We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and ...atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky core falls in to the core of the target. However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer. Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ∼3 R⊕. This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field. We verify the results from the single previous study of lower resolution simulations that an impactor with a mass of at least 2 M⊕ can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta. At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 M⊕ impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (∼3 × 1036 kg m2 s−1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 M⊕ impactors than 3 M⊕ ones, because it requires tidal disruption that is suppressed by the more massive impactors.
ABSTRACT
We perform simulations of giant impacts on to the young Uranus using smoothed particle hydrodynamics (SPH) with over 100 million particles. This 100–1000 × improvement in particle number ...reveals that simulations with below 107 particles fail to converge on even bulk properties such as the post-impact rotation period, or on the detailed erosion of the atmosphere. Higher resolutions appear to determine these large-scale results reliably, but even 108 particles may not be sufficient to study the detailed composition of the debris – finding that almost an order of magnitude more rock is ejected beyond the Roche radius than with 105 particles. We present two software developments that enable this increase in the feasible number of particles. First, we present an algorithm to place any number of particles in a spherical shell such that they all have an SPH density within 1 per cent of the desired value. Particles in model planets built from these nested shells have a root-mean-squared velocity below 1 per cent of the escape speed, which avoids the need for long precursor simulations to produce relaxed initial conditions. Secondly, we develop the hydrodynamics code sph with interdependent fine-grained tasking(swift) for planetary simulations. swift uses task-based parallelism and other modern algorithmic approaches to take full advantage of contemporary supercomputer architectures. Both the particle placement code and swift are publicly released.
We examine the mechanisms by which the atmosphere can be eroded by giant impacts onto Earth-like planets with thin atmospheres, using 3D smoothed particle hydrodynamics simulations with sufficient ...resolution to directly model the fate of low-mass atmospheres. We present a simple scaling law to estimate the fraction lost for any impact angle and speed in this regime. In the canonical Moon-forming impact, only around 10% of the atmosphere would have been lost from the immediate effects of the collision. There is a gradual transition from removing almost none to almost all of the atmosphere for a grazing impact as it becomes more head-on or increases in speed, including complex, nonmonotonic behavior at low impact angles. In contrast, for head-on impacts, a slightly greater speed can suddenly remove much more atmosphere. Our results broadly agree with the application of 1D models of local atmosphere loss to the ground speeds measured directly from our simulations. However, previous analytical models of shock-wave propagation from an idealized point-mass impact significantly underestimate the ground speeds and hence the total erosion. The strong dependence on impact angle and the interplay of multiple nonlinear and asymmetrical loss mechanisms highlight the need for 3D simulations in order to make realistic predictions.
We present a new scaling law to predict the loss of atmosphere from planetary collisions for any speed, angle, impactor mass, target mass, and body composition, in the regime of giant impacts onto ...broadly terrestrial planets with relatively thin atmospheres. To this end, we examine the erosion caused by a wide range of impacts, using 3D smoothed particle hydrodynamics simulations with sufficiently high resolution to directly model the fate of low-mass atmospheres around 1% of the target's mass. Different collision scenarios lead to extremely different behaviors and consequences for the planets. In spite of this complexity, the fraction of lost atmosphere is fitted well by a power law. Scaling is independent of the system mass for a constant impactor mass ratio. Slow atmosphere-hosting impactors can also deliver a significant mass of atmosphere, but always accompanied by larger proportions of their mantle and core. Different Moon-forming impact hypotheses suggest that around 10%-60% of a primordial atmosphere could have been removed directly, depending on the scenario. We find no evident departure from the scaling trends at the extremes of the parameters explored. The scaling law can be incorporated readily into models of planet formation.
Abstract
The Moon is traditionally thought to have coalesced from the debris ejected by a giant impact onto the early Earth. However, such models struggle to explain the similar isotopic compositions ...of Earth and lunar rocks at the same time as the system’s angular momentum, and the details of potential impact scenarios are hotly debated. Above a high resolution threshold for simulations, we find that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth’s Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon’s Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon’s early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.
Abstract
We simulate the collision of precursor icy moons analogous to Dione and Rhea as a possible origin for Saturn’s remarkably young rings. Such an event could have been triggered a few hundred ...million years ago by resonant instabilities in a previous satellite system. Using high-resolution smoothed particle hydrodynamics simulations, we find that this kind of impact can produce a wide distribution of massive objects and scatter material throughout the system. This includes the direct placement of pure-ice ejecta onto orbits that enter Saturn’s Roche limit, which could form or rejuvenate rings. In addition, fragments and debris of rock and ice totaling more than the mass of Enceladus can be placed onto highly eccentric orbits that would intersect with any precursor moons orbiting in the vicinity of Mimas, Enceladus, or Tethys. This could prompt further disruption and facilitate a collisional cascade to distribute more debris for potential ring formation, the re-formation of the present-day moons, and evolution into an eventual cratering population of planetocentric impactors.
ABSTRACT
We simulate the hypothesized collision between the proto-Earth and a Mars-sized impactor that created the Moon. Among the resulting debris disc in some impacts, we find a self-gravitating ...clump of material. It is roughly the mass of the Moon, contains $\sim 1{{\ \rm per\ cent}}$ iron like the Moon, and has its internal composition resolved for the first time. The clump contains mainly impactor material near its core but becomes increasingly enriched in proto-Earth material near its surface. The formation of this Moon-sized clump depends sensitively on the spin of the impactor. To explore this, we develop a fast method to construct models of multilayered, rotating bodies and their conversion into initial conditions for smoothed particle hydrodynamical (SPH) simulations. We use our publicly available code to calculate density and pressure profiles in hydrostatic equilibrium and then generate configurations of over a billion particles with SPH densities within 1 per cent of the desired values. This algorithm runs in a few minutes on a desktop computer, for 107 particles, and allows direct control over the properties of the spinning body. In comparison, alternative relaxation or spin-up techniques take hours on a supercomputer and the structure of the rotating body cannot be known beforehand. Collisions that differ only in the impactor’s initial spin reveal a wide variety of outcomes: a merger, a grazing hit-and-run, or the creation of an orbiting proto-Moon.
ABSTRACT
Density discontinuities cannot be precisely modelled in standard formulations of smoothed particles hydrodynamics (SPH) because the density field is defined smoothly as a kernel-weighted sum ...of neighbouring particle masses. This is a problem when performing simulations of giant impacts between proto-planets, for example, because planets typically do have density discontinuities both at their surfaces and at any internal boundaries between different materials. The inappropriate densities in these regions create artificial forces that effectively suppress mixing between particles of different material and, as a consequence, this problem introduces a key unknown systematic error into studies that rely on SPH simulations. In this work, we present a novel, computationally cheap method that deals simultaneously with both of these types of density discontinuity in SPH simulations. We perform standard hydrodynamical tests and several example giant impact simulations, and compare the results with standard SPH. In a simulated Moon-forming impact using 107 particles, the improved treatment at boundaries affects at least 30${{\ \rm per\ cent}}$ of the particles at some point during the simulation.
Permanently shadowed locations at the lunar poles are potential sites for significant concentrations of cold‐trapped volatiles, including water ice. Hydrogen enhancements are seen at the poles, but ...the physical form, abundance and distribution of this hydrogen remains controversial. Using a pixon‐based image reconstruction algorithm to effectively improve spatial resolution, we derive maps of the lunar south polar water‐equivalent hydrogen concentration that are fully consistent with the orbital neutron measurements, with abundances greater than 0.5 wt% in some permanently shadowed locations. This is much greater than the highest solar wind hydrogen abundance in returned lunar samples, and may indicate ice between regolith grains. If the hydrogen distribution is inhomogeneous within a permanently shadowed crater, then even higher abundances are implied. In Shackleton crater, for example, the derived count rates are consistent with 10% of the crater floor area having 20‐wt% water‐equivalent hydrogen, and the remainder at 0.25 wt%.
A detailed comparison is made of results from the Lunar Prospector Neutron Spectrometer (LPNS) and the Lunar Exploration Neutron Detector Collimated Sensors for Epithermal Neutrons (LEND CSETN). ...Using the autocorrelation function and power spectrum of the polar count rate maps produced by these experiments, it is shown that the LEND CSETN has a footprint that is at least as big as would be expected for an omnidirectional detector at an orbital altitude of 50 km. The collimated flux into the field of view of the collimator is negligible. A dip in the count rate in Shoemaker crater is found to be consistent with being a statistical fluctuation superimposed on a significant, larger‐scale decrease in the count rate, providing no evidence for high spatial resolution of the LEND CSETN. The maps of lunar polar hydrogen with the highest contrast, i.e., spatial resolution, are those resulting from pixon image reconstructions of the LPNS data. These typically provide weight percentages of water‐equivalent hydrogen that are accurate to 30% within the polar craters.
Key Points
How well do we know the polar hydrogen distribution
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