Surface Science 500 (2002) 793–822 www.elsevier.com/locate/susc Cosmic dust and our origins J. Mayo Greenberg * Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden University, Leiden, The Netherlands Received 20 November 2000; accepted for publication 28 June 2001 Abstract The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the pro- genitors of life to the earth is summarized. Ó 2001Published by Elsevier Science B.V. Keywords: Molecule–solid reactions; Photochemistry; Semi-empirical models and model calculations; Desorption induced by photon stimulation; Surface diffusion; Surface chemical reaction; Chemical vapor deposition; Porous solids 1. Introduction the cosmic scheme of things is fundamental to comets and other primitive solar system objects Pervading all the space between the stars are and is believed by many to provide the key to life’s clouds of gas and dust which often appear as dark origins. The ‘large’ tenth micron sized particles are nebulae such as that which appears in the shape of generally very cold with temperatures approaching a horse’s head in the region of the constellation as low as 5–6 K ðÀ268 °CÞ. Interactions on their Orion (see Fig. 1). Over 100 organic and inorganic surfaces with atoms, molecules, energetic photons molecules have been detected in the gas but it is the and cosmic rays provide a major source of the dust that contains the largest and most complex chemical evolution in space. This is as much as we organic molecules; these dust grains are very, very know today. But the subject of dark nebulae and small. Even the large ones are only tenths of mi- what makes them dark has a patchy history. The crons in size and there are myriads of smaller ones earliest relevant ideas go back 100 years. Looking which may be more like large molecules, such as at the sky in the direction of the constellation polycyclic aromatic hydrocarbons (PAHs) and Sagittarius, it is clear that there are tremendously perhaps fullerenes. The role of interstellar dust in dark lanes especially in the region toward the center of our Milky Way. This observation in- spired Herschel in 1884 to say that there was un- * Deceased. doubtedly a hole in the sky. Barnard started to 0039-6028/01/$ - see front matter Ó 2001Published by Elsevier Science B.V. PII: S0039-6028(01)01555-2 794 J.M. Greenberg / Surface Science 500 (2002) 793–822 Fig. 1. The Horsehead nebula, so called because in projection it looks like a horse’s head, is a dark cloud of dust and gas in the Orion nebula in the region where much star formation is currently taking place. Its darkness is simply explained by the dust blocking of the starlight behind it. take pictures and reported vast and wonderful published measurements of star colors versus cloud forms with their remarkable structure, lanes, spectral types over a wavelength range from about holes and black gaps some of which are now called 350 nm (ultraviolet) to the near infrared. Spectral Bok globules. Clerke [1], in an astrophysics text types characterize the size and temperature of she authored around the turn of the century, sta- stars. The relation was not the expected straight ted. ‘‘The fact is a general one, that in all the forest line, but rather, it showed curvature at the near of the universe there are glades and clearings. How ultraviolet and infrared regions. This was later they come to be thus diversified we cannot pretend updated in 1958 [4]. Things were beginning to say; but we can see that the peculiarity is to make some physical sense from the point of structural that it is an outcome of the fundamental view of small particle scattering. In 1935 Lindblad laws governing the distribution of cosmic matter. [5] published an article in Nature indicating that Hence the futility of trying to explain its origin, interstellar elemental abundances made it seem as a consequence, for instance, of the stoppage of reasonable to grow particles in space. Eddington light by the interposition of obscure bodies, or had long before hypothesized that it was so cold in aggregations of bodies, invisibly thronging space’’. space that anything that hits a small particle will It was not until the work of Trumpler in 1930 [2] stick. But the earliest attempts to explain inter- that the first evidence for interstellar reddening stellar extinction were not in terms of something was found; which would imply that many stars that could grow in space but rather in terms of were cooler than predicted. This reddening has meteors [6,7]. Interstellar extinction is the apparent nothing to do with the Doppler shift of receding reduction of starlight. objects. His observations indicated reddening even It was in the 1940s that van de Hulst [8] broke where he saw no clouds. In 1948 Whitford [3] with tradition and published the results of making J.M. Greenberg / Surface Science 500 (2002) 793–822 795 particles out of atoms that were known to exist in fact that there had to be a very wide range of space: H, O, C, and N. He assumed these atoms particle sizes and types to account for the blocking combined on a surface of some kind to form fro- of the starlight. Secondly, the infrared provided zen saturated molecules. The surface was left as ‘to a probe of some of the chemical constituents of be determined’ (TBD). This is what later became the dust. Thirdly, laboratory techniques were ap- known as the ‘dirty ice’ model and was the first plied to the properties and evolution of possible application of surface physics to interstellar dust. grain materials. It was the advent of infrared The dirty ice model of dust was a logical follow up techniques that made it possible to demonstrate to the then existing information about the inter- conclusively that something like rocks (but very stellar medium and contained the major idea of small, of course) constituted a large fraction of the surface chemistry leading to the ices H2O, CH4 interstellar dust. However the initial attempt to and NH3. But it was not until the advent of in- find the 3.1 lm feature of H2O predicted by the frared astronomical techniques, making it possible dirty ice model was unsuccessful [12]. This was, at to observe silicate particles emitting at their char- first, a total surprise to those of us who had ac- acteristic 10 lm wavelength in the atmospheres of cepted the dirty ice model. However, it supplied cool stars, that we had the cores (surfaces) on the incentive to perform the early experiments on which the matter could form. TBD was now de- the ultraviolet photoprocessing of low temperature termined. It is interesting to note that their pres- mixtures of volatile molecules which would simu- ence was predicted on theoretical grounds by late the ‘original’ dirty ice grains [13]. By doing this Kamijo in 1963 [9]. As van de Hulst said, he chose we hoped to understand how and why the pre- to ignore the nucleation problem and just go ahead dicted H2O was not clearly present. These experi- (where no one had gone before) with the as- ments gave rise to predictions of a new component sumption that ‘something’ would provide the seeds of interstellar dust in the form of complex organic for the mantle to grow on. What a great and cre- molecules, as well as mantles of all sorts of ices on ative guess! So it was that by 1945 we had many of the silicates. The grain surfaces were becoming the theoretical basics to help us understand the much more interesting. sources of interstellar dust ‘ices’ but it was not Present studies of interstellar dust lead us in until about 1970 that the silicates were ascertained. many directions; from chemical evolution of the However, having a realistic dust model, van de space between the stars, to comets, to solar sys- Hulst developed the scattering tools to provide a tems. I will try to present an overview of our good idea of dust properties. current knowledge of the dust which brings many After the extinction curve––how the light related astrophysical problems to the fore. Where blockage increases from the red to the blue––and and how small particles form and grow and evolve the inferred particle size had been well established, in the space between the stars, involve interactions two investigators, Hall [10] and Hiltner [11], in- between the solid dust and the gas atoms, mole- spired by a prediction of Chandrasekhar on in- cules, the ultraviolet photons, and cosmic rays trinsic stellar polarization, discovered instead, the which drive the processes. The questions posed by general interstellar linear polarization. It turns out our present information and the suggested needs that the blockage of starlight by the dust produces for the future are discussed in the last section. The a partial linear polarization of the starlight as if the space age has ushered in some of the most dra- clouds were polarizers.The implication of the lin- matic developments and will ultimately give us the ear polarization was that the extinction was caused data we need to understand how solar systems are by nonspherical particles aligned by magnetic born out of collapsing clouds; how comets are fields.