Evolution of Morphological and Physical Properties of Laboratory Interstellar Organic Residues with Ultraviolet Irradiation

TL;DR

This study investigates how ultraviolet irradiation alters the morphology and physical properties of laboratory-simulated interstellar organic residues, revealing potential pathways for nanoglobule formation and implications for dust grain aggregation in space.

Contribution

It provides experimental evidence on the morphological and viscoelastic evolution of interstellar organic analogs under UV irradiation, linking laboratory results to astrophysical observations.

Findings

Irradiated residues become highly porous and rigid, forming amorphous nanospherules.

The elastic modulus of irradiated organics is about 100 MPa, lower than minerals and ice.

Irradiation may explain the formation of organic nanoglobules in primitive meteorites and comets.

Abstract

Refractory organic compounds formed in molecular clouds are among the building blocks of the solar system objects and could be the precursors of organic matter found in primitive meteorites and cometary materials. However, little is known about the evolutionary pathways of molecular cloud organics from dense molecular clouds to planetary systems. In this study, we focus on the evolution of the morphological and viscoelastic properties of molecular cloud refractory organic matter. We found that the organic residue, experimentally synthesized at about 10 K from UV-irradiated H2O-CH3OH-NH3 ice, changed significantly in terms of its nanometer- to micrometer-scale morphology and viscoelastic properties after UV irradiation at room temperature. The dose of this irradiation was equivalent to that experienced after short residence in diffuse clouds (equal or less than 10,000 years) or irradiation in outer protoplanetary disks. The irradiated organic residues became highly porous and more rigid and formed amorphous nanospherules. These nanospherules are morphologically similar to organic nanoglobules observed in the least-altered chondrites, chondritic porous interplanetary dust particles, and cometary samples, suggesting that irradiation of refractory organics could be a possible formation pathway for such nanoglobules. The storage modulus (elasticity) of photo-irradiated organic residues is about 100 MPa irrespective of vibrational frequency, a value that is lower than the storage moduli of minerals and ice. Dust grains coated with such irradiated organics would therefore stick together efficiently, but growth to larger grains might be suppressed due to an increase in aggregate brittleness caused by the strong connections between grains.