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Pages 130-163

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From page 130...
... 130 ORIGINS, WORLDS, AND LIFE Q2.1b How Did Uranus and Neptune Form and What Prevented Them from Becoming Gas Giants? There are several challenges to explaining the origin of Uranus and Neptune.
From page 131...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 131 FIGURE 5-2  A cartoon of the formation of a gas giant in a circumplanetary disk in Keplerian rotation. In the inner regions of the disk, thermal ionization imparts conductivity to the disk, coupling the planetary magnetic field to the quasi-Keplerian motion of the gas.
From page 132...
... 132 ORIGINS, WORLDS, AND LIFE Strategic Research for Q2.1 • Determine the atmospheric composition of Saturn, Uranus, and Neptune via in situ sampling of noble gas, elemental, and isotopic abundances, and remote sensing by spacecraft and ground/space-based telescopes. • Determine the bulk composition and internal structure of Uranus and Neptune via gravity, magnetic field, and atmospheric profile measurements by spacecraft, as well as Doppler seismology.2 • Constrain physical properties and boundary conditions (i.e., tropospheric temperatures, shapes, and rotation rates)
From page 133...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 133 ? 100.0 Uranus, Neptune Planetary to Protosolar Elemental Abundance Ratios ?
From page 134...
... 134 ORIGINS, WORLDS, AND LIFE p­ rotosolar-abundance mixture would produce more ice than rock, if the condensation temperature is low enough, but verifying a high (or any) ice-to-rock ratio in the giant planets is challenging because rock-forming species condense in deep cloud layers inaccessible to observations (although evidence for such clouds abounds in hot exo-Jupiters)
From page 135...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 135 Strategic Research for Q2.2 • Determine the atmospheric composition of Saturn, Uranus, and Neptune via in situ sampling of noble gas, elemental and isotopic abundances, and remote sensing by spacecraft and ground- or space-based telescopes. • Understand how compositional gradients in the atmosphere and interior of Jupiter, Saturn, Uranus, and Neptune affect the determination of bulk planetary composition based on observed atmospheric composition, using gravity, magnetic field, and atmospheric profile measurements by spacecraft, Doppler seismology, and laboratory/theoretical studies of physical processes (e.g., turbulent diffusion, moist convection, precipitation, and helium rain)
From page 136...
... 136 ORIGINS, WORLDS, AND LIFE Q2.3a How Did Protosatellite Disks Form, and How Did Disk Structure and Composition Evolve During the Accretion of Primordial Satellite Systems? Formation of the gas giants Jupiter and Saturn is thought to have been accompanied by that of a CPD surrounding each (e.g., Peale and Canup 2015; see Q2.2)
From page 137...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 137 FIGURE 5-5 Schematic cross section of some hypothetical processes within circumplanetary, satellite-forming nebulae. Material flows in from the protosolar nebula and forms a flared disk around a gas giant such as Jupiter.
From page 138...
... 138 ORIGINS, WORLDS, AND LIFE lacking. Improved understanding of these satellites (including, e.g., their moments of inertia and differentiation states; compositions; geological character and signs of past/current activity; constraints on orbital evolution)
From page 139...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 139 moons also, generally, increase in mass with distance from the planet. The saturnian satellites, with their mixed character, clearly challenge formation scenarios.
From page 140...
... 140 ORIGINS, WORLDS, AND LIFE • Determine Callisto's state of differentiation to constrain the accretional conditions of large icy moons via spacecraft geodesy (shape) , gravity, pole position, and magnetic field and associated charged particle measurements (the latter necessary for proper interpretation)
From page 141...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 141 Vokrouhlický 2016; Morbidelli and Nesvorný 2020)
From page 142...
... 142 ORIGINS, WORLDS, AND LIFE FIGURE 5-6  Gas-free giant planet migration and dynamical instability. The giant planets scatter each other, while Neptune migrates into a massive outer disk of cometesimals (i.e., primordial comets)
From page 143...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 143 FIGURE 5-7  A possible orbital history of the giant planets. Five planets were started in a 3:2, 4:3, 2:1, and 3:2 mean-motion resonant chain along with a 20 Earth-mass planetesimal disk between 23 AU and 30 AU.
From page 144...
... 144 ORIGINS, WORLDS, AND LIFE • Characterize the basic properties of TNOs of diverse size, binarity, and dynamical subpopulations with flyby(s) /orbital/landed missions to the outer solar system and through remote sensing by ground-/ space-based telescopes (including surveys)
From page 145...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 145 that these bodies can teach us a great deal about planetesimal formation (Stern et al. 2019; McKinnon et al.
From page 146...
... 146 ORIGINS, WORLDS, AND LIFE The orbit, masses, and compositions of Pluto-Charon all point to a giant impact origin (Canup et al.
From page 147...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 147 Early differentiation of a Titan-like satellite is a requirement of one hypothesis for the formation of Saturn's rings, via tidal stripping of an icy mantle (Canup 2010)
From page 148...
... 148 ORIGINS, WORLDS, AND LIFE Strategic Research for Q2.5 • Characterize the basic properties (size, mass, shape, cratering, rings, and binarity) of diverse TNOs and related bodies (Centaurs and comets)
From page 149...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 149 The resonant TNOs were trapped within Neptune's mean motion resonances as Neptune migrated outward (e.g., Nesvorný and Vokrouhlický 2016)
From page 150...
... 150 ORIGINS, WORLDS, AND LIFE gravitational influence and not on a Neptune-crossing orbit, has several sub-groups, with the extreme TNOs having perihelion distances between ~40 and ~55 AU and the Inner Oort cloud objects having perihelia beyond ~65 AU (Nesvorný 2018)
From page 151...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 151 hot/cold classicals (Jewitt 2018)
From page 152...
... 152 ORIGINS, WORLDS, AND LIFE Strategic Research for Q2.6 • Determine the rotational, physical, chemical, geological, and interior properties of a diversity of primitive small bodies (TNOs) in the outer solar system with spacecraft and/or ground-/space-based observations.
From page 153...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 153 Canup, R.M., K.M. Kratter, and M
From page 154...
... 154 ORIGINS, WORLDS, AND LIFE McKinnon, W.B., D Prialnik, S.A.
From page 155...
... QUESTION 2: ACCRETION IN THE OUTER SOLAR SYSTEM 155 Rufu, R., and R.M. Canup.
From page 156...
... Q3 PLATE: An enhanced-color image mosaic of Mercury acquired by the MESSENGER spacecraft in 2013. The colors reveal different surface compositions.
From page 157...
... 6 Question 3: Origin of Earth and Inner Solar System Bodies How and when did the terrestrial planets, their moons, and the asteroids accrete, and what processes determined their initial properties? To what extent were outer solar system materials incorporated?
From page 158...
... 158 ORIGINS, WORLDS, AND LIFE is a critical component for constraining both the physical and chemical characteristics of the terrestrial planets, as well as for understanding the processes that occurred in the protoplanetary gas nebula. However, many fundamental questions remain unanswered.
From page 159...
... QUESTION 3: ORIGIN OF EARTH AND INNER SOLAR SYSTEM BODIES 159 The innermost planet, Mercury, has a highly reduced surface, with low iron contents and unexpectedly high sulfur contents, consistent with a planet formed from highly reduced materials. Information about the composition of Venus is extremely limited, a fundamental gap in our understanding of the compositional variations among the terrestrial planets.
From page 160...
... 160 ORIGINS, WORLDS, AND LIFE obtained or recognized if they exist within our meteorite collection. Refined in situ geochemical characterization of surface materials at Mercury and Venus would transform our knowledge of these bodies, especially in the absence of samples.
From page 161...
... QUESTION 3: ORIGIN OF EARTH AND INNER SOLAR SYSTEM BODIES 161 treating many stages of evolution and planetary size scales simultaneously in numerical models, which may be needed to ultimately link large-scale planet accretion to the meteoritic record and small body populations. We also may not fully understand the dynamic interplay between nebular gas, dust, nascent planetesimals, and the Sun.
From page 162...
... 162 ORIGINS, WORLDS, AND LIFE Strategic Research for Q3.1 • Determine the compositional diversity of the terrestrial planets and inner solar system feedstocks by obtaining mineralogical, geochemical, and isotopic data from the surfaces and atmospheres of Mercury, Venus, the Moon, and the less explored regions of the Moon and Mars, as well as the currently unsampled small body population. • Determine the diversity of compositions and nature of remnant planetesimals residing in the inner solar system and establish links between the small body taxonomy and meteorite types through Earth based and spacecraft-based remote sensing, in situ measurements, and laboratory analyses of meteorites and returned samples.
From page 163...
... QUESTION 3: ORIGIN OF EARTH AND INNER SOLAR SYSTEM BODIES 163 in distinct chemical reservoirs (Q1.3)

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