Are There Cometary Meteorites?

Comets have been often considered as potential sources of meteorites [e.g., 1], but there are no meteorites currently believed to have come from comets. Recent developments, including the identification of comet-asteroid transition objects, new information on the collisional history of Jupiter-Family comets, and on the composition of cometary solids, provide new insights into the topic of cometary meteorites. We have revisited this question [2] and conclude that comets do indeed yield meteorites, which have either not been found or recognized. Here, we will summarize the arguments that we should have comets from meteorites, and consider the expected characteristics of cometary meteorites. Jupiter-Family comets as parent bodies Jupiter-Family (JF) comets are those with low inclinations and short period orbits, dominated by the gravitational influence of Jupiter [3]. A number of arguments summarized by Wetherill [1] suggest that if cometary meteorites exist, they are most likely derived from the nonvolatile residues of JF comets. Two of the main predictions reviewed by [1] have been confirmed. First, several arguments support the notion that JF comets leave behind nonvolatile residues strong enough to survive as meteorites if they have low Earth-encounter velocities. Observations of the nuclei of three low-activity comets have shown that the active fraction of their nuclear surface was less than 1% in each case. Analysis of fireballs believed to be of cometary origin suggests a small fraction (perhaps 10) of cometary material is relatively strong (~ 10 dynes/cm), and is abundant enough to account for the number of recovered weak carbonaceous meteorites [4]. In addition, [5] argues that the densities of many shower meteors have been underestimated, and can be as high as carbonaceous chondrites or even ordinary chondrites. Second, the identification of 4015 1979VA as Comet Wilson-Harrington 1949 III [ 6], and 3200 Phaethon as the (probably cometary) parent body of the Geminid meteor shower, confirms that some fraction of Earth-crossing asteroids have a cometary origin. Orbital characteristics make 4015 W-H particularly interesting, even among the comet-asteroid transition objects. It has a low inclination orbit (2.6 degrees), perihelion just inside Earth's orbit (0.995 AU), and aphelion in the asteroid belt (4.3 AU). This orbit has the potential of delivering meteoroids to Earth at lower velocities than most other near-Earth objects; hence, 4015 W-H is a potentially meteorite producing comet [7]. Anders [8] argued against a cometary origin for all stony meteorites classes that have gas-rich members. He used the implanted solar wind noble gases and the cosmic ray exposure ages to infer the approximate heliocentric distance (4-8 AU) and cratering rate (10-10 higher than for the Moon) where the regolith formed, and concluded they were inconsistent with Oort Cloud comets. However, JF comets most likely originate in the Kuiper belt [3], where the cratering rate is approximately the same as in the asteroid belt, but 10-10 greater than in the Oort cloud [9]. In addition, the uncertainties in solar wind implanted gas concentrations and surface residence times used by [8] may be sufficiently large to include at least the inner Kuiper belt. Besides, these arguments do not apply to rare classes without gas-rich members. Expected characteristics Based on studies of cometary dust [10], interplanetary dust particles (IDPs) [11], and cometary meteors and fireballs [4], we have compiled a list of the main characteristics we expect in cometary meteorites: 1. Rare (as rare as CI carbonaceous chondrites) 2. Dark (~5% geometric albedo) 3. Weak (~10 dynes/cm) 4. High porosity ($35%), low density (#2g/cm) 5. Highly unequilibrated Fe/(Mg+Fe) in silicates 6. Nearly Solar elemental abundances 7. High abundance of C, N, and organics 8. Anhydrous silicates 9. More likely than asteroidal meteorites to contain interstellar grains with peculiar isotopic ratios 10. Likely not to have chondrules 11. Unremarkable cosmic ray exposure ages (~10 years) 12. Presence of polymerized hydrocarbons resulting from galactic cosmic ray exposure of ices, and different types of organics in cometary material than in asteroidal meteorites 13. Presence of cosmogenic nuclides from galactic cosmic ray exposure of ices