Spectral emissivity of Pluto and Charon at millimeter wavelength inferred from spatially resolved observations of ALMA

Although Pluto and its moon Charon may have undergone a process of co-evolution, their surfaces exhibit distinct characteristics in that Pluto is predominantly covered with nitrogen and methane ice, while Charon contains a greater abundance of water ice. Their albedo, surface emissivity, and thermal inertia—which all influence surface temperature—also differ significantly [Grundy et al., 2016, Science 351, 6279; Lellouch et al., 2016, A&A 588, A2]. Further investigation of the individual surface properties of Pluto and Charon will provide deeper insights into the evolutionary history of the Pluto system.

Separating Charon from Pluto (by an angular separation of ~0.9 arcseconds) has long been a challenge in observational studies. Most past ground-based and space-based observations (except for the New Horizons spacecraft) have lacked the spatial resolution to separate the two bodies, and the data have typically been analyzed as the combined Pluto system. However, radio interferometry in the submillimeter and millimeter wavelength ranges offers a solution to this problem. High spatial resolution observations of Pluto and Charon have previously been conducted with the Submillimeter Array (SMA) and Karl G. Jansky Very Large Array (VLA). In addition, the Atacama Large Millimeter/submillimeter Array (ALMA) enables us to achieve even higher spatial resolution. Furthermore, these long-wavelength observations are capable of probing thermal emissions from a few centimeters to several tens of centimeters beneath the surface.

In this study, we analyzed continuum emission images of Pluto and Charon at a wavelength of 1.2 mm, obtained from the ALMA Science Archive (Project code: 2016.1.01100.S), which was reported preliminarily in the conference presentations [Butler et al., 2019, Pluto System After New Horizons 2019 (LPI Contrib. No. 2133)]. The observations had been carried out on September 27 and October 14, 2017. Using the Second Levels Quality Assurance (QA2) data, we found that observations achieved a spatial resolution of approximately 0.025 arcseconds—sufficient to resolve Pluto, whose apparent diameter is about 0.1 arcseconds. Thus, as expected, Pluto and Charon are clearly separated in the images. The disk-averaged brightness temperatures, averaged over three data sets, were measured as 22.9 ± 2.9 K for Pluto and 25.3 ± 3.0 K for Charon. These values are nearly 10 K lower than those reported in previous studies, which suggests a potential calibration error in the archived ALMA images.

We calculated the spatial distribution of thermal emission at 1.2 mm using a three-dimensional polygonal surface model for both Pluto and Charon. This model solves one-dimensional heat diffusion equation, each location on Pluto’s and Charon’s disks, from their orbital and rotational motions. Due to Pluto’s long orbital period and the significant tilts of its orbital plane (~17°) and rotational axis (~120°), latitudes above 60° experience polar days lasting more than one Earth year. By representing the surface as a mesh of polygons, it becomes possible to efficiently determine the areas illuminated by the Sun and those visible to the observer. This modeling approach also allows for straightforward application of the heat diffusion equation on a per-polygon basis, making it well suited for comparison with high-spatial-resolution thermal emission data, such as those provided by ALMA. While previous studies treated Pluto's albedo as uniformly 0.46 [Lellouch et al., 2016, A&A 588, A2] and Charon's albedo as uniformly 0.25 [Lellouch et al., 2011, Icarus 214, 701], we assigned spatially varying Pluto and Charon albedo values to each surface polygon based on latitude and longitude using New Horizons images [Buratti et al., 2017, Icarus 287, 207]. In this presentation, we report spectral emissivities derived from the ALMA measurements.