title: The possible formation of a hydrogen coma around comets at large heliocentric distances.
abstract: An observational test--the detection of a hydrogen coma around comets at large heliocentric distances--is proposed for determining whether comets were formed by the agglomeration of unaltered, ice-coated, interstellar grains.  Laboratory experiments showed that amorphous water ice traps H2, D2, and Ne below 20 K and does not release them completely until the ice is heated to 150 K.  Gas/ice ratios as high as 0.63 are obtainable.  Thus, if the ice-coated interstellar grains were not heated above approximately 110 K, prior to their agglomeration into cometary nuclei, the inward propagating heat waves should release from the comets a continuous flux of molecular hydrogen.  This flux would exceed that of water molecules at approximately 3 AU preperihelion and approximately 4 AU postperihelion.
authors.affiliation: Department of Geophysics and Planetary Sciences, Tel Aviv University.
authors.forename: A
authors.initials: A
authors.lastname: Bar-Nun
authors.forename: D
authors.initials: D
authors.lastname: Prialnik
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D003903:Deuterium / Q000032:analysis; D004391:Dust / Q000032:analysis; D005118:Extraterrestrial Environment; D006859:Hydrogen / Q000032:analysis*; D007053:Ice / Q000032:analysis; D019110:Meteoroids; D008962:Models, Theoretical; D009356:Neon / Q000032:analysis; D013696:Temperature; D014867:Water / Q000032:analysis
chemical_list: D004391:Dust; D007053:Ice; D014867:Water; D009356:Neon; D006859:Hydrogen; D003903:Deuterium