1. ----------------------------
By - J. Durech et al, 2008
Summary - For Apollo, observations match the theoretical YORP value, while for Itokawa, observations do not match the theoretical YORP value. Lightcurves of both asteroids are shared in the paper.
2. ----------------------------
Title - Rubble pile asteroids are forever
By - Fred Jourdan et al, 2023
Summary - This study is based on 3 dust particles asteroid Itokawa & has an illustration depicting its formation. Argon isotope study, along with thermal and diffusion models, constrain the formation age of Itokawa to over 4.2 billion years ago. Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created. The texturally and compositionally equilibrated nature of the particles show that they were initially located deep in the monolithic parent asteroid, hence protected from ambient bombardment and shock heating processes. In order to be affected or subsequently affectable by impact-related thermal events at 4.2 Ga, the particles would need to be brought near the surface, either by total disruption of the parent body or by deep crater excavations. Importantly, impact shock pressures less than 25 GPa will not raise the temperature by more than 50 °C in compacted material, clearly insufficient to raise the rock above its Ar closure temperature of 250 to 300 °C for plagioclase of this grain size.
News - Universe Magazine, Phys.org, Space.com
3. ----------------------------
By - Katherine Burgess & Rhonda Stroud, 2021
Summary - Samples from asteroid 25143 Itokawa returned by the Hayabusa mission have been identified as LL 4-6 ordinary chondrite materials and have shown it to be a rubble pile that aggregated after break-up of a parent body. We identify a cubanite-chalcopyrite-troilite-pyrrhotite assemblage, the phases and structure of which are indicative of low-temperature, aqueous alteration. Cubanite is stable only at temperatures below around 250 °C and has thus far only been identified in CI carbonaceous chondrites and the comet 81P/Wild2 sample suite. Chalcopyrite is also very rare in the meteorite record and is found mostly in R chondrites and some CK chondrites.
Because the Itokawa parent body experienced significant thermal alteration with little evidence of low-temperature equilibration or aqueous alteration, we propose that the assemblage we identify is most likely exogenous (external origin) and represents a component of an impacting body.
The sulfide grains in Itokawa particle RB-CV-0038 are inconsistent with the primary formation on the Itokawa parent body, and must instead be a microxenolith of carbonaceous chondrite or cometary origin. Cubanite and 4C-pyrrhotite together, which has been identified in CI chondrites, indicate temperatures less than 200 C. By contrast, LL6 particles among the Itokawa sample set, equilibrated at temperatures of 800 C, with slow cooling to only 600 C. Cubanite, a Cu-Fe-sulfide, is known only in terrestrial ore deposits, comet 81P/Wild2, and CI-chondrite materials.
Low-temperature aqueous alteration did occur on ordinary chondrite parent bodies, as evidenced by bleached chondrules in ordinary chondrites of all metamorphic grades. Additionally, the cubanite in the comet Wild2 sample suite may have formed during alteration of the comet, which in turn could mean that cometary material is also a possible source for the cubanite on Itokawa, although such alteration is not thought to be pervasive on comets. The presence of exogenous material on Bennu and Ryugu, and now on Itokawa, confirms that asteroids with material on their surface from unrelated parent bodies are the norm rather than an exception.
No comments:
Post a Comment