Thermal metamorphoses of cosmic dust aggregates: Experiments by furnace, electrical gas discharge, and radiative heating
© The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan 2010
Received: 30 July 2008
Accepted: 3 October 2008
Published: 7 February 2015
We experimentally investigated thermal modifications of porous dust aggregates composed of micrometersized grains by furnace, electrical discharge, and laser radiation heating. In the furnace, porous SiO2 aggregates of 95% porosity at first underwent surface diffusion sintering, which led to progressively increasing necking between adjacent particles. Subsequently, viscous flow dissolved the particulate structure of the still porous sample, and finally melting occurred. Exposing aggregates of various grain types to electrical discharges dispersed most of the sample and left it thermally unprocessed. Nevertheless, some material was thermally processed to sintered aggregates and a tiny fraction to solidified melt spherules with diameters of less than 180 μm and most with interior bubbles. In comparison, radiative laser heating turned out to be a much more efficient process to produce melt spherules of chondrule size, and voids were rarer than in discharge heating. Besides providing material data for further applications, our work also allows a direct conclusion to be drawn on chondrule formation. Low energetic efficiency and aggregate destruction exclude chondrule formation from loosely-bound aggregates inside hypothetical nebular lightning channels. However, radiative heating of whatever origin, including possible lightning, remains a candidate process of chondrule formation.
We assume that loosely-bound aggregates of micrometersized cosmic grains underwent heating events in the early solar system. This assumption is supported by several aspects of today’s understanding of the early solar system. At first, an important fraction of the dust condensed from the gas phase, a process which required an initially hot solar nebula. In this environment, a dust aggregation started; such processes of aggregation have been investigated in detail in order to describe terrestrial planet formation (e.g., Weidenschilling and Cuzzi, 1993). Moreover, meteoritic chondrule formation indicates that there were heating events in the same period and in the same material reservoir that transformed dust aggregates to melt spherules. Today, frictional heating by shocks in the solar nebula is favored among the numerous hypotheses on chondrule formation (Boss, 1996; Ciesla, 2005). With respect to the following, we add that the hypothesis that heating happened by lightning-like gas discharges is still under serious discussion. Finally, a general hint for heating events of cosmic dust in the early solar system are FU Orionis outbursts (e.g., Bell et al., 2000), which are an increase of luminosity for years or decades which solar-type stars undergo in their period of formation.
Consequently, thermal transformations of dust grain aggregates are of interest, and we therefore exposed cosmic dust analogous to furnace (Poppe, 2003), electrical discharge (Güttler et al., 2008), and radiative heating (Springborn et al. in preparation). Here, we give a comparative overview over the manifold outcomes and also discuss aspects of chondrule formation as chondrules must be regarded as an important sink of cosmic dust in the early solar system.
2. Heating in a Furnace
3. Heating in an Electrical Gas Discharge
The hypothesis of chondrule formation by lightning motivated Wdowiak (1983) to pioneer experimental work on this, and he exposed ground material of the Allende meteorite to a 5-kJ electrical gas discharge. Most of the sample dispersed, and some melt spherules were formed, which solidified as spherules with up to 200 μm diameter. The spherules contained bubbles which led Wdowiak (1983) to the conclusion that chondrules did not form by nebular lightning because chondrules were regarded as voidless. The argument of voids against the lightning hypothesis meanwhile appeared weaker than in the past because voids could be caused by low viscosity as a consequence of lower degree of melting than in reality (Maharaj and Hewins, 1993), and voids in chondrules may be rare but are known to exist (Tsuchiyama et al., 2003). Besides this, a progress compared to Wdowiak (1983) seemed possible by performing a series of experiments with varying experimental parameters.
The solidified melt spherules were rare, small, and porous. While the porosity, as mentioned above, may be a weak argument against formation in nebular lightnings and one could be tempted to attribute the small size to different length scales in experimental and real situations, an analysis of energetic efficiency shows what “rarity” means in this context. The ratio of energy consumed for melting spherules devided by the discharge energy, yielded values between 0.006% and 0.06%, depending on the material. Even considering the ultimate source of energy the protoplanetary disk had, which is its potential energy, it is by orders of magnitude too small to explain chondrule formation by nebular lightning. Furthermore, the destruction of aggregates in the gas discharge is a strong argument against chondrule formation inside hypothetical nebular lightning channels. Loosely-bound aggregates would not have survived the even more violent conditions in possible nebular lightning than in the experiments. These are strong and independent arguments against chondrule formation inside nebular lightning channels. However, one must be careful not to misunderstand these arguments. Not only they do not exclude the possibility of nebular lightnings and whatever effect they may have had to dust aggregates, but also they do not exclude that nebular lightning melted chondrule precursors by some mechanism (e.g. radiation or hot gas) outside the discharge channel.
The destruction of loosely-bound aggregates in the electrical gas discharge stresses that, in general, a quiescent environment is important for the transformation of dust aggregates to chondrule-like spherules. This reminds us that there is a corresponding challenge to the currently favored hypothesis of chondrule fomation due to frictional heating with gas after passing a shock front. Recently, Sirono (2006) treated this problem.
4. Radiative Heating by IR Laser
We exposed loosely-bound aggregates of micrometersized dust, which represented properties of early solar system dust, to heating by furnace, electrical discharge, and infrared radiation. This led to manifold outcomes. The comparatively exact setting of heating temperature and time in the furnace allowed the thermal transformation to be resolved as a sequence of neck formation between adjacent particles due to surface diffusion sintering, restructuring, and dissolution of particulate structure by viscous flow, and finally, melting. For several possible applications, the results can be applied to predict morphological changes of dust aggregates according to their heating temperature and time. In contrast to the other experiments, the heating in electrical discharges turned out to be energetically inefficient and violent, dispersing most of the material unprocessed. Only a very few tiny and porous melt spherules or some aggregates with sinter features were found. In contrast, radiative heating was experimentally shown to efficiently convert dust samples of loosely-bound grains into chondrule-sized melt spherules with far fewer or no voids than those found in discharge heating experiments. Concerning chondrule formation, low energetic efficiency and violent conditions in electrical gas discharges showed that chondrules did not form inside hypothetical lightning channels in the solar nebula, while radiative heating of whatever origin, including lightning, remains a candidate mechanism for chondrule formation. In general, the experiments stress that a quiescent environment is necessary to transform loosely-bound aggregates into chondrules.
We thank Dominik Hezel for taking the image in Fig. 4 (left) and John Wasson for support concerning the image in Fig. 4 (right). We also thank the referees Y. Kimura and S. Sirono for their useful comments. Finally, we thank DFG for funding part of the work (grant no Po 817).
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