On the possibility of H2-near-infrared detection in comets with large gas production

Comets with large gas production offer a unique chance to observe a H₂-flux of about 10⁵ photon cm^–2 s^–1 sr^–1 (1 Rayleigh) at wavelengths 8497.4 Å, 8560.2 Å and 8747.9 Å-i.e., where photon counting methods are still applicable. In the following it will be shown that population of the vibrational levels, giving rise to these quadrupole overtone transitions, is dominated by photodissociation of methane, and that the emission even of quadrupole lines is not attenuated by collisional quenching. Wavelength scanning by ±1 Å is shown to be enough to discriminate between cometary and atmospheric emissions by phase-sensitive subtraction techniques. Solid angle of <10^–7 sr has to be used, whence follows that a large ground-based telescope combined with a tilting Fabry-Perotfilter is best suited for detection of the near-infrared H₂-emissions at reasonable counting rates and sufficient rejection of the atmospheric background.Since H₂ is supposed to be formed mainly by the photolysis of CH₄, the optimum time for detection will be during approach to perihelion when, because of its high vapor pressure, methane will vaporize from the cometary nucleus. Variation of the source strength of both CH₄ and its photolysis product H₂ with time are particularly valuable indicators for the structure of the nucleus, its thermal history and conditions of formation. A high-resolution tilting filter photometer, which allows phase-sensitive background subtraction was used for the first time for near-infrared observations on the dust coma of Comet Kohoutek (Barbieriet al., 1974). The same technique was successfully used for the determination of an upper limit for CH₄ production at 3.3 by airborne observations on the same comet (Cosmoviciet al., 1974).