4. FUTURE

There are quite a few unsolved or partially solved problems related to interstellar dust which will be demanding close attention in the future (see below for a list). A number of new remote observational facilities which will be available early in the new millennium (Atacama Large Millimeter Array [MMA/LSA], Far Infrared and Submillimeter Telescope [FIRST], Next Generation Space Telescope [NGST], Space Infrared Telescope Facility [SIRTF], Stratospheric Observations for Infrared Astronomy [SOFIA], Submillimeter Wave Astronomy Satellite [SWAS]) will permit further tests of current dust models and promise new observational breakthroughs.

1. What is the source and nature of the Diffuse Interstellar Bands?
2. What is the carrier of the 2175 Å extinction hump?
3. What is the carrier of the "Unidentified Infrared Bands"? if it is PAHs, where are they formed? are they mainly from carbon star outflows or formed in situ by ion-molecule reactions (Herbst 1991) or from the organic refractories derived from photoprocessing of ice mixtures (Greenberg et al. 2000)?
4. What is the carrier of the Extended Red Emission?
5. What are all the sources and sinks (destruction) of interstellar dust? where are interstellar grains made? are they mainly made in the cold ISM (Draine 1990) or are the silicate cores mainly stardust (serving as "condensation seeds") while the organic mantles are formed in the ISM (Greenberg 1982a; Greenberg & Li 1999a)?
6. What are the exact composition and morphology of interstellar dust? are they separate bare silicate and graphite grains or silicate core-carbonaceous mantle grains or composite grains composed of small silicates, carbon and vacuum? if most of interstellar grain mass is condensed in the cold ISM, how can pure silicate and graphite grains form (see Draine 1995)?
7. What are the sizes of large dust grains (> 0.25 µm)? how much can we learn from X-ray halos and from spacecraft in situ dust detections?
8. Why are crystalline silicates not seen in the ISM while they are present in stardust and cometary dust? how do cometary silicates become crystalized?
9. How do molecular hydrogen and other simple molecules form on grain surfaces? although considerable progress has been made in recently years in studies of the diffusion rates of adsorbed hydrogen atoms on the surfaces of variable dust materials, the recombination reactions, and the restoration of the new molecules to the gas phase (Pirronello et al. 1997; Pirronello et al. 1999; Manicò et al. 2001), the formation of molecular hydrogen is still not well understood (Herbst 2000; Pirronello 2002).
10. How do interstellar grains accrete and deplete mantles in dense molecular clouds? we need high spatial resolution observations of molecule distributions in the gas and in the solid as function of depth in the cloud - interiors of clouds as well as regions of low and high mass star formation.
11. How does dust evolve in protosolar regions? we need higher spatial resolution and sensitivity. Improvements in the theory of dust/grain chemistry, particularly in collapsing clouds leading to star formation as well as in quiescent molecular clouds.
12. Will the chemical and morphological analysis of comet nuclei and dust material reveal the true character of interstellar dust? will they provide further answers to the question of life's origin?
13. How can we resolve the evolution of interstellar matter leading to the material measured and analyzed in meteorites, in interplanetary dust particles?
14. What is the true atomic composition of the interstellar medium? how variable is it in time and space? are there global variation over distances of kiloparsecs?
15. When did dust first form in a galaxy? what are the composition and sizes of dust in extragalactic environments?

Acknowledgements

A. Li was deeply saddened by the passing away of Prof. J. Mayo Greenberg on November 29, 2001. As a pioneer in the fields of cosmic dust, comets, astrochemistry, astrobiology and light scattering, Mayo's passing was a great loss for the astro-community. Mayo had been scientifically active till his very last days. Just a few weeks before Mayo passed away, A. Li discussed future collaboration plans with him on dust in high-z galaxies. It was a great experience for A. Li to work with Mayo in Leiden. He will be remembered forever, as a great astrophysicist and as a great mentor. A. Li is also grateful to Profs. Bruce T. Draine and Ewine F. van Dishoeck for their continuous advice, encouragement and support. A. Li thanks Profs. Lou J. Allamandola, Bruce T. Draine, Jonathan Lunine, Valerio Pirronello for valuable discussions, comments and suggestions. Some of the materials of Section 2 (dust history) were taken from Greenberg & Shen (1999) and the "Introduction" Chapter of A. Li's PhD thesis (Leiden, 1998); of Section 3.2.1 (dust luminescence) from Li & Draine (2002a); and of Section 3.2.2 (dust IR emission) from Li & Draine (2001b). This work was supported in part by NASA grant NAG5-7030 and NSF grant AST-9988126 and by a grant from the Netherlands Organization for Space Research (SRON).