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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 solar system reservoirs that contributed to that flux. In the case of cometary nuclei, it is expected that most of the contained organic compounds 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 altogether as a result of aqueous processes within mildly heated parent bodies.

The CI1 (e.g., the meteorite Orgueil) and CM2 (e.g., the meteorite Murchison) assemblages contain significant amounts of extra-terrestrial organic matter, as well as elemental carbon and water-bearing minerals. Today they represent highly prized meteorites, but such material has been falling to Earth over the entire history of the solar system. Based on compositional patterns seen in the asteroid belt, these objects formed in a limited heliocentric zone. The critical conditions for forming these organic- and water-bearing assemblages is that during planetesimal accretion, their parent bodies incorporated a component of water ice and that their parent bodies underwent moderate heating sufficient to melt that ice without attaining high temperatures which would have dehydrated object. In this widely accepted model, the parent bodies of the CI1- and CM2-type assemblages underwent internal aqueous alteration which converted anhydrous mafic silicates (primarily olivine) into hydrated phyllosilicates (clay minerals). During this process, pre-existing nebular organic molecules apparently underwent alteration and/or abiotic carbon species were processed to produce relatively high concentrations of prebiotic molecules in the more altered meteorites.

Based on observations of compositional gradients in the asteroid belt, this formation zone appears to have extended from ~2.4 AU (inside of which water ice shouldn’t be stable and wouldn’t be incorporated into accreting planetesimals) to ~3.2 AU (beyond which heating of the planetesimals was not sufficient to melt contained water ice). This region covers the outer portion of the main asteroid belt. While all portions of the planetesimal population across the region of the asteroid belt would contribute to the population of Earth- and Mars-impacting objects, there sizes and orbital locations which will dominate the flux of objects that deliver prebiotic molecules to the early Earth and early Mars. (In the subsequent discussion, we focus on material delivered to the early Earth, but the same general considerations apply to material delivered to 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 fall 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. These resonances are produced primarily by the gravitational effects of Jupiter (e.g., Wisdom 1985). Of these resonances, the 3:1 proper motion resonance at 2.5 AU, the 5:2 proper motion resonance at 2.82 AU, and the ν6 secular resonance at ~2.1 AU (at low inclinations) are the fastest pathways, and are likely to have dominated the asteroidal flux onto the early Earth (e.g. Gladman et al. 1997).

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). The 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 resonance 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.

The question that we’ve addressed is the abundance of CI1- and CM2-type materials in the “feeding zones” of these resonances. Combined with models that predict the fraction of meteoroids from each resonance that impact the Earth (e.g., Gladman et al. 1997), and estimates of the initial mass of the asteroid population and its depletion rate with time, we can estimate the amount of CI1- and CM2-type material delivered to Earth during the time period (perhaps 4.2 to 3.9 Gyr) when sustainable life arose.
During the past year, we have completed 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.

Although there is a taxonomic classification designated as “C”, which naively is often taken to mean “carbonaceous”, no such inference can be made with any confidence. The C-taxonomic type includes a diverse suite of assemblages, a subset of which are CI1- or CM2-type assemblages. The most diagnostic test for such assemblages would be spectra in the ~2.5 – 4 μm spectral interval, where the fundamental water and C-H features can be detected. Unfortunately, observations in this spectral interval from ground-based observatories are exceedingly difficult due to atmospheric interference, so that only a few bright objects have been observed.

Instead, we have used a less direct indicator of CI1- and CM2-type assemblages which can be applied to the large number of visible and near-infrared (VNIR) CCD spectra (~0.4 – 0.96 μm spectral coverage) obtained in the Small Mainbelt Asteroid Survey (SMASS). Figure 1 shows the VNIR spectral of three different types of CM2 meteorites. All are characterized by one or more broad shallow absorption features in the interval between ~0.6 and ~1.0 μm. These features arise from Fe²⁺ and/or Fe³⁺ crystal field and/or charge transfer absorption in the phyllosilicates which make up the bulk of these meteorites.

We have completed our classification of 1323 SMASS spectra into three groups: “Yes” the features are present; “No” the features are absent; and “Maybe” which is self-explanatory. Based on ongoing work on CI1 chondrites (Cloutis et al. 2010) and on planned work on CM2 chondrites, we might revisit this issue at some future time but not as part of the present effort.
The result of our census of mainbelt asteroids is shown on Figure 2. In the feeding zone of the 3:1 and 5:2 proper motion resonances, approximately 15% of the objects are CI1- and CM2-type assemblages. (The 8:3 resonance is less efficient than the 3:1 and 5:2 resonances, but would intercept some meteoroids and transfer them into Earth-crossing orbits.)

Final analysis will be completed in the next several months, but some initial numbers are interesting. We will refine the following estimates, but for the moment these include:
Initial Mass of asteroid belt between 2.4 & 3.0 AU ~1 Earth mass ~ 6 × 1024 kg
Fraction of mass lost from this region between 4.2 and 3.9 Gyr ~10%
Abundance of prebiotic molecules in CI1/CM2 assemblages ~0.2%
Fraction of asteroids in 2.4-3.0 AU interval that are CI1/CM2-type ~15%
Fraction of asteroid fragments escaping through the 3:1, 5:2 & 8:3 resonances ~50%
Percent of meteoroids from the 3:1, 5:3 & 8:3 resonances that impact Earth ~1%
Percent of meteoroid mass surviving atmospheric entry ~2%

Taking these at face value, they would indicate that ~10¹⁹ kg of material from this region rained down on the early Earth. Spread equally over the surface of the Earth this would be ~20,000 kg / m², equivalent to a layer ~10 meter deep, containing ~2 × 10¹⁶ kilograms of prebiotic molecules, or about 40 kilograms of prebiotic molecules per square meter of the Earth’s surface.

There are significant uncertainties in some of these values, but overall the result is probably a lower limit. Nevertheless, it is hard to dismiss the conclusion that the asteroid belt must have been a significant source of prebiotic molecules to the Earth during the period in which life arose and persisted.

REFERENCES:
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.
Cloutis E. A., T. Hiroi, M. J. Gaffey, and P. Mann (2010) Spectral Reflectance Properties of CI1 Carbonaceous Chondrites. 73rd Annual Meteoritical Society Meeting, New York City, Abstract #5120 and manuscript in preparation.
Gaffey, M.J. (1976). Spectral reflectance characteristics of the meteorite classes. J. Geophys. Res. 81, 905-920.
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.