A thermophysical model for airless bodies and evaluation of impact risk

Introduction. The characterization of the surface material of airless bodies is important for the comprehension of the geology and evolution, and also for mission decisions as the selection of the sample site. The thermophysical properties affect the surface temperature curve, that is the temperature as function of time; the thermal inertia, defined by (kρc)^1/2 (where k is the thermal conductivity,  the density and c the specific heat), is the key parameter that controls the maximum daytime temperature and the time at wich the maximum occurs. The comparison between observed and calculated temperature of an airless body allows the retrieval of thermophysical properties of the object. Furthermore, the thermal inertia causes an asimmetric thermal emission and non gravitational perturbation on orbital parameters called Yarkovsky  effect; a proper evaluation of  this is required in order to calculate the impact risk of a potentially hazardous object.

 

Methods. We developed a thermophysical model for the calculation of surface temperature as function of thermal inertia and emissivity [1, 2, 3]. The code numerically solves the 1D heat equation taking into account the solar illumination given by orbital and topographic conditions. We have included in our thermophysical model the effects of the particle size distribution, using the results of Ryan et al. (2020) [4]; numerical simulations by these authors indicate that the thermal conductivity of a polydisperse soil is approximated by the thermal conductivity of a monodisperse soil with a particle diameter equal to Sauter mean D32, that is the diameter of a particle with the same volume-to-area ratio. Corrections for non-isothermality of the particles are also included.

The temperatures can be projected on the target shape model with the MATISSE tool [5].

 

Results and future work. The code is beeing checked on Ryugu with the data provided by the TIR instrument onboard the Hyabusa 2 mission [6] of JAXA, and some theoretical lunar temperatures have been checked with the measurements of the DIVINER instrument of NASA's Lunar Reconnaissance Orbiter [7]. The comparison theoretical-observed temperature for Ryugu also will allow to properly modelize the rubble-pile asteroids. We want to apply our model for 99942 Apophis, that will safely pass close to Earth on April 13, 2029.

 

References.

[1] Rognini, E., et al. (2019), Journal of Geophysical Research: Planets, 125

[2] Rognini, E., et al. (2022), Planetary and Space Science, 212

[3] Capria, M. T., et al. (2014), Journal of Geophysical Research, 41, 1438-1443

[4] Ryan, A., et al. (2020), Journal of Geophysical Research, 125

[5] Zinzi, A., et al. (2016), Astronomy and Computing, 15

[6] Okada, T., et al. (2017), Space Sci. Rev., 208

[7] Paige, D. A., et al. (2010), Space Sci. Rev., 150

 

 Figure 1: Occator's thermal map calculated with the model, image created with MATISSE (Multi-purpose Advanced Tool Instrument for the Solar System Exploration, https://tools.ssdc.asi.it/matisse.jsp, Zinzi et al. 2016)