Can the Charon-forming giant impact generate elongated dark areas on Pluto?

Abstract:

The New Horizons spacecraft has found elongated dark areas in the equatorial region of Pluto, which were informally called “the Whale”. Here we examine the possibility that the dark areas on Pluto were formed by thermal alterations and subsequent polymerization of interstellar volatiles by a Charon-forming giant impact. Based on the satellite-to-planet mass ratio, the Pluto-Charon system is suggested to be of a giant impact origin [Canup, 2005]. However, the occurrence of the giant impact has been poorly constrained by factual evidence. On the other hand, Pluto is one of the largest Kuiper belt objects, which is highly likely to contain various interstellar volatiles, including aldehyde and ammonia. The previous studies show that these soluble volatiles are thermally polymerized in solutions at high temperatures, leading to a formation of complex insoluble organic solids [Cody et al., 2011; Kebukawa et al., 2013]. Thus, impact-induced heating could have melted the icy crust and converted the interstellar volatile into complex organic solids near the impact site.

In the present study, we produce complex organic solids by thermal polymerization of formaldehyde and ammonia in solutions for various temperatures. By comparing the colors (in UV-VIS range) of the produced organic solids, we constrain possible temperatures achieved in the dark areas on Pluto, assuming that they were formed by thermal alterations of interstellar volatiles. In addition, we perform hydrodynamic simulations of a giant impact onto Pluto in order to examine whether both the formations of a Charon-sized moon and the spatial distribution and achieved temperatures of heated materials can be reproduced. Based on the combination of laboratory experiments and numerical simulations, we will examine the giant-impact hypothesis for the origin of the Pluto-Charon system.