A Composite Phase Function for Cometary Dust Comae

Comets represent the most preserved planetesimals we can nowadays study and dust is one of their major components. Once emitted in the coma, cometary dust particles represent anisotropic scatterers of the incident solar light and their nature can be investigated with remote sensing studies. Among them, the measurement of the phase function curve has a key importance in several scientific aspects. It can be inverted with theoretical and laboratory studies to derive hints on the intimate nature of the emitted dust. It is also needed in adjusting cometary dust production rates for phase angle effects when data obtained throughout large time intervals are correlated. Finally, it is useful for space instruments planning since it provides inputs for optimal exposure times for remote sensing sensors which observe the coma spanning a large range of phase angles during close approaches. This will be particularly valuable in the framework of the future ESA Comet Interceptor mission which is going to fly-by a Dynamically New Comet entering our Inner Solar System for the very first time, carrying instruments which will image the coma with different observing geometries and phase angles in a short amount of time. In order to provide an useful tool to address the aforementioned scientific topics, we used available literature data to build a new composite phase function for cometary dust comae. This was obtained fitting Henyey-Greenstein functions to the original data of 11 comets and then connecting them in a continuous way as all data values were coming from a single average comet. We then fitted our result with a compound Henyey-Greenstein curve and compared it with previous models which were not including recent literature data constituting fine follow-ups of comets at small and large phase angles. The main difference is found in the description of the forward scattering surge, where our model depicts intensity one order of magnitude larger than previous ones. This finding is extremely important since it shows that the choice of the model may have severe consequences when interpreting, or instrumentally planning, forward scattering data.