FM10: Nanodust in Space and Astrophysics

Presolar Grains

Schematic illustration of the history of cosmic dust. (A. Li & I. Mann 2012)

The size of nanodust particles falls between that of molecules and bulk matter. Because of their small size and large surface-to-volume ratio, their properties — for example, their heat capacity, melting temperature, surface energy, diffusion coefficient and optical properties — are often peculiar and quite different from those of bulk materials. Specifically, clusters of 1-10 nanometers are expected to reveal strongly variable, size-dependent properties such as geometric and electronic structure, binding energy and dielectric function, which determine how they interact with gas particles and electromagnetic radiation. Carbon nanoparticles include “bucky onions” and fullerenes such as C60 that account for less than 1% of cosmic carbon. Larger clusters, with many thousands of atoms and diameters in the range of 10 nm or larger, exhibit behavior that varies smoothly with size and approaches bulk-matter properties as their size increases.

While the role of nanodust is not fully understood yet, nanoclusters should be very important because, given their large surface area relative to their small mass, they interact more efficiently with particles and fields. Interstellar nanodust dominates far-ultraviolet extinction as well as the near- and mid-infrared emission of the interstellar medium of the Milky Way and external galaxies. The heating of interstellar gas and the surface layers of protoplanetary disks is dominated by nanodust through photoelectrons that form on their surfaces. Nano-sized (or smaller) polycyclic aromatic hydrocarbon (PAH) C60 diamonds also reveal their presence in astrophysical regions through their characteristic vibrational spectral features. The presence of charged nanodust similarly influences other space plasma, also leading to dusty plasma effects such as waves and instabilities.

For many years nanodust has been detected with in-situ instruments from spacecraft in different regions of the solar system. A notable recent finding is that nanodust in the heliosphere is deflected and accelerated by the solar wind. Sounding rockets detect nanodust in Earth’s upper atmosphere (mesosphere), where it forms from the re-condensation of metallic compounds produced from ablating meteoroids. Meteoric smoke is also implicated in the formation and freezing of stratospheric clouds, resulting in polar ozone depletion, as well as in the chemistry of clouds and atmospheres of Mars, Venus and Titan and in the vicinity of comets. Planetary volcanic plumes and impacts on planetary objects are sources of interplanetary nanodust, as observed, for example, near the surface of the Moon.

The vast majority of our universe is plasma in which heavy chemical elements are often contained in small solid dust particles that carry electric surface charges. A large fraction of the plasma is therefore dusty. Examples include the interstellar medium, Earth’s ionosphere, the ring systems of planets the surface layers of moons and other solar system objects that lack atmospheres. Although dusty plasma has been studied extensively, only a few observations in space are fully described with existing theory.

NASA’s Parker Solar Probe, now en route to the Sun, and ESA’s forthcoming Solar Orbiter, now expected to lift off in 2020, will provide valuable data on the formation and dynamics of nanodust in the inner heliosphere. Observations suggest the presence of nanodust in circumstellar debris disks, which are among the targets of the James Webb Space Telescope (JWST), now scheduled for launch in 2021. Laboratory astrophysics continues to play a role in studies of cosmic dust, including nanodust, and is poised to make particularly valuable contributions through studies of samples returned from comets and asteroids.

IAU Focus Meeting 10, Nanodust in Space and Astrophysics, will bring together space physicists who study nanodust in the heliosphere and specialists from physics, astrophysics and atmospheric science to make progress in understanding nanodust particles by combining their knowledge on dust under a wide range of near-space and deep-space conditions.

INGRID MANN is a professor in the Dept. of Physics and Technology at UiT, the Arctic University of Norway, and President of IAU Commission E3, Solar Impact Throughout the Heliosphere.

 

Kyoko K. Tanaka is a theoretical physicist in Tohoku University (visiting researcher) / Nihon University (Lecturer) in Japan. Her research interests are cosmic dust formation and evolution.

 

Aigen Li is a professor of astrophysics at the University of Missouri in the USA, with research interest in the interstellar medium and interstellar dust, comets, planet-forming dust disks, dust-making evolved stars, and dust in external galaxies, active galactic nuclei, and gamma-ray bursts.