Bifurcation of planetary building blocks during Solar System formationOPEN ACCESS
Tim Lichtenberg, Joanna Dra̧żkowska, Maria Schönbächler, Gregor J. Golabek, Thomas O. Hands
Science, 22 Jan 2021
Vol. 371, Issue 6527, pp. 365-370
PDF (OPEN ACCESS) Published 21 January 2021; authors’ version
“Geochemical and astronomical evidence demonstrates that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermochemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets.”
VIDEO Summary (Tim Lichtenberg)
Bifurcation of planetary building blocks during Solar System formation (Tim Lichtenberg, Oxford U.) (ICS Theoretical Astrophysics seminar, 19 March 2021)
Fig. 1 Formation of two distinct planetesimal populations in the disk simulation.
The rate of planetesimal formation (change in surface density per timestep) is shown on logarithmic scale in centimeter–gram-second and natural units. The solid purple line indicates the orbital migration of the snow line; red and blue areas highlight the formation regions of the two planetesimal populations (Reservoir I and Reservoir II). The dashed gray lines (labeled “Terrestrial planet migration corridor”; compare to fig. S9) bound the range of possible planet migration scenarios consistent with the present-day orbits of the terrestrial planets. Although smaller embryos move slowly inward, more rapidly accreting (and thus more massive) embryos would migrate faster toward the proto-Sun. The light gray arrows on the left side indicate the transition from the Class I to Class II disk stage.
Fig. 2 Pebble flux and planetesimal growth time scales in the disk simulation.
(A) Pebble flux at 2 (red) and 15 (blue) au over time. During the disk build-up stage, the pebble flux is dominated by outward-moving dust (dotted lines), after which the pebbles start drifting inward. Reservoir I and II in the simulation progressively diverge in pebble flux by more than one order of magnitude owing to accretion of Reservoir II (black arrows, labeled “Reservoir separation”). (B) Comparison of growth time scales for birth-sized planetesimals of 300-km radius by either pebble or planetesimal (collisional) accretion (10) within the approximate lifetime of the solar protoplanetary disk (gray horizontal band, labeled “≈ disk lifetime”). The red shaded area in the bottom indicates the time interval during which pebble accretion is more effective than collisional growth (labeled “Pebble-aided growth”; compare to fig. S11).
Fig. 3 Simulated thermochemical evolution of planetesimals.
(A) Highest mean internal temperature (color bar), interpolated from simulated planetesimal radii and formation times (dots). Reservoir I and II in the disk model are indicated by red and blue dots, respectively (compare to Fig. 1). Green and yellow lines (labeled H1 to C2) indicate bodies for which 50 vol % (dashed lines) or 90 vol % (solid lines) of their interiors undergo hydrothermal activity (H1, H2) and iron core formation (C1, C2). (B) Schematic illustration of the scenarios indicated by the lines in (A) at peak heating and the threshold parameters [see text and (10)] to distinguish between regimes. H1, H2: Dark and light blue represent water ice and liquid water, respectively. Silicates are colored yellow to orange, indicating increasing temperature. Dashed black lines indicate the thresholds for water ice melting (Thydr) and hydrous rock decomposition (Tdecomp). The light blue arrows indicate hydrothermal circulation (H1) or degassing of volatiles (H2). C1, C2: Dark gray represents Fe,Ni–S metals. Silicates are colored yellow to red, indicating increasing temperature (yellow to orange) and progressive melting (red). Dashed black lines indicate the thresholds for metal percolation (Tperc) and rain-out (ϕrain). Black arrows indicate core formation by percolation (C1) or rain-out (C2). Dark red dashed arrows (C2) indicate convection in the internal magma ocean.
Fig. 4 Comparison of metal-silicate separation times in simulated planetesimal populations with the meteorite record.
(A) Metal-silicate separation times in NC and CC meteorite classes (table S1). (B) Timing of core formation in Reservoir I (red) or Reservoir II (blue). Solid lines with colored shading represent the fraction of material undergoing metal-silicate separation [compare Fig. 2 and (10)]. Dashed lines show the total volumetric fraction for each scenario over time. Light and dark red or blue indicate percolation or rain-out, respectively.
Fig. 5 Comparison of hydrothermal activity in simulated planetesimal populations with aqueous alteration times in the meteorite record.
(A) Aqueous alteration times in NC and CC meteorite classes (table S1). (B) Fraction of planetary materials that retain primordial water ice (dashed lines) or undergo hydrothermal activity and retain water in hydrous silicate phases (solid lines with colored shading) in the simulations.
Fig. 6 Schematic illustration of our proposed chronology of early Solar System accretion.
Nucleosynthetic isotope variability (left) across the disk due to varying composition of infall material is retained by the pile-up of inward-drifting dust grains at the snow line. The formation of two distinct planetesimal populations initiates divergent evolutionary pathways of inner and outer Solar System (right) due to the secular variation of local material composition, internal radiogenic heating, and dominant mode of planetary growth.
