  The H2 molecule has two nuclear spin isomers, the so-called ortho and para
isomers. Nuclear spin conversion (NSC) between these states is forbidden in the
gas phase. The energy difference between the lowest ortho and para states is as
large as 14.7 meV, corresponding to ~170 K. Therefore, each state of H2
differently affects not only the chemistry but also the macroscopic gas
dynamics in space, and thus, the ortho-to-para abundance ratio (OPR) of H2 has
significant impacts on various astronomical phenomena. For a long time, the OPR
of nascent H2 upon formation on dust grains has been assumed to have a
statistical value of three and to gradually equilibrate in the gas phase at the
temperature of the circumstances. Recently, NSC of H2 was experimentally
revealed to occur on water ice at very low temperatures and thus incorporated
into gas-dust chemical models. However, H2 molecules should form well before
dust grains are coated by water ice. Information about how the OPR of H2
behaves on bare silicate dust before ice-mantle formation is lacking. Knowing
the influence of the OPR of H2 if the OPR changes even on a bare silicate
surface within an astronomically meaningful time scale is desirable. We report
the first laboratory measurements of NSC of H2 physisorbed on amorphous
silicate (Mg2SiO4) at temperatures up to 18 K. The conversion was found to
occur very rapidly.
