  Thermal radiation becomes a prominent feature in the continuum spectrum of
Venus longwards of $\sim$3 $\mu$m. The emission is traceable to the upper cloud
and haze layers in the planet's mesosphere. Venus' thermal radiation spectrum
is punctuated by CO$_2$ bands of various strengths probing into different
atmospheric depths. It is thus possible to invert measured spectra of thermal
radiation to infer atmospheric temperature profiles and offer some insight into
the cloud and haze structure. In practice, the retrieval becomes complicated by
the fact that the outgoing radiation is multiply scattered by the ubiquitous
aerosol particles before leaving the atmosphere. We numerically investigate the
radiative transfer problem of thermal radiation from the Venus night side
between 3 and 5 $\mu$m with a purpose-built model of Venus' mesosphere. Special
emphasis is laid on the significance of scattering. The simulations explore the
space of model parameters, which includes the atmospheric temperature, cloud
opacity, and the aerosols' size and chemical composition. We confirm that
aerosol scattering must be taken into account in a prospective temperature
retrieval, which means an additional complication to the already ill-posed
retrieval problem. We briefly touch upon the degeneracy in the spectrum's shape
associated with parameterization of the Venus clouds. Reasonable perturbations
in the chemical composition and size of aerosols do not significantly impact
the model simulations. Although the experiments are specific to the technical
characteristics of the Visual and Infrared Thermal Imaging Spectrometer on the
Venus Express spacecraft, the conclusions are generally valid.
