Compositional Analysis of Haumea and Varuna with Spitzer Space Telescope

The trans-Neptunian region contains objects of different shapes, compositions and sizes, ranging from few kilometers to dwarf planets sizes. Due to their history and evolution, the TNOs (trans-Neptunian objects) are excellent candidates for having retained ices, silicates and volatile materials. With the aim of better understanding and restricting the surface composition of TNOs, we analyze two large TNOs with elongated shapes, being one categorized as a dwarf planet, Haumea and other, Varuna, as a candidate to be one. The novelti of our analisys is that we use Spitzer data, merging them with those of ground-based telescope. We present here new observations obtained in 2016 on the channels 1 (3.6 microns) and 2 (4.5 microns) of IRAC on board of Spitzer Space Telescope during a whole rotational period of these bodies. In order to achieve our goals we compared visible rotational light curves (RLC) obtained from the literature with the ones derived from Spitzer and we found that they are correlated, which indicate that the behavior of the RLCs are dominated by the shape of the bodies. We also look for color variations between Spitzer channels that could be indicative of the presence of patches of different composition, but we did not found hints of heterogeneities in the range of our data. Additionally, we used the albedo from 0.5 to 4.5 microns to restrict the composition of Varuna and Haumea models available in the literature. Our results confirm the results of previous models of Haumea’s surface composition, moreover, we found a new limit to the presence of minor components that should be confirmed with James Webb Space Telescope. We reproduce the previous compositional models of Varuna and both a change in the relative amount of the components proposed in the literature and the addition of new components are needed to properly fit the IRAC/Spitzer data, in particular, the presence of hydrocarbon as methane, previously suggested by Lorenzi et al. 2014, is confirmed by our models. In summary, we present results for two large TNOs, one of them a confirmed dwarf planet, both with elongated shapes. The study of their RLC and of their surface composition is, yet another proof of the diversity present in the trans-Neptunian belt.