15:45 〜 16:00
[SIT18-07] CORE COOLING IN PLANETESIMALS: CONSTRAINING THE TIMING OF PARENT BODY IMPACTS AND SOLAR NEBULA DYNAMICS.
★Invited Papers
キーワード：protoplanetary disk evolution, Pd-Ag chronology, asteroid core evolution, iron meteorites, gas dissipation, giant planet migration
Iron meteorites sample the metallic cores of some of the earliest-accreted planetesimals in our Solar System and hence survived the processes that shaped Solar System architecture, including giant planet migrations and gas disk dissipation. Additionally, iron meteorites preserve a record of core thermal evolution, such as core-mantle differentiation and subsequent cooling. Their parent bodies underwent core formation < 3 Myr after the condensation of the first Solar System solids (calcium-aluminium rich inclusions; CAIs) [1]. Metallographic cooling rates imply subsequent rapid cooling of many planetesimal cores, attributed to the impact removal of their mantles [2]. The short-lived 107Pd-107Ag decay system (t1/2 ~6.5 Myr) provides a tool for dating core crystallisation and exploring asteroid thermal histories. Here, we present core cooling timescales for inner Solar System asteroids represented by iron meteorite groups IAB, IIAB and IIIAB [3].
The newly determined 107Pd/108Pd initial ratios for these 3 asteroids are within uncertainty of each other, indicating the Pd-Ag system closed in a similar time-frame on these bodies. The IVA group, an additional iron meteorite parent body, yields a similar value [4]. The Pd-Ag cooling ages depend on the Solar System initial 107Pd/108Pd. We calculate Pd-Ag ages relative to the value determined for carbonaceous chondrites [5]. Pd-Ag ages for the IIAB, IIIAB and IVA bodies equate to cooling between ~7.8-11.7 Myr, and this dates the timing of mantle-stripping impacts. Additionally, the IAB parent body cooled quickly after an impact between ~11 – 13.6 Myr.
Our new data therefore suggest a highly energetic inner Solar System between ~7.8-11.7 Myr. This may date the dissipation the gas disk, after which the damping effect of gas drag ceases. Planetesimals embedded in the disk became gradually excited, leading to energetic collisions [6]. A giant planet instability (‘Nice’ model) could also cause reorganisation of the inner Solar System and impacts. Although the precise timing of the instability is difficult to determine, models that predict the small size of Mars suggest an instability ~5–14 Myr after CAI [e.g., 7]. Core cooling resulting from impacts on the iron parent bodies between ~7.8-11.7 Myr agrees well with this. Additionally, gas dissipation may trigger an early giant planet instability [8] and hence these two mechanisms may have acted together to create an energetic, collision-rich Solar System between ~7.8-11.7 Myr after CAI.
References: [1] Kruijer et al. (2017) PNAS 114, 6712-6716. [2] Yang et al. (2007) Nature 446, 888. [3] Hunt et al. (2022) Nat. Astron. 6, 812–818, https://doi.org/10.1038/s41550-022-01675-2. [4] Matthes et al. (2018) GCA 220, 82-95. [5] Schönbächler et al. (2008) GCA 72, 5330-5341. [6] Davison et al. (2013), MAPS 48, 1894-1918. [7] Clement et al. (2018) Icarus 311, 340-356. [12] Liu et al. (2022) Nature 604, 643-646.
Figure 1. Cartoon showing the evolution of iron meteorite parent bodies. Top: Parent bodies differentiate before ~3Myr after CAI. The disk is truncated in the region of Jupiter. Later, the cores of the parent bodies were exposed by impacts between ~7.8 - 11.7 Myr. Two mechanisms, alone or in combination, generated an energetic inner Solar System at this time. Scenario A: When the gas disk dissipates, planetesimals are gradually excited until they begin to undergo high velocity impacts [6]. Scenario B: An early giant planet instability is triggered, leading to reorganisation of the inner Solar System and impacts [7].
The newly determined 107Pd/108Pd initial ratios for these 3 asteroids are within uncertainty of each other, indicating the Pd-Ag system closed in a similar time-frame on these bodies. The IVA group, an additional iron meteorite parent body, yields a similar value [4]. The Pd-Ag cooling ages depend on the Solar System initial 107Pd/108Pd. We calculate Pd-Ag ages relative to the value determined for carbonaceous chondrites [5]. Pd-Ag ages for the IIAB, IIIAB and IVA bodies equate to cooling between ~7.8-11.7 Myr, and this dates the timing of mantle-stripping impacts. Additionally, the IAB parent body cooled quickly after an impact between ~11 – 13.6 Myr.
Our new data therefore suggest a highly energetic inner Solar System between ~7.8-11.7 Myr. This may date the dissipation the gas disk, after which the damping effect of gas drag ceases. Planetesimals embedded in the disk became gradually excited, leading to energetic collisions [6]. A giant planet instability (‘Nice’ model) could also cause reorganisation of the inner Solar System and impacts. Although the precise timing of the instability is difficult to determine, models that predict the small size of Mars suggest an instability ~5–14 Myr after CAI [e.g., 7]. Core cooling resulting from impacts on the iron parent bodies between ~7.8-11.7 Myr agrees well with this. Additionally, gas dissipation may trigger an early giant planet instability [8] and hence these two mechanisms may have acted together to create an energetic, collision-rich Solar System between ~7.8-11.7 Myr after CAI.
References: [1] Kruijer et al. (2017) PNAS 114, 6712-6716. [2] Yang et al. (2007) Nature 446, 888. [3] Hunt et al. (2022) Nat. Astron. 6, 812–818, https://doi.org/10.1038/s41550-022-01675-2. [4] Matthes et al. (2018) GCA 220, 82-95. [5] Schönbächler et al. (2008) GCA 72, 5330-5341. [6] Davison et al. (2013), MAPS 48, 1894-1918. [7] Clement et al. (2018) Icarus 311, 340-356. [12] Liu et al. (2022) Nature 604, 643-646.
Figure 1. Cartoon showing the evolution of iron meteorite parent bodies. Top: Parent bodies differentiate before ~3Myr after CAI. The disk is truncated in the region of Jupiter. Later, the cores of the parent bodies were exposed by impacts between ~7.8 - 11.7 Myr. Two mechanisms, alone or in combination, generated an energetic inner Solar System at this time. Scenario A: When the gas disk dissipates, planetesimals are gradually excited until they begin to undergo high velocity impacts [6]. Scenario B: An early giant planet instability is triggered, leading to reorganisation of the inner Solar System and impacts [7].