Surface Properties Of Asteroids

Our solar system contains a myriad of bodies with sizes ranging from a few meters to hundreds of kilometers. We call them asteroids. The most numerous population (by the number of discoveries) exists between the orbits of Mars and Jupiter, the so-called main-belt asteroids (MBAs). Moreover, we also know of thousands of asteroids with orbits within the orbit of Mars or the Earth. These include, for example, the profound near-Earth asteroids.

Most asteroids are so far away from the Earth and so small that we can only see them as moving points of light. Only a few asteroids have been directly visited by spacecraft, and several were big enough to be resolved by the largest ground-based telescopes (W.M. Keck, VLT) or NASA’s Hubble Space Telescope, or came close enough to be imaged by radar facilities (such as Arecibo).

Therefore, asteroids are usually observed by optical telescopes. We obtain the reflected sunlight (optical photometry), from which we can infer basic orbital and physical characteristics, such as the orbit, a rough estimate of the size, an approximation of the shape, and a brief idea of the surface composition.

However, to learn more about asteroids’ physical properties and surfaces, the emitted component of the light is highly valuable. In particular, the surface of an asteroid absorbs part of the visible sunlight, gets heated, and then re-radiates the energy in infrared wavelengths. This thermal radiation reflects the physical properties of the surface material such as the thermal conductivity, heat capacity, density, or the macroscopic surface roughness. The thermal radiation also allows us to infer the asteroid’s size and surface reflectivity (albedo).

The thermal radiation is difficult to observe from the ground-based facilities because Earth’s atmosphere is almost opaque for the typical infrared wavelengths (10-30 micrometers). Fortunately, NASA’s Near-Earth Object Wide-field Infrared Survey Explorer, or NEOWISE spacecraft, which orbits the Earth, provided precious thermal infrared observations for more than 100,000 asteroids.

In our recent work in Icarus, we analyzed NEOWISE thermal data by the means of a thermophysical model for more than 300 asteroids. This number reflects the two necessary inputs for the detailed thermophysical modeling — high quality thermal infrared observations and an approximation of a shape model. The latter is the main limiting factor: multi-faceted shape models with large-enough resolution are available for fewer than 1,000 asteroids.

The basic idea behind the thermophysical model is to compute the temperature map on an asteroid’s surface by solving the 1D heat conduction equation in a surface layer of each shape’s facet. Once we know the temperature, we can then derive the flux emitted towards the observer for any wavelength. The temperature map depends on several physical properties, such as thermal inertia (the resistance of the surface material toward temperature change, a function of thermal conductivity, heat capacity, and density), macroscopic surface roughness, size, or albedo. We vary these properties to find the best agreement with the observed thermal infrared fluxes.

Our work more than doubles the number of MBAs with determined thermophysical properties, especially thermal inertias. We confirmed the trend that thermal inertia increases with decreasing size, however, a large range of thermal inertia values is observed within the similar size ranges between D∼10–100 km. The value of thermal inertia is believed to be related to the structure of the surface material that we call regolith. It consists of broken rocks and dust, and thermal inertia tends to be larger if there is less dust in the regolith.

On a timescale of millions of years, the rocks on the surface are continuously destroyed by micrometeorites and/or thermal fatigue, which drives the thermal inertia to lower values. Therefore, older asteroids, which usually implies larger as well, should have lower thermal inertia values. We also derived unexpectedly low thermal inertias for several asteroids with sizes 10 < D < 50 km, indicating a very fine and mature regolith on these small bodies (they might be older). Furthermore, our work indicates that the majority of small asteroids, as well as fast-rotating asteroids, have little, if any, dust covering their surfaces.

We now have a much better idea of the properties of the surface regolith for MBAs in the size range of D∼10–100 km. Currently, we continue to improve our knowledge of asteroid shape models, which will allow us to apply the thermophysical model to many additional asteroids. Once the number of detailed thermophysical solutions grows sufficiently, we will have the opportunity to study surface properties within smaller asteroidal groups characterized by their similar composition or past collisional origin.

These findings are described in the article entitled Thermophysical modeling of main-belt asteroids from WISE thermal data, recently published in the journal Icarus. This work was conducted by J. Hanuš and J. Ďurech from Charles University, M. Delbo’ from the Université Côte d’Azur, and V. Alí-Lagoa from the Max-Planck-Institut für extraterrestrische Physik.