|
|
Astrophysics |
Our research seeks to test the hypothesis that organic molecules relevant to the origin of life are ubiquitous to interstellar condensations from which planetary systems are born. The principal carriers of the biogenic elements in interstellar clouds are submicron sized dust grains. The physical and chemical properties of these particles, and their evolution with respect to time in regions of active star formation, are important and topical issues in modern astrophysics which have potentially far-reaching implications for studies of the origin of life.Research on star formation has been greatly stimulated in recent years by advances in infrared astronomy, providing a means of studying stellar populations deep within molecular clouds in regions hidden from view at visible wavelengths. Infrared observations also provide an extremely powerful technique for investigating the nature of interstellar dust: solid state spectral features contain a wealth of information on the composition, internal structure and thermal history of the grains. The spectral resolving power required to extract all information inherent in the profiles is now routinely available on both ground-based telescopes and the Infrared Space Observatory.
We are carrying out a systematic study of infrared dust features in a carefully selected sample of embedded young stellar objects and distant field stars viewed through molecular clouds. The primary aim is to explore the evolution of icy grain mantles in the cocoons of low-mass protostars, using the field stars as the 'control experiment' delineating dust properties in undisturbed molecular-cloud material.
Results are interpreted with reference to the optical properties of compounds synthesised in the laboratory under simulated interstellar conditions. Our observations provide detections of not only simple ices such as H2O, CO and CO2, but also organic molecules of various degrees of complexity and oxidation state, including CH4, CH3OH, H2CO, HCOOH and HCN. This program of research will lead to a clear understanding of the evolution of organic matter in dust in the environments of objects which are realistic analogs of the early Solar System.
This research is being carried out in collaboration with groups at NASA Ames Laboratory, the Laboratory for Space Research, Groningen, and Leiden University.
Chemistry and Physics of the Solar Nebula
Synthesis of Biogenic Compounds by Shocks in the Solar NebulaVirtually all of the material which eventually formed the protoplanetary disk was processed by strong shock waves: matter accreted from the parent molecular cloud was accelerated by gravity to highly supersonic infall velocities before striking the disk surface, where it passed through an accretion shock. This shock heated the accreting material to temperatures which ranged from about 5000K in the outer solar system to above 10,000K in the inner solar system. Although the chemistry of these shocks is largely unexplored, studies of the relevant physics show that the chemical effects must have been profound for the biogenic materials:
- Shock heating causes qualitative changes in the gas-phase chemistry. The exothermic ion-neutral reactions which dominate the chemistry of cold (10K), quiescent gas are supplanted by neutral-neutral reactions with energy barriers 1000K. These reactions are important sources of species that are not synthesized efficiently by ion-molecule reactions, including the simplest hydrocarbons. Thus methane and acetylene, which account for a negligible fraction of the gas-phase carbon in cold regions, may be quite abundant behind a shock. Shocks also transform virtually all of the gas-phase oxygen into water.
- Fast shocks with velocities above 30km/s produce copious UV radiation, mainly by collisional excitation of the Lyman alpha line of atomic hydrogen. UV photons ionize most of the heavy elements, and this presumably has a significant affect on the gas-phase chemistry. The absorption of UV radiation by icy mantles on dust grains drives photolytic reactions which lead to the formation of organic material on the grain surfaces.
- Shocks can vaporize dust grains. Ice mantles are returned to the gas in shocks with speeds above 10km/s. Faster shocks can vaporize volatile organics, refractory organics, and even the refractory mineral cores of the grains.
The goal of our research is to model the gas and grain surface chemistry of shock-heated material in the primitive solar nebula. Principal questions to be addressed include the following:
- How much of the material initially locked up in grain mantles (ices and organic material) is returned to the gas phase by the accretion shock? How does the answer depend on distance from the center of the nebula? Will the mantles reform before the incorporation of grains into planetesimals and, if so, how is the chemistry of the re-formed mantles different from the chemistry of ``primitive'' material (e.g., in comets) which has not been subjected to shock processing.
- Were shocks responsible for the large abundance of methane in the giant planets?
- What biogenic compounds are synthesized by gas-phase chemistry in shocks? Are these compounds subsequently incorporated into grain mantles or do they stay in the gas phase?
- How does UV photochemistry affect the gas-phase and grain surface chemistry of the protoplanetary disk?
- How does shock chemistry affect the fractionation of species in the gas and solid phases?
- For what species, if any, do shock waves ``reset the chemical clock?'' That is, which abundances lose all memory of their values prior to star formation? Conversely, which abundances are still linked to the chemistry of the parent molecular cloud?
This project is being carried out in collaboration with Dr. Alexander Tielens of NASA/Ames.
Constraints on Planetary Habitability by Ultraviolet RadiationIntroduction Previous work on habitable zones around solar-type stars has been concerned primarily with the constraint that liquid water must exist on the surface of a planet suitable for life, a condition currently satisfied only by Earth in our own solar system. An important additional constraint, generally overlooked in previous work, is the effect of solar ultraviolet (UV) radiation. UV light may have had a profound influence on the early Earth environment, before the establishment of the ozone layer some 2.5Gyr ago. If, for example, it can be shown that UV exposure at the surface was sufficient to be fatal to primitive organisms, this would give support to shielded environments (such as subterranean hydrothermal systems) as more realistic scenarios for the origin of life. More generally, it is important to know UV flux levels at the surface and in the atmosphere of the primitive Earth, as UV radiation may be a vital energy source driving prebiotic chemical reactions.
With the first detection of extrasolar planets and the intensification of the search for Earth-like planets orbiting other stars, constraints on habitability are of increasingly direct relevance to exobiology.
Methods and preliminary results The first goal is to determine detailed mean stellar spectral energy distributions as a function of spectral type and evolutionary status, to quantify the flux entering planetary atmospheres. The archive of the International Ultraviolet Explorer satellite (IUE) is the primary resource as a database of stellar UV fluxes. This archive contains data in the form of flux vs. wavelength over the range 320-120nm for many thousands of stars of all spectral types. For a preliminary study we selected three representative single (non-binary) main sequence stars of spectral type F, G and K from the IUE Low Dispersion Spectral Reference Atlas; such stars seem most likely to provide a suitable and stable environment to support life on any associated planets.
We plan to extend our study to include a much larger sample of stellar spectral types. This will enable us to (1) obtain fuller sampling of spectral subclasses amongst main sequence stars; (2) check for consistency amongst stars of the same subclass; (3) investigate evolutionary effects amongst main sequence stars of different ages; and (4) compare UV flux levels emitted by zero-age main-sequence and pre-main-sequence stars of similar mass. It is well known that the luminosity and spectral energy distribution of a star evolve during its main-sequence lifetime, and may depend on composition (metallicity). It will be important to determine to what extent these factors affect the UV flux emitted by the star. We will therefore carry out a careful assessment of the evolutionary status of all the stars included in our sample.
Our stellar data provide input to model calculations that determine the flux incident on the surface of a planet as a function of assumed atmospheric composition. To calculate atmospheric ozone concentrations, we use a photochemical model similar to that described by Kasting et al. (1985, J. Geophys. Res., 90, 10497), but containing Cl chemistry in addition to that of C, H, O and N. Our model takes into account the effect of ambient temperature on the rates of the various chemical reactions.
In our preliminary study, we considered the affect of different UV fluxes on the ozone layer of an Earth-like planet with the current terrestrial atmospheric composition. We found that an Earth-like planet orbiting an F-type or K-type star may well receive LESS harmful UV radiation than one orbiting a G-type star like the Sun. The next step is to apply the calculation to models of the primitive Earth atmosphere, where the abundance of free oxygen is much lower and likely to be limited by the photodissociation rates of molecules such as water and carbon dioxide.
This research is being carried out in collaboration with Dr. James Kasting (Penn State University)