Response to the comment by B. Hapke on “A critical assessment of the Hapke photometric model”

In this note we address critical comments given by Hapke (in this issue) [1] on our paper “A critical assessment of the Hapke photometric model” (Shkuratov et al., 2012 [2]) elaborating further on weak points of this model. One of the main problems of the H-model is the too free juggling with formulas, leading, in particular, to violation of energy conservation. The model is filled with ungrounded approximations and unsupported conclusions. Some of the model assumptions are mutually contradictory, e.g., it considers anisotropic single-particle phase functions to describe incoherent multiple scattering and simultaneously uses the isotropic function in order to calculate coherent backscattering. Another well-known problem of the H-model is that some of the model parameters have similar effects on the reflectance, which makes it difficult to retrieve unique parameter values. The model uses empirical parameters and, hence, is empirical.

► The comment provides no satisfactory answers to our criticisms of the Hapke model. ► The model is self-inconsistent, e.g., it treats single scattering as both isotropic and anisotropic. ► At least two of the model implications are in violation of energy conservation law. ► The model cannot reproduce parameters of well-characterized surfaces. ► The model requires very cautious use.