El Mir, Charles - ORIGIN AND EVOLUTION OF REGOLITH ON AIRLESS BODIES: THE ROLE OF THERMAL FATIGUE

Abstract: 
The surface of planetary bodies is of interest as it is the part observed with remote sensing instruments, and the part that will be sampled by sample return missions. Images obtained through space missions [1,2], infrared and radar observations [3,4] and returned samples [5] re-veal these surfaces are covered by a layer of loose, unconsolidated, debris referred to as ‘rego-lith’. Understanding the formation and evolution of regolith as well as the variation of these pro-cesses will provide insight into the origin, age, and properties of these bodies.
Regolith generation and evolution is typically attributed to re-accumulation of ejecta as well as continuous micrometeoroid impact leading to the breakdown of boulders [6, 1]. While this model can explain lunar regolith, it may not be applicable to kilometer-sized asteroids. Im-pact velocities on main-belt asteroids (~5km/s) are much lower than on the Moon (~15 km/s) and are therefore less efficient in shattering asteroidal rocks. Laboratory experiments [7] and impact models [8] show that crater ejecta velocities tend to exceed the escape velocity of asteroids. An alternative mechanism of regolith formation is fracturing due to thermal fatigue by diurnal cy-cling. Such a phenomenon was noted as a mechanism for fracturing of rocks in cold and hot ter-restrial environments [9]. Dombard et al. [10] attributed the formation of ponds on Eros to ther-mally disaggregated boulders, and Delbo et al. [11] demonstrated the importance of thermal fragmentation for regolith production and surface rejuvenation on asteroids. Thermal fragmenta-tion also has the effect of weakening surface rocks, thus increasing the efficiency of rock commi-nution by micrometeoritic impact.
This study aims to characterize the thermal fragmentation by examining its viability in producing regolith. Thermal fragmentation is studied as a function of temperature range, rate, petrology, grain size, and size-frequency distribution of the resulting particles. It is noted that areas of lunar permanent shadow [12] provide the potential to test the relative importance of thermal fragmentation models. These areas do not experience a diurnal thermal cycle; thus, fragmentation would be exclusively due to micrometeorite bombardment. Surface properties and fragment distributions can be measured and compared with areas that experience thermal cycling. In this context, fatigue experiments will be conducted to provide the necessary input parameters for the thermal fragmentation mechanism in a regolith formation and evolution model. Digital Image Correlation (DIC) will be used to obtain full-field strain maps as a consequence of fatigue crack growth, and these strain maps will be used to help determine the driving force on the crack tip. The recorded images by DIC technique will be also utilized to measure fatigue crack growth rate.