Interstellar Origins of Preplanetary Matter
Funded by the NASA Exobiology & Evolutionary Biology Research Program and the NASA Astrobiology Institute
Organic molecules that originated in interstellar space are known to exist in our solar system: they are detected in meteorites – remnants from the time when the planets were formed – that fall to the Earth today. The goal of this project is to test the hypothesis that molecules relevant to the origin of life are ubiquitous within the interstellar condensations that give birth to planetary systems. We seek to understand not only how these molecules came to exist but also to explore how universal they are: do the same processes lead to a rich supply of organic molecules in other emerging solar systems in other parts of our Galaxy?
To answer these questions, we are using telescopes on Earth and in space that are designed to obtain spectroscopic data in the infrared region of the electromagnetic spectrum. We are analyzing spectra from the archives of past space missions such as the Infrared Space Observatory and the Spitzer Space Telescope. Current and future missions such as the Stratospheric Observatory for Infrared Astronomy and the James Webb Space Telescope will add new data of unprecedented quality over the next decade. The observations enable us to determine not only what molecules are present but also to explore their evolution. We find, for example, subtle difference in the spectral features observed in a cold interstellar cloud compared with a warm “protoplanetary” disk surrounding a newly formed star. The data are used to constrain theoretical models of the physics and chemistry of the clouds and disks, allowing us to identify key chemical pathways and determine organic inventories at different stages of evolution. Analogs of the young Sun are studied in a variety of star-forming environments, enabling us to compare possible scenarios for the birth of our own solar system and to examine the range of initial conditions that might give rise to habitable planets elsewhere. Our research also leads to important comparisons between interstellar processes and the remnants of planet formation (the comets, asteroids and meteorites) in our solar system. Interplanetary materials that fall to Earth are known to contain key biomolecules such as amino acids and sugars, and our research will shed light on how they are formed and what significance they may have for the origin of terrestrial life.
This research is being carried out in collaboration with groups at the SETI Institute, the SOFIA Science Center, the University of Missouri at St Louis, the University of Virginia, and the University of Helsinki.
The Physics of Interstellar Dust
The radiation from stars observed through interstellar matter in our Galaxy is typically partially plane-polarized, as the result of linear dichroism (angle-dependent extinction) caused by aligned, anisotropic dust grains in the intervening medium. The spectral dependence of interstellar polarization has long been recognized as an important tool for studying grain physics, placing useful constraints on the optical properties of the aligned grains and on the nature of the mechanism(s) responsible for their alignment. Observations of linear polarization in different lines of sight demonstrate the occurrence of variations in both grain size and degree of alignment with physical conditions. The observations provide evidence for systematic trends toward larger average grain sizes in dense clouds where new stars are born. Two distinct growth processes – aggregation by grain-grain coagulation and mantle growth by accumulation and chemical interaction of atoms and molecules adsorbed from the gas – may occur sequentially or in parallel. The growth of micron-sized dust particles by these processes, in essence, the initial step toward planet formation, and in recent years much research has focused on developing realistic models and laboratory simulations of grain growth over environments that range from diffuse regions of the interstellar medium to dense clouds and protoplanetary disks.
The aim of this long-term research program is to reassess the available observational data with respect to the most recent models of grain growth and alignment. The database includes a wealth of data from both the published literature and archival observations not yet in the public domain. The goal is to place new constraints on the evolution of grain properties with environment, and to reassess the role of magnetic field in their alignment.