Space Weathering of Airless Bodies (2015) 2005.pdf

EFFECTS OF SOLAR HEATING ON ASTEROIDS. G. Libourel1, M. Delbo1, J. Wilkerson2, C. Ganino3, P. Michel1, 1Laboratoire Lagrange, Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Blvd de l'Observatoire, 06304 Nice, France, [email protected]; 2Department of Mechanical Engineering Engineering Building OneUTSA, Circle, San Antonio, TX 78249-0670, USA; 3Laboratoire Géoazur, Université Côte d'Azur, Observatoire de la Côte d'Azur, 250 rue Albert Einstein, Les Lucioles 1, Sophia-Antipolis, 06560 Valbonne, France.

Introduction: The surfaces of atmosphere-less so- Here we will describe laboratory experiments and lar system bodies are weathered in space by the action numerical modeling devoted to investigating whether of several processes, such as the implantation of ions thermal fatigue is active on asteroid surfaces. Labora- of the solar wind and the bombardment of micromete- tory experiments of thermal cycling (Fig. 1) were per- orites. It is well known that these phenomena alter the formed on meteorites - taken as analogues of asteroid spectroscopic properties of asteroids [1,2]. Here we surface material - to study under which conditions rock describe the great effects of another process, whose cracking on NEAs occurs and how this process may importance has been considered only recently, that can alter asteroid surfaces. alter the original surface nature of asteroids: the radia- tive heating by the light from the Sun. In particular the surfaces of Near-Earth objects (NEOs) can easily reach temperatures >400 K, and the heat penetration depth is of the heat is in the order of some centimetres [3]. At perihelion distances (q), smaller than 0.5 AU temperatures can be >550 K lead- ing to the breakup of organic components (e.g. 300– 670 K [4,5]). The knowledge of the temperature range of materials at different depth over the orbital evolu- tion of space mission target asteroids is important for Fig. 1. CO2 pulsed laser thermal fatigue experiments on defining sampling strategies that ensure the likelihood the same Murchison CM2 carbonaceous chondrite sample that unaltered and pristine material will be brought after 2 and 128 thermal cycles (∆T≈ 180K). back to Earth [6]. Discussion: Our results [11] demonstrate that There are for instance NEOs with extremely close thermal fatigue is a dominant process governing rego- Sun approaches, such as the asteroids 3200 Phaethon lith generation on small asteroids. We find that thermal and 1566 Icarus. Temperature on these asteroids can fragmentation induced by the diurnal temperature vari- reach 1000 K, inducing mineralogical changes, thermal ations breaks up rocks larger than a few centimetres fracture and/or desiccation cracking and the production more quickly than do micrometeoroid impacts. Be- of dust particles. In particular, 3200 Phaethon is the cause thermal fragmentation is independent of asteroid parent body of the Geminids meteors and activity near size, this process can also contribute to regolith pro- perihelion has been detected for this asteroid [7]. duction on larger asteroids. Production of fresh rego- Moreover, the surfaces of NEOs are also subject to lith originating from thermal fatigue fragmentation large temperature variations (e.g. 150 K): these are due may be an important process for the rejuvenation of the to the change of the insolation intensity caused by the surfaces of near-Earth asteroids. The efficiency of diurnal cycle between day and night, by seasonal ef- thermal fragmentation is controlled by the amplitude of fects, by the orbital eccentricity or simply by a shad- the temperature cycles and by the temperature change owing effect. Are the surface make up and mineralogy rate, which in turn depend on heliocentric distance, of these asteroids altered by these strong temperature rotation period, and the surface thermal inertia. References: [1] Hapke B. (2001) JGR, 106, 10039. [2] variations ? Sasaki S. et al. (2001) Nature, 410, 555. [3] Spencer J. R. et Experimental and theoretical analysis: It is al. (1989) Icarus, 78, 337. [4] Frost, R. et al. (2000) Thermo- known that temperature cycles can lead to mechanical chim. Acta, 346, 63. [5] Kebukawa Y. et al. (2010) Meteoritis load cycles inducing stresses in surface rocks. Cracks and Planetary Science, 45, 99. [6] Michel P. and Delbo M. can thus form and propagate due to temperature varia- (2010) Icarus, 209, 520. [7] Jewitt et al. (2013) AJL 771:L36. tions and the resulting temperature gradients set up by [8] Duennebier, F. and Sutton, G.H. (1974) Journal of Geo- physical Research 79, 4351. [9] Molaro, J. L. and Byrne, S. the thermal cycles. Several works have suggested that (2011) 42nd LPSC, 1494. [10] Dombard, A.J. et al. (2010) such thermal fatigue may play an important role in the Icarus 210, 713. [11] Delbo, M. et al. (2014) Nature, evolution of airless landscapes on bodies such as the doi:10.1038/ nature13153. Moon, Mercury, and on the asteroid Eros [8,9,10].