Lunar-forming collisions with pre-impact rotation

doi:10.1016/j.icarus.2008.03.011

Icarus

Volume 196, Issue 2, August 2008, Pages 518-538

https://doi.org/10.1016/j.icarus.2008.03.011 Get rights and content

Abstract

Prior models of lunar-forming impacts assume that both the impactor and the target protoearth were not rotating prior to the Moon-forming event. However, planet formation models suggest that such objects would have been rotating rapidly during the late stages of terrestrial accretion. In this paper I explore the effects of pre-impact rotation on impact outcomes through more than 100 hydrodynamical simulations that consider a range of impactor masses, impact angles and impact speeds. Pre-impact rotation, particularly in the target protoearth, can substantially alter collisional outcomes and leads to a more diverse set of final planet–disk systems than seen previously. However, the subset of these impacts that are also lunar-forming candidates—i.e. that produce a sufficiently massive and iron-depleted protolunar disk—have properties similar to those determined for collisions of non-rotating objects [Canup, R.M., Asphaug, E., 2001. Nature 412, 708–712; Canup, R.M., 2004a. Icarus 168, 433–456]. With or without pre-impact rotation, a lunar-forming impact requires an impact angle near 45 degrees, together with a low impact velocity that is not more than 10% larger than the Earth's escape velocity, and produces a disk containing up to about two lunar masses that is composed predominantly of material originating from the impactor. The most significant differences in the successful cases involving pre-impact spin occur for impacts into a retrograde rotating protoearth, which allow for larger impactors (containing up to 20% of Earth's mass) and provide an improved match with the current Earth–Moon system angular momentum compared to prior results. The most difficult state to reconcile with the Moon is that of a rapidly spinning, low-obliquity protoearth before the giant impact, as these cases produce disks that are not massive enough to yield the Moon.

Section snippets

Background

The leading theory for the Moon's origin is that it formed as a result of the impact of a Mars-sized object with the early Earth (Cameron and Ward, 1976). Key strengths of the giant impact theory include its ability to account for the Earth–Moon system angular momentum (which implies a terrestrial day of only about 5 h when the Moon formed close to the Earth), and the Moon's relatively low iron abundance compared to other inner Solar System objects. In addition, dynamical models of the final

Constraints

Basic properties that must be accounted for by a successful impact include: (i) a total Earth–Moon system angular momentum, $L_{EM} \equiv 3.5 \times 10^{41} {g-cm}^{2} / s$ , (ii) a Moon whose mass is $M_{L} = 7.35 \times 10^{25} g = 0.012 M_{\oplus}$ , and (iii) a bulk lunar mass abundance of elemental iron in the few to 10% range (e.g., Lucey et al., 1995, Jones and Delano, 1989, Jones and Palme, 2000; Canup, 2004a, Canup, 2004b).

If the target and impactor are not rotating prior to impact, the angular momentum delivered by an impactor of mass $M_{i} \equiv γ M_{T}$

Approach

Smooth particle hydrodynamics, or SPH (e.g., Lucy, 1977, Benz et al., 1989, Canup and Asphaug, 2001; Canup, 2004a, Canup, 2005), is used here to model giant impacts. In SPH, matter is represented by spherically symmetric overlapping ‘particles’ whose individual evolutions are calculated as a function of time. Each particle represents a quantity of mass of a given composition, whose 3-dimensional spatial extent is specified by a density weighting function known as the kernel, and the

Results

In this section, I begin by describing general trends in the outcomes of impacts between non-rotating Mars-sized impactors and target protoearths. I then consider a single collision to which pre-impact rotation in the target or the impactor has been added, and use these results to identify which pre-impact spin orientations have the largest effects on impact outcome for a given impactor mass, impact speed and impact angle. Finally I describe several series of simulations involving pre-impact

Summary and discussion

Prior works consider non-rotating targets and impactors, assuming that the lunar-forming impact delivered the entire angular momentum of the Earth–Moon system. In this case, a range of successful candidate impacts has been identified that produces favorable conditions for forming the Moon (Canup and Asphaug, 2001; Canup, 2004a, Canup, 2004b). However, planet formation models suggest that the protoearth and impactor were likely rotating rapidly during the late stages of their growth. This work

Acknowledgments

This paper is dedicated to Alastair Cameron, in recognition of his fundamental work on lunar origins and in appreciation for his having inspired my interest in impact simulations. I thank Eiichiro Kokubo and an anonymous referee for their helpful and thorough reviews, Jay Melosh and Betty Pierazzo for providing me with M-ANEOS and the associated material parameter constants used here, and Amy Barr for comments on an earlier draft of the paper. This work benefited from Peter Tamblyn's assistance

References (33)

C.B. Agnor et al.
On the character and consequences of large impacts in the late stage of terrestrial planet formation
Icarus
(1999)
W. Benz et al.
The origin of the Moon and the single impact hypothesis III
Icarus
(1989)
R.M. Canup
Simulations of a late lunar forming impact
Icarus
(2004)
R.M. Canup et al.
A scaling relationship for satellite-forming impacts
Icarus
(2001)
J.H. Jones et al.
A three-component model for the bulk composition of the Moon
Geochim. Cosmochim. Acta
(1989)
E. Kokubo et al.
Evolution of a circumterrestrial disk and formation of a single moon
Icarus
(2000)
K. Ohtsuki et al.
Planetary rotation by accretion of planetesimals with non-uniform spatial distribution formed by the planet's gravitational perturbation
Icarus
(1998)
K. Pahlevan et al.
Equilibration in the aftermath of the lunar-forming giant impact
Earth Planet. Sci. Lett.
(2007)
C.B. Agnor et al.
Accretion efficiency during planetary collisions
Astrophys. J.
(2004)
A.G.W. Cameron
Higher-resolution simulations of the giant impact

A.G.W. Cameron et al.

The Origin of the Moon

Lunar Planet. Sci.

(1976)

R.M. Canup

Formation of the Moon

Annu. Rev. Astron. Astrophys.

(2004)

R.M. Canup

A giant impact origin of Pluto–Charon

Science

(2005)

R.M. Canup et al.

Origin of the Moon in a giant impact near the end of the Earth's formation

Nature

(2001)

R.N. Clayton

Oxygen isotopes in meteorites

Annu. Rev. Earth Planet. Sci.

(1993)

P. Goldreich

History of the lunar orbit

Rev. Geophys.

(1966)

Cited by (141)

Trace element volatility and the conditions of liquid-vapor separation in the proto-lunar disk
2022, Icarus
The Moon is thought to have formed from material ejected by a giant impact that took place at the end of Earth's accretion. The material ejected to space generated a large hot structure where material beyond the Roche limit accreted to form the Moon. It has long been known that the Moon is characterized by abundances in moderately volatile elements (MVE) lower than that of the Earth, while more recent studies have established that the concentrations in refractory elements are similar to the bulk Silicate Earth. The thermodynamic conditions that prevailed after this impact are poorly known and understanding the origin of the Moon-Earth differences in MVE requires a knowledge of the volatility of elements under these conditions. In this study, we reexamine the volatility of a large set of geochemically relevant elements and attempt to determine the P-T conditions under which volatiles were putatively separated from the liquid material. Our model predicts very different condensation temperatures due to higher pressures, compared with the conditions of the Solar Nebula and we extend the values of these temperatures to a wide number of trace elements (Se, Ag, Pt, Mo, W, Zn, Sn, Sb, Rb, Cs, U, Th, Cr, Ni, Co, Ga, Ge, Cu, and P). Our modeling shows that the observed lunar compositions cannot be explained by a single set of P and T conditions. Rather, it is best explained by a mixture between high-temperature condensates (~4000 K) and low temperature condensates (2000-2500 K). An important constraint is that for the low temperature condensates, liquid metal must have been stable and this is crucial for matching the abundance of volatile siderophile elements in the bulk Moon.
The Diffusion Limit of Photoevaporation in Primordial Planetary Atmospheres
2024, arXiv
A Systematic Survey of Moon-forming Giant Impacts. I. Nonrotating Bodies
2023, Astrophysical Journal
Outcomes of Planetary Collisions: Importance of Gravity and Material Properties
2023, arXiv
A Systematic Survey of Moon-Forming Giant Impacts: Non-rotating bodies
2023, arXiv
Spin of protoplanets generated by pebble accretion: Influences of protoplanet-induced gas flow
2023, Astronomy and Astrophysics

View all citing articles on Scopus

View full text

Lunar-forming collisions with pre-impact rotation

Abstract

Section snippets

Background

Constraints

Approach

Results

Summary and discussion

Acknowledgments

Icarus

Icarus

Icarus

Icarus

Geochim. Cosmochim. Acta

Icarus

Icarus

Earth Planet. Sci. Lett.

Accretion efficiency during planetary collisions

Astrophys. J.

Higher-resolution simulations of the giant impact

The Origin of the Moon

Lunar Planet. Sci.

Formation of the Moon

Annu. Rev. Astron. Astrophys.

A giant impact origin of Pluto–Charon

Science

Origin of the Moon in a giant impact near the end of the Earth's formation

Nature

Oxygen isotopes in meteorites

Annu. Rev. Earth Planet. Sci.

History of the lunar orbit

Rev. Geophys.