The one-dimensional diffusion accelerated neutral-particle transport (ONEDANT) code is augmented to explore the effects of gravity on neutron flux spectra near planetary surfaces. The lifetime of the neutron is also explicitly accounted for. The results show a qualitatively new feature in planetary neutron leakage spectra in the form of a component of returning neutrons having kinetic energies less than the gravitational binding energy (0.132 eV for Mars). The net effect is an enhancement in flux at the lowest energies that is largest at and above the outermost layer of planetary matter. This effect diminishes with increasing depth. Fluxes for kinetic energies larger than the gravitational binding potential are minimally changed by gravity. All energy spectra can be well characterized by a model consisting of the super position of relatively simple thermal and epithermal functions that are completely specified by four parameters; the thermal and epithermal amplitudes, α and β, respectively; the thermal temperature, Tα; and the epithermal power law exponent, p. Instrumental parameters for the initial version of the neutron mode of the Mars observer gamma ray spectrometer are used to demonstrate the ability to use measured count rates to determine thermal and epithermal amplitudes. ¿ American Geophysical Union 1989