  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.
