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Planetary Accretion in the Inner Solar System Earth and Planetary Science Letters 223 (2004) 241–252 www.elsevier.com/locate/epsl Frontiers Planetary accretion in the inner Solar System John E. Chambers* Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road N.W., Washington, DC 20015, USA Received 15 December 2003; received in revised form 23 April 2004; accepted 27 April 2004 Available online 4 June 2004 Abstract Unlike gas-giant planets, we lack examples of terrestrial planets orbiting other Sun-like stars to help us understand how they formed. We can draw hints from elsewhere though. Astronomical observations of young stars; the chemical and isotopic compositions of Earth, Mars and meteorites; and the structure of the Solar System all provide clues to how the inner rocky planets formed. These data have inspired and helped to refine a widely accepted model for terrestrial planet formation—the planetesimal hypothesis. In this model, the young Sun is surrounded by a disk of gas and fine dust grains. Grains stick together to form mountain-size bodies called planetesimals. Collisions and gravitational interactions between planetesimals combine to produce a few tens of Moon-to-Mars-size planetary embryos in roughly 0.1–1 million years. Finally, the embryos collide to form the planets in 10–100 million years. One of these late collisions probably led to the formation of Earth’s Moon. This basic sequence of events is clear, but a number of issues are unresolved. In particular, we do not really understand the physics of planetesimal formation, or how the planets came to have their present chemical compositions. We do not know why Mars is so much smaller than Earth, or exactly what prevented a planet from forming in the asteroid belt. Progress is being made in all of these areas, although definitive answers may have to wait for observations of Earth-like planets orbiting other stars. D 2004 Elsevier B.V. All rights reserved. Keywords: earth; terrestrial planets; asteroids; accretion; solar nebula 1. Introduction planets. This is in a most unfamiliar place: orbiting a pulsar, the extinct remnant of a supernova explosion We are witnessing a revolution in planetary science. [1]. Remarkably, the pulsar planets bear a striking The discovery of about a hundred other planetary resemblance to the inner planets of the Solar System systems has provided a wealth of new information to in terms of their orbits and masses, although they may afieldthatwaspreviouslyfocussedononlyone. have originated under very different conditions. Cur- However, the new planets are probably all gas giants, rently, we lack a way to detect terrestrial planets in orbit akin to Jupiter and Saturn, so they tell us relatively little around ordinary stars [2], so we have almost no notion about the nature and origin of small, rocky planets like of how common or otherwise Earth-like planets may Earth. We know of only one other system of terrestrial be. As tantalizing as the new planetary discoveries are, we must look elsewhere for clues to the origin of the * SETI Institute, 2035 Landings Drive, Mountain View, CA Sun’s terrestrial planets. 94043, USA. Tel.: +1-650-604-5514; fax: +1-650-604-6779. Astronomical observations of newborn stars show E-mail address: [email protected] (J.E. Chambers). that many are surrounded by a disk-shaped region of 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.031 242 J.E. Chambers / Earth and Planetary Science Letters 223 (2004) 241–252 gas clouded with fine dust [3]. The disks generally have up the rock, metal and ice that resides in the planets, radii at least as large as the Sun’s planetary system, and and adding enough hydrogen and helium to give a contain at least as much mass as our planets. These composition similar to the Sun. This minimum mass observations, together with the planar geometry of the nebula contains a few percent of a solar mass—a Solar System, and the fact that the Sun’s planets all number that fits snugly within the range of values orbit in the same direction, suggest the planets formed estimated for circumstellar disks. However, this ma- in a similar disk environment. This is often called the terial did not quietly metamorphose into the planets protoplanetary disk or protoplanetary nebula. If this we see today. Most of the gas has gone, somewhere. disk had the same composition as the Sun, roughly Some of the dust has disappeared too, or at least it has 0.5% of the mass in Earth’s locale would have existed moved around a great deal. The giant planets probably in solid grains of rock and metal. The remaining 99.5% contain tens of Earth masses of rocky and icy mate- would have been gas: hydrogen, helium and volatile rial, while the vast expanse of the asteroid belt, materials such as water and carbon monoxide. Since between 2 and 4 astronomical units (AU) from the Earth is made almost entirely of rock and metal, it is Sun, has only enough stuff to make a planet 1% the clear that planet formation in the inner Solar System mass of Mercury. Just as curiously, 90% of the total was a sideshow compared to the evolution of the more mass of the inner Solar System now resides in a massive protoplanetary nebula itself, although the narrow strip between 0.7 and 1 AU. conventional viewpoint is just the opposite. Much of what we know about the early history of Interestingly, most known giant planets orbit stars the Solar System comes from studying primitive (the Sun included) that contain above average meteorites. These rocks come from asteroids that amounts of dust-forming elements such as iron [4]. never became hot enough to melt. Thus, primitive One way to interpret this correlation is that giant meteorites and their parent bodies act as a kind of planets form most readily where solid materials are archaeological site, preserving the detritus formed in abundant. The same may be true of rocky planets. It is the first few million years of the Solar System. Most also apparent that planet formation is an inefficient primitive meteorites are composed largely of chon- process, because even stars with dust-poor disks drules—beads of rock typically about 1 mm in size. contain enough material to form a respectable system The composition and texture of most chondrules of planets. Stars older than a few million years (Myr) suggests they were once balls of dust floating in apparently lack massive gas-rich disks [5]. If massive the solar nebula that were strongly heated and cooled disks are essential in order to generate planets then over the space of a few hours [9]. The heating melted planet formation must begin within a few million the chondrules but was not sufficiently protracted to years. Some older stars possess tenuous disks con- allow all the more volatile elements such as sulfur to taining some dust but apparently little gas [6,7]. Dust escape. Particular types of meteorite contain chon- in these systems should be ground down to small sizes drules with distinctive sizes and compositions. This and pushed out of the system on time scales of 104 – may mean that chondrules formed in small regions of 106 years by the gentle but insistent pressure of light the protoplanetary nebula in a series of separate from the star itself [8]. This may be second-generation events. Theories abound for the origin of chondrules dust formed by high speed collisions between solid [9]. Models in which dust is melted by shock waves bodies orbiting these stars or dust evaporating from in the nebula are currently in vogue [10,11], although the surface of comets [8]. If so, this suggests that dust the source of these shock waves is unclear. Chon- is able to accumulate into large solid bodies in a drule formation may be intimately tied to other variety of protoplanetary disks. events in the Solar System. In particular, shock driven chondrule formation could require the early formation of Jupiter [11,12]. A small fraction of 2. Physical and cosmochemical constraints chondrules appear to have formed as a result of impacts on asteroids [13], which implies that large We can make a crude estimate of the minimum bodies had already accreted by the time these chon- mass of the Sun’s protoplanetary nebula by totalling drules formed. J.E. Chambers / Earth and Planetary Science Letters 223 (2004) 241–252 243 Primitive meteorites also contain refractory com- the decay of U isotopes to Pb, and also the short-lived ponents, similar in size to chondrules. These calcium isotope 182Hf, which decays to 182W with a half-life of aluminium rich inclusions (CAIs) are a minor com- 9 Myr. These isotope systems are useful because the ponent of meteorites, and their origin is even more parent nuclei are lithophile (silicate loving) while the enigmatic than that of chondrules. They are important daughter isotopes are more siderophile. Assuming core here because their age places timing constraints on formation happened continuously and that accretion planet formation. The relative ages of CAIs and tailed off roughly exponentially over time, the lead chondrules can be estimated from the abundances of isotopes indicate that Earth accreted/differentiated with the decay products of short-lived isotopes. Particularly a mean life of 15–40 Myr [22]. Somewhat confusing- useful are, 41Ca, 26Al and 53Mn, with half lives of 0.1, ly, the Hf–W isotopes provide a shorter mean life of 0.7 and 3.7 Myr respectively [14]. The absolute ages about 11 Myr [23].
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