A method to distinguish between micro- and macro-granular surfaces of small Solar system bodies

Abstract

The surface granularity of small Solar System bodies is diverse through the different types of planetary bodies and even for specific objects it is often not known in detail. One of the physical properties that strongly depends on the surface structure is the surface temperature. In highly porous media with large voids, radiation can efficiently transport heat, whereas more compact, micro-porous structures transport the heat primarily by conduction through the solid material. In this work, we investigate under which conditions a macro-porous surface can be distinguished from a micro-porous one by simply measuring the surface temperature. In our numerical simulations, we included circular and elliptical orbits with and without obliquity and varied the rotation period of the considered objects. We found that daily temperature cycles are rather insensitive to the specific surface granularity. However, the surface temperature at sunrise shows significant dependency on the material structure and this effect becomes even more pronounced when the solar intensity increases. By measuring the sunrise temperature as a function of insolation at noon, a differentiation between micro- and macro-granular surface structures is possible. In this paper, we provide a strategy how remote sensing can be used to derive the surface structure of small Solar System bodies.

Formation of Comets

“…For pebbles, heat conduction is reduced further because of the granularity inside the pebbles. Therefore, and due to the large void spaces between the pebbles, thermal radiation contributes to the energy transport and can even dominate for large pebbles and high temperatures, as shown in Bischoff et al [149] (their Figure 1). Only for low temperatures (<100 K for pebbles with 5 mm radius) does heat conduction dominate, but then the thermal conductivity is extremely small (<10 −3 W/(K m)).…”

“…Strong positive correlation between the surface temperature at sunrise and the insolation at local noon for a subsurface made of pebbles, in contrast to no such dependency for a makeup without large void spaces [149], measurable by thermal IR mapping. Such measurements would deliver, from remote observations only, invaluable information about the presence and size of pebbles in a shallow subsurface layer.…”

“…They found a best-fitting thermal inertia of 85 J/(K m 2 s 0.5 ) [210]. Assuming a heat capacity of 560 J/(kg K) (as used in Bischoff et al [149]) and a density of 532 kg/m 3 (for the bulk nucleus, [169]), these thermal inertia values translate into thermal conductivities of ∼0.02 W/(K m). As described before, the uncertainty in heat capacity and density of the measurement spots is forwarded into uncertainties of the thermal conductivity.…”

“…It is the pebble-pile structure that makes planetesimals formed by gentle gravitational collapse so interesting for the study of their thermal evolution, because (i) pebbles possess low thermal conductivity due to their porous structure, (ii) the thermal contact between pebbles is minimal, and (iii) radiative heat transport is very inefficient at low temperatures. The heat-transport model of Gundlach and Blum [148] shows that at low temperatures and for pebbles with radii of 5 mm, the thermal conductivity is less than 10 −3 W m −1 K −1 (see also Figure 1 in [149]) and, thus, is more than three orders of magnitude lower than that of solid (i.e., non-porous, non-hierarchical) materials with the same bulk composition.…”

“…Thus, such a value alone cannot be used to conclude the kind of structure of the cometary surface. The method proposed by Bischoff et al [149], which allows the distinction between micro-and macro-porous subsurface structures by relating the sunrise temperatures to the insolation at noontime, could not be used with measured data of comets due to the lower limit of available temperature data. However, in future missions to comets, such as ESA's Comet Interceptor [211], the application of this method should be kept in mind when measuring surface temperatures.…”