Sunday, July 22, 2012

Solar System Archaeology, Part II

Figure 9. Sailboats in the harbor of Nice, a seaside city at the epicenter of theoretical research
into the evolution of the Solar System.

the grand tack

For a long time, the circular, evenly-spaced orbits of the Solar System were interpreted as relics of a placid evolutionary history. Each planet seemed to have assembled very near its present orbit and maintained long-term dynamical harmony with its neighbors. In addition, the distribution of mass among the eight planets seemed to support a scenario of orderly growth: the smallest, Mercury, travels on the shortest orbit, where planetesimals were least abundant in the primordial nebula; the largest, Jupiter, occupies the nebula’s sweet spot, where solids were plentiful and growth was fastest; the next planet outward, Saturn, is only one-third as massive as Jupiter; and the combined mass of the two outer planets, Uranus and Neptune, is only one-third of Saturn’s.

This simple scenario, as it turns out, has fatal flaws.

  1. Our system’s mass distribution exhibits two striking discontinuities. Considering its position in the planetary line-up, Mars should be a few times the mass of Earth, yet it is only one-ninth as massive (Figure 12); while Neptune, the outermost planet, should be less massive than Uranus, its inner neighbor, yet it is almost 20% heavier.
  2. The skewed configurations of several planetary orbits testify to violent histories. Although Venus and Neptune have orbital eccentricities smaller than 1%, and Earth is under 2%, three adjacent giant planets – Jupiter, Saturn, and Uranus – have eccentricities in the vicinity of 5%, while Mars has 9% and Mercury has more than 20%.
  3. Both Jupiter and Neptune share their orbits with large populations of so-called Trojan asteroids, which occupy the stable Lagrange points located 60 degrees ahead of and behind each planet. A relatively static model of system evolution cannot explain these accumulations of small bodies.
  4. The axis of rotation of Uranus is tilted more than 90 degrees to the plane of the Sun’s equator (Between Earth & Uranus, Part I), a configuration that is often explained as resulting from a collision between Uranus and an otherwise unknown rogue planet in the Solar System’s infancy (Slattery et al. 1992, Parisi et al. 2008).

Taken together, these discontinuities, eccentricities, and irregularities offer incontrovertible proof of a complex evolutionary process.

The Nice model (Figure 9), which has become the most widely endorsed scenario of Solar System evolution, took as its starting point the moment when the Solar nebula had completely evaporated (Gomes et al. 2005, Tsiganis et al. 2005). At that time the four (or five) massive outer planets were assumed to be collected in a much more compact configuration than they are today, orbiting inside a semimajor axis of 15 AU.

However, the original Nice model could not explain how our giants happened to arrive in such a snug relationship. Over the past few years this gap in our understanding has been filled by a series of studies outlining a remarkable sequence of events known as the Grand Tack (Morbidelli et al. 2007, Batygin & Brown 2010, Morbidelli et al. 2011, Pierens & Raymond 2011, Walsh et al. 2011). The “tack” in this nickname has nothing to do with sharp pointy objects. Instead, it refers to a sailing maneuver (Figure 10) in which a ship sails against the wind by zigzagging back and forth (tacking) across her forward path, thereby changing the direction from which the wind hits her sails at the turning point of each zigzag (tack).


Figure 10. 2011 Extreme Sailing Series, Nice, Côte d’Azur

In the Grand Tack model, the rapid accretion of solids in the inner system during the first few million years after the Sun ignited was paralleled by an even more rapid accumulation of rocky and icy particles beyond 3 AU. The problem of narrow feeding zones, which made most inner-system embryos stall somewhere between the masses of the Moon and Mars, did not pose a barrier to growth in this outer region. Two large protoplanets assembled between 3 and 5 AU, with the inner object quickly reaching the phase of runaway growth, during which it accreted a deep hydrogen atmosphere. This object was proto-Jupiter, the first gas giant to emerge from the Sun’s protoplanetary cloud.

Having opened a gap in the dissipating nebula, proto-Jupiter began to spiral toward the Sun, carried along with the flow of accreting gases in a process known as Type II migration. The proto-giant plowed through broad fields of planetesimals orbiting inward of 3 AU, pitching them into the Sun, scattering them on eccentric orbits, or ejecting them from the system altogether.

Right behind proto-Jupiter came the second proto-giant, the ancestor of Saturn, which initiated runaway accretion not long after its bigger sibling and began to spiral inward even faster. When proto-Jupiter reached a semimajor axis of 1.5 AU (equivalent to the current orbit of Mars) the two objects entered into a mean motion resonance (MMR), such that their orbital periods could be expressed as the ratio of two small integers. Large suites of numerical simulations have specified the likely resonance as 3:2, such that proto-Jupiter completed three orbits for every two of proto-Saturn (Morbidelli et al. 2007, Batygin & Brown 2010, Pierens & Raymond 2011). The 2:1 and 5:3 resonances have also been shown to result in similar outcomes, at least in some iterations of the model (Batygin & Brown 2010).


Figure 11. The Grand Tack. Migration of the four giant planets through the Solar nebula over a period of 600,000 years. Distance is marked in astronomical units (AU). The narrow belt of planetesimals at 1 AU (the present orbit of Earth) later coalesced into the four terrestrial planets.

The establishment of this orbital resonance was in effect a “marriage” between the two proto-giants, such that their dynamical trajectories thereafter became inextricably linked. Their inward migration now halted and abruptly reversed. This striking reversal is the Grand Tack itself, which brought both giants back into the outer system, maintaining their resonant configuration the whole time (Figure 11). Jupiter’s tacking maneuver permitted a considerably diminished belt of planetesimals and planetary embryos to reassemble around 1 AU, the current orbit of the Earth. Meanwhile, the outward passage of Saturn brought the inner of the newly-formed gas dwarfs under its gravitational influence, such that Saturn captured the young dwarf (either Uranus or Neptune) in another MMR (Morbidelli et al. 2007, Batygin & Brown 2010). This gas dwarf, in turn, captured the next dwarf out.

In one promising model developed by Morbidelli and colleagues (2007), Neptune (identified as the outer gas dwarf) participated in a 3:4 MMR with Uranus, Uranus participated in a 2:3 MMR with Saturn, and Saturn participated in another 2:3 MMR with Jupiter. In this model, the ratio of the four orbital periods, from Neptune to Jupiter, is 3:4:6:9.

Having attained their mature masses, Jupiter and Saturn paused in their flight from the Sun. A likely explanation for their journey’s end is the attenuation of the Solar nebula, to the point where it could no longer sustain gas-driven migration: as if an ebbing flood had stranded these two giants in its wake. Jupiter landed near its present semimajor axis of 5.2 AU, Saturn achieved an orbit near 7 AU, while the brood of gas dwarfs – Neptune, Uranus, and perhaps “Amphitrite” – were confined to a region between 9 and 15 AU. Beyond these cold orbits stretched the solid remnants of the protoplanetary disk: a field of rocky and icy objects extending to a radius of about 30 AU or so. This population would eventually contribute to the Kuiper Belt.

Intuitively, the titanic sequence of events involved in the Grand Tack might seem to require many aeons to reach its climax. As a point of reference, the Mesozoic Era on Earth lasted 180 million years, during which the supercontinent Pangaea broke up and the dinosaurs evolved, flourished, and died out. Yet the span of time required for the Grant Tack was actually shorter by a few orders of magnitude. According to recent simulations, only 600,000 years elapsed from the accretion of Jupiter’s atmosphere, through the migration of both gas giants across the Solar nebula, to the establishment of the four-way orbital resonance that provisionally stabilized the outer system (Walsh et al. 2011). Thereafter, according to this model, the four giant planets ran like clockwork for hundreds of millions of years in precisely matched revolutions around the Sun.

Although the duration of the Grand Tack can be estimated, uncertainty surrounds both its absolute chronology (in terms of time since the Sun’s ignition) and its relative chronology (in terms of its temporal relationship with other key system events). For example, Vesta formed about 2 or 3 million years after the Sun, but did its birth occur before, during, or after Jupiter’s migration? The same questions can be asked of Mars, whose accretion was complete about 10 million years after the condensation of the Solar nebula.

Two synchronisms seem clear: the Grand Tack finished when the nebula dispersed, and the process of dispersal coincided with the formation of the regular satellites of Jupiter and Saturn in “gas-starved” accretion disks around their parent planets (Canup & Ward 2006). Given what we know about the lifetimes of gas disks, these outcomes transpired no earlier than 3 million years and probably not much later than 6 million years after the Sun’s first light. 


Figure 12. Runts of the Litter. Mercury & Mars are dramatically smaller
and less massive than their cloudy siblings, Venus & Earth.

worlds in collision

The evaporation of the gas nebula ended the tumultuous first act of our Solar System’s origin story. Jupiter’s brief incursion to a perihelion of 1.5 AU had devastated the inner system. All that remained of the earlier population of planetesimals and embryos was a narrow ring of rocky and icy objects orbiting around 1.0 AU, near the present orbits of Venus and Earth. Much of the second act of the drama focuses on events in this inner region, although its finale involves the whole system.

The notion that the four terrestrial planets (Mercury through Mars) originated in a narrow annulus between 0.7 and 1 AU (Hansen 2009), instead of in a much broader field of planetesimals extending from ~0.5 AU to ~3.5 AU, is perfectly congruent with the scenario of the Grand Tack (Brasser 2012, Morbidelli et al. 2012). Yet it was originally advanced by Brad Hansen on the basis of numerical simulations whose sole purpose was to reproduce the current architecture of the inner Solar System (Hansen 2009). In fact, Hansen conceded that, although his model nicely explained the masses, orbits, and formation timescales of the four inner planets, it could not account for the origin of their depleted birth environment or for existence of the current Asteroid Belt.

Serendipitously, the Grand Tack fills both gaps. We have already seen that it explains the annulus as a leftover from Jupiter’s marauding days. The Asteroid Belt then becomes a by-blow of the accretion of the terrestrial planets, a junkyard that collected all the debris ejected from the inner system by gravitational scattering and collisions between planetary embryos.

After Jupiter withdrew from the inner system, the remaining mass of planetesimals accreted into a population of planetary embryos that underwent violent impacts. This process assembled Mars within about 10 million years after the birth of the Sun, and simultaneously ejected the young planet from the main annulus onto a wider and somewhat eccentric orbit (Hansen 2009). Mercury was probably produced on a similar timescale and ejected onto an even more eccentric orbit closer to the Sun, although its formation has not yet been explored in detail. Both of these planets are considerably smaller and less massive than Earth and Venus (Figure 12): Mars has only 0.11 of Earth’s mass (Mea), while Mercury has only 0.06 Mea. Their stunted growth is readily explained by their early expulsion from the “birth ring.” At 0.81 Mea, Venus required the equivalent of five Mercuries and five Marses to attain her current bulk, while Earth needed approximately six of each.

All scenarios agree that Earth is the youngest of the terrestrial planets, and thus the youngest planet in the whole system. According to the most widely accepted models, its formation was not complete until it suffered a glancing collision with a Mars-sized object late in its accretion history. The resulting debris endowed the Earth with its final mass and contributed to a circumplanetary ring from which the Moon rapidly assembled (Canup 2004). Although debate continues over the timing of this Moon-forming event, a date between 30 and 50 million years after the condensation of the Solar nebula has broad support (Canup 2004, Jacobsen 2005, Jacobsen et al. 2009, Hansen 2009).

A remaining problem in inner system evolution is the origin of the Earth’s oceans, sometimes discussed under the heading of  “water delivery.” Numerous studies have argued that proto-Earth was dry, since its birth environment inside the ice line was too hot to sustain freely orbiting volatiles. Water must have arrived from cooler regions of the system, with the Asteroid Belt favored over the Kuiper Belt as its source on the basis of hydrogen isotopes in Earth’s current oceans (Morbidelli et al. 2000). However, the delivery of just the right amount of water at just the right time may have required an implausible degree of fine-tuning. 

Not to worry, because along with so much else, the Grand Tack can also explain the origin of our biogenic oceans (Brasser 2012, Morbidelli et al. 2012). Jupiter’s early migration through the inner system scattered plenty of wet planetesimals from beyond the ice line into the terrestrial birth ring, making them available for accretion by all four protoplanets. In the end, Earth was the only one with the right stuff – mass, temperature, magnetic field, atmosphere – to retain its precious inheritance. The match between the isotopic content of our oceans and that of carbonaceous chondritic meteorites is then explained, not by a bombardment of such objects after Earth formed, but by their presence among the building blocks that originally assembled it.

Figure 13. Late Heavy Bombardment. a. At the end of the Grand Tack, two gas giants and two ice giants orbited inside 15 AU (Jupiter = green, Saturn = orange, Neptune = blue, Uranus = turquoise).
b. All four giants suffer orbital instabilities, and Neptune and Uranus switch orbits, scattering asteroids and comets throughout the Solar System. c. The four giants achieve their final orbits amid a dramatically depleted field of leftover planetesimals. Credit: Wikimedia

late heavy bombardment

After the Moon-forming event, the inner Solar System settled into a long period of relative stability. For several hundred million years, asteroid and comet strikes happened intermittently, while the four (or more) outer giants revolved in their clockwork orbits. Trouble was brewing, nevertheless. The presence of a large, encircling field of planetesimals – ancestor of the present-day Kuiper Belt – resulted in a steady enlargement of the orbits of the gas dwarfs (Uranus, Neptune, and maybe “Amphitrite”) located beyond Saturn. Eventually, interactions between Saturn and the next planet out resulted in a scattering event that disrupted the outer system (Morbidelli et al. 2007, 2009a). As Morbidelli and colleagues describe it:

The migration of the giant planets, induced by the interaction with this [planetesimal] disk, increases the ratios of the orbital periods between each pair of planets. Thus, the planets are extracted from their mutual quadruple resonance. Because the system is very compact, new resonances are crossed during the migration. These resonances excite the eccentricities of the planets, triggering a global instability of the system. The orbits of the planets are eventually stabilized by the dynamical friction exerted by the planetesimal disk during its dispersal. (Morbidelli et al. 2007)

During this violent epoch, Uranus and Neptune may have switched orbits, and “Amphitrite” may have collided with Uranus, producing the extreme axial tilt we observe today. Thereafter, any extra gas dwarfs must have been ejected from the system, since we have only two left.  

Regardless of the details, Neptune migrated steadily outward across a distance of 15 AU, to its present semimajor axis of 30 AU. In the process, it scattered planetesimals far and wide and dramatically reduced the overall mass of the Kuiper Belt. The storm of eccentric comets and asteroids launched by Neptune is the Late Heavy Bombardment, whose scars are still visible as craters on the surfaces of the Moon and the larger satellites of Jupiter and Saturn. Earth and the giant planets must also have experienced an intense hail of impactors, but their dynamic local environments have erased all evidence of this cataclysm.

The starting point of the Late Heavy Bombardment has been variously placed between 600 and 800 million years after the birth of the Solar System, while its duration was in the vicinity of 10 to 150 million years (Gomes et al. 2005). By the time the dust settled, the eight major planets had established the orbital architecture we see today, and the second act of our drama of origin had given way to the third.

life

The Earth as we know it was born in the Moon-forming event, meaning that the Earth and the Moon share the same birthday. The epoch between that date and the Late Heavy Bombardment is known as the Hadean, after the hellish conditions once thought to prevail then. Recent evidence, however, demonstrates that Earth cooled rapidly after the Moon’s formation, so that a global ocean was established within 100 million years (Arndt & Nisbet 2012), or about 4.4 billion years ago. That ocean has persisted to the present day, rising and falling ceaselessly in response to lunar and solar tides. (Update August 2014: for a more recent a far less benevolent picture of the Hadean, see A Billion Years of Hell on Earth.)

Already in the Hadean, then, Earth was a blue planet, with an even smaller ratio of dry land to water than at present. Volcanism was common on land and undersea, but no significant mountain ranges had yet developed (Arndt & Nisbet 2012). The global temperature at that epoch remains a matter of debate, with many researchers favoring a climate cool enough for flotillas of icebergs to populate the seas. Some have argued that the first microbial life may have emerged during the Hadean, perhaps in the vicinity of submerged volcanoes (Arndt & Nisbet 2012).

The Late Heavy Bombardment that ended the era was a catastrophe of literally cosmic proportions, yet it did not evaporate the planetary ocean. Nor was it sufficient to sterilize our planet, if any life already existed (Abramov & Mojzsis 2009). In fact, the first signs of life appear right after this event, leading to some interesting speculations. Perhaps the bombardment triggered the evolution of the first organisms on Earth, or perhaps the impactors themselves brought dormant organisms with them, or at least the amino acids that became their building blocks. In any event, microbial life was abundant on Earth by 3.8 billion years ago, when the Solar System had reached an age of about 800 million years (Arndt & Nisbet 2012). It has survived ever since.

The emergence of multicellular organisms, however, is a far more recent phenomenon. The Ediacaran Period, which was under way just 575 million years ago, when the Earth was already 4 billion years old, marks “the appearance of the first large and architecturally complex organisms in Earth history” (Narbonne 2005). The Ediacaran was immediately followed by the better-known Cambrian Period around 540 million years ago. Modern humans, of course, are no older than about 100,000 years or so. All our works represent a thin veneer on a very ancient world.

Because simple organisms appeared in the Solar System well within the first billion years after its creation, similar life forms may also have emerged elsewhere in the system. For example, the early environments of Venus and Mars may have been much more biophilic than they are at present, even if nothing could survive on either planet today. Still more hopeful speculations have addressed the icy moons of Jupiter and Saturn, some of which – Europa, Ganymede, Enceladus – may support bodies of liquid water beneath their frozen crusts. If they have water today, then maybe they have microbes also. Finally, the unique environment of Titan – Saturn’s largest moon, and the only Solar System object besides Earth that is known to support surface bodies of liquid – may hold still more surprises in store for us. Nothing resembling Earthly organisms could live in such a frigid place, whose rivers run with ethane, and whose lakes are liquid hydrocarbons. Nonetheless, it is conceivable that some as yet unimagined form of life may be drinking that liquid or floating down those streams.

Then there are the hundreds and hundreds of exoplanetary systems waiting for us in the Sun’s back yard. It’s becoming progressively clearer that very few of them resemble our home system (see How Weird Is Our Solar System?). The formation scenarios presented here – Grand Tack, Terrestrial Birth Ring, Late Heavy Bombardment – explain why our system is so unusual. These theoretical constructs detail a highly specific evolutionary history that is unlikely to be repeated with much regularity around other stars. Again, however, the sheer abundance of worlds points to a similar abundance of potential sites for life – life that is weird, unearthly, perhaps even unrecognizable – throughout the Milky Way.

Figure 14. Dickinsonia costata. Fossil imprint of an Ediacaran organism,
dating from about 560 million years ago. Credit: Wikimedia

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