits, each one corresponding to an HST orbit filled with 3 ACS exposures of either 800 or 857 s duration with the F606W broadband filter. These very deep exposures revealed a star-like object, without any apparent coma. The light curve, defined by 49 data points, is characterized by a mean apparent V magnitude of 21.8 and an amplitude of 0.5 mag, indicating that we were viewing the varying cross-section of a rotating, elongated body. The periodicity was analyzed with seven different techniques yielding a rotational period in the range 39.40 to 43.00 h, and a mean value of 41.27±1.85 h (1σ). Using an albedo pV=0.04 and a linear phase law with a coefficient , we determined an effective radius of 3.01 km; a possible prolate spheroid solution has semi-axes a=3.71 km, b=2.36 km and a minimum axial ratio a/b∼1.57. By comparing the light curves obtained in 1997 and in 2004, we were able to constrain the phase function of the nucleus. Finally, an upper limit of Afρ<0.04 cm is set based on the non-detection of the coma.     

Asymmetries in the distribution of H2O and CO2 in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact

Prior to the impact event, Deep Impact monitored the ambient inner coma of Comet 9P/Tempel 1 at high spatial resolution in July 2005. Gaseous H₂O and CO₂ are unambiguously detected in the infrared spectra collected with the HRI-IR spectrometer aboard Deep Impact. Detailed distribution maps of these volatiles in the inner coma, within 60 km from the nucleus, are produced from the integrated emission bands of H₂O (2.66 μm) and CO₂ (4.26 μm). Uncorrelated asymmetries are determined in the spatial distribution of both species indicating chemical heterogeneities within the nucleus. Although present at some abundance surrounding the entire nucleus, H₂O has a pronounced enhancement in abundance in the sunward direction rotational phases, evidence that the dominant process of subliming water ice from the nucleus is solar heating. In contrast, CO₂ is enhanced in the regions near the negative rotational pole of the nucleus, suggesting localized outgassing there. Both species show an increase in radiance above the limb of the nucleus toward Ecliptic North. The distribution maps also suggest that the process of dust removal from the nucleus is strongly connected to the outgassing of volatiles. Detailed study of these coma asymmetries gives insight to the relative abundances of the dominant molecular components of the inner coma, source regions of the native volatiles, anisotropic outgassing of the nucleus, and the formation and evolution of the nucleus. A quiescent water production rate for Tempel 1 on July 4, 2005, is estimated to be .