Thermal inertia and roughness of the nucleus of comet 67P/Churyumov–Gerasimenko from MIRO and VIRTIS observations

Abstract

Aims. Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal inertia (TI) and roughness in several regions on the largest lobe of the nucleus. Methods. We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014. Results. The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK−1 m−2 s−0.5 for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1 m−2 s−0.5 and values in the range 40–160 JK−1 m−2 s−0.5 in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.

The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta

“…With a value of 22 J K -1 m -2 s -1/2 , they check their modelled value against diurnal phase curves for the temperature and find good agreement between the data and model. Finally, Marshall et al (2018) also analyse data from September 2014 and calculate bounds for the thermal inertia in five spots on the nucleus surface. Results in the mm channel imply that the thermal inertia is <80 J K -1 m -2 s -1/2 .…”

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

“…67P exhibits a low surface albedo ∼ 4% , is dominated by organic materials and has an extremely high porosity Hérique et al 2016;Pätzold et al 2016Pätzold et al , 2019. These compositional and structural characteristics result in a low thermal inertia Marshall et al 2018), which promotes large temperature gradients over a few cm and extreme temperature swings following rapid changes in illumination. Little to no ice is directly detected on the surface, except: 1) in specific shadowed areas where ice can remain stable for extended periods of time (Filacchione et al 2016a), 2) as surface frost associated to a diurnal cycle where ice is mobilized from the subsurface during the day and then is recondensed on the surface during the night (De , and 3) when the activity level increased enough towards perihelion for nucleus erosion to expose fresher materials .…”

“…For modeling such heat transfer processes, one has to know the thermal and mechanical properties of the materials involved. Rosetta instruments (MIRO, VIRTIS, OSIRIS, RSI, MUPUS, and SESAME) provided some physical characteristics that are related to the thermal inertia: density, strength, diurnal period, and thermal conductivity (for a review, see Marshall et al (2018) and , this issue). The thermal inertia was derived from Rosetta measurements with considerable error bars.…”

Comet 67P/Churyumov-Gerasimenko rotation changes derived from sublimation-induced torques

“…However, this scenario is unlikely, since the response of sublimation rates to radiation is almost immediate as seen by short (< 1 h) delays of jet outbreaks (Lai et al (2016); Shi et al (2016)) and the good agreement of inner dust structures with illumination driven dust release (Kramer & Noack (2016); Kramer et al (2018)). Measurements of VIRTIS and MIRO found that the thermal inertia is lower than 320 JK −1 m −2 s −0.5 when including the error bars (see Marshall et al (2018) for an overview). Such a small thermal inertia is not able to provide the needed phase lag of several hours of the maximum sublimation with respect to the maximum irradiation as thermal simulations show and measurements of the activity maxima compared to noon time show Shi et al (2016).…”