1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

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The Exoplanet Handbook

The age of Jupiter, the largest planet in our Solar System, is still unknown. Gas-giant planet formation likely involved the growth of large solid cores, followed by the accumulation of gas onto these cores. Thus, the gas-giant cores must have formed before dissipation of the solar nebula, which likely occurred within less than 10 My after Solar System formation. Although such rapid accretion of the gas-giant cores has successfully been modeled, until now it has not been possible to date their formation. Here, using molybdenum and tungsten isotope measurements on iron meteorites, we demonstrate that meteorites derive from two genetically distinct nebular reservoirs that coexisted and remained spatially separated between ∼1 My and ∼3-4 My after Solar System formation. The most plausible mechanism for this efficient separation is the formation of Jupiter, opening a gap in the disk and preventing the exchange of material between the two reservoirs. As such, our results indicate that Jupiter's core grew to ∼20 Earth masses within <1 My, followed by a more protracted growth to ∼50 Earth masses until at least ∼3-4 My after Solar System formation. Thus, Jupiter is the oldest planet of the Solar System, and its solid core formed well before the solar nebula gas dissipated, consistent with the core accretion model for giant planet formation.

https://www.pnas.org/content/pnas/114/26/6712.full.pdf

Age of Jupiter inferred from the distinct genetics and formation times of meteorites.

Fundamentals of Rock Mechanics

The Late Heavy Bombardment lasted much longer than previously thought, up to 1.7 billion years ago on Earth, with impacts on the Moon and Earth coming mostly from the E-belt-survivor Hungaria asteroids. The Late Heavy Bombardment was a period of time, generally put at about 4.1 billion to 3.8 billion years ago, when the inner planets of the Solar System were subjected to a high-frequency barrage of asteroids. This left its mark on the Moon, but on Earth the craters quickly disappeared owing to tectonic processes and erosion. In the first of two papers on the bombardment, Brandon Johnson and Jay Melosh determine the properties of the asteroids by looking at spherule beds: layers of debris ejected during the impacts. The thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit Earth. This historical record of impacts indicates that the number of projectiles colliding with Earth was substantially higher 3.5 billion years ago than it is today, with a gradual decline in the number of strikes after the Late Heavy Bombardment. Bottke et al. modelled the evolution of an asteroid belt that extended farther towards Mars than the present one. They find that most of the impactors traced by the spherule beds probably originated in this 'E-belt', which was disrupted during migrations of some of the giant planets. The barrage of comets and asteroids that produced many young lunar basins (craters over 300 kilometres in diameter) has frequently been called the Late Heavy Bombardment1 (LHB). Many assume the LHB ended about 3.7 to 3.8 billion years (Gyr) ago with the formation of Orientale basin2,3. Evidence for LHB-sized blasts on Earth, however, extend into the Archaean and early Proterozoic eons, in the form of impact spherule beds: globally distributed ejecta layers created by Chicxulub-sized or larger cratering events4. At least seven spherule beds have been found that formed between 3.23 and 3.47 Gyr ago, four between 2.49 and 2.63 Gyr ago, and one between 1.7 and 2.1 Gyr ago5,6,7,8,9. Here we report that the LHB lasted much longer than previously thought, with most late impactors coming from the E belt, an extended and now largely extinct portion of the asteroid belt between 1.7 and 2.1 astronomical units from Earth. This region was destabilized by late giant planet migration10,11,12,13. E-belt survivors now make up the high-inclination Hungaria asteroids14,15. Scaling from the observed Hungaria asteroids, we find that E-belt projectiles made about ten lunar basins between 3.7 and 4.1 Gyr ago. They also produced about 15 terrestrial basins between 2.5 and 3.7 Gyr ago, as well as around 70 and four Chicxulub-sized or larger craters on the Earth and Moon, respectively, between 1.7 and 3.7 Gyr ago. These rates reproduce impact spherule bed and lunar crater constraints.

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An Archaean heavy bombardment from a destabilized extension of the asteroid belt

This chapter reviews our current knowledge of the gravity and topography of the terrestrial planets and describes the methods that are used to analyze these data. A general review of the mathematical formalism that is used in describing gravity and topography is first given. Next, the basic properties of Earth, Venus, Mars, Mercury, and the Moon are characterized. Following this, the relationship between gravity and topography is quantified, and techniques by which geophysical parameters can be constrained are detailed. Analysis methods include crustal thickness modeling, geoid/topography ratios, spectral admittance and correlation functions, and localized spectral analysis and wavelet techniques. Finally, the major results that have been obtained by modeling the gravity and topography of Earth, Venus, Mars, Mercury, and the Moon are summarized.

Gravity and Topography of the Terrestrial Planets

High-resolution gravity data from the Gravity Recovery and Interior Laboratory spacecraft have clarified the origin of lunar mass concentrations (mascons). Free-air gravity anomalies over lunar impact basins display bull's-eye patterns consisting of a central positive (mascon) anomaly, a surrounding negative collar, and a positive outer annulus. We show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool. We used a hydrocode to simulate the impact and a self-consistent finite-element model to simulate the subsequent viscoelastic relaxation and cooling. The primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy, the lunar thermal gradient at the time of impact, the crustal thickness, and the extent of volcanic fill.

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The Origin of Lunar Mascon Basins

The origin of most chondrules (small, previously molten spherules inside meteorites) is shown to be impact jetting; chondrules form from the shock-melted material ejected from a protoplanet on impact, making meteorites a byproduct of planet formation. The origin of chondrules, the millimetre-scale, once-molten droplets found in most meteorites, has been an enduring mystery in the science of meteoritics. Using a hydrocode model and a dynamical accretion model to simulate protoplanetary impacts, Brandon Johnson et al. show that large-scale accretionary impacts can produce massive sprays of millimetre-scale molten droplets in sufficient quantities during the first 5 Myr of planetary accretion to explain the observed abundance of chondrules. This finding supports an impact origin for chondrules and implies that meteorites are a byproduct of planet formation rather than leftover building material. Chondrules are the millimetre-scale, previously molten, spherules found in most meteorites1. Before chondrules formed, large differentiating planetesimals had already accreted2. Volatile-rich olivine reveals that chondrules formed in extremely solid-rich environments, more like impact plumes than the solar nebula3,4,5. The unique chondrules in CB chondrites probably formed in a vapour-melt plume produced by a hypervelocity impact6 with an impact velocity greater than 10 kilometres per second. An acceptable formation model for the overwhelming majority of chondrules, however, has not been established. Here we report that impacts can produce enough chondrules during the first five million years of planetary accretion to explain their observed abundance. Building on a previous study of impact jetting7, we simulate protoplanetary impacts, finding that material is melted and ejected at high speed when the impact velocity exceeds 2.5 kilometres per second. Using a Monte Carlo accretion code, we estimate the location, timing, sizes, and velocities of chondrule-forming impacts. Ejecta size estimates8 indicate that jetted melt will form millimetre-scale droplets. Our radiative transfer models show that these droplets experience the expected cooling rates of ten to a thousand kelvin per hour9,10. An impact origin for chondrules implies that meteorites are a byproduct of planet formation rather than leftover building material.

Impact jetting as the origin of chondrules.

Impact craters are the most obvious indication of asteroid impacts, but craters on Earth are quickly obscured or destroyed by surface weathering and tectonic processes. Earth’s impact history is inferred therefore either from estimates of the present-day impactor flux as determined by observations of near-Earth asteroids, or from the Moon’s incomplete impact chronology. Asteroids hitting Earth typically vaporize a mass of target rock comparable to the projectile’s mass. As this vapour expands in a large plume or fireball, it cools and condenses into molten droplets called spherules. For asteroids larger than about ten kilometres in diameter, these spherules are deposited in a global layer. Spherule layers preserved in the geologic record accordingly provide information about an impact even when the source crater cannot be found. Here we report estimates of the sizes and impact velocities of the asteroids that created global spherule layers. The impact chronology from these spherule layers reveals that the impactor flux was significantly higher 3.5 billion years ago than it is now. This conclusion is consistent with a gradual decline of the impactor flux after the Late Heavy Bombardment.

Brandon C. Johnson

Papers

The Origin of Lunar Mascon Basins

Impact jetting as the origin of chondrules.

Impact spherules as a record of an ancient heavy bombardment of Earth

South Pole–Aitken basin ejecta reveal the Moon’s upper mantle

Formation of spherules in impact produced vapor plumes