Infrared Radiation from Comet Hale-Bopp

Summary

Observations of infrared radiation emitted by small particles in the coma of Comet Hale-Bopp have recently been published (Crovisier et al. 1997). Intensity values in units of Janskys (10^-26 W m^-2 Hz^-1 steradian^-1) were given at wavelengths from about 7.5 to about 45 micrometers for observations made on 6 October, 1996 when the comet had a heliocentric distance of 2.9 AU. These observations are reproduced without change in Figure 1.

Also given in Figure 1, is a curve calculated for submicron particles that are a mixture of two distinct components. An olivine component at a temperature of 175 K, as suggested by Crovisier et al. (1997), and a general bioculture, such as one obtains in springtime in great abundance from rivers and lakes in the U.K., at a temperature of 200 K. The extinction spectrum of the latter over the wavelength range of interest in the present context is dominated by diatom silica, which is a siliceous material that forms the frustules of a large class of diatomaceous algae. The actual spectrum of the bioculture that we used was taken from a paper by Wickramasinghe, Hoyle and Majeed (1989). Diatom silica occurs in this mixed culture in a mass fraction of about 20%. (See also spectra on page 227 of our book The Theory of Cosmic Grains (Hoyle and Wickramasinghe, 1991)).

The proportions of the two components, olivine and bioculture, for which the dashed curve of Figure 1 was calculated, were defined by the requirement that the olivine component should produce 25% of the total intensity count at 24 micrometers. Mass-for-mass, olivine is about three times more effective at this wavelength than the bioculture. Thus, the mass fraction of olivine in the mixture for which the curve of Figure 1 was calculated was about 10%. About 90% of the mass was the bioculture.

Figure 2 compares the laboratory extinction spectrum of olivine (solid curve) (Hasegawa and Koike 1984) with the extinction required for particles at 200 K to produce the observed intensity counts of Hale-Bopp (dashed curve). The latter is just the Hale-Bopp spectrum of Figure 1 divided by the Planck function Fn(T) for a temperature of 200 K. If the particles in Hale-Bopp were olivine only, as Crovisier et al. (1997) suggest, the two spectra in Figure 2 should be the same, except for small differences due to the temperature of the particles differing somewhat from 200 K. But the margin of difference is vastly too large to be attributed to a temperature effect. The olivine spectrum is much more jagged than the comet spectrum. The latter demands a main contributing material which has an oscillator strength at all wavelengths that is constant to within a factor of about 2, whereas the oscillator strength of olivine falls by a factor of 10 between 20 and 30 micrometers. Similarly, between 10 and 13 micrometers, the oscillator strength falls by a factor of 2 for the comet, but by a factor of 40 for olivine. Between 27 and 40 micrometers oscillator strength falls by only about 1.25 for the comet, but by the large factor of 8 for olivine.

What olivine can validly do, however, is to influence the details of the calculation leading to the curve of Figure 1 at its peaks. The olivine does so at its own major peaks, as may also other minor constituents. Something further is needed at 11.5 micrometers for example, most probably a hydrocarbon. However, the generally very good agreement in Figure 1 shows that the main emission from the comet is well explained by a mixture that is overwhelmingly dominated by a bioculture.

It is the disordered arrangement of its constituent SiO₄ tetrahedra (Hecky et al. 1973) that permits diatom silica to maintain a roughly constant oscillator strength over a wide band of wavelengths. In this, it differs from most minerals in which the basic molecule, MgSiO₃ or Mg₂SiO₄ in olivine, is arranged in repeated structures that resonate and confine permitted oscillator frequencies to comparatively narrow bands.

An alternative explanation of the Hale-Bopp infrared spectrum, should one wish to reject our biological interpretation, is to assume that the bulk of the particles from Hale-Bopp consist of ground-up Pyrex^tm glass to which must be added a generous quantity of inconsequential organic junk.¹

¹ Pyrex glass is produced commercially by a very rapid quenching of fused silica. The quenching prevents the SiO₄ tetrahedra from reorganizing themselves, producing a disordered structure that is generally similar to diatom silica.

References