Formation Of Uranus Research Articles

The role of ice lines in the formation of Uranus and Neptune.

We aim at investigating whether the chemical composition of the outer region of the protosolar nebula can be consistent with current estimates of the elemental abundances in the ice giants. To do so, we use a self-consistent evolutionary disc and transport model to investigate the time and radial distributions of H2O, CO, CO2, CH3OH, CH4, N2 and H2S, i.e. the main O-, C-, N and S-bearing volatiles in the outer disc. We show that it is impossible to accrete a mixture composed of gas and solids from the disc with a C/H ratio presenting enrichments comparable to the measurements (approx. 70 times protosolar). We also find that the C/N and C/S ratios measured in Uranus and Neptune are compatible with those acquired by building blocks agglomerated from solids condensed in the 10-20 arb. units region of the protosolar nebula. By contrast, the presence of protosolar C/N and C/S ratios in Uranus and Neptune would imply that their building blocks agglomerated from particles condensed at larger heliocentric distances. Our study outlines the importance of measuring the elemental abundances in the ice giant atmospheres, as they can be used to trace the planetary formation location, the origin of their building blocks and/or the chemical and physical conditions of the protosolar nebula. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

The Formation of Uranus and Neptune: Fine-tuning in Core Accretion

Abstract Uranus and Neptune are ice giants with ∼15% atmospheres by mass, which places them in a category intermediate between rocky planets and gas giants. These atmospheres are too massive to have been primarily outgassed, but they never underwent runaway gas accretion. The ice giants never reached critical core mass ( ) in a full gas disk, but their cores are , suggesting that their envelopes were mainly accreted at the end of the disk lifetime. Pebble accretion calls into question traditional slow atmospheric growth during this phase. We show that the full-sized ice giants predominantly accreted gas from a disk depleted by at least a factor of ∼100. Such a disk dissipates in ≲105 years. Why would both cores stay subcritical for the entire ∼Myr disk lifetime, only to reach in the final 105 years? This is fine tuned. Ice giants in the outer disk have atmospheric mass fractions comparable to the disk gas-to-solid ratio during the bulk of their gas accretion. This point in disk evolution coincides with a dynamical upheaval: the gas loses its ability to efficiently damp the core random velocities, allowing them to be gravitationally excited by Jupiter and Saturn. We suggest that the ice giants’ cores began growing on closer-in orbits (staying subcritical), and migrated out during this dynamical instability. There, their orbits circularized after accreting much of their mass in solids. Finally, they accreted their envelopes from a depleted nebula, where the sparseness of feeding-zone gas prevented runaway.

Delay of planet formation at large radius and the outward decrease in mass and gas content of Jovian planets

A prominent observation of the solar system is that the mass and gas content of Jovian planets decrease outward with orbital radius, except that, in terms of these properties, Neptune is almost the same as Uranus. In previous studies, the solar nebula was assumed to preexist and the formation process of the solar nebula was not considered. It was therefore assumed that planet formation at different radii started at the same time in the solar nebula. We show that planet formation at different radii does not start at the same time and is delayed at large radii. We suggest that this delay might be one of the factors that causes the outward decrease in the masses of Jovian planets. The nebula starts to form from its inner part because of the inside-out collapse of its progenitorial molecular cloud core. The nebula then expands outward due to viscosity. Material first reaches a small radius and then reaches a larger radius, so planet formation is delayed at the large radius. The later the material reaches a planet's location, the less time it has to gain mass and gas content. Hence, the delay tends to cause the outward decrease in mass and gas content of Jovian planets. Our nebula model shows that the material reaches Jupiter, Saturn, Uranus and Neptune at t = 0.40, 0.57, 1.50 and 6.29 × 106 yr, respectively. We discuss the effects of time delay on the masses of Jovian planets in the framework of the core accretion model of planet formation. Saturn's formation is not delayed by much time relative to Jupiter so that they both reach the rapid gas accretion phase and become gas giants. However, the delay in formation of Uranus and Neptune is long and might be one of the factors that cause them not to reach the rapid gas accretion phase before the gas nebula is dispersed. Saturn has less time to go through the rapid gas accretion, so Saturn's mass and gas content are significantly less than those of Jupiter.

The Formation of Uranus and Neptune among Jupiter and Saturn

The outer giant planets, Uranus and Neptune, pose a challenge to theories of planet formation. They exist in a region of the Solar System where long dynamical timescales and a low primordial density of material would have conspired to make the formation of such large bodies ($\sim$ 15 and 17 times as massive as the Earth, respectively) very difficult. Previously, we proposed a model which addresses this problem: Instead of forming in the trans-Saturnian region, Uranus and Neptune underwent most of their growth among proto-Jupiter and -Saturn, were scattered outward when Jupiter acquired its massive gas envelope, and subsequently evolved toward their present orbits. We present the results of additional numerical simulations, which further demonstrate that the model readily produces analogues to our Solar System for a wide range of initial conditions. We also find that this mechanism may partly account for the high orbital inclinations observed in the Kuiper belt.

On the accretion of Uranus and Neptune

In this paper, we discuss some problems concerning the formation of Uranus and Neptune. We find that the adoption of reduced Hill spheres as the region of close interaction between planetesimals introduces an enhancement of the gravitational cross-sections in previous numerical simulations. We also discuss a way to make possible the formation of Uranus and Neptune on time-scales shorter than the age of the Solar system.

Could the Lunar “Late Heavy Bombardment” Have Been Triggered by the Formation of Uranus and Neptune?

We investigate the hypothesis that the so-called Late Heavy Bombardment (LHB) of the Moon was triggered by the formation of Uranus and Neptune. As Uranus and Neptune formed, which we assume occurred at the epoch of the LHB, they scattered neighboring icy planetesimals throughout the Solar System. Some of these objects hit the Moon. Our integrations show that the Moon would have accreted about 6×10 21 g, if we assume that the Uranus–Neptune region initially contained 5 times the current mass of these planets in the form of small solid objects. In addition, Mars would have accumulated ∼6×10 22 g of icy material, which could have supplied its putative early massive atmosphere. However, Earth would likely have accreted only ∼7×10 22 g of water, or ∼5% of its oceans, through the mechanisms studied here. The numerical experiment that we have performed on the behavior of Uranus–Neptune planetesimals shows very good agreement with current constraints on the LHB. The influx of Uranus–Neptune planetesimals onto the Moon could have lasted for a time as short as 10 or 20 million years. The dynamical transport of the Uranus–Neptune planetesimals during this process would have caused Jupiter and Saturn to migrate. This migration, in turn, would have destabilized objects in the jovian Trojan swarms and the asteroid belt. Thus, not only would Uranus and Neptune planetesimals have struck the Moon, but asteroids would have as well. We find that the Trojan asteroids of Jupiter could not have contributed a large percentage of material to the LHB, but the asteroid belt could, in principle, have contributed to, or even dominated, the LHB. Although this model appears to explain the LHB well, it requires that fully formed Uranus and Neptune not appear in the trans-saturnian region until some 700 million years after the formation of the Earth.

The formation of Uranus and Neptune in the Jupiter-Saturn region of the Solar System.

Planets are believed to have formed through the accumulation of a large number of small bodies. In the case of the gas-giant planets Jupiter and Saturn, they accreted a significant amount of gas directly from the protosolar nebula after accumulating solid cores of about 5-15 Earth masses. Such models, however, have been unable to produce the smaller ice giants Uranus and Neptune at their present locations, because in that region of the Solar System the small planetary bodies will have been more widely spaced, and less tightly bound gravitationally to the Sun. When applied to the current Jupiter-Saturn zone, a recent theory predicts that, in addition to the solid cores of Jupiter and Saturn, two or three other solid bodies of comparable mass are likely to have formed. Here we report the results of model calculations that demonstrate that such cores will have been gravitationally scattered outwards as Jupiter, and perhaps Saturn, accreted nebular gas. The orbits of these cores then evolve into orbits that resemble those of Uranus and Neptune, as a result of gravitational interactions with the small bodies in the outer disk of the protosolar nebula.

Exchange of condensed matter among the outer and terrestrial protoplanets and the effect on surface impact and atmospheric accretion

One current picture of the accretion process of Uranus and Neptune invokes extensive gravitational scattering of the icy planetesimals by the photoplanets as they grew to significant masses. This phase should last a few 100 million years during which the planetary scattering effect could lead to implantation of comets in the Oort cloud. The injection of the scattered planetesimals into the inner Solar System also leads to a gain of volatile materials by the terrestrial planets. The potential H 2O input to primordial Earth, for instance, could be on the order of 10 25 – 10 26 g even though a large part of it might have been subsequently lost via atmospheric escape process. This early planetesimal impact event was followed by a much more gradual process, i.e., the return of the icy cometary cores from the distant Oort cloud via stellar perturbations. The total amount of volatile material as accreted by the terrestrial planets was quite small as compared with the earlier episode accompanied by the formation of Uranus and Neptune. As an example, the Earth would have recieved about 6 × 10 20g of cometary material in total over the last 4 billion years if the cometary nucleus density is as high as 0.5 g cm −3. Within the context of H 2O inventory (the terrestrial ocean mass is 1.3 × 10 24 g) the cometary influx should have minor effects. On the other hand, because of the paucity of H 2O content in the atmospheres of Mars and Venus, cometary impact could produce strong time variation in their water contents. As for Titan, its nitrogen atmosphere could be derived from cometary impact only if the early phase of comet bombardment was very intense. A comparison of the adopted comet flux model with the crater count statistics indicates a certain discrepancy (a factor of 10–50) in the estimated comet impact rates. Possible sources of errors could come from the size distribution of the long-period comets, their average density, albedo, and other factors. A more complete survey of the population of subkilometer comets in the outer Solar System by Space Telescope and other instruments will be critical importance in clarifying the interrelation among the original mass of the Oort cloud, the volatile accruement by the terrestrial planets, and the impact crater production rates on planetary and satellite surfaces.