Context. The Nice model predicts that the trans-planetary planetesimal disk made a large or even dominant contribution to the cratering in the inner solar system during the late heavy bombardment ...(LHB). In the presence of evidence that lunar craters and mare basins may be mainly of asteroidal origin, there is a dilemma of the missing comets that is not yet resolved. Aims. We aim to revisit the problem of cometary impact rates on the Moon and the terrestrial planets during the LHB with a flexible model, allowing us to study the influences of physical destruction of comets, the mass of the primordial disk, and the distribution of this mass over the entire size range. Methods. We performed a Monte Carlo study of the dynamics of the cometary LHB projectiles and derive the impact rates by calculating individual collision probabilities for a huge sample of projectile orbits. We used Minimum Orbit Intersection Distances (MOIDs) according to a new scheme introduced here. Different calculations were performed using different models for the physical evolution of comet nuclei and for the properties of the primordial, trans-planetary disk. Results. Based on the capture probability of Jupiter Trojans, we find a best fit radius of the largest LHB comet impacting the Moon for a low-mass primordial disk. For this disk mass, the LHB cratering of the Moon, Mercury and Mars were dominated by asteroids. However, some smaller lunar maria were likely preceded by comet impacts. The volatile delivery to the Earth and Mars by LHB comets was much less than their water inventories. Conclusions. There is no excessive cometary cratering, if the LHB was caused by a late planetary instability in the Nice Model. The Earth and Mars obtained their water very early in their histories. The Noachian water flows on Mars cannot be attributed to the arrival of LHB-related H2O or CO2.
We present Herschel observations of water isotopologues in the atmosphere of the Jupiter-family comet 45P/Honda-Mrkos-Pajdusakova No HDO emission is detected, with a 3sigma upper limit of 2.0 x 10 ...super(-4) for the D/H ratio. This value is consistent with the earlier Herschel measurement in the Jupiter-family comet 103P/Hartley 2. The canonical value of 3 x 10 super(-4) measured pre-Herschel in a sample of Oort-cloud comets can be excluded at a 4.5sigma level. The observations presented here further confirm that a diversity of D/H ratios exists in the comet population and emphasize the need for additional measurements with future ground-based facilities, such as CCAT, in the post-Herschel era.
176P/LINEAR is a member of the new cometary class known as main-belt comets (MBCs). It displayed cometary activity shortly during its 2005 perihelion passage, which may be driven by the sublimation ...of subsurface ices. We have therefore searched for emission of the H2O 110–101 ground state rotational line at 557 GHz toward 176P/LINEAR with the Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory on UT 8.78 August 2011, about 40 days after its most recent perihelion passage, when the object was at a heliocentric distance of 2.58 AU. No H2O line emission was detected in our observations, from which we derive sensitive 3-σ upper limits for the water production rate and column density of <4 × 1025 mol s-1 and of <3 × 1010 cm-2, respectively. From the peak brightness measured during the object’s active period in 2005, this upper limit is lower than predicted by the relation between production rates and visual magnitudes observed for a sample of comets at this heliocentric distance. Thus, 176P/LINEAR was most likely less active at the time of our observation than during its previous perihelion passage. The retrieved upper limit is lower than most values derived for the H2O production rate from the spectroscopic search for CN emission in MBCs.
ABSTRACT We report observations at millimeter and submillimeter wavelengths of comet C/2009 P1 (Garradd) from 2011 December 28 to 2012 April 24, using the Arizona Radio Observatory submillimeter ...telescope (SMT) and the James Clerk Maxwell Telescope (JCMT). Garradd is a dynamically young long-period comet from the Oort Cloud, with a periodicity of 127,000 yr, that reached perihelion on 2011 December 23 (at = 1.55 AU and Δ = 20.1 AU) and made its closest approach to the Earth on 2012 March 05 (at = 1.84 AU and Δ = 1.26 AU). We obtained gas production rates, and molecular abundances relative to water for HCN, ortho- , CS, CO and . A rotational temperature, K, was determined by observing multiple methanol lines with the JCMT. By averaging the abundance ratio relative to water from the SMT and the JCMT we derive: CO: 7.03% 1.84%, HCN: 0.04% 0.01%, o- : 0.14% 0.03% as a parent molecule (and 0.28% 0.06% as an extended source), CS: 0.03% 0.01% and : . We concluded that Garradd is normal in , depleted in HCN, o- and CS and slightly enriched in CO with respect to typically observed cometary mixing ratios. We also studied the temporal evolution of HCN and CO and find that the production of HCN has a trend similar to water (but with short-term variation), with a decrease after perihelion, while that of CO shows contrary behavior: remaining constant or increasing after perihelion.
The Jupiter-family comet 10P/Tempel 2 was observed during its 2010 return with the Herschel Space Observatory. We present here the observation of the JK (10–00) transition of NH3 at 572 GHz in this ...comet with the Heterodyne Instrument for the Far Infrared (HIFI) of Herschel. We also report on radio observations of other molecules (HCN, CH3OH, H2S and CS) obtained during the 1999 return of the comet with the CSO telescope and the JCMT, and during its 2010 return with the IRAM 30-m telescope. Molecular abundances relative to water are 0.09%, 1.8%, 0.4%, and 0.08% for HCN, CH3OH, H2S, and CS, respectively. An abundance of 0.5% for NH3 is obtained, which is similar to the values measured in other comets. The hyperfine structure of the ammonia line is resolved for the first time in an astronomical source. Strong anisotropy in the outgassing is present in all observations from 1999 to 2010 and is modelled to derive the production rates.
Context. Hydrogen chloride (HCl) and hydrogen fluoride (HF) are expected to be the main reservoirs of fluorine and chlorine over a wide range of conditions, wherever hydrogen is predominantly ...molecular. They are found to be strongly depleted in dense molecular clouds, suggesting freeze-out onto grains in such cold environments. We can then expect that HCl and HF were also the major carriers of Cl and F in the gas and icy phases of the outer solar nebula, and were incorporated into comets. Aims. We aimed to measure the HCl and HF abundances in cometary ices as they can provide insights on the halogen chemistry in the early solar nebula. Methods. We searched for the J(1−0) lines of HCl and HF at 626 and 1232 GHz, respectively, using the Heterodyne Instrument for the Far-Infrared (HIFI) onboard the Herschel Space Observatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1 (Garradd), whereas observations of HF were conducted in comet C/2009 P1 (Garradd). In addition, observations of H2O and H218O lines were performed in C/2009 P1 (Garradd) to measure the H2O production rate at the time of the HCl and HF observations. Three lines of CH3OH were serendipitously observed in the HCl receiver setting. Results. HCl is not detected, whereas a marginal (3.6-σ) detection of HF is obtained. The upper limits for the HCl abundance relative to water are 0.011% and 0.022%, for comet 103P/Hartley 2 and C/2009 P1 (Garradd), respectively, showing that HCl is depleted with respect to the solar Cl/O abundance by a factor more than 6\hbox{$^{+6}_{-3}$}+6-3 in 103P/Hartley 2, where the error is related to the uncertainty in the chlorine solar abundance. The marginal HF detection obtained in C/2009 P1 (Garradd) corresponds to an HF abundance relative to water of (1.8 ± 0.5) × 10-4, which is approximately consistent with a solar photospheric F/O abundance. The inferred water production rate in comet C/2009 P1 (Garradd) is (1.1 ± 0.3) × 1029 s-1 and (0.75 ± 0.05) × 1029 s-1 on 17 and 23 February 2012, respectively. CH3OH abundances relative to water are 2.7 ± 0.3% and 3.4 ± 0.6%, for comets 103P/Hartley 2 and C/2009 P1 (Garradd), respectively. Conclusions. The observed depletion of HCl suggests that HCl was not the main reservoir of chlorine in the regions of the solar nebula where these comets formed. HF was possibly the main fluorine compound in the gas phase of the outer solar nebula. However, this needs to be confirmed by future measurements.
Context. Cometary activity at large heliocentric distances is thought to be driven by outgassing of molecular species more volatile than water that are present in the nucleus. The long-period comet ...C/2006 W3 (Christensen) was an exceptional target for a detailed study of its distant gaseous and dust activity. Aims. We aimed to measure the H2O and dust production rates in C/2006 W3 (Christensen) with the Herschel Space Observatory at a heliocentric distance of ~5 AU and compared these data with previous post-perihelion Herschel and ground-based observations at ~3.3 AU from the Sun. Methods. We have searched for emission in the HO and NH3 ground-state rotational transitions, JKaKc (110–101) at 557 GHz and JK (10–00) at 572 GHz, simultaneously, toward comet C/2006 W3 (Christensen) with the Heterodyne Instrument for the Far Infrared (HIFI) onboard Herschel on UT 1.5 September 2010. Photometric observations of the dust coma in the 70 μm and 160 μm channels were acquired with the Photodetector Array Camera and Spectrometer (PACS) instrument on UT 26.5 August 2010. Results. A tentative 4σ H2O line emission feature was found in the spectra obtained with the HIFI wide-band and high-resolution spectrometers, from which we derive a water production rate of (2.0 ± 0.5) × 1027 molec s-1. A 3σ upper limit for the ammonia production rate of <1.5 × 1027 molec s-1 is obtained taking into account the contribution from all hyperfine components. The dust thermal emission was detected in the 70 μm and 160 μm filters, with a more extended emission in the blue channel. We fit the radial dependence of the surface brightness with radially symmetric profiles for the blue and red bands. The dust production rates, obtained for a dust size distribution index that explains the fluxes at the photocenters of the 70 μm and 160 μm PACS images, lie in the range 70 kg s-1 to 110 kg s-1. Scaling the CO production rate measured post-perihelion at 3.20 AU and 3.32 AU, these values correspond to a dust-to-gas production rate ratio in the range 0.3–0.4. Conclusions. The blueshift of the water line detected by HIFI suggests preferential emission from the subsolar point. However, it is also possible that water sublimation occurs in small ice-bearing grains that are emitted from an active region on the nucleus surface at a speed of ~0.2 km s-1. The dust production rates derived in August 2010 are roughly one order of magnitude lower than in September 2009, suggesting that the dust-to-gas production rate ratio remained approximately constant during the period when the activity became increasingly dominated by CO outgassing.
► We present models that retrieve the dependence of cometary gas flux on direction. ► We determine the rotation rate of Comet 2P/Encke from observations of its CN coma. ► We determine the sense of ...rotation of Comet 2P/Encke from HCN spectra. ► We find evidence in Comet 2P/Encke for gas flow across its terminator. ► Comet 2P/Encke’s outgassing before perihelion is likely to come from deeper layers.
Axisymmetric models of the outgassing of a cometary nucleus have been constructed. Such models can be used to describe a nucleus with a single active region. The models may include a solar zenith angle dependence of the outgassing. They retrieve the outgassing flux at distances from the nucleus where collisions between molecules are unimportant, as function of the angle with respect to the outgassing axis. The observed emissions must be optically thin. Furthermore the models assume that the outflow speed at large distance from the nucleus does not depend on direction. The value of the outflow speed is retrieved. The models are applied to CN images and HCN spectra of Comet 2P/Encke, obtained nearly simultaneously in November 2003 with the 2
m optical telescope on Mount Rozhen, Bulgaria, and with the 10
m Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona, USA. According to
Sekanina (1988), Astron. J. 95, 911–924, at that time a single outgassing source was active. Input parameters to the models like the rotation period of the nucleus and a small correction to Sekanina’s rotation axis are determined from a simpler jet position angle model. The rotation is prograde with a sideric period of 11.056
±
0.024
h, in agreement with literature values. The best fit model has an outflow speed of 0.95
±
0.04
km
s
−1. The same value has been derived from the corkscrew appearing in the CN images. The location of the outgassing axis is at colatitude
δ
a
=
7.4°
±
2.9° and longitude
λ
a
=
235°
±
17° (a definition of zero longitude is provided). Comet Encke’s outgassing corresponds approximately to the longitudinally averaged solar input on a spherical nucleus (i.e. very likely comes from deeper layers) but with some deficiency of outgassing at mid-latitudes and non-zero outgassing from the dark polar cap. The presence of gas flow from the dark polar cap is explained as evidence of gas flow across the terminator. The models rely mostly on the CN images. The HCN spectra are more noisy. They provide information how to determine the best fit outflow velocity and the sense of rotation. The model HCN spectra are distinctly non-Gaussian. Within error limits they are consistent with the observations. Models based solely on the HCN spectra are also presented but, because of the lower quality of the data and the unfavorable observing geometry, yield inferior results. As a by-product we determine the CN parent life time from our CN observations. The solar EUV and Ly
α radiation field at the time of our observations is taken into account.
We investigated the influence of the non-gravitational effects on the orbital motion of 81P/Wild 2. First, the non-gravitational accelerations on cometary nucleus throughout all five revolutions ...around the sun were analysed using both the symmetrical and the asymmetrical g(r)-function. Next, the symmetric model was used to examine the past and future dynamical evolution of Wild 2 within the time interval of c8 thousand years. We then introduced a statistical approach. In the most probable scenario for Wild 2 history, a few thousand years ago, this comet was an object with its perihelion close to the Jupiter orbit with an aphelion distance greater than the Neptune orbit: JN class, JE class, JT class, or even larger. Due to the extremely close approach to Jupiter in 1974, its future evolution differs substantially from earlier, so most probably, comet Wild 2 will survive as a typical Jupiter-family comet during the few thousand years that follow. After 8000 yr, however, the chance that the comet Wild 2 will still be a typical JF comet (with q < 3 AU) drops to 29%. Similarities between the orbital histories of Wild 2 and comets 16P/Brooks 2 and 67P/Churyumov-Gerasimenko are also discussed.
Aims. The non-gravitational perturbations in the orbital motion of comet 81P/Wild 2 are investigated using a two-source model of outgassing. Methods. In the process of orbit improvement the nucleus ...orientation and the location of two active regions are found from numerical fitting of the non-gravitational acceleration model to positional observations of the comet. Two different approaches to the lag angle of the outgassing behind the subsolar meridian are considered, assuming this angle to be either constant or varying with heliocentric distance. Results. The derived spin axis orientation (I sim 60 logical or circ, phi sim 155 logical or circ) as well as the location of two active regions, the northern (\beta=84 logical or circ) and the southern one (\beta=-42 logical or circ), agree with determinations based on different observations by other authors. Orbital linkages of two and three successive apparitions of 81P/Wild 2 indicate an evolution of the change in the orbital period, possible time variations of the spin axis of the comet and an increase of the source areas during the last five revolutions. The non-gravitational perturbations were also used to constrain the sizes of the two active areas as well as the mass and bulk density (\sim 400 kg\,m super(-3)) of the comet nucleus. The modeled two-source water production curve is compared with the activity data represented by the observed water production curve and the brightness curve.
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