Introduction

The knowledge about the properties of cosmic materials has improved thanks to the advent of new ground-based observational instruments and of space-born telescopes, such as the Hubble Space Telescope and the Infrared Space Observatory. Thanks to such tools we have gained unprecedented new information about the spatial distribution and the spectral signatures of cosmic gas and dust components in the most relevant space environments outside (e.g., star forming regions, evolved stars, circumstellar and interstellar medium) and inside (e.g., planets, comets, asteroids) our Solar System. With specific concern to the solid component (dust), attention has been focused on the two main families of materials that are present in space: carbons and silicates. Interesting relations have been identified among the specific compounds of the two classes observed in different space conditions.

Infrared spectroscopic observations of carbon-based materials in the ISM [1] and in the Proto-Planetary Nebula CRL618 [2] present similar absorption features around 3.4 p.m, typical of aliphatic solids. On the other hand, sharp emission bands around stellar sources rich in UV flux indicate the presence of aromatic compounds, such as Polycyclic Aromatic Hydrocarbons (PAHs) [3]. Emission spectra of comets present a convolution of bands probably due to volatile components (e.g., methane and ethane) and solid carbon [4].

As far as silicates are concerned, the information derived by ISO observations is quite innovative, as sharp emission bands in the 10 - 20 |im spectral range have been clearly identified as due to crystalline compounds, both in the circumstellar medium [5,6] and in comets [7], On the other hand, both the diffuse and the dense ISM seems rich in amorphous silicates, as testified by the observation of broad absorption bands around 10 p.m [8], The main puzzle in this case is to find a reasonable evolution path from circumstellar to interstellar media and, then, to comets that might account for the observed structural variations.

The previous remarks evidence the complexity in interpreting the observations about cosmic materials. Actually, it is well understood that intimately different materials may provide similar features and that, on the other hand, the "intrinsic nature" of materials is closely related to their optical behaviour. Moreover, materials react to space environment conditions as different agents, such as thermal, UV and ion processes, are active with different efficiency depending on local conditions. Therefore, materials evolve in space and change in terms of physical, chemical and structural properties. A difficult task is to trace such evolution based on evidence coming from astronomical observations only.

In the case of comets, much help came from in situ analyses performed by the GIOTTO spacecraft during the fly-by of comet lP/Halley. We know that comet grains present different chemical phases [9], Organic (CHON) particles represent about 30 % in mass, while silicates are mainly Mg rich. Grains present densities between 2.5 g cm"3 (silicates) and 1 g cm"3 (mixture) and have a so-called core-mantle or fluffy structure. The overall composition (in mass) is [10]: 26 % silicates (Si, Mg, Fe, ...), 23 % organic material (carbon), 9 % small carbonaceous grains / PAH's, while the rest is ice (water and other condensed volatiles). Despite this direct evidence, also in the case of comets many questions are open about the pristine degree of the composing materials.

In this scenario, a fundamental contribution to the identification of physical and chemical characteristics of cosmic materials comes from laboratory investigation. In fact, the analysis of natural and synthetic compounds allows us to evidence relations between material properties and optical behaviour. This is a fundamental step for the quantitative interpretation of astronomical observations. Moreover, the new frontier of laboratory experiments is oriented to analyse the evolution of analogues due to the most important mechanisms which may alter materials in space (e.g., thermal annealing, UV irradiation, ion bombardment, hydrogenation). The experimental results obtained so far offer a solid background to interpret observations and to trace the changes induced in space. We are today tackling long standing astrophysical questions, which may find a definite clarification thanks to the contribution of deep experimental analyses.

In section 2 we will summarise the laboratory approach and some of the most recent and relevant results obtained in the laboratory concerning materials of interest in the view of applications to the interpretation of space observations. Section 3 will be devoted to demonstrate how the laboratory data can be fitted to observations to derive information about the actual status of cosmic materials. Finally, in section 4 we will indicate what are the most promising future steps both in laboratory and in space to achieve a deeper understanding on the main species populating key space environments.

Was this article helpful?

0 0

Post a comment