Spitzer Space Telescope observations of the nucleus of comet 67P/Churyumov-Gerasimenko (original) (raw)

Observations of Comet 103 P / Hartley 2

2011

We report results based on mid-infrared photometry of comet 103P/Hartley 2 taken during 2010 May 4–13 (when the comet was at a heliocentric distance of 2.3 AU, and an observer distance of 2.0 AU) by the Wide-field Infrared Survey Explorer. Photometry of the coma at 22 μm and data from the University of Hawaii 2.2 m telescope obtained on 2010 May 22 provide constraints on the dust particle size distribution, d log n/d log m, yielding power-law slope values of alpha = −0.97 ± 0.10, steeper than that found for the inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. The extracted nucleus signal at 12 μm is consistent with a body of average spherical radius of 0.6 ± 0.2 km (one standard deviation), assuming a beaming parameter of 1.2. The 4.6 μm band signal in excess of dust and nucleus reflected and thermal contributions may be attributed to carbon monoxide or carbon dioxide emission lines and provides limits and estimates of species production. Derived carbon di...

Shape, density, and geology of the nucleus of Comet 103P/Hartley 2

Icarus, 2013

Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33 km in length. Surface features are primarily small mounds <40 m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a ''waist'' between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200-400 kg m À3. Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object's complex rotation.

The Nucleus of Comet Hyakutake (C/1996 B2)

Icarus, 1999

Infrared, optical, and radio continuum observations were made of the long-period comet C/Hyakutake 1996 B2 during its close approach to Earth in March 1996. Using these observations to characterize the comet's nucleus, we find an estimated nuclear radius of 2.4 ± 0.5 km (1σ) from photometric resolution of the nucleus in the thermal infrared at 8-20 µm on 25 March 1996, no detectable optical nuclear emission above that of the coma on 19-23 March 1996, and a 3σ upper limit to the radius of 2.7 km (assuming an emissivity of 0.9) in the radio at 3.6 cm on 27 March 1996. The infrared color temperature of the nucleus was consistent with a 320-K blackbody, and assuming a 2.4-km radius, the maximum effective temperature at 3.6 cm was 230 K. We explain the optical nondetection of the nucleus as due to excess emission from a halo of small, cold, and high optical albedo dust particles surrounding the nucleus; such particles would have low emissivity in the infrared and radio. A surrounding halo of icy dust grains emitting water in addition to the nucleus accounts for the small nuclear size but large production rate of water from C/Hyakutake; otherwise an anomalously large fraction of the nuclear surface, nearly 100%, must be active. A fast rotation period of 6.30 ± 0.03 h due to coma dust cross-sectional variations was found from optical and infrared imaging on March 20-23. The minimum bulk tensile strength required to stabilize the comet against centrifugal breakup due to this rotation, ∼10 3 dynes cm −2 , is similar to that found for other comets.

Physical Properties of the Nucleus of Comet 2P/Encke

Icarus, 2000

We report a new study of the nucleus of Comet 2P/Encke, which the CONTOUR spacecraft is scheduled to encounter in November 2003. During the comet's close approach to Earth in July 1997, we measured the mid-infrared thermal and optical scattered continua with data from the TIMMI instrument (imaging) at the ESO 3.6-m telescope (wavelength λ from 8 to 12 µm), the ISOPHOT instrument (photometry) aboard ISO (3.6 µm ≤ λ ≤ 100 µm), and the STIS instrument (imaging) aboard HST (5500Å ≤ λ ≤ 11000Å ). The optical images show the nucleus with very little coma contamination, and the ISO photometry allowed us to separate the comatic and nuclear contributions to the ESO images. We used the Standard Thermal Model for slow rotators to calculate an effective nuclear radius of 2.4 km ± 0.3 km. The comet's mid-IR light curve implies a nuclear rotation period of 15.2 h ± 0.3 h, although some subharmonics of this also satisfy the data. If we assume that the nucleus is a triaxial ellipsoid in principal short axis rotation with the axis direction in 1985 as derived by Sekanina (1988, Astron. J. 95, 911), then by combining our data with light curves from the 1980s we find that the nucleus' angular momentum vector migrates, making a would-be circle in less than 81 years, and that one axial ratio is at least 2.6. The nucleus' optical linear phase coefficient is 0.06 mag/degree, making it one of the most phasedarkened objects known. The surface is also rougher than that of 146 FERNÁNDEZ ET AL. most asteroids. The visual geometric albedo is 0.05 ± 0.02, within the range found for other cometary nuclei. c 2000 Academic Press Key Words: comets; infrared observations. 12 µm. The images have 64 2 pixels and cover (21.8 ) 2 . Each pixel width covered 65 to 87 km at the comet during the observing run. The plate scale was measured using the known relative positions of α Cen A and B . The pointspread function's (PSF) full width at half-maximum (FWHM) varied from 0.7 to 1.0 arcsec. Chopping of the secondary mirror northward and nodding of the telescope westward, with typical throws of 30 arcsec, were employed. An array flat field was created by measuring the relative photometry of a bright star at 23 different locations on the array and then interpolating a surface with a minimum of curvature. We observed the comet at three wavelengths, but only at λ = 10.7 µm was the comet bright enough to let us build a well-sampled time series of data. Absolute flux calibration was done using α Cen A, whose 10.7-µm magnitude is −1.56 ± 0.05, interpolating from photometric data given by . The magnitude scale zero point is 35.7 Jy. Color corrections were done and were at most

Thermodynamic model of the activity of the comet 103P/Hartley

Research in Astronomy and Astrophysics, 2021

The paper presents three processes related to the dynamics of cometary particles. The following thermodynamic mechanisms were taken into account: quiet sublimation, emission of cometary matter via jet and migration of particles on the surface of the comet 103P/Hartley. Based on the first two mechanisms, the maximum particle size that can be lifted into the coma was determined. Additionally, in the case of a jet, the angle at which it is emitted from inside the cometary nucleus was determined. However, in the case of migration, the maximum width of individual belts within which a given particle can move was determined. In the context of the discussed mechanisms related to the activity of comet 103P/Hartley, the coefficient of friction and the structure of the solid or porous particles are of key importance.

Temporal and spatial aspects of gas release during the 2010 apparition of comet 103P/Hartley 2

2011

We report measurements of eight primary volatiles (H 2 O, HCN, CH 4 , C 2 H 6 , CH 3 OH, C 2 H 2 , H 2 CO, and NH 3) and two product species (OH and NH 2) in comet 103P/Hartley-2 using high dispersion infrared spectroscopy. We quantified the long-and short-term behavior of volatile release over a three-month interval that encompassed the comet's close approach to Earth, its perihelion passage, and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission. We present production rates for individual species, their mixing ratios relative to water, and their spatial distributions in the coma on multiple dates. The production rates for water, ethane, HCN, and methanol vary in a manner consistent with independent measures of nucleus rotation, but mixing ratios for HCN, C 2 H 6 , & CH 3 OH are independent of rotational phase. Our results demonstrate that the ensemble average composition of gas released from the nucleus is well defined, and relatively constant over the three-month interval (September 18 through December 17). If individual vents vary in composition, enough diverse vents must be active simultaneously to approximate (in sum) the bulk composition of the nucleus. The released primary volatiles exhibit diverse spatial properties which favor the presence of separate polar and apolar ice phases in the nucleus, establish dust and gas release from icy clumps, and from the nucleus, and provide insights into the driver for the cyanogen (CN) polar jet. The spatial distributions of C 2 H 6 & HCN along the near-polar jet (UT 19.5 October) and nearly orthogonal to it (UT 22.5 October) are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water was 2.85 ± 0.20; the lower bound (2.65) defines T spin > 32 K. These values are consistent with results returned from ISO in 1997.

Comet 103P/Hartley 2 at perihelion: gas and dust activity

Astronomy & Astrophysics, 2011

Context. The comet 103P/Hartley 2, target of the EPOXI mission (NASA), was supposed to be observed for 3 days around its perihelion, from October 27 to 29, 2010, but photometric data were obtained only on October 27 and 29, 2010. On both dates, the comet visibility was not optimal due to its proximity to the Moon, as projected on the plane of the sky, whereas on October 28, the comet could not be observed at all. Aims. The goal of the campaign was to give ground support to the EPOXI mission by establishing a baseline of activity at perihelion to be compared with in situ activity observed by the space mission about 7 days later on Nov. 4, 2010. We aimed to assess gas and dust production rates, to study the gas and dust coma morphology, to investigate the behaviour of the refractory component by analysing the dust colour variations with date and with projected cometocentric distance, ρ, and to determine the slope of the surface brightness profiles, B, as a function of ρ. Methods. Long-slit spectra and optical broad-and narrowband images were acquired with the instrument ACAM mounted on the William Herschel Telescope (WHT) at La Palma Observatory. We investigated the evolution of the dust coma morphology from the images acquired with specific continuum cometary filters (in the blue and red wavelength region) with image-enhancing techniques. We studied (1) the gas and dust production rates; (2) the dust radial brightness profiles; (3) the profiles of the CN, C 2 , C 3 and NH 2 column densities, and (4) the CN and C 3 coma morphologies. The dust and gas profiles were azimuthally averaged, as well as measured in both the E-W direction (∼Sun-antisolar direction) and in a direction defined by the slit orientation at PA 70 to 250 degrees. Results. The morphological analysis of the dust coma reveals only one structure. Aside from the dust tail in the west direction, a bright jet is detected in images acquired on October 27 at 03:00-04:00 UT. This jet turns on and off and it is not clearly detected at any time on the images obtained during October 29. This structure is enhanced by making use of the radial renormalization and the Larson-Sekanina method. It is also confirmed by the distortion of the isophotes at the same position angle (PA). The Afρ parameter, a proxy to the dust production rate, and the gas (CN, C 3 , C 2 , and NH 2) production rate, Q i , have been measured at perihelion, r h ≈ 1.058 AU. The quotient Q C 2 /Q CN ∼ 1.3 places 103P/Hartley 2 as a typical comet in terms of long-chain hydrocarbon abundance. The gas-to-dust mass ratio is ∼3−6, indicating that 103P/Hartley 2 is a relatively gas-rich comet. At perihelion, Afρ, as measured in a circular aperture of ∼4700 km ranges from ∼60 cm in the blue to ∼110 cm in the red, which indicates an overall change in the optical properties of the dust grains. On the other hand, the Afρ is rather stable in the innermost coma when it is computed from the spectroscopic measurements within several continuum spectral ranges from 482-685 nm. Both 2D dust colour maps and profiles in the directions imposed by the slit indicate that there are variations with ρ with a trend towards bluer dust colour with increasing ρ. This could indicate sublimation of ices as the cameras on board the EPOXI mission have shown. The average dust reddening is ∼24%/100 nm. The azimuthally averaged surface brightness profiles of the continuum from the broad band images can be well fitted with m ∼ 1 in the tail direction, whereas in the opposite direction the dust profiles are much flatter at ρ ≤ 10 000 km. The azimuthally averaged profiles of the comet images acquired with the blue and red continuum cometary filters show a nominal behaviour of log B ∼ −m log ρ with m ∼ 1.

Water ice and dust in the innermost coma of Comet 103P/Hartley 2

Icarus, 2014

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05-4.83 µm wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ∼10 kilometers of the coma collected 7 minutes and 23 minutes after closest approach. The extracted reflectance spectra include welldefined absorption bands near 1.5, 2.0, and 3.0 µm consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke's radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 µm in size. The strong correlation between the water ice, dust, and CO 2 spatial distribution supports the

A distribution of large particles in the coma of Comet 103P/Hartley 2

Icarus, 2013

The coma of Comet 103P/Hartley 2 has a significant population of large particles observed as point sources in images taken by the Deep Impact spacecraft. We measure their spatial and flux distributions, and attempt to constrain their composition. The flux distribution of these particles implies a very steep size distribution with power-law slopes ranging from −6.6 to −4.7. The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their exact sizes depend on their unknown light scattering properties. We consider two cases: bright icy material, and dark dusty material. The icy case better describes the particles if water sublimation from the particles causes a significant rocket force, which we propose as the best method to account for the observed spatial distribution. Solar radiation is a plausible alternative, but only if the particles are very low density aggregates. If we treat the particles as mini-nuclei, we estimate they account for < 16 − 80% of the comet's total water production rate (within 20.6 km). Dark dusty particles, however, are not favored based on mass arguments. The water production rate from bright icy particles is constrained with an upper limit of 0.1 to 0.5% of the total water production rate of the comet. If indeed icy with a high albedo, these particles do not appear to account for the comet's large water production rate. Erratum: We have corrected the radii and masses of the large particles of comet 103P/Hartley 2 and present revised conclusions in the attached erratum.