Classification of cometary orbits based on the concept of orbital mean temperature

The orbital mean temperature (T_m) for periodic cometary orbits has been calculated as a function of eccentricity e for 0 ≤ e ≤ 0.97, the semimajor axis a for 1 ≤ a ≤ 100 AU, and for an albedo A = 0. If water ice is the major constituent of comets, it can be concluded from these results that d'Arrest, Encke, Oterma, Schwassmann-Wachmann 2, and Tempel 2 contain crystalline ice. In the past, they must have experienced a short period of high activity due to the phase transition from amorphous to cubic ice. The actual low activity of those comets is not necessarily due to depletion of volatiles. Schwassmann-Wachmann 1 and Halley are expected to contain amorphous ice in the inner part of the nucleus. Schwassmann-Wachmann 1 will conserve its erratic activity for a long time until all its ice is in the cubic state. Halley will have an important activity at small heliocentric distance until final depletion of volatiles as no thick crystalline crust can be built up. The present statements are also valid for albedos A ≤ 0.3. THe results are generalized to other periodic orbits.