Cosmic dust
[7][8][9] A smaller fraction of dust in space is "stardust" consisting of larger refractory minerals that condensed as matter left by stars.
[17] These disparate research areas can be linked by the following theme: the cosmic dust particles evolve cyclically; chemically, physically and dynamically.
The astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the Universe's complicated recycling steps.
[18][19] NASA collects samples of star dust particles in the Earth's atmosphere using plate collectors under the wings of stratospheric-flying airplanes.
Dust samples are also collected from surface deposits on the large Earth ice-masses (Antarctica and Greenland/the Arctic) and in deep-sea sediments.
Don Brownlee at the University of Washington in Seattle first reliably identified the extraterrestrial nature of collected dust particles in the latter 1970s.
Dust detectors in the past flew on the HEOS 2, Helios, Pioneer 10, Pioneer 11, Giotto, Galileo, Ulysses and Cassini space missions, on the Earth-orbiting LDEF, EURECA, and Gorid satellites, and some scientists have utilized the Voyager 1 and 2 spacecraft as giant Langmuir probes to directly sample the cosmic dust.
Infrared light can penetrate cosmic dust clouds, allowing us to peer into regions of star formation and the centers of galaxies.
During its mission, Spitzer obtained images and spectra by detecting the thermal radiation emitted by objects in space between wavelengths of 3 and 180 micrometres.
The scattering of light from dust grains in long exposure visible photographs is quite noticeable in reflection nebulae, and gives clues about the individual particle's light-scattering properties.
For example, the heavy elements within the silicon carbide (SiC) grains are almost pure S-process isotopes, fitting their condensation within AGB star red giant winds inasmuch as the AGB stars are the main source of S-process nucleosynthesis and have atmospheres observed by astronomers to be highly enriched in dredged-up s process elements.
The outflowing 44Ti nuclei were thus still "alive" (radioactive) when the SUNOCON condensed near one year within the expanding supernova interior, but would have become an extinct radionuclide (specifically 44Ca) after the time required for mixing with the interstellar gas.
Stardust itself (SUNOCONs and AGB grains that come from specific stars) is but a modest fraction of the condensed cosmic dust, forming less than 0.1% of the mass of total interstellar solids.
[32] This was once thought impossible, especially in the 1970s when cosmochemists were confident that the Solar System began as a hot gas[33] virtually devoid of any remaining solids, which would have been vaporized by high temperature.
For example, grains in dense clouds have acquired a mantle of ice and on average are larger than dust particles in the diffuse interstellar medium.
Most of the influx of extraterrestrial matter that falls onto the Earth is dominated by meteoroids with diameters in the range 50 to 500 micrometers, of average density 2.0 g/cm3 (with porosity about 40%).
That cyclic process of growth and destruction outside of the clouds has been modeled[36][37] to demonstrate that the cores live much longer than the average lifetime of dust mass.
Those refractory cores are also called stardust (section above), which is a scientific term for the small fraction of cosmic dust that condensed thermally within stellar gases as they were ejected from the stars.
Those molecular clouds are very cold, typically less than 50K, so that ices of many kinds may accrete onto grains, in cases only to be destroyed or split apart by radiation and sublimation into a gas component.
Finally, as the Solar System formed many interstellar dust grains were further modified by coalescence and chemical reactions in the planetary accretion disk.
In infrared light, emission at 9.7 micrometres is a signature of silicate dust in cool evolved oxygen-rich giant stars.
The metallic elements: magnesium, silicon, and iron, which are the principal ingredients of rocky planets, condensed into solids at the highest temperatures of the planetary disk.
Stardust once more provides an exception to the general trend, as it appears to be totally unprocessed since its thermal condensation within stars as refractory crystalline minerals.
Most of the matter present in the original solar nebula has since disappeared; drawn into the Sun, expelled into interstellar space, or reprocessed, for example, as part of the planets, asteroids or comets.
Due to their highly processed nature, IDPs (interplanetary dust particles) are fine-grained mixtures of thousands to millions of mineral grains and amorphous components.
The arrows in the adjacent diagram show one possible path from a collected interplanetary dust particle back to the early stages of the solar nebula.
[45] In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".
"[46][47] In February 2014, NASA announced a greatly upgraded database[48][49] for detecting and monitoring polycyclic aromatic hydrocarbons (PAHs) in the universe.
[49] In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites.
Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in the Universe, may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists.
