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Pages 102-129

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From page 102...
... 102 ORIGINS, WORLDS, AND LIFE FIGURE 4-1  Schematic of four basic phases of the protoplanetary disk evolution and their relationship to Question 1. From top to bottom: collapse of the molecular cloud core to form an embedded protostar (Q1.1)
From page 103...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 103 Q1.1a What Initiated the Collapse of the Molecular Cloud? The solar system likely formed from the collapse of a molecular cloud core within a cluster of perhaps ~103−104 stars (Adams 2010)
From page 104...
... 104 ORIGINS, WORLDS, AND LIFE then prevented subsequent mixing between NC and CC groups, although a pressure bump in the gas disk near the location of Jupiter's formation may have provided an even earlier barricade (Kruijer et al.
From page 105...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 105 FIGURE 4-2  The isotopic composition of planetary materials in the solar system -- hydrogen (H2) , carbon monoxide (CO)
From page 106...
... 106 ORIGINS, WORLDS, AND LIFE ∆17O composition (where ∆17O ≡ δ17O − 0.56 δ18O; see Figure 4-2) , would permit a test for a genetic relationship with documented solar system reservoirs, as already achieved for Earth and Mars.
From page 107...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 107 FIGURE 4-3 Sources of isotopic anomalies in the protosolar nebula from hydrogen (H2) , carbon monoxide (CO)
From page 108...
... 108 ORIGINS, WORLDS, AND LIFE space-based telescopic observations of the composition (gas, ice, dust) of comets, Kuiper belt objects, and protoplanetary disks.
From page 109...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 109 Chondrules are tiny, 0.1–1 mm igneous rocks and are the main structural component of chondrites. Various models have been proposed to explain the transient heating necessary for formation of chondrules from their precursor dust particles.
From page 110...
... 110 ORIGINS, WORLDS, AND LIFE Q1.2b How, Where, and When Did Radial Mixing and Segregation of Solids and Gas Occur in the Nebula? During the protoplanetary disk phase as the Sun continued to accrete, the first solids formed and planetesimals assembled and agglomerated into protoplanets.
From page 111...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 111 were transported inward. As a result of this bidirectional transport, many regions of the solar nebula may have served as the source materials for the terrestrial planets, the giant planets, and small bodies like asteroids, comets, Centaurs, and Kuiper belt objects.
From page 112...
... 112 ORIGINS, WORLDS, AND LIFE occurrence of secondary minerals that require precipitation from fluid and varying degrees of fluid alteration for chondrites with the same bulk composition. Melting of accreted ice to facilitate these water-rock reactions likely stemmed from heating from the decay of 26Al.
From page 113...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 113 Q1.3 WHAT PROCESSES LED TO THE PRODUCTION OF PLANETARY BUILDING BLOCKS -- THAT IS, PLANETESIMALS? A common assertion in classical planet formation models is that the initial size of planetesimals -- that is, planetary building blocks, was approximately a kilometer, and that they grew from pairwise collisions between smaller objects.
From page 114...
... 114 ORIGINS, WORLDS, AND LIFE Q1.3b How and When Did Grains Grow from Centimeter-Size Objects to ~100 Kilometer-Size Planetesimals? The formation of planetesimals is a necessary step to making the terrestrial planets and giant planet cores, but precisely how this happened is still debated.
From page 115...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 115 the high porosity and weak strength of comets and Kuiper belt objects, particularly Arrokoth (see Figure 2-20) , a contact binary in the cold classical Kuiper belt with modestly flattened lobes (22 × 20 × 7 km and 14 × 14 × 10 km)
From page 116...
... 116 ORIGINS, WORLDS, AND LIFE protoplanetary disk, where water ice would have been in an amorphous form and cold enough that the trapping efficiency of volatiles was uniform. Alternatively, Jupiter's elemental enrichments could be telling us more about the evolution of nebular gas rather than the thermal history of small particles and planetesimals.
From page 117...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 117 Strategic Research for Q1.3 • Infer the compositions and locations of nebular source reservoirs by return of samples from, especially, comet surfaces as well as from asteroids; measuring the elemental and stable isotopic compositions of refractory and volatile elements with lander and orbiter missions (especially for the ice giants, Centaurs, and Mercury and also Venus, comets, Saturn, and Kuiper belt objects) ; and ground- and space-based telescopic observations of atmospheric and/or sublimated volatiles toward small bodies in outer solar system, planets across the solar system and their moons; and laboratory petrological, elemental, and isotopic analyses of returned and terrestrially collected samples.
From page 118...
... 118 ORIGINS, WORLDS, AND LIFE the final compositions of the terrestrial planets (Grossman 1972) , giant planets (Guillot and Hueso 2006; Monga and Desch 2015)
From page 119...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 119 2 million years (Mamajek 2009)
From page 120...
... 120 ORIGINS, WORLDS, AND LIFE yet understand the role of hydrodynamic winds and/or magnetic fields in a dispersing gas in different regions of the nebula, nor do we know how the composition of the gas and the ratio of gas to dust evolved with time in the same regions. Answers to these questions would enable us to constrain which mechanisms (e.g., photoevaporation, magnetized disk winds, or other mechanisms)
From page 121...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 121 Bergin, E., Y Aikawa, G.A.
From page 122...
... 122 ORIGINS, WORLDS, AND LIFE Donahue, T.M., J.H. Hoffman, R.R.
From page 123...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 123 Joung, M.K.R., M.-M.
From page 124...
... 124 ORIGINS, WORLDS, AND LIFE Nesvorný, D., R
From page 125...
... QUESTION 1: EVOLUTION OF THE PROTOPLANETARY DISK 125 Weisberg, M.K., T.J.
From page 126...
... Q2 PLATE: An enhanced-color image mosaic of Pluto and Charon taken by the New Horizons spacecraft in 2015. SOURCE: Courtesy of NASA/JHUAPL/SwRI.
From page 127...
... 5 Question 2: Accretion in the Outer Solar System How and when did the giant planets and their satellite systems originate, and did their orbits migrate early in their history? How and when did dwarf planets and cometary bodies orbiting beyond the giant planets form, and how were they affected by the early evolution of the solar system?
From page 128...
... 128 ORIGINS, WORLDS, AND LIFE Q2.1 HOW DID THE GIANT PLANETS FORM? The main research themes of outer planet formation have been known for decades, but developments in observations and theory continually revise our understanding of how these themes relate and how the planets formed.
From page 129...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 129 FIGURE 5-1  A sketch of the planetary growth in the core accretion model. Shown is the planet's mass (in Earth masses)

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