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Comets Nature and Nomenclature of Comets • A comet is a small ice-rich body, about 1 km size, in an eccentric orbit about the Sun. The nuclei have low albedo (0.03) and red color. The mass density is 1g/cm3 or less. • The phenomena that may make the comet spectacular are associated with the progressively rapid evaporation of ices from its solid nucleus during its rapid fall toward the Sun. • These gases, and some entrained dust, expand outward around the nucleus to form a roughly spherical envelope (coma) about the nucleus. At a distance of 1,000 - 100,000km from the nucleus, the expanding coma becomes so tenuous that the gas and dust become uncoupled and begin to stream systematically away from the nucleus in different direction. The dust roughly follows ballistic trajectories, whereas the gas flows in the direction radially outward from the Sun (tail). Size of comet nuclei Meech et al. (2004) Hale-Bopp --> Largest observed comet 1P/Halley 17P/Holmes Figure 1 5.5km 1.7km January 8, 2005 v Anti-solar direction Figure 2 C/2004 Q2 (courtesy of Tai, NHAO) Nature and Nomenclature of Comets • Many comets have orbits that lie well within the Solar System. Frequently the apherlia of their orbits are near the Jupiter or Saturn. Their orbital periods are below 200 years (corresponding to the semimajor axis < 34AU). These are called “Short-period comets”. • The other comets have orbits that are nearly randomly distributed in space, have enormous orbital eccentricities and semimajor axes, and typically pass perihelion only about once every few million years. These comets are referred to as “Long-period comets”. Recently, some dynamical families were found among these long-period comets (e.g. Kreutz sun-grazer). Nature and Nomenclature of Comets • Historically, when a comet is discovered it has been named after the discoverer (e.g. Comet Holmes was discovered by Edwin Holmes). • The prefix C/ denotes a comet. When a comet discovered, it is given preliminary designation such as C/2001 Q3 (SOHO). In this example, “2001” is the date of discovery and “Q” designates the half-month in which the discovery was made (A for the first half of January, B for the second half, and so on, skipping “I”). The name of the discoverer is appended in parentheses. • Periodic comets are assigned the prefix P/ (for periodic). Since there are many observers who have found more than one comets, it has become convenient to affix a catalog number to each periodic comet. It is not unusual for a comet to be discovered by two or more observers on the same night, an event that produces names such as 29P/Schwassmann-Wachmann 1, 31P/Schwassmann-Wachmann 2, 73P/Schwassmann-Wachmann 3. • Occasionally a comet goes dormant or is lost due to the inaccurate orbital data. The comets that have suffered such a fate bear the designation D/.(e.g. D/Shoemaker-Levy) 1. Orbits Cometary Orbits 1: Short-Period Comet • The orbits of the short-period comets are ellipses of moderate eccentricity and inclination (Figure 3). Most of comets have inclinations of less than 30 relative to the ecliptic plane. Their eccentricities lie mostly 0.2-0.7. The perihelion distances of most comets are1-2AU. • Several comets have retrograde orbits (e.g. 1P/Halley, 55P/Tempel-Tuttle). The shortest orbital period is 3.3 years for 2P/Encke, a body whose orbit fall within the range of near-Earth asteroid orbits. 96P/Machholtz 1 has the shortest perihelion distance of 0.12AU. 29P/Schwassmann-Wachamann 1 have perihelion beyond the Jupiter orbit (5.77AU). • The longitudes of the node and the longitudes of the perihelia of short-period cometary orbits are roughly evenly distributed around the ecliptic. 2P/Encke 23P/Brorsen-Metcalf 8P/Tuttle (Ursids) 1P/Halley (Orionids) 55P/Tempel-Tuttle (Leonid) 17P/Holmes 29P/Schwassmann-Wchamann 1 Figure 3. Eccentricity-inclination plot for short period comets. (updated 2007.11.5) Figure 4. Eccentricity-Period plot for short period comets. (updated 2007.11.5) 2P/Encke P/Read (S3) Figure 5. e-Q (aphelion distance) plot for short period comets. P/Read (S3) Main-Belt 29P/Schwassmann-Wachamann 1 Figure 6. e-q (perihelion distance) plot for short period comets. Halley-type comets Jupiter-family comets Figure 7. i-Q (aphelion distance) plot for short period comets. Figure 8. plot for short period comets. Sun-grazing comet http://soho.esac.esa.int Figure 9. Orbital Elements of Comets (Tp>200 years) Kreutz Sungrazers Kreutz Sungrazers Figure 8. • The origin of the long-period comets is an interesting problem. It would be helpful to explore the distribution of the long-period comets over their entire range of semimajor axes a from 100 AU. Figure 12. N The noticeable features are that there is a strong peak in the distribution of observed comets near 1/a=0, parabolic heliocentric orbit and a lack of comets with energies greater than parabolic must be rare. The right side, the energy constant for an elliptical orbit, is a function of its semi-major axis alone and is independent of the eccentricity. Similar quantities can be defined for parabolic and hyperbolic orbits. It can be shown that Cpara=0 and Chyper=/2a. 1/a (AU-1) Dones et al. (2004) • The most common stars are not Sunlike stars but are M9 dwarfs of mass 0.07 M. These stars perturbed the Oort cloud every 11 million years. A combination of proper motion and parallax data by Hipparcos and radial velocity data by ground-based telescope tells us stellar encounters with the Oort cloud (Garcia-Sanchez et al. 1999) Figure 14. • The huge spheroidal cloud of long-period comets with the radius of ≈104 AU is first suggested by Dutch astrophysicist, Jan Oort in 1950, and it is referred to as “Oort cloud” • The orbital velocity of a comet near aphelion can be calculate from æ2 1ö v = GM ç - ÷ è r aø 2 whence the orbital velocity of a comet of a= 104 AU is found to be 0.3 km/s. • Long-period comets have very small velocities aphelion. They spend virtually all of their lives at distance from the Sun, stored at extremely temperatures. The blackbody temperature by illumination at r 104 (AU) is near great low solar æ r ö-1/ 2 T(r) = 280ç ÷ è AU ø = 280 ´ (10 4 )-1/ 2 = 2.8 [K] • Nowadays, the Kuiper belt is believed to be the main source for short-period comets. It is a region of the Solar System between 30 AU to ~55 AU from the Sun. The Kuiper belt is similar to the main-belt asteroid that consists of small bodies. • The most widely-accepted hypothesis of its formation is that the Oort cloud's objects initially formed much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptical or parabolic orbits First Kuiper-belt object, Jewitt and Luu (1993) Heating by Passing Stars • • • The prospect that long-period comets may be pristine, unaltered fossils that have escaped from significant heating since the time of their formation suggests that they may be the best probes of ancient Solar System processes available to us. But the passage of other stars through the Oort cloud might give these comets the chance to be heated to much higher temperatures than could be provided by the Sun. Most stellar encounter involve M-class red dwarfs. In order to achieve temperatures high enough for the loss of very volatile species such as CH4 (~30K), the encounter distance of the star from the comet must be rather small. The black body temperature at distance r from a star of luminosity L (in units of the solar luminosity) is roughly, 1/ 4 æ r ö-1/ 2 æ L ö T(r) = 280ç ÷ ç ÷ è AU ø è L ø é æ 10-4 L ö1/ 4 ù 280 r =ê ç ÷ ú = 0.9AU êë 30 è L ø úû 2 • Thus, the comet located around 1AU from the star can be altered by the heat of the star. However, we have to consider the orbital change of the comet during the encounter. A star of mass 0.1M passing at a distance D=1 AU from a comet nucleus at a speed of 20 km/s can import a velocity change of 10 km/s, which exceeds the escape velocity 0.3m/s at 104 AU from the Sun. Space Mission to Comets Target Comet type Mission type Year 19P/Borrelley JF Flyby 2001 Stardust 81P/Wild 2 JF Sample return 2004 Deep Impact 9P/Tempel 1 JF Impact 2005 67P/Churyumov-Gerasimenko JF Rendezvous 2014 1P/Halley H Deep Space 1 Rosetta Vega, Giotto, Suisei, Sakigake 1986 v [1] Soderblom et al. et al. “Observations of Comet 19P/Borrelly by the Miniature Integrated Camera and Spectrometer Aboard Deep Space 1”, Science 296, 1087 (2002) v [2] Boice et al. “The Deep Space 1 Encounter with Comet 19P/Borrelly”, Earth, Moon and Planets 89, 301 (2002) v [3] A’Hearn et al. “Deep Impact: Excavating Comet Tempel 1”, Science 310, 258 (2005) v [4] Brownlee et al. “Surface of Young Jupiter Family Comet 81P/Wild 2: View from the Stardust Spacecraft”, Science 304, 1764 (2004) [5] Sunshine et al. “Exposure Water Ice Deposits on the Surface of Comet 9P/Tempel 1”, Science 311, 1453, (2006) Deep Space 1 Mission and 19P/Borrelly • Deep Space 1 (DS1) is a part of NASA’s program to test technologies to lower the cost and risk. It succeeded in the flyby of its target, 19P/Borrelly, on September 22, 2001. • At encounter, 25 visible-wavelength images and 45 short-wave infrared spectra (1.3-2.6 m) were obtained. The images cover solar phase angles from 88o to 52o. • There are two classes of dust feature: “jets” and “fans (or very wide jets)”. By comparing the seven images, two sets of collimated dust jets were distinguished. The jet is aligned at the core of the main jet, and the jet are offset by about 15o from the direction of the jet. The fan-shaped dust feature is centered on the Sun line. It is found that the main jet is nearly aligned with the rotational axis of the nucleus. Main Jet Main Jet jet jet Boice et al. (2002) • There are two classes of dust feature: “jets” and “fans (or very wide jets)”. By comparing the seven images, two sets of collimated dust jets were distinguished. The jet is aligned at the core of the main jet, and the jet are offset by about 15o from the direction of the jet. The fan-shaped dust feature is centered on the Sun line. It is found that the main jet is nearly aligned with the rotational axis of the nucleus. 19P/Borrelly nucleus topography • The nucleus can be divided into two fundamental units: Smooth Terrain and Mottled Terrain. • The smooth terrain occupies the broad basin that dominates the central part of the comet. It shows higher albedo. Areas of smooth terrain appear to be the general source region of both the main jets and the fans. Several mesa-like features are found within the smooth terrain, and appear to be associated with the active jets. *Mesa: an elevated area of land with a flat top and sides that are usually steep cliffs. Soderblom et al. (2002) Smooth terrain on 19P/Borrelly • A possible scenario for mesa formation could be that – Thick, dusty lag deposit insulate the comet’s volatile-rich substrate. – Mesa slopes are area of thinner deposits and the source of volatile loss. – Volatile loss erode the mesa tops and collapses the lag material to the lower floor of the smooth terrain. Boice et al. (2002) Dust and Gas Mesa scarp Smooth terrain Heavy rocks 1.55 1.75 1.95 2.15 2.35 2.55 Wavelength (m) Reflectance (normalized at 2.359m; offsets +0.02) Reflectance (normalized at 1.7m) 1.35 1.3 1.5 1.7 1.9 2.1 2.3 2.5 Wavelength (m) •No water ice •High temperature •Unknown absorption at 2.39 m • No evidence of impact crater with diameter larger than 200 m, are present. It indicate a young and active surface compared to typical asteroids and most small planetary satellites. • The entire nucleus is extremely dark with geometric albedo of 0.007 - 0.035. • Short-wavelength infrared spectra (1.3-2.6 m) show a red-ward slope and reveal that the surface is hot (345K) and dry. Stardust Mission and 81P/Wild 2 • Stardust was launched in 1999 by NASA and returned to the Earth on January 2006 to release a sample material capsule. It is the first sample return mission to collect cosmic dust and return the sample to the Earth. • The mission target of Stardust, 81P/Wild 2 passed within only about one million kilometers of Jupiter, whose strong gravitational pull altered the comet's orbit and brought it into the inner solar system. Its orbital period changed from 40 years to about 6 years, and its perihelion is now about 1.59 AU. • On July 3, 2007, a second mission was approved to re-visit the 9P/Tempel 1, under the designation of New Exploration of Tempel 1 (NExT). NExT will take images of crater formed by Deep Impact mission. NExT is scheduled to fly by Tempel 1 on February 14, 2011. Brownlee et al. (2004) Pit-halo Flat-floor • There are two distinguish morphologies for circular depression: pit halo and flat floor. Pit-halo feature have a rounded central pit surrounded by an irregular and rough region of excavated material, whereas flat-floor feature do not have halo region and bounded by steep cliffs. • The authors argued that most of depression results from the compression of the weakly cohesive, porous target. They experimentally reproduced these two depression observed on 81P/Wild 2 by hypervelocity impacts. Pit-halo structures formed when a loose sand layer covered a harder underlying layer, and flat-floor crater resulted when the unconsolidated surface layer was modestly baked to give it some cohesion. Brownlee et al. (2004) Hemenway Mayo Sekanina et al. (2004) Triangulation of 20 jets shows that 2 spread out from the dark side and 7 coincide with relatively bright surface spots. Deep Impact Mission and 9P/Tempel 1 • Deep Impact is NASA space probe launched on January 12, 2005 that was designed to study the composition of the interior of the comet by colliding a section of the spacecraft into the comet. • The mission target is 9P/Tempel 1. At 5:52 UTC on July 4, 2005, the impactor of the Deep Impact probe successfully impacted the comet's nucleus, excavating debris from the interior of the nucleus. • 9P/Tempel 1 is a member of Jupiterfamily comets. The perihelion distance is 1.5AU and the orbital period is 5.5 years. It was discovered in 1867. 9P/Tempel 1 1 km Composite of ITS images. Arrows a and b point to the large, smooth areas. The impact site is indicated by the other large arrow. The small arrows highlight the scarp, bright due to the illumination angle. Celestial north is near the rotational pole; ecliptic north is more nearly upward. (A’Hearn et al. 2005) • The mean radius is estimated to have been 3.0±0.1 km. • There are several dozen circular features, ranging from 40 to 400 m. The cumulative size-frequency distribution of these features is consistent with impact crater population. • Two region of smooth surface exist. One smooth region is bounded to the north by a scarp (급사면) ~20 m in height. The Rougher terrain (east), smooth terrain (west), and the scarp strongly suggests removal by backwasting of a 20m layer, leaving an exhumed surface containing the circular features. • Albedo variations are within 50% of an average of 0.04 in the visible wavelength. Color ratios between 300 nm and 1000 nm are uniform to <2%. No exposures of clean ice or frost have been identified on the basis of albedo or color. -A’Hearn et al. (2005) scarp Rough terrain Smooth terrain 9P/Tempel 1: Temperature Map R=1.5AU With the near infrared spectrometer, the two dimensional spectral data were obtained. The temperature is derived by fitting the thermal component. The maximum temperature is around the sub-solar point. There are no areas on the sunlit surface colder than 260 K. The obtained temperature is above the temperature expected for sublimation of volatile on the surface. Therefore, volatiles probably sublimate below the surface. Exposed Water Ice Deposits on the Surface of Comet 9P/Tempel 1 Sunshine et al. (2006) 81P/Wild 2 19P/Borrelly 9P/Tempel 1 The appearances of three comets explored by the spacecrafts are so different from one another. Both 9P/Tempel 1 and 19P/Borrelly have had long times in the inner solar system, whereas 81P/Wild 2 has not. 81P/Wild 2 have lost only a meter or so of surface after 1974 apparition. Deep Impact: Cratering Experiment • Impactor struck the nucleus near its southern limb. At about 20 s and 10s before impact, large dust particles hit the impactor, causing the pointing to turn away. • The impact time is 05:44:35.4 ~ 05:44:36.2. The final crater has not been observed because of a ejecta. Blue dotted line is the position of the spectrograph slit. 1st flash 700-ms2nd flash First flash lasted <200 ms. This is probably associated with vaporization of the impactor and part of the comet. Second flash may be associated with the first eruption of material at the surface. A hot material moved at a projected velocity of 5km/s. Ejecta Impactor (364kg, 10km/s) 2nd flash shadow A’Hearn et al. 2005 285K850K Brightness of the peak pixel in the plume versus time in MRI. The origin of time is based on extrapolating the motion of the plume back to its origin at constant velocity. The dashed curve is what is expected for uniformly expanding particles reflecting sunlight. 1.7 km/s Total mass = 4000kg Initial temperature=3500K Liquid silicate droplet Spectra 3km beyond the limb of the nucleus Pre-impact Radiance (W m-2 sr-1 m-1) 0.0025 Post-impact 0.015 0.0020 0.010 0.0015 0.0010 0.005 0.0005 0.000 0.0000 -0.0005 -0.005 2.5 4.5 3.0 3.5 4.0 Wavelength (m) 2.5 3.0 3.5 4.0 Wavelength (m) 4.5 The most dramatic difference was the large increase in organics. The feature at 4.4 m is CN stretch band. Methyl cyanide could be present in the ejecta. Chemical Composition (1) • The information about the chemical composition of the nucleus comes from the observed chemical composition of the gaseous coma. • The elements H, C, N and O present in the spectra of comets by the telescopic observations. • The in-situ measurements of Comet 1P/Halley with ionization mass spectrometer on board the Vega spacecraft have given the elemental composition of dust. -Solar -Halley’s dust -Comet gas -Chondrite Anders & Grevesse (1989), Geochim. Cosmochim. Acta. 53, 197 Jessberger et al. (1988), Nature 332, 21, 691 Swamy, in Physics of Comets Andrers & Ebihara, Geochim. Cosmochim. Acta. 46, 2363 (1982) Chemical Composition (2) • There was a standing problem on the depletion of carbon by a factor of 4. The observations of 1P/Halley helped in resolving this carbon depletion problem as carbon is mostly tied up in organics, called the CHON particles. • A direct comparison of the abundances of 1P/Halley and in the Sun shows that the elemental abundances in comet material are very similar to the solar values except for hydrogen. • The detailed results derived from 1P/Halley make it possible to build a consistent model for the volatiles escaping from the nucleus. A heuristic model is shown below. 92.0% with O 5.6% with N 2.2% Hydrocarbon 0.2% with S 78.5 H2O 2.6 N2 1.5 C2H2 0.1 H2S 4.5 HCO OH 1.0 HCN 0.5 CH4 0.05 CS2 4.0 H2CO 0.8 NH3 0.2 C3H2 0.05 S2 3.5 CO2 0.8 N2H4 1.5 CO 0.4 C4H4N2 Delsemme 1991, In Comets in the Post-Halley Era Evolution of Comets into Asteroids In Asteroids III, page 669-685 Paul R. Weissman Willam F. Bottke Jr. Harold F. Levison Introduction • Solar system astronomers have long speculated on the possible existence of extinct or dormant comets, objects that appear asteroidal but in truth are icy objects that had their origins as comets. • The chaotic nature of the dynamical evolution of objects in planet-crossing orbits, as well as nongravitational accelerations on comets caused by outgassing, make it impossible to track orbits accurately backward (or forward) in time more than a few decades or centuries. • In the past decade, several lines of research have provided new data (e.g. NEO findings and physical observations of comets and asteroids) and new tools (numerical simulation) with which to pursue these questions. • In this chapter, the authors discuss how these new tools have made it possible to argue for, at least statistically, the existence of extinct or dormant comets among the observed asteroid population, in particular among the NEOs. Cometary Definitions • Active comet: a comet nucleus losing volatiles and dust in a detectable coma. • Inactive comet: a comet nucleus that is active during part of its orbit, but presently is in a part of the orbit where volatile loss is negligible and there is no detectable coma. • Dormant comet: a comet nucleus that, although once active, has lost the ability to generate a detectable coma in any part of its present orbit. A dormant comet perturbed to a smaller perihelion distance might be reactivated. Or an impact might remove an overlying nonvolatile crust and expose fresh icy materials to sublimation, reactivating the comet. • Extinct comet: a comet nucleus that has lost its ices or has its ices so permanently buried under a nonvolatile crust that it is incapable of generating a coma. • Asteroid: an interplanetary body formed without significant ice content, and thus incapable of displaying cometary activity. • Exceptions: Kuiper belt objects, Trojan asteroids, some outer main-belt asteroids Physical End States of Cometary Nuclei (1) • Many cases of comets that disappeared can likely be attributed to poorly determined orbits. • But in other cases, well-observed comets seem to have simply disappeared (e.g. 3D/Biela). Note that the designation “D” refers to “defunct” comets. Several of these comets displayed unusual brightness changes on their last apparition (outbursts and/or fading). • Three physical mechanisms have been proposed to explain the disappearance of comets: (1) random disruption, (2) loss of all volatiles, and (3) formation of a nonvolatile crust or mantle on the nucleus surface. Unfortunately, none of these mechanisms are well understood or quantified. • An example of (1) is comet LINEAR, D/1999 S4. Recently, this comet was observed to completely disrupt as it passed through perihelion in July 2000. • Weissman (1980) compiled records of observations of disrupted or split comets and showed that 10% of dynamically new comets from the Oort cloud split, 4% for returning long-period comets, and only 1% for short-period comets (per orbit). The splitting events did not show any correlation with perihelion distance, distance from the ecliptic plane, or time of perihelion passage. 1999 S4 (LINEAR) i = 149º q = 0.765 AU e = 0.9999970 Weaver et al. (2001) Physical End States of Cometary Nuclei (2) • Formation of a nonvolatile crust or mantle on the nucleus surface is the one process that would presumably lead to asteroid-like objects. This was first proposed by Whipple (1950). There are two ways that such crusts may form. • First, irradiation of comets by galactic cosmic rays and solar protons will sputter away volatiles and transform organic molecules to more refractory forms during the comets’ long storage in the Oort cloud or Kuiper belt (Johnson et al., 1987; Moore et al., 1983; see also Weissman and Stern, 1997). This irradiated crust would extend ~1/ρ m below the nucleus surface, where ρ is the density of the cometary materials in g cm–3. An interesting and unsolved problem is how such a crust is removed when the comet re-enters the planetary region on it first perihelion passage, allowing it to become active. • A second method for forming a cometary surface crust is through the lag deposit of nonvolatile grains, left behind or launched on suborbital trajectories that do not achieve escape velocity, as water and more volatile ices sublimate from the nucleus surface during perihelion passage. It is not clear how such large non-volatile grains, which presumably are too heavy to be lifted off the nucleus by evolving gases, might form. However, if they did, thermal models have shown that a layer only a few to perhaps 10 cm thick would be sufficient to insulate the underlying ices from sublimation (Brin and Mendis, 1979; Fanale and Salvail, 1984; Prialnik and Bar-Nun, 1988). Luu (1994) Physical End States of Cometary Nuclei (3) • 1P/Halley and 19P/Borrelly images appear to support the idea that comets can develop crusts and thus evolve to dormant or extinct objects. • Further support comes from estimates of the active fraction of other cometary nuclei. A’Hearn (1988) showed that many Jupiter-family comets have active fractions of only a few percent. For comets 49P/Arend-Rigaux and 28P/Neujmin 1, the active fractions are estimated as only 0.08% and 0.1% respectively. • This suggests that comets may slowly age and evolve toward total inactivity, either by developing non-volatile crusts on their surfaces or by some other as-yet-unrecognized mechanism. Thus, crust formation does appear to provide a mechanism for evolving cometary nuclei to dormant or extinct states. Cometary Dynamics: Evolution to Asteroidal Orbits (1) • Comets have traditionally been divided into two major dynamical groups: long-period (LP) comets with orbital periods >200 yr, and short-period (SP) comets with periods <200 yr. Long-period comets typically have random orbital inclinations while short-period comets typically have inclinations relatively close (within ~35°) to the ecliptic plane. • In addition it has become common to divide the SP comets into two subgroups: Jupiter-family comets (JFC), with orbital periods <20 yr and a median inclination of ~11°, and Halley-type comets (HTC), with periods of 20–200 yr and a median inclination of ~45°. • A more formal dynamical definition of the difference between the JFC and HTC comets was proposed based on the Tisserand parameter. The Tisserand parameter is an approximation to the Jacobi constant, which is an integral of the motion in the circular restricted three-body problem. It was originally devised to recognize returning periodic comets that may have been perturbed by Jupiter and is given by T= aJ a 2 +2 1e cos(i) ( ) a aJ where aJ is Jupiter’s semimajor axis, and a, e, and i are the comet’s semi-major axis, eccentricity, and inclination, respectively. An example of such an encounter for Jupiter family comet 81P/Wild 2 is shown below. A close approach to Jupiter alters the orbital elements of the comet with the semi-major axis from 4.95 AU to 1.49 AU. 81P/Wild 2 in early 1970’s Cometary Dynamics: Evolution to Asteroidal Orbits (2) • Jupiter-family comets have T > 2 while long-period and Halley-type comets have T < 2. Levison (1996) proposes that comets with T > 2 be known as ecliptic comets, while comets with T < 2 be known as nearly isotropic comets. HTC HTC JFC • Comets on planet-crossing orbits are transient members of the solar system. Close and/or distant encounters with the giant planets, in particular Jupiter, limit their mean dynamical lifetimes to ~0.4–0.6 m.y. (Weissman, 1979; Levison and Duncan, 1997). Thus, they must be continually re-supplied from long-lived dynamical reservoirs. • The different inclination distributions of the ecliptic and nearly isotropic comets reflect their different source reservoirs. Nearly isotropic comets (Long-Period and Halley-Type Comets) are believed to originate from the nearly spherical Oort cloud. Ecliptic comets (Jupiter-family comets) are fed into the planetary system from the highly flattened Kuiper belt. • Fernández et al. (2001) compared measured albedos for 14 cometary nuclei and 10 NEOs with Tisserand parameters T<3, and 34 NEOs with T > 3. They showed that all of the comets and 9 out of 10 of the NEOs with T < 3 had low albedos, ≤0.07, while most of the NEOs with T >3 had albedos >0.15. This result suggests that the T <3 objects have a cometary origin. From Interstellar Material to Cometary Particles and Molecules P. Ehrenfreund S.B. Charnley and D.H. Wooden In Comet II, Univ. of Arizon Press (2004) Introduction of molecules Is There Interstellar Cloud Material in Comets? • The gases observed in cometary comae originate from the nuclear ices and offer insight into the nucleus composition. • Coma molecules can either have – a “native” source, which have been sublimed directly from the nucleus itself, – An “extended” source, due to the decomposition of large organic particles or molecules. • A comparison with the list of interstellar ices and gases found in dark molecular clouds and in regions of massive and low-mass star formation, suggests a direct link. • The abundances of the spices in star forming-regions are similar to those in comets (see Table). ISO spectrum of high-mass star Comparison of molecules in star-forming region and comets Star-forming regions Comets High-mass stars Solar-type stars 1P/Halley Hyakutake Hale-Bopp H2O 100 100 100 100 100 CO 9—20 5—50 15 6—30 20 CO2 12—20 12—37 3 2—4 6—20 CH3OH 0—22 0—25 1—1.7 2 2 CH4 1—2 <1 0.2—1.2 0.7 0.6 H2CO 1.5—7 — 0—5 0.2—1 1 OCS 0—0.3 <0.08 — 0.1 0.5 NH3 0—5 <9.2 0.1—2 0.5 0.7—1.8 0.4—3 — — — 0.06 C2H6 <0.4 — — 0.4 0.3 HCN <3 — 0.1 0.1 0.25 C2H2 — — — 0.5 0.06—0.1 H2S — — 0.04 0.8 1.5 HCOOH Ehrenfreund & Charnley, Annu, Rev. Astron. Astrophys. 2000, 38:427 Ehrenfreund, Charnley & Wooden, in Comet II, Arizon Press Paucity of CO2 • H2O, CO, CO2 and CH3OH are important ice species that can be used as a tracer for the interstellar/cometary link. • ISO identified CO2 ice as one of the major components in interstellar ice mantles with an average abundance of ~15–20% relative to water ice. CO2 appears to be much less abundant in comets than in molecular clouds (Feldman et al., 2004). • Whereas CO2 ice is ubiquitous in the interstellar medium, the measured abundance of CH3OH is highly variable and in fact is undetected toward many sources. The cometary abundance of methanol is appreciable but generally much smaller (~2%) when compared to interstellar ices (5–25%). • The possible explanation is that these molecules are interstellar but have had their original populations partially depleted in the nebula. For example, CO2 is very susceptible by H atoms in warm gas and partially destroyed at the accretion shock or in nebular shock waves. In the hot gas, the CO2 abundance is quickly diminished through reactions which proceed efficiently at temperature above ~1000K. H2S and CS2 in comets • The problem in comparing the abundances of S-bearing compounds is that the cometary parents CS2 and H2S are not detected in interstellar ices. • On the other hand, the most abundant of the interstellar S-bearing compounds have all been detected in comets (e.g. OCS). It is very unlikely that these sulfuretted species represent pristine interstellar material. Probably, these are photoproducts of other compounds or may have a distributed (polymeric?) source. HNC in comets • The difficulties in connecting observed cometary volatiles with interstellar molecular cloud material can be illustrated for the case of the HNC molecule. • Cometary HNC was originally discovered in the coma of Hyakutake (Irvine et al., 1996); the high HNC/HCN ratio in the comet Hyakutake is similar to that in the interstellar materials, which suggests that this HNC was preserved interstellar material. • However, subsequent observation showed that the HNC/HCN ratios in HaleBopp varied as a function of the heliocentric distance. Furthermore, Hale-Bopp showed two HNC components: one correponding to nuclear emission and another corresponding to extended (or jet) source. The HNC/HCN ratio of the nuclear component is similar to hose of Hyakutake and comet Lee, whereas the extended component is almost 2-3 times as large (Blake et al. 1999). • The most likely source of HNC in the coma is the decomposition of a large organic, perhaps polymeric, compound (Rodgers and Charnley, 2001a; Irvine et al., 2003). C chain radicals in comets • The C chain radicals, C2 and C3 are widespread in comets. • Helbert et al. (2002) have shown that the abundances of these molecules could be derived from C2H2, CH33CH, and C3H8 in a coma chemistry driven by electron-impact dissociations. However, methylacetylene (CH33CH) and propane (C3H8) have not yet been detected in comets. • Synthesis of long, unsaturated, C-chain molecules appears to be one signature of interstellar organic chemistry. There have been tentative detections of C4H and its possible parent C4H2 in Halley and Ikeya-Zhang, respectively (Geiss et al., 1999; Magee-Sauer et al., 2002). C4H2 and C6H2 were found in the protoplanetary nebula CRL 618. These detections of long C-chain molecules in comets would be strong evidence for an interstellar origin of these organics H-C º C-C º C-H