Space dust spectroscopy

Ever since organic matter was discovered in meteorites that crash-landed on Earth, it has been thought that it might have contributed to the prebiotic chemistry, leading to biomolecules and to early life. The confirmation of organic compounds in space, specifically in the dust grains of Halley's comet and in the ice grains in the outer rings of Saturn, leads us to the obvious question.

If organic matter from space contributed to life on Earth, could it have done the same thing elsewhere in the universe?

Until we actually detect signs of extraterrestrial life, scientists are continuing to gather evidence both on our own planet and in space. On Earth, experiments are being carried at the Heidelberg Dust Accelerator facility in Germany to find out what happens when superfast dust particles crash into objects. The results will be used to calibrate dust sensors aboard space vehicles to help interpret on-board experiments, such as those aboard the Cassini mission to Saturn.

The Dust Accelerator fires particles at velocities up to 100 km/s and fires them onto targets variously described as metals, planetary analogues and interplanetary dust collector materials for use in space. These impacts generate plasmas which can be studied by time-of-flight mass spectrometry.

In a recent set of experiments, the particle impact mass spectra were measured using a newly developed Large Area Mass Analyser with a target area of 0.1 m². It used a reflectron to produce spectra with a mass resolution of about 200, which is described as exceptionally high for such a target size and is far higher than that currently achieved on instruments such as the cosmic dust analyser aboard Cassini.

The results were published in Rapid Communications in Mass Spectrometry by Ralf Srama and colleagues in a seven-centre study involving the Max Planck Institute for Nuclear Physics, Heidelberg, the universities of Stuttgart, Heidelberg and Braunschweig in Germany, the University of Sheffield, UK, the University of Colorado, USA and the Institute for Meteorology and Climate Research, Karlsruhe, Germany.

Organic micrograins were prepared consisting of a latex core with an ultrathin layer of an organic conducting polymer that ensures sufficient accumulation of surface charge. The overall particle diameters ranged from 0.2-1.8 µm and they were directed at speeds up to 35 km/s onto the LAMA target which was coated with silver foil or silver black.

Polypyrrole-coated (PPA) and polyaniline-coated polystyrene (PAS) particles and polypyrrole-coated poly[bis(4-vinylthiophene)sulphide] (PPV) particles were tested. Good quality mass spectra were obtained. In all cases, the expected ¹⁰⁷Ag and ¹⁰⁹Ag ions from the foil were detected, along with others from impurities on the surface such as sodium and potassium ions in positive mode and chloride ions in negative mode.

These ions were supplemented by hydrocarbon fragments, carbon clusters, silver clusters and mixed target-particle clusters, all of which were singly charged. Peaks were observed from the latex core material but not from the shell, probably due to the low fraction (<10%) of the overall particle mass. At the lower velocities of 5-8 km/s, the spectra of PPA and PAS were dominated by ions from polystyrene, such as the tropylium ion at m/z 91.

For velocities higher than 10 km/s, the spectra changed completely, losing peaks due to the tropylium ion and related species and becoming dominated by hydrogenated carbon clusters over the entire mass range. Structures based on polyynes and cumulenes were proposed for these species.

For the PPV particles at 15 km/s, hydrogenated carbon clusters with odd numbers of hydrogens were preferred but at 20 km/s, the C₄ cluster was the largest and peaks due to silver clusters were observed.

PPV was also analysed in negative-ion mode above 12 km/s. Bare carbon clusters with up to 8 atoms were observed up to m/z 96 and peaks due to chloride, sulphide and hydrogenated sulphide were found.

The organic-type ions in these spectra reflected the composition of the parent molecule, permitting that molecule to be identified. Lower impact velocities yielded larger ions and fragments and higher velocities led to more efficient bond cleavage and smaller species.

This structural relationship convinced the researchers that velocities lower than 35 km/s would be suitable for identifying organic material in micrometeroids and space dust, with lower speeds giving more detailed information. "These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The Heidelberg Dust Accelerator

The accelerator vacuum chamber with a diameter of 1.4 m