Comets as Parent Bodies of CI1 Carbonaceous Meteorites.
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The CI1 carbonaceous meteorites are jet-black stones that contain indigenous extraterrestrial water. The albedo of the Orgueil meteorite is extremely low (~0.05) and comparable to that of the very dark C-type asteroids and the nuclei of comets. This is blacker than asphalt which has an albedo of ~ 0.07. The European Space Agency Halley Multicolor Camera aboard the Giotto Spacecraft obtained images at the closest approach (00:03:01.84 UT on March 14, 1986) at a distance of 596 km from the centre of the nucleus revealing detailed topographic features on the black (albedo 0.04) surface and jets Lamarre et al. (1986) reported that IKS-Vega data indicated the temperature of nucleus of comet Halley was 420 K +/- 60K at 0.8 A.U which was consistent with “a thin layer of porous black material covering the comet nucleus.” The Deep Space 1 spacecraft found the 8 km long nucleus of Comet 19P/Borrelly to be very hot (~345 K) with prominent jets aligned with the orientation of the rotation axis of the nucleus and albedo of 0.01 to 0.03 (Soderbloom et al. 2002). Ices of water, carbon dioxide, methane and other volatiles in the cold nucleus in proximity to the hot crust would melt and then boil to produce high pressure beneath the crust if gas is released faster than it can escape through the porous crust. In regions where the pressure exceeds the strength of the crust, localized failure of portions of the crust could result in explosive release of the gas giving rise to the observed flaring of comets and the dramatic jets.
Once a comet enters the inner solar system, it becomes hot from solar radiation on the black nucleus and loses mass rapidly. The European Space Agency Infrared Space Observatory (ISO) showed that water was the primary volatile (75-80 %) of the 40-50 km diameter nucleus of Comet Hale-Bopp. Minor volatile fractions detected (CH₄, NH₃ and H₂CO) could have come from clathrates (H₂O ice with simple gasses like CO₂ and NH₃ in a stable lattice structure) or result from atmospheric chemistry. ISO found that Hale-Bopp released water vapor, carbon monoxide and carbon dioxide at a rate of 2 x 10⁹ kg/sec and detected olivine in the dust. Olivine is commonly encountered in meteorites. As comets lose ices they develop an inert outer crust from the less volatile material. The nuclei of comets are extremely complex – they exhibit rugged terrain, smooth rolling plains, deep fractures and are composed of very dark material. This black crust becomes very hot while the comet is in the inner regions of the Solar System.


Figure 7.a. Deep Space 1 image of Comet P/Borrelly with jets of gas and dust; b. Deep Impact image of nucleus of Comet 9P/Temple 1 shows regions of exposed water ice and c. temperature map from Deep Impact IR spectra d. Giotto Halley Multicolor Camera (HMC) image showing jets emanating from of the 0.04 albedo nucleus of Comet P/Halley Image Courtesy: Max Plank Institute for Solar System Research http://www.mps.mpg.de/en/projekte/giotto/hmc/; e. Deep Impact spacecraft extended mission (EPOXI) image of the nucleus of comet Hartley 2 showing jets of dust and gas. Image Courtesy: NASA/JPL UMD). Figure 7.a. is a NASA Deep Space 1 spacecraft composite false color image showing geyser-like jets erupting from the long prolate nucleus (8 km) of comet 19P/Borrelly on Sept. 22, 2001. (The colors indicate three orders of magnitude in light level (red is 1/10, blue 1/100 and purple 1/1000 the intensity of the comet nucleus). The red bumps on the nucleus are real and show where the main jet resolves into three distinct narrow jets coming from distinct sources on the comet nucleus. These narrow jets are entirely consistent with the hypothesis that internal pressures generated by steam produced by melting of internal ices which then boil into gases as they are vaporized as heat conducts through hot crust. The NASA Deep Impact probe obtained the valuable data about the nature of comets as it approached and when the impactor collided with the nucleus of comet 9/P Temple 1 on July 4, 2005. Fig. 7.b is a Deep Impact image of the nucleus of comet Temple 1. The regions shown in blue are where exposed deposits of water ice that were detected on the surface of the comet nucleus Sunshine et al. (2005). These water ice regions ere observed to be ~30% brighter than the surrounding areas and probably were exposed when portions of the black crust was blown off into space by the explosive eruptions such as were recorded in a video by the spacecraft. The Deep Impact measurements of the temperature profile of comet P/Temple 1 nucleus at 1.5 AU is shown in Figure 7.c. Even as far away from the Sun as Mars the jet-black comet nucleus reaches temperatures as high as 330 K (57 oC). Furthermore, the lowest temperatures measured on the crust were ~ 280 K (7 oC) which is slightly above the temperature at which water ice changes from solid to liquid phase. Prior to the impact, the ambient outgassing of Temple 1 was ~6x10²⁷ molecules/s of water. However, the free sublimation of ice calculated above (~200 K) was only ~4.5 x 10²¹ molecules/m²/s indicating that the ambient outgassing had significant subsurface sources. The Deep Impact spacecraft also observed numerous events of flaring of the nucleus and eruption of geyser-like jets as the comet was approached and before the collision of the impactor. On November 4, 2010, the NASA EPOXI extended mission of the Deep Impact Spacecraft passed within 435 miles of the 2.2 km long nucleus of comet Hartley 2 and revealed bright jets of carbon dioxide gas and dust.
These observations of comets are consistent with the hypothesis that the comet crust impedes the flow of gasses such that pressures develop as ices melt and vaporize in pockets and cavities beneath the crust. This provides the pressures needed to allow water to transition from the solid to the liquid state and then into the gaseous state. This would create micro-niches with pools of liquid water trapped within pockets in rock and ice, very much analogous to the cryoconite and ice bubble ecosystems contained psychrophilic microbial extremophiles such as those described from the glaciers and frozen Pleistocene thermokarst ponds of Alaska and Siberia and the glaciers and perennially ice covered lakes of the Schirmacher Oasis and Lake Untersee in East Antarctica (Hoover, 2008; Hoover and Pikuta, 2010; Pikuta et al. 2005). If gas is produced faster than it can escape through the porous crust, it could high pressures resulting in localized failure of weaker portions of the crust and the violent eruption into space of carbon dioxide, water vapor and chunks of crust and particles of ice and dust propelled into space and directed into the dust tail of the comet. These dust particulates could give rise to meteor showers as the comet passes through the tail. From time to time, larger chunks of the ejected may survive passage through the Earth’s atmosphere and this could be the link between comets and the CI1 (and possibly the CM2) carbonaceous meteorites. The fact that the CI1 meteorites contain minerals that were extensively altered by liquid water on the parent body and that the stones have been found to contain a large amount of indigenous extraterrestrial water clearly establishes that their parent bodies were most likely comets or water-bearing asteroids. It is now well known that the black nuclei of comets get very hot (significantly above >273 K where water ice melts) as they approach the Sun.
Gounelle et al. (2006) used the eyewitness accounts to compute the atmospheric trajectory and orbit of the Orgueil meteoroid and concluded that the orbital plane was close to the ecliptic and that entry into the atmosphere took place at a height of approximately 70 km and an angle of ~20°. Their calculations indicated the meteoroid terminal height was ~20 km and the pre-atmospheric velocity was > 17.8 km/sec. They found the aphelion to be 5.2 AU (the semi-major axis of orbit of Jupiter) and perihelion ~0.87 AU, which is just inside the Earth's orbit as would be expected for an Earth-crossing meteorite. This calculated orbit suggests the Apollo Asteroids and the Jupiter-family of comets are likely candidates for the Orgueil parent body include (although Halley-type comets are not excluded).
The cosmochemistry data for a cometary parent body is entirely consistent with the composition and characteristics of the CI1 meteorites. This suggestion that the parent body of the CI1 carbonaceous meteorites were possibly comets is significant with regard to possible existence of indigenous microfossils in the Alais, Ivuna and Orgueil meteorites. From the extensive evidence of aqueous alteration on the Orgueil parent body and the presence of indigenous water in the Orgueil meteorite it is clear that the parent body was either a water-bearing asteroid or a comet. However the Giotto and Vega observations of Halley and the Deep Impact Observations of the nucleus of 9P/Temple-1 have clearly established that these bodies get very hot as they enter the inner regions of the Solar System. It is now clear that any water bearing asteroid with an albedo of the Orgueil meteorite would reach a temperature above 100 C at 1AU. At these temperatures, water ice and other volatiles would be converted to liquid water, steam, and produce an expanding cloud of gas and expelled particulates. Any planetessimal orbiting the Sun and possessing a gaseous envelope and dust tail is traditionally refered to as “comet” rather than an asteroid, and therefore it seems logical that comets represent the most probable parent bodies for these water rich, black meteorites that travel in trajectories that cross the orbit of planet Earth.
4.6 Role of Comets and Carbonaceous Meteorites in the Origin and Evolution of the Earth’s Atmosphere, Hydrosphere, and Biosphere The relationship of comets with carbonaceous meteorites and their role in the origin and evolution of the atmosphere, hydrosphere, and biosphere of Earth has become better understood during the past few decades. The cratered surface of the moon provides clear evidence of the intense Hadean bombardment of the inner planets and moons by comets, asteroids and meteorites during the early history of the Solar System. Watson and Harrison (2005) interpreted the crystallization temperatures of 4.4 Ga Zircons from Western Australia as providing evidence that liquid water oceans were present on the early Earth within 200 million years of the formation of the Solar System. It has recently become more widely recognized that comets played a crucial role in the formation of the atmosphere and oceans of early Earth during the Hadean bombardment (Delsemme, 1997; Steel, 1998; Owen, 1997).
In 1978, Sill and Wilkening proposed that comets may have delivered life-critical biogenic elements carbon and nitrogen trapped within clathrate hydrates in their icy nuclei. In the same year, Hoyle and Wickramasinghe (1978, 1981, 1982, 1985) have proposed that comets delivered not only water, biogenic elements and complex organic chemicals to the surface of planet Earth, but that they also delivered intact and viable microorganisms. The detection of microfossils of cyanobacteria and other filamentous trichomic prokaryotes in the CI1 carbonaceous meteorites (which are likely cometary crustal remnants) may be interpreted as direct observational data in support of the Hoyle/Wickramasinghe Hypothesis (Wickramasinghe 2011) of the role of comets in the exogenous origin of terrestrial life.

Eberhardt et al. (1987) measured the deuterium/hydrogen ratios in the water of comet P/Halley. Delsemme (1998) found that that the D/H ratio of the water molecules of comets Halley, Hale–Bopp and Hyakutake were consistent with a cometary origin of the oceans. Dauphas et al., (2000) interpreted the deuterium/hydrogen ratios indicate that the delivery of water and ice to the early Earth during the late Hadean heavy bombardment by comets, asteroids and meteorites helped to cool the Earth’s crust and form the early oceans. Table V shows data extracted from the Robert et al. (2000) compilation of Deuterium/Hydrogen ratios of selected components of the Cosmos.
When these bodies are grouped in accordance with their D/H ratio it is easily seen that the telluric inner planets and the LL3 (stony) and SNC (Mars) meteorites have high (~500-16,000) ratios and the gas giants, protosolar nebula, ISM and Galaxies are very low (~15-65). The D/H ratios of the comets (~290-330) and carbonaceous meteorites (~180-370) are much closer to that of Earth (~149) and support the hypothesis that they may have made significant contributions to the formation of the oceans of our planet. It is interesting that the D/H ratios of comets are very similar to the ratios measured in the kerogen, amino acids and carboxylic acids of the Orgueil (CI) and other (CM, CV, and CR) carbonaceous meteorites. This supports the view that although stony meteorites are most probably derived from rocky asteroids, the carbonaceous meteorites most probably are derived from water-bearing asteroids or the nuclei of comets. The 30 m diameter fast-spinning carbonaceous asteroid 1998 KY26 that was discovered on June 2, 1998 has been found to contain 10-20% water. However, the small carbonaceous, water-rich asteroid 1998 KY26 also has color and radar reflectivity similar to carbonaceous meteorites and it may be a spent comet. Near IR observations indicated the presence of crystalline water ice and ammonia hydrate on the large Kuiper Belt object (50000) Quaoar with resurfacing suggesting cryovolcanic outgassing. The Cassini/Huygens spacecraft has recently obtained data indicating that a vast liquid water ocean may also exist beneath the thick frozen crust of Titan. Cassini/Huygens has also detected evidence for cryovolcanic water-ice geysers on Titan and Saturn’s moon Enceladus.