Gas-grain chemistry in protosolar nebula and composition of comets

Abstract

The time dependent chemistry of dense interstellar clouds is applied in order to estimate the possible composition of the volatile component of a comet nucleus originated in the protosolar nebula. Both gas‐phase and grain‐surface chemistry are considered and initial gas‐phase atomic abundances are assumed to be protosolar. Physical conditions in the protosolar nebula in the phase of dust grains formation are assumed to be similar to those in dense, quiescent interstellar clouds. The gas‐phase and grain‐surface chemistry scheme is based on similar models discussed other authors, namely by Hasegawa et al. It is assumed that resulting surface abundances of molecular species in grain mantles at times 106 years, when the gas phase is substantially depleted of its heavy molecules, remain preserved in cometary dust and volatile constituents. Therefore the comparison of the modeled molecular composition of cometary material with abundances derived from infrared observations and data provided by measurements ‘‘in situ’’ on the VEGA spacecraft may throw a new light on the processes prededing the formation of the solar system. The results indicate that water, as expected from generally accepted comets composition, is a dominant volatile constituents. However other relative highly abundant species according here discussed model in the grain mantles are CH4 and CO2. This is in contradition to observational data of comets in which these molecules tend to be only marginal even as parent compounds. More consistent with observed abundances seems to be models in which the desorption induced by the cosmic rays and with so called ‘‘normal’’ initial gas phase abundance. The preliminary results combined with another evidences concerning the cometary dust composition suggest that the dust grains coming from the interstellar environments with relatively stable mantles and are preserved in their pristine form in those primitive solar system bodies as are represented by comets.