Georgia Tech | EPICS Lab

Energetic Processing and Stimulated Chemistry of Pre-Cometary Ices and the Primordial Surfaces of Comet Nuclei

Since Fred Whipple first proposed the "dirty snowball" hypothesis,¹ researchers have viewed comets as a tantalizing glimpse of the chemistry and composition of the early Solar System. Prior to being accreted into comets, primordial materials such as grains and molecules formed in gaseous molecular clouds, were altered to varying extents by chemical and physical processes in the solar nebula. Thermochemical reactions, ultraviolet photochemistry, ion-molecule reactions, and physical mixing with other nebular materials all probably occurred.² The hypothesis that the interiors of comet nuclei contain pristine material from that time seems plausible since those that we observe receive most of their thermal and radiation processing only after they have approached the inner solar system. Until then, they sit in a dark cosmic "deep freeze" far away from the sun. For instance, comets in the Oort cloud, orbiting roughly 50,000 AU away are far outside the bow shock of our solar system. In this region, they experience exposure to galactic cosmic rays and deep UV photons.

The Oort cloud at ~10⁵ AU has ~ 10¹² comets. These have been stored for > 10⁹ years and thus exposure times are similar to those in the Kuiper belt. Pre-cometary grains may have been radiation processed for > 10⁶ yrs before being incorporated into meter-sized or larger bodies and then into the final cometary nucleus.³ Thus, comets can still retain a considerable amount of information about the primordial processes that affected the formation of the solar system. However, they may not be unaltered "snapshots" of that time in history. In fact, certain notable discrepancies between the composition of interstellar gas and cometary materials exist and represent the key challenge in drawing the line from past to present in our solar system. First, there are apparent differences in the relative abundances of materials in comets compared to the ISM ices,⁴ as shown in Table 1. This comparison is complicated by the fact that the ISM molecules are identified by broad infrared bands that are dependent on guest-host interactions, while comet materials are observed only during sublimation. However, these estimates are the best available data and if accurate, indicate that some form of thermal or non-thermal processing has occurred.

Comets are regarded as the most pristine samples from the formation of our solar system. Their elemental abundances, isotope ratios and ortho-para spin temperatures contain information concerning the composition and dynamics of the early evolution of the solar system. Several missions (e.g. Deep Impact, Star Dust, CHANDRA, Hubble, ROSAT, EUVE, IUE, ISO, Voyager 1 and FUSE) and ground-based IR studies have or will provide a wealth of information concerning the composition of comets, and the cosmic ray, far ultraviolet and x-ray fluxes permeating the solar system. This proposal seeks to provide further information on the physics and chemistry involved in the radiation processing and stimulated chemical reactions within pre-cometary ices and in the outer layer of current comet nuclei. Recent studies have brought to light several means by which non-thermal processing of cometary material can cause disproportionate and non-equilibrium changes to their hydrogen-deuterium ratio and ortho-para ratio through substrate-mediated and organic contaminant-mediated mechanisms. Such processes may alter the long term apparent composition of comets, and affect the interpretation of observations. We propose to investigate the detailed molecular mechanisms by which radiation induced effects, carbonaceous substrates, and mixed molecular ices alter the isotope fractionation and spin disproportionation of ice as cometary analogs. Specifically, we will address the following issues: What are the detailed dynamics of isotopic substitution on fast electronic dissociation and hot-atom reactive scattering events in ice and mixed ices? What is the role of carbonaceous grains on deuterium isotope fractionation and selective ortho-para spin adsorption and interconversion? Are there mechanisms in electronic dissociation that are nuclear-spin state selective? These effects in cometary ice over long periods may alter the apparent composition and hence affect the accuracy of extrapolation backward to the origins of comets and our solar system.

Comets are frequently considered time capsules which hold important clues regarding the formation and early evolution of the outer planets 4 to 4.5 billion years ago. This is a time when the presolar dense molecular cloud was condensing to form the protoplanetary disk. Consequently there has been much interest in the chemical transformations that may skew any extrapolation backward to this time. An important, but often-overlooked process is radiation-induced isotope enrichment. In our studies of electron-beam induced processing of ice surfaces, we have measured large isotope effects leading to enrichment of deuterium in the condensed phase. This effect can be enhanced due to pores and the presence of co-adsorbed species. These phenomena may be potentially relevant to understanding the deuterium/hydrogen ratios observed in comets Halley, Hyakutake and Hale-Bopp (and in future comets). Since comets are related to early planet formation models, we plan to study the radiation processing, reactions and release of gas from mixed ices as surrogates of comet cores.

Several missions (e.g. Deep Impact, Star Dust, CHANDRA, Hubble, ROSAT, EUVE, IUE, ISO and FUSE) and ground-based IR studies have or will provide a wealth of information concerning the composition of comets, and the cosmic ray, far ultraviolet and x-ray fluxes permeating the solar system. This proposal seeks to provide further information on the physics and chemistry involved in the radiation processing and stimulated chemical reactions within pre-cometary ices and in the outer layer of current comet nuclei.

Some of the most important clues concerning the origin of Earth's oceans come from the isotope abundances of comets and cometary nuclei.¹⁰ For example, our presolar nebula contained roughly 6 x 10²⁷ kg of water, most likely possessing the cosmic D/H ratio of 1.5 x 10^-4. Today, there are an estimated 10¹³ comets that are remnants of this source material, but the D/H ratio has been measured only in three comets (they possess about twice the D/H ratio of Earth), and other indicators suggest they formed in the Uranus-Neptune range. While this value is much closer to Earth's than to the cosmic ratio, and comets are the prime candidate for a post-cooling delivery mechanism, the question remains: where did the deuterium go?