A Collective Trigger for Widespread Planetesimal Formation Revealed by Accretion Ages

Abstract

The formation of planetesimals was an integral part of the cascading series of processes that built the terrestrial planets. To illuminate planetesimal formation, here we develop a refined thermal evolution model to calculate the formation ages of meteorite parent planetesimals. This model includes chemical reactions and phase changes during heating, as well as natural variations in the proportions of the constituent phases of these planetesimals. We find that the parent bodies of non-carbonaceous (NC) and carbonaceous (CC) iron meteorites start forming at very similar times (~0.95 Myr after calcium-aluminium-rich inclusion [CAI] formation) and occupy overlapping time windows. NC and CC chondrite parent bodies formed later during non-overlapping periods. We combine these ages with proportions of isotopic end-members we recover from mixing models to construct records of motion throughout the protoplanetary disk. These records argue that NC and CC material traversed the barrier in the disk after ~0.95 Myr after CAI formation. The onset of this motion coincided with planetesimal formation, indicating that the phenomenon that drove motion also triggered planetesimal formation. We argue that this feature also served as the semi-permeable barrier in the disk. Although its identity is uncertain, the effects this phenomenon had on the timing of planetesimal formation and motion through the disk can now serve as constraints on models of disk evolution. Models that reproduce these effects would elucidate the nature and implications of this phenomenon, which is key to unlocking a holistic model of terrestrial planet building.

Figures (6)

• Figure 1: Calculated accretion ages ($t_\mathrm{a})$ for NC (red) and CC (blue) meteorites. The error bars are smaller than the points in some cases. The grouped iron meteorites and chondrites are ordered by their accretion age within each reservoir. The ungrouped iron meteorites are ordered by their accretion ages within each reservoir and are placed between the grouped iron meteorites and grouped chondrites in their respective reservoir. Ages calculated for NC iron meteorites with 95% matrix (closed symbols) and 5% matrix (open symbols) in their chondritic progenitor are included. Our adopted start time of planetesimal formation is marked by the vertical dashed line. The grey line distinguishes NC and CC meteorites.
• Figure 2: Recovered proportions of CI material ($P_{\mathrm{CI}}$) for NC (red) and CC (blue) meteorites. The error bars are smaller than the points in some cases. The order of meteorites is the same as Fig. \ref{['Ta']}. The grey line distinguishes NC and CC meteorites.
• Figure 3: Recovered proportions of CAI material ($P_{\mathrm{CAI}}$) for CC meteorites. The error bars are smaller than the points in some cases. The order of meteorites is the same as Fig. \ref{['Ta']}.
• Figure 4: a $P_{\mathrm{CI}}$ as a function of $t_{\mathrm{a}}$ for CC meteorites. The trends we identify are highlighted by arrows that evolve from blue to red as the amount of NC and CAI material in the CC reservoir increases. The names of CC iron meteorites are not included to avoid overcrowding the figure. b $P_{\mathrm{CAI}}$ as a function of $t_{\mathrm{a}}$ for CC meteorites. The error bars are smaller than the size of the points in some cases. Our recovered start time of planetesimal formation is marked by the vertical dashed line. The lines are linear best fits to iron meteorite (excluding the four ungrouped examples that did not fall on the main trend) and chondrite (excluding CI chondrites) data points. The ungrouped iron meteorite data points that as discussed in the text as falling off the main trend are shown as open symbols.
• Figure 5: Schematic of disk evolution and planetesimal formation. Timings are Myr after CAI formation. At early times ($\sim$0.9 Myr after CAI formation) the disk is divided by a barrier feature that creates an inner reservoir that is divided into three sub-reservoirs---NC-$\alpha$, NC-$\beta$, and NC-$\gamma$---and an outer reservoir composed of CI material. CAIs are localised at the position of this barrier. At $\sim$0.95 Myr after CAI formation, the nature of the barrier changes, becoming semi-permeable. After this time (labelled nominally as $\sim$1.0 Myr after CAI formation), this feature has three affects: it transports NC material outwards and CC material inwards (shown by the arrows); it causes planetesimals to form on both sides (shown by circles); and it moves CAIs overwhelmingly outwards (shown by the green hexagons). The locations of planetesimals within the NC-$\alpha$ and NC-$\beta$ sub-reservoirs are arbitrary. By $\sim$2 Myr after CAI formation, more NC and CI material has crossed the feature and more planetesimals have formed. Between $\sim$2-4 Myr after CAI formation, NC material no longer mixes into the CC reservoir, and distal CI material starts mixing into the innermost CC reservoir. More planetesimals form during this period in the CC reservoir. The NC reservoir may have dissipated by this time. Throughout this figure, the feature that separates the NC and CC reservoirs is depicted by a thick grey line that is solid when impermeable and dashed when semi-permeable. The condensation lines within the NC reservoir are shown by thin grey lines that are solid when impermeable and dashed when semi-permeable.