2009 Annual Science Report
Rensselaer Polytechnic Institute Reporting | JUL 2008 – AUG 2009
Project 3: Pathways for Exogenous Organic Matter to the Early Earth and Mars
Comets are rich in ices and organic molecules and were almost certainly important sources of biogenic elements to early Earth and Mars. However, because of their relatively high encounter speeds (averaging some 50 km/s), comets may be relatively inefficient sources of organic compounds to these planets. In contrast asteroids, although less rich in organics, may have been more important because their much lower encounter speeds (15 – 20 km/s) allow significant quantities of unaltered material to reach the surfaces of the terrestrial planets. A major question we propose to investigate is the relative contributions of the thermally-altered asteroidal organics versus relatively pristine cometary organics to early Earth and Mars.
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 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 present study focuses on identifying the locations and relative abundances of organic-bearing source bodies in the early solar system. Although cometary bodies – including interplanetary dust grains – are generally richer in organic compounds than asteroidal bodies, the lower relative atmospheric entry velocity of asteroid materials biases the pristine material reaching the surfaces of Earth and Mars toward asteroid sources. In particular, our initial work is aimed at determining the current inventory of asteroids that include organic compounds. This information, combined with models of meteoroid delivery mechanisms and the mass depletion rate of the early asteroid belt, will allow constraints on the amount of organic-bearing asteroid material deposited on the prebiologic Earth and in the same time interval on Mars.
During the period of activity since funding was received, we have classified the spectra of the ~1650 main belt in the SMASS data archive into organic-bearing and organic-free classes. Although this data set had previously been classified into several taxonomies (including the misleadingly named “C-type”), our effort is the first to focus on identifying assemblages analogous to CI1 and CM2 carbonaceous chondrite meteorites. Among the several score of distinct meteorite types, only these two types contain significant amounts of organic compounds, including amino acids. Fortunately, the mild aqueous alteration within their parent bodies that produced the organics and/or that altered nebular and prenebular organics also produced a narrow suite of mineral assemblages which exhibit a subtle but readily identifiable spectral pattern.
We have taken these results and mapped the proportion of these bodies across the present asteroid belt. In particular, we have noted their distribution relative to the major resonances (Kirkwood Gaps) which have represented the primary escape hatches from the belt since Jupiter acquired its current mass and orbital location. Theory suggested that the proportion of such organic-bearing asteroids should maximize within a limited interval of heliocentric distance. This is the result of two competing effects. The formation of such assemblages requires the incorporation of water ice into their parent bodies and subsequent moderate heating to melt the ice and aqueously alter the original anhydrous assemblage. Ice only becomes a significant nebular constituent beyond some heliocentric distance, while the magnitude of post-accretionary heating declines with increasing distance.
The results of our survey of the ~1650 SMASS asteroids indicate that the relative proportion of CI1 and CM2-type assemblages maximizes between the 3:1 and 5:2 resonances at 2.5 and 2.82 AU. A significant proportion of the organic-bearing asteroid material reaching the early Earth and Mars will derive from this heliocentric interval. Ongoing work will estimate the initial total mass in this region – and in the adjacent less organic-rich intervals – in order to calculate the amount of CI1 and CM2-type material delivered to the early Earth and Mars in the time interval before the earliest life on Earth.