Fine-grained rim formation – High speed, kinetic dust aggregation in the early Solar System

Abstract

Type 3 chondritic meteorites often contain chondrules and refractory inclusions that are coated with accretionary, fine-grained rims (FGRs). FGRs are of low porosity, were subject to centrally directed pressure, may contain high temperature products like microchondrules and there is a linear relationship between the rim thickness and the radius of the enclosed object.FGRs are thought to have formed by the gentle adhesion of dust onto the central object with the subsequent compression of this fluffy rim within the parent body. However, this model does not explain the low porosity, micro-chondrules and centralized pressure. This model also has difficulties explaining the linear relationship between rim thickness and object size including the existence of a non-zero constant in that linear relationship.We propose that FGRs formed by the relatively high-speed interaction between dust and the object, where high initial impact speed produced abrasion and, possibly, microchondrules. FGR formation occurred over a range of lower speeds aided by vacuum adhesion of fragments from the impacting dust particles. This model naturally produces the rim thickness linear relationship with non-zero constant, low porosity and centrally directed pressure. We call this process kinetic dust aggregation (KDA), which is another name for the aerosol deposition processes used in industry. KDA may be a tentative, part explanation of how dust aggregation occurs in protostellar disks on the pathway from dust to planets.

Nebular thermal processing of accretionary fine-grained rims in the Paris CM chondrite

“…Chondrules, metal grains and calcium aluminum inclusions (CAIs) formed at high temperature, while the fine-grained matrix in which they are embedded contains low temperature material, including hydrated silicates and organic compounds as well as abundant volatile elements. The origins and the mixing processes of the high and the low-temperature reservoirs are still poorly understood and several models of accretion have been proposed (Liffman and Toscano, 2000;Laibe et al, 2008;Morbidelli et al, 2012;Gonzalez et al, 2015;Johansen et al, 2015a;Johansen et al, 2015b;Gonzalez et al, 2017;Pignatale et al, 2018;Liffman, 2019).…”

“…Usually, FGRs are considered to be formed with dust collision speeds of order of tens of centimeters per second (Chokshi et al, 1993;Dominik and Tielens, 1997;Cuzzi, 2004;Blum and Wurm, 2008). Other authors argue that slow speed particle collisions produce a dust rim that is far too porous in comparison to what is observed in the different groups and proposed an alternative scenario, the Kinetic Dust Aggregation (KDA) model, where FGRs are formed through relatively highspeed (order of meters per second to kilometers per second) collisions (Liffman and Toscano, 2000;Liffman, 2019). Careful microscale study of the FGRs should bring new insights on the speed collision.…”

Dependence of the initial internal structure of chondrule rim on dust size distribution

“…FGRs may have been formed soon after or during chondrule forming events (e.g. Liffman, 2019). If it is true, we have to consider the formation of FGRs in the context of chondrule formation.…”

Formation of rims around chondrules via porous aggregate accretion

“…There are two types of rims: fine-grained rims (FGRs, e.g., King and King, 1981) and coarse-grained igneous rims (e.g., Rubin, 1984;Matsuda et al, 2019). One plausible explanation for the formation of FGRs is the accretion of tiny dust grains by chondrules in the solar nebula (e.g., Morfill et al, 1998;Liffman, 2019). After FGRs formed, some of them may be transformed into igneous rims by remelting events (Rubin, 2010).…”

“…Equation (2) indicates that when = 0, no dust grains are accreted, and hence rim = 0. After ≫ acc , almost all dust grains are accreted, and the rim thickness becomes The above estimation has been improved by several studies, where the more realistic treatments of turbulence, the sticking efficiency, the internal structure of rims, and/or erosive collisions were adopted (Cuzzi, 2004;Carballido, 2011;Xiang et al, 2019a,b;Liffman, 2019). However, such improvements still do not allow for accurate estimation of the rim thickness.…”