  Quantum mechanical tunneling inversion transition in ammonia NH3 is actively
used as a sensitive tool to study possible variations of the electron-to-proton
mass ratio, mu = m_e/m_p. The molecule H3O+ has the inversion barrier
significantly lower than that of NH3. Consequently, its tunneling transition
occurs in the far-infrared (FIR) region and mixes with rotational transitions.
Several such FIR and submillimiter transitions are observed from the
interstellar medium in the Milky Way and in nearby galaxies. We show that the
rest-frame frequencies of these transitions are very sensitive to the variation
of mu, and that their sensitivity coefficients have different signs. Thus, H3O+
can be used as an independent target to test hypothetical changes in mu
measured at different ambient conditions of high (terrestrial) and low
(interstellar medium) matter densities. The environmental dependence of mu and
coupling constants is suggested in a class of chameleon-type scalar field
models - candidates to dark energy carrier.
