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Project Progress

This research focuses on: (a) identifying the pathways by which extraterrestrial prebiotic organic compounds reached the early Earth and Mars, and (b) estimating the flux of such material as a function of time. Asteroids and comet nuclei (which include both Kuiper Belt and Oort Cloud objects) represent the primary potential solar system reservoirs that contributed to that flux. Our knowledge of the nature of organic compounds in comet nuclei is limited because physical samples are very limited. However, it is expected that most of the organic compounds contained within comet nuclei will be essentially pristine nebular and/or pre-nebular molecules. In the case of asteroids – as indicated by organic-bearing CI1 and CM2 meteorites – the organic compounds will commonly have been either modified or synthesized as a result of aqueous processes within mildly heated asteroid parent bodies. Thus both asteroids and comet nuclei are potential sources of prebiotic molecules to the early Earth and Mars. However, these are independent sources with different delivery rates. Comet nuclei are generally rich in carbon and carbon compounds, and are commonly assumed to also be richer in organic compounds than the asteroid derived carbon-rich CI & CM type meteorites. However, there is a significant difference in the delivery efficiency of comet and asteroid materials. On average the atmospheric entry velocity of meteoroids released by comet nuclei is ~50 km/sec, while the atmospheric entry velocity of meteoroids derived from asteroids is ~20 km/sec. Only the small fraction of cometary meteoroids with entry velocities below ~25 km/sec have any chance of delivering intact organic compounds to the Earth’s surface. Thus, although cometary material is certainly richer in carbon and carbon compounds than asteroidal material, the high average entry velocity strongly attenuates the cometary contribution of intact prebiotic compounds, which would be destroyed by vaporization of the infalling meteoroid. This attenuation should be less intense for early Mars.

Size matters, at least if you want to deliver prebiotic molecules to the Earth. Dust sized-particles released by comets and by asteroid collisions are likely to undergo significant bleaching by solar ultraviolet light, which reduces their delivery efficiency for prebiotic molecules. Larger bodies (e.g., larger than a few tens of meters) either detonate in the atmosphere vaporizing most of their mass or reach the surface while still retaining a substantial portion of their cosmic velocity, vaporizing most of their mass upon impact. The maximum delivery efficiency for unaltered material is for bodies in the meter to 10-meter size range. Most meteorites falls are from meteoroids in this size range.

The second effect of size relates to the delivery of asteroidal material into Earth-crossing orbits. There are several routes that an asteroid or asteroid fragment can follow that will lead to a close encounter with the Earth or Mars. However, the fastest and most productive pathway is via the chaotic zones associated with the proper motion and secular resonances in the asteroid belt (e.g. Gladman et al. 1997). These resonances are produced primarily by the gravitational effects of Jupiter (e.g., Wisdom 1985). We note the importance of distinguishing between the “early Earth” and the “earliest Earth”. Outside the zones cleared by planetary accretion, a large residual planetesimal population would have remained. Due to multiple gravitational perturbations, these objects would have rained down on the earliest Earth, and could be regarded as the very last stages of accretion. Once this population was depleted, the probability of impact sterilization decreases precipitously. It is in that subsequent period in which asteroidal sources of prebiotic molecules become potentially important. During this “early Earth” period, size again becomes important, this time because of the Yarkovsky effect (e.g., Bottke et al. 2006). Dynamical theory indicates that rapid orbital evolution of objects in the chaotic regions associated with these proper motion and secular resonances will clear out objects quickly, resulting in the depleted zones known as Kirkwood Gaps. The Yarkovsky effect provides a means of rapidly migrating 1 to 10 meter asteroid fragments into these resonances. Thus, fragments ejected from larger asteroids near these resonances continuously resupply the resonances resulting in a high flux of meteoroids to Earth crossing orbits; meteoroids in a size range which maximizes their delivery efficiency of prebiotic molecules.

In the previous year, we quantified the heliocentric distribution of CI1- and CM2-type materials in the “feeding zones” of these resonances from our analysis of more than 1300 asteroid spectra from the SMASS survey (Xu et al. 1995, Bus and Binzel 2002a). Although most of these objects had been classified (e.g., Bus and Binzel 2002b) into the various asteroid taxonomies, these classifications were inadequate for our purposes. In particular, our goal was to determine the fractional abundance of CM2- and CI1-like assemblages across the asteroid belt. We utilized diagnostic spectral parameters of CI1 and CM2 meteorites to identify these assemblages within the sample population. This work showed that CI1 and CM2-type assemblages were located within the feeding zones of the critical resonances (a pattern preserved from the early solar system), and that dynamical theory would predict that such materials were delivered into Earth-crossing orbits in rough proportion to the abundance adjacent to the resonances.

During the past year, our efforts have focused on testing the prediction of the dynamical models of meteoroid delivery from the feeding zones of the resonances to Earth-crossing orbits. In particular we have focused on characterizing the asteroids adjacent to the 3:1 proper motion resonance located at 2.5 AU, and which dynamical models indicate should provide a large fraction of Earth-approaching meteoroids. We have utilized the data obtained from an ongoing observational program to investigate specific asteroids from which dynamical models predict relatively large fluxes of Earth-crossing meteoroids. Various parts of our initial results in this investigation have been described in Gaffey (2011), Fieber-Beyer and Gaffey (2011), Fieber-Beyer et al. (2011a, b), and Reddy et al. (2011). The results are distributed between several papers because we start with a specific asteroid or asteroids which dynamical models have identified as significant potential sources of Earth-crossing meteoroid and then test that prediction. Although work is still ongoing, the results to date generally confirm the dynamical predictions, giving us confidence that similar dynamical models can be used to confidently estimate the flux of organic-bearing asteroid meteoroids to the early Earth.

Papers supported in part by this research grant:
Fieber-Beyer S. K. and M. J. Gaffey (2011) Near-infrared Spectroscopy of 3:1 Kirkwood Gap Asteroids (3760) Poutanen and (974) Lioba. Icarus 214, 645-651.
Fieber-Beyer S. K., M. J. Gaffey, M. S. Kelley, V. Reddy, C. M. Reynolds, and T. Hicks (2011) The Maria Asteroid Family: Genetic Relationships and a Plausible Source of Mesosiderites near the 3:1 Kirkwood Gap. Icarus 213, 524-537.
Fieber-Beyer S. K, M. J. Gaffey, and P. A. Abell (2011) Mineralogical Characterization of Near Earth Amor Asteroid 1036 Ganymed. Icarus 212, 149-157.
Gaffey M. J. (2011) Mineralogy of Asteroids. In XV Special Courses at the National Observatory of Rio de Janeiro. AIP Conf. Proc. 1386, 129-169. doi: 10.1063/1.3636041
Reddy V., J. M. Carvano, D. Lazzaro, T. A. Michtchenko, M. J. Gaffey, M. S. Kelley, T. Mothe-Diniz, A. Alvarez-Candal, N. A. Moskovitz, E. A. Cloutis, and E. L. Ryan (2011) Mineralogical characterization of Baptistina Asteroid Family: Implications for K/T impactor source. Icarus 216, 184–197.

Other literature cited:
Bottke W. F. Jr., D. Vokrouhlický, D. P. Rubincam, and D. Nesvorný (2006) The Yarkovsky and YORP Effects: Implications for Asteroid Dynamics. Annu. Rev. Earth Planet. Sci. 34, 157–191.
Bus S. J. and R. P. Binzel (2002a) Phase II of the small main-belt asteroid spectroscopic survey – The observations. Icarus 158, 106-145.
Bus S. J. and R. P. Binzel (2002b) Phase II of the small main-belt asteroid spectroscopic survey – A feature based taxonomy. Icarus 158, 146-177.
Gladman B. J., F. Migliorini, A. Morbidelli, V. Zappalà, P. Michel, A. Cellino, C. Froeschlé, H. F. Levison, M. Bailey and M. Duncan (1997) Dynamical lifetimes of objects injected into asteroid belt resonances. Science 277, 197-201.
Wisdom J. (1985) A perturbative treatment of motion near the 3/1 commensurability. Icarus 63, 272-289.
Xu S., R. P. Binzel, T. H. Burbine and S. J. Bus (1995) Small main-belt asteroid spectroscopic survey: Initial results. Icarus 115, 1-35.