Evolutionary Twist

Figure 1. The Sun-like star WASP-47 hosts a transiting Hot Jupiter flanked by two low-mass planets, and all three have periods shorter than 10 days. This was the first and so far the only detection of such a configuration. Above, an artist’s rendering shows a Hot Jupiter in transit across the face of its star. Credit: European Southern Observatory

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A few months back I wrote about WASP-47b, the first Hot Jupiter ever found with low-mass planets on nearby orbits. I noted my eagerness to see a theoretical study of the possible formation mechanisms behind this unusual system, while venturing that the orbital architecture of WASP-47 could not have resulted from in situ accretion.

Then last month I took a close look at a study by Konstantin Batygin & Greg Laughlin (2015), who offered a novel explanation for the formation of the inner planets of the Solar System. Their approach folds the evolutionary history of the four terrestrial planets into the Nice Model and the Grand Tack. I observed in passing that, although Laughlin was one of the earliest proponents of in situ theory (which underlies the scenario that he and Batygin proposed), he hadn’t written anything purely theoretical on this topic for quite a while.

Two new preprints respond serendipitously to those conjectures and observations. One is a theoretical study by Batygin, Bodenheimer & Laughlin (2015) on the potential for Hot Jupiters – Hot Jupiters, no less! – to form in situ. The authors develop a new formation scenario for star-hugging gas giants and use numerical simulations to test it. Then they appeal to the WASP-47 system as possible evidence that this mechanism really works.

The second preprint is also highly theoretical. Nathan Kaib and John Chambers (2015) tersely present the results of a large ensemble of numerical simulations designed to test the Nice Model of Solar System evolution. Their findings question the utility of this approach as an explanation for the Late Heavy Bombardment. They can also be interpreted as evidence that our four terrestrial planets formed after, not before, the dynamical instability addressed by the Nice model. Although Kaib & Chambers don’t mention the recent articles by Batygin & Laughlin or Volk & Gladman on the inner Solar System, their conclusions call for rethinking both the Nice model and the Grand Tack.

a burning cradle for baby giants?

The new study by Batygin & colleagues (hereafter B15) is called “In situ formation and dynamical evolution of Hot Jupiter systems.” The title is a misnomer, however, as they don’t actually model the in situ accretion of solids on orbits where Hot Jupiters are observed. Instead, as often happens in simulation studies, they use simplified initial conditions to examine a narrowly defined question: Can a dense planet of sufficient mass, orbiting at or near the inner edge of a protoplanetary nebula, accrete enough ambient hydrogen to balloon into a gas giant?

B15 offer four representative evolutionary sequences in which solid planetary cores at a range of masses follow circular orbits around a Sun-like star at a semimajor axis of 0.05 AU. They “purposely adopted an agnostic viewpoint” regarding formation mechanisms, explaining that their results “are largely independent of how the solid core arises.” The simulations seem equally friendly to x) strict in situ formation involving the accretion of local mass only; y) migration of solids from cooler orbits into the hot inner nebula and subsequent accretion at the new location; or z) migration of fully-formed cores from cooler orbits.

In units of Earth mass (Mea), the core masses they select are 4 Mea, 10 Mea, and 15 Mea. The object of 15 Mea is used in two different set-ups, one with low ambient gas density and the other with higher density. As a reality check, I note that atmosphere-free planets of 10 Mea or more are not attested (Jontof-Hutter et al. 2015), while realistic models of planet formation have great difficulty forming an object of 4 Mea or more at 0.05 AU, where silicate dust sublimates (Dullemond & Monnier 2010).

Regarding gas accretion, B15 concede that insufficient mass in ambient gas would be available in the local feeding zone to build a gas giant at 0.05 AU. Therefore, they assume that the local supply of hydrogen/helium is constantly replenished by the regular flow of gas through the nebula onto the infant star. Their model seems to hinge on the capacity of the short-period planet to capture this inrushing gas.

Across simulations, objects of 15 Mea were the only ones that managed to accrete gas envelopes equal in mass to their solid cores and thereby trigger runaway gas accretion. Thus, they find that the minimum core mass capable of growing into a gas giant in the vicinity of a Sun-like star is 15 Mea.

Figure 2. Light curves of the three inner planets of WASP-47

Phase-folded light curves of the three transiting planets detected by the K2 mission: Becker et al. 2015

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B15 conclude that their scenario will regularly produce gas giants, so that in situ formation might “represent the dominant channel for hot Jupiter generation.” They proceed to a discussion of WASP-47b as an example of what this channel might bring.

But I’m not convinced. The likeliest origin for an object of 15 Mea with a semimajor axis around 0.05 AU is migration from a wider orbit where accretion can more readily produce such massive objects. As far as I know, a gas giant can migrate into a hot orbit just as easily as a Neptune-mass object can. So why resort to a two-step process – first migrate the core, then capture the atmosphere – when a well-theorized one-step process (Type II migration) works even better? B15 appear to be offering a solution in search of a problem.

They cite WASP-47 as a rare system in which a compact family of Super Earths somehow managed to promote one of their number to giant status without changing any semimajor axes or otherwise disrupting the rest of the family. But it seems that considerable acrobatics would be needed to achieve this remarkable outcome. For example, how did only one planet get pumped up? What kept the others down?

I suggest that, along with exploring novel in situ explanations for the evolution of WASP-47, the exoplanet community should test whether migration scenarios could explain this architecture with fewer contortions.

Figure 3. The wreck of the Kronan

In 1676, the Swedish warship Kronan capsized while trying to execute a sharp turn under too much sail, causing an explosion in the gunpowder magazine. Few survived. Illustrated is an imaginative (and historically inaccurate) rendering of the disaster by Claus Moinichen in 1686.

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does the sailing master have no clothes?

The study by Kaib & Chambers is conducted on a grander scale than B15. They ran almost 300 high-resolution simulations of Solar System evolution to test three different versions of the Nice model and explore the effects of the resulting dynamical instability on the four terrestrial planets. A simulation was considered successful if the final system met three criteria after the instability concluded: A) all four outer planets from Jupiter to Neptune survived, with the orbits of Saturn and Jupiter lying between a 2:1 and 3:1 mean motion resonance; B) all four inner planets also survived on stable orbits; and C) the angular motion deficit of the four inner planets was less than or equal to its present value after their orbits were integrated for an additional billion years.

In all three versions of the initial set-up, the four terrestrial planets were assigned their present masses and orbits. In two versions, the outer system contained Jupiter, Saturn, and three Neptune-like planets. In the third, it contained Jupiter, Saturn, two Neptune-like planets, and two “Super Earths” of 8 Mea each. Among all iterations of these set-ups, the success rate according to Criterion A was 13% to 16%. However, the set-up with six outer planets failed both of the other criteria, while the other two models produced just 1 and 2 systems, respectively, that met Criterion B. Only one iteration of one of these models also met Criterion C. All other iterations lost at least one terrestrial planet (usually Mercury), and many lost two (usually Mercury and Mars).

These results are very likely to inform future discussions of the Nice Model. As Kaib & Chambers conclude:

[W]e find a probability of 1% or less that the orbital architectures of the inner and outer planets are simultaneously reproduced in the same system. These small probabilities raise the prospect that the giant planet instability occurred before the terrestrial planets had formed. This scenario implies that the giant planet instability is not the source of the Late Heavy Bombardment and that terrestrial planet formation finished with the giant planets in their modern configuration.

Notably, the recent study by Volk & Gladman (2015) proposed an approach to the evolution of the four terrestrial planets that dispensed with the Nice Model altogether, suggesting an alternative explanation for the Late Heavy Bombardment. Although I don’t find their alternative persuasive, I hope these efforts inspire further evolutionary studies to address the problem of our denuded inner system.

In the meantime, the findings of Kaib & Chambers might benefit from reframing. As discussed in earlier blog posts, the phenomenon known as the Late Heavy Bombardment or Lunar Cataclysm can be understood in different ways. In one view, it was a self-contained period of heavy cometary impacts that were preceded and followed by long stretches of relative calm; its peak came about 3.8 billion years ago (Gya), at a system age of ~800 million years (Gomes et al. 2005). In another, it was simply one episode in a long, saw-toothed series of bombardments that began with Earth’s accretion and subsided only toward the end of the Archean period, about 2.5 – 3 Gya (Morbidelli et al. 2012, Marchi et al. 2014). Instead of triggering a late and relatively short-lived catastrophe, maybe the dynamic instability happened in the first 100 million years of Solar System history and inaugurated that long and violent epoch of bombardments. The Nice Model would still explain how the massive outer planets nudged and jostled one another into their mature (and usually wider) orbits.

Figure 4. Overview of Earth history

Again, maybe the failure of a single model to explain the final architecture of the inner as well as the outer Solar System is another clue that we need a new picture of inner system evolution. What if the Grand Tack triggered an inner system catastrophe, as modeled by Batygin & Laughlin (2015), but the resulting mess did not immediately resolve into a four-planet system? Maybe the battle of oligarchs that followed Jupiter’s retreat from the Sun’s inner territories was still raging while Neptune was decimating the outer realms. Maybe lasting peace in the inner system was contingent on resolving tensions among the outer worlds, a resolution that evidently hinged on the exile of a Uranus-type planet. Maybe the final detente between Jupiter and Saturn was what enabled Earth to dominate its three smaller siblings.

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REFERENCES

Batygin K, Laughlin G. (2015) Jupiter’s decisive role in the inner Solar System’s early evolution. Proceedings of the National Academy of Sciences 112, 4214-4217. Abstract: 2015PNAS..112.4214B

Batygin K, Bodenheimer P, Laughlin G. (2015) In situ formation and dynamical evolution of Hot Jupiter systems. In press. Abstract: 2015arXiv151109157B

Becker J, Vanderburg A, Adams F, Rappaport S, Schwengler H. (2015) WASP-47: A Hot Jupiter system with two additional planets discovered by K2. Astrophysical Journal Letters 812, L18. Abstract: 2015ApJ...812L..18B

Dullemond CP, Monnier JD. (2010). The inner regions of protoplanetary disks. Annual Review of Astronomy & Astrophysics 48, 205-239.

Gomes R, Levison HF, Tsiganis K, Morbidelli A. (2005) Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature, 435: 466-469. Abstract: 2005Natur.435..466G

Jontof-Hutter D, Ford EB, Rowe JF, Lissauer JJ, Fabrycky DC, Christa Van Laerhoven5, Agol E, Deck KM, Holczer T, Mazeh T. (2015) Robust TTV mass measurements: Ten Kepler exoplanets between 3 and 8 M_earth with diverse densities and incident fluxes. In press.

Kaib NA, Chambers JE. (2015) The fragility of the terrestrial planets during a giant planet instability. Monthly Notices of the Royal Astronomical Society. In press. Abstract: http://arxiv.org/abs/1510.08448

Marchi S, Bottke WF, Elkins-Tanton LT, Bierhaus M, Wuennemann K, Morbidelli A, Kring DA. (2014) Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. Nature 511, 578-582. 2014Natur.511..578M

Morbidelli A, Marchi S, Bottke WF, Kring DA. (2012) A sawtooth-like timeline for the first billion years of lunar bombardment. Earth and Planetary Science Letters 355, 144-151. Abstract: http://adsabs.harvard.edu/abs/2012E%26PSL.355..144M

Volk K, Gladman B. (2015) Consolidating and crushing exoplanets: Did it happen here? Astrophysical Journal Letters, 806: L26. Abstract: 2015ApJ...806L..26V

Mighty Throxeus

Imaginary view of ancient Mars, ca. 4.4 billion years ago, with clouds and an ocean.
Composite of NASA and NOAA images.

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Just catching up with the big news about that ocean on Mars! As many of you probably know already, successive robotic missions have returned plenty of evidence that water once flowed on the Red Planet. The evidence includes dry riverbeds, deltas, and gullies photographed by orbiters; sedimentary rocks photographed by surface rovers; and hydrated minerals detected through direct analysis by rovers. In fact, it’s widely believed that Gale Crater, the landing site of the Curiosity rover, was once a freshwater lake.

Nevertheless, most of these surface features could be explained by intermittent episodes when surface water flowed briefly and then sublimated or froze. Such activity might be related to asteroid impacts or volcanic eruptions. Given this possibility, the evidence so far available has not offered unambiguous proof that Mars ever supported open bodies of water for biologically significant stretches of time.

Channels of ancient riverbeds on Mars. (Source unknown)

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Now an international team led by Geronimo Villanueva argues that ancient Mars had an ocean that persisted for hundreds of millions of years. They base their findings on an analysis of the relative abundance of water isotopes in the Martian atmosphere, as measured through spectrographic observations by telescopes on Earth. Compared to water down here, water on Mars is enriched in deuterium. Villanueva and colleagues interpret this enrichment as evidence that most of the original reservoir of water on Mars has dissipated into space. They propose that, not long after Mars formed out of the Solar nebula, it supported an ocean covering about 20% of the surface, and as deep as the Mediterranean Sea. They estimate that the ocean was near its full extent about 4.5 billion years ago, but had lost a substantial amount of water by 4.1 billion years ago. On the basis of the current topography of Mars, they hypothesize that water covered much of the planet’s northern hemisphere, concentrated on a region called the Vastitas Borealis.

Layered buttes on the slopes of Mount Sharp on Mars. Credit: NASA/JPL-Caltech

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The study appeared in Science magazine and was accompanied by lots of media activity. A prime example is this nifty NASA video on YouTube, in which Villanueva and his colleague Michael Mumma speak optimistically about the habitability of ancient Mars. Below are examples of the many related image captures that flooded the Internet:

Two views of ancient Mars in the heyday of the northern ocean, based on the findings of
Villanueva et al. 2015. Credit: NASA/JPL-Caltech

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Two prerequisites for surface water on any planet are appropriate temperatures to maintain the liquid state and sufficient atmospheric pressure to prevent sublimation. Currently, the atmospheric pressure on Mars is less than 1% of Earth’s, so liquid water is impossible even at optimal temperatures. How ancient Mars might have harbored an atmosphere thick enough to produce the pressure necessary to sustain an ocean is an open question.

Another key question is the timing of events either known or hypothesized for the early history of Mars. Several lines of evidence indicate that all four terrestrial planets formed in a ring of planetesimals located between 0.7 and 1.0 astronomical units (AU) from the Sun (Hansen 2009). Exactly how this “birth ring” acquired its unique geometry is uncertain, but one hypothesis is that the formation and migration of Jupiter and Saturn during the first few million years of their existence depleted the inner regions of the Solar nebula and sculpted this narrow annulus (Brasser 2013). After the dissipation of the nebular gas – a process that occurred around 4 to 6 million years after the Sun’s ignition – protoplanets forming in the birth ring engaged in violent interactions that led to orbital perturbations and collisions. The small mass and eccentric orbit of Mars indicate that the planet was ejected from the ring relatively soon after the nebular gas dispersed, preventing further accretion of planetesimals. Accordingly, Ramon Brasser describes Mars as “essentially a planetary embryo that never grew to a full-fledged planet.” The planet acquired most of its mass during the first 4 million years of Solar System history, while the nebular gas was still present, and reached its final mass at a system age of about 10 million years. Thus Mars is older than the Earth, which evidently did not achieve its final mass until the Moon-forming event at a system age of about 30 to 50 million years.

By studying the effect of Jupiter and Saturn’s orbital excursions on the Solar birth ring, Brasser concludes that abundant water was available in the ring’s constituent planetesimals. Many of these would have been scattered inward from regions of the Solar nebula where ices were abundant. Given its wider orbit, Mars likely formed with an even larger bulk fraction of water than Earth, about 0.1% to 0.2% of its overall mass. Since Mars is only 11% as massive as Earth, it would have cooled much faster, enabling the condensation of a global ocean as soon as atmospheric conditions permitted. Nevertheless, Brasser notes that Mars’ gravity was too weak to retain all that water over billion-year time scales, although he does not address the process of mass loss.

As Villanueva and colleagues argue, the global Martian ocean was largely confined to the northern hemisphere, where the crust is 30 kilometers thinner than in the south and the average elevation is much lower (Marinova et al. 2008). This long-recognized north/south asymmetry is typically known as the Martian Hemispheric Dichotomy (or some variation thereof). Steep scarps and concentric massifs mark the elliptical boundary between the southern highlands and the northern basin, which is centered on a point 67 degrees N, 208 degrees E (Andrews-Hanna et al. 2008).

It is now widely accepted that this dichotomy was created by a “mega-impact” early in Martian history. Marinova and colleagues (2008) propose that the impactor struck at an angle of about 45 degrees and had a diameter of 1600-2700 km; it was notably smaller than our Moon (diameter 3474 km/2154 mi) and intermediate in size between Titania (1578 km), a moon of Uranus, and Triton (2706 km), Neptune’s largest moon. As for the timing of this catastrophe, Andrews-Hanna and colleagues describe the dichotomy as “one of the most ancient features on the planet.” Nimmo and colleagues (2008) infer that the collision occurred within the first 100 million years of Mars’ existence, “within the same time window as the Moon-forming impact on Earth.” They further argue that the southern highlands of Mars solidified out of the planet’s primordial magma ocean, while the northern lowlands arose from shock melting in the mantle. If the Martian mantle was rich in water, as Brasser concludes, it could have been the source of the northern sea.

In any event, it’s worth speculating how an ocean on Mars would have fared during the Noachian aeon (4.5-3.5 billion years ago), the Martian period that coincided roughly with the Hadean (4.5-4.0 billion years ago) and Archean (4.0-2.5 billion years ago) aeons on Earth, as well as the system-wide cataclysm known as the Late Heavy Bombardment (4 billion years ago). Two different pictures are available for this period on Earth.

In the happier picture, a global ocean formed within a few hundred million years of the Earth’s cooling and persisted ever afterward. Although asteroids frequently struck during the Hadean, the impacts were insufficient to resurface the Earth, and even the Late Heavy Bombardment was never intense enough to boil away the ocean. Thus life could have formed early and survived the epoch of asteroid strikes. In the harsher picture, however, a global ocean formed quickly, but was constantly subjected to cataclysms from the sky. Between 4.5 and 4.1 billion years ago – the era when the northern sea billowed on Mars, according to Villanueva and colleagues – Earth was probably struck by one to four objects with diameters of 1000 km (620 miles) or more, and by three to seven with diameters of 500 km (310 miles) or more (Marchi et al. 2014). Each of the larger objects would have resurfaced the whole planet and sterilized the biosphere; each of the smaller ones would have evaporated all surface water. It’s difficult to reconcile the long-lasting Martian ocean proposed by Villanueva’s group with the cruel bombardment envisioned by Marchi’s group, since the impactors originated in the asteroid belt and would have hit Mars just as hard as Earth.

As I look forward to more research that might solve these mysteries, I want to suggest a name for the hypothetical Martian ocean, regardless of its ultimate relation to reality. Although Latin could provide Mare Boreale, the North Sea, or Oceanus Borealis, the Northern Ocean, I prefer to use a bona fide Martian source: the Barsoomian language, spoken by the imaginary inhabitants of Mars in a series of books by Edgar Rice Burroughs (1875-1950). Burroughs’ Martians preserved historical records of the existence of five seas on Barsoom, back in the days when a pale race known as the Orovar circumnavigated the planet and built ornate treasure cities. Of all these marinered seas, everyone agreed that the mightiest was Throxeus. What better name for the vast body of water proposed by Villanueva’s group?

Planisphere of ancient Barsoom, the imaginary version of Mars created by Edgar Rice Burroughs in the eleven novels of his John Carter series (originally published 1912-1941). The largest body of water is Throxeus, mighty ocean of the north. Credit: Oberon Zell and Ralph Aeschliman.

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References to Throxeus appear many times in the Martian series. Below is a typical example from The Chessmen of Mars (1922). Believe it or not, these words were spoken by a diamond-studded prince from the wealthy land of Gathol while attempting to flirt with a haughty, raven-haired princess of Helium:

“Your ancient history has doubtless told you that Gathol was built upon an island in Throxeus, mightiest of the five oceans of old Barsoom. As the ocean receded Gathol crept down the sides of the mountain, the summit of which was the island upon which she had been built, until today the bowels of the great hill are honeycombed with the galleries of her mines.”

All that remained of Throxeus in Burroughs’ universe were dead sea bottoms overgrown with ochre moss. But in the map shown above, which offers a glimpse of ancient Barsoom, Gathol is visible as an offshore island amid a still-thriving Throxeus. Now that we’ve found traces of that lost ocean, maybe it’s time to send a robot and see if the diamond mines are there, too.

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REFERENCES

Andrews-Hanna JC, Zuber MT, Banerdt WB. (2008) The Borealis basin and the origin of the martian crustal dichotomy. Nature Letters 453, 1212-1215.

Brasser R. (2013) The Formation of Mars: Building Blocks and Accretion Time Scale. Space Science Reviews 174, 11-25.

Burroughs ER. (1922) The Chessmen of Mars. New York: A.C. McClurg.

Hansen B. (2009) Formation of the terrestrial planets from a narrow annulus. Astrophysical Journal 703, 1131-1140. Abstract: http://adsabs.harvard.edu/abs/2009ApJ...703.1131H

Marchi S, Bottke WF, Elkins-Tanton LT, Bierhaus M, Wuennemann K, Morbidelli A, Kring DA. (2014) Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. Nature 511, 578-582. Abstract:  http://adsabs.harvard.edu/abs/2014Natur.511..578M

Marinova MM, Aharonson O, Asphaug E. (2008) Mega-impact formation of the Mars hemispheric dichotomy. Nature Letters 453, 1216-1219.

Nimmo F, Hart SD, Korycansky DG, Agnor CB. (2008) Implications of an impact origin for the martian hemispheric dichotomy. Nature Letters 453, 1220-1223.

Villanueva GL, Mumma MJ, Novak RE, Käufl HU, Hartogh P, Encrenaz T, Tokunaga A, Khayat A, M. D. Smith MD. (2015) Strong water isotopic anomalies in the martian atmosphere: Probing current and ancient reservoirs. Science Express 5 March 2015 / 10.1126/science.aaa3630 

A Billion Years of Hell on Earth

Figure 1. Artist’s impression of Earth during the Hadean aeon, when bombardment by asteroids and comets was frequent. Image credit: BBC

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The Earth is 4.55 billion years old, but microbial life is not attested until about 3.8 billion years ago. What was happening between those two milestones, during the period known as the Hadean? The very name derives from Hades, evoking a hellish environment with constant asteroid strikes and blazing lava flows. Yet an earlier posting on this blog reported studies arguing for more benign conditions in this aeon.

Both Arndt & Nisbet (2012) and Abramov & Mojzsis (2009) have argued that the Earth had cool seas and landmasses during the later Hadean. Thereafter came the Late Heavy Bombardment, a dynamical instability in the outer Solar System that created a system-wide storm of asteroids and comets. Their impact scars are still visible on the surface of the Moon, Mars, and Mercury. Nevertheless, according to several studies, none of the asteroid strikes were sufficient to boil off the global ocean or sterilize the entire crust. Arndt & Nisbet even offered a figure to illustrate this appealing revision of Earth history.

Figure 2. The first figure in Arndt & Nisbet’s 2012 review of life on the young Earth. Panel a is a detail of a painting by Chesley Bonestell, a noted space artist of the mid-twentieth century. 

Figure 3. The painting by Bonestell, shown above left, was featured on the cover of Life Magazine (with a horizontal flip) in 1952. The glowing sphere on the horizon is the newborn Moon. 

Now a new study of the Hadean returns us to a picture very similar to Chesley Bonestell’s rendition. The Hadean really was Hell on Earth, and both the convulsions on land and the rain of fire from the sky continued – with irregular breaks – for at least half a billion years after the birth of the Earth and Moon. Last month in Nature, Simone Marchi and colleagues reported the results of impact simulations based on the cratering record of the inner Solar System during the Earth’s first billion years. (Check out this related video.) They argued that between 4.5 and 4.1 billion years ago (Gya), our planet was struck by one to four objects with diameters of 1000 km (620 miles) or more, and by three to seven with diameters of 500 km (310 miles) or more. The larger objects were similar to Ceres in size; the smaller were similar to Vesta. Each of the Ceres-sized objects would have resurfaced the planet and sterilized the entire biosphere, if any existed. Each of the Vestas would have evaporated all surface water.

The Hadean cataclysms gave way to the Late Heavy Bombardment, the final major episode of resurfacing for our Moon, and by extension the final major asteroid catastrophe for Earth. Most studies over the past twenty-odd years have characterized this epoch as a relatively brief spike in impacts around 3.9 Gya (e.g., Ryder 1990, Hand 2008). However, Marchi’s group places its onset around 4.1 Gya and argues that, rather than a single spike, both large and small impacts continued intermittently for hundreds of millions of years. Even the small impacts in this new model were on the scale of the Chicxulub event (popularly known as the Dinosaur Killer). In this picture, the Earth had no long-lasting geological peace until well into the Archean. 

Figure 4. Basic timeline of Earth history

from cruel to kind and back again: evolving views of the hadean

Before the Apollo mission of the later twentieth century, scientific views of Earth’s infancy were simple. Having coalesced out of crashing planetesimals, the newborn planet radiated heat until it cooled enough to support oceans and continents. Thereafter life evolved, most likely in the sea, and eventually colonized the land. (Continental drift and plate tectonics were still crackpot ideas in the 1960s.)

Given these assumptions, analyses of Moon rocks collected by the Apollo astronauts brought a big surprise. Isotopic dating indicated that the Moon was completely resurfaced by asteroid strikes around 3.8 Gya. Whatever bombardment the Moon suffered must have been even worse on Earth, a much larger and more gravitationally attractive target. The simple picture of a steadily cooling planet had to be discarded. At least two replacements are available: 1) after the Earth formed, asteroid impacts steadily declined over half a billion years, then climaxed in an intense but brief bombardment, or 2) an irregular but unrelenting rain of impactors battered the Earth until the outer planets settled into stable orbits, and then the storm concluded with a final roar, or maybe just a slow retreat.

The first alternative has been widely endorsed; Morbidelli et al. 2012b provide a summary of perspectives since the 1970s. Several publications over the past few decades present a consistently benign view of the Hadean (Ryder 1990, 2002; Gomes et al. 2005; Hand 2008; Arndt & Nisbet 2012). Between 4.4 and 4.0 Gya, according to these studies, impacts were infrequent and the global environment was probably friendly to the emergence of life. A more cataclysmic bombardment began around 3.9 Gya, but even then the flux of impactors was “comparatively benign” (Ryder 2002), lasting only about 80 million years. This bombardment was insufficient to boil off the oceans or sterilize the crust. Thus life could have evolved at any point after 4.4 Gya, and once it did, it would likely survive all subsequent bombardments.

A seminal study by Gomes and colleagues (2005) wove the lunar cataclysm into the dynamical history proposed by the Nice model of Solar System formation. At the time of publication, all four authors were associated with the Observatoire de la Côte d’ Azur in Nice. Using numerical simulations, they argued that Jupiter and Saturn migrated into wider orbits during the Hadean, ultimately triggering a gravitational instability in the outer system. Objects from the Kuiper belt beyond Neptune and the asteroid belt inside Jupiter’s orbit were scattered into the inner Solar System, producing the Late Heavy Bombardment. The Nice group placed the peak of this event around 3.8 Gya and estimated its duration between 30 and 150 million years. From beginning to end, this instability depleted the asteroid belt by a factor of 10.

In 2011, the same group – now augmented by David Nesvorny – presented a revised version of the model, which they dubbed Nice II (Levison et al. 2011). Although it changes the initial orbital configuration of the outer Solar System, Nice II retains a similar timeline for the gravitational instability of the giant planets, as well as for the Late Heavy Bombardment.

Nonetheless, in the same year, Morbidelli and Nesvorny contributed to another study of the Late Heavy Bombardment that offered a very different perspective on this event (Bottke et al. 2011). The bombardment was no longer a brief spike but an extended cataclysm that continued throughout the Archean aeon. A key element in this new model was a putative extension of the classical asteroid belt – dubbed the “E-belt” – millions of kilometers inward to the vicinity of Mars. This E-belt was proposed as the source of the impactors that resurfaced the Earth and Moon. With successive revisions, including the addition of Simone Marchi to the author group (Bottke et al. 2011; Bottke et al. 2012a, 2012b; Morbidelli et al. 2012a, 2012b), this model developed into the headline-grabber just published in Nature.

time’s toothy saw

In place of a curve or spike, Morbidelli’s group offers a saw – more specifically, a sawtooth-shaped timeline in which impacts were intermittent, rarely cataclysmic, and progressively milder. In 2012, they presented this summary:

“In our sawtooth view, big impactors hit over an extended period, with more lulls and therefore more opportunities for the Hadean-era biosphere to recover.” (Morbidelli et al. 2012b) “Life might have formed early in the Earth’s history and survived from that time.” (Morbidelli et al. 2012a).  

Two years later, though, their perspective has become less sanguine. Marchi et al. (2014) argue that every centimeter of the Earth’s surface was jolted by massive impactors before 4 Gya, and that potential sterilizing events recurred throughout the Hadean. Early Earth is once again an alien and threatening place.

Figure 5. The latest perspective on Hadean Earth. Image credit: Simone Marchi

REFERENCES

Abramov O, Mojzsis SJ. (2009) Microbial habitability of the Hadean Earth during the late heavy bombardment. Nature 459, 419-422.

Arndt NT, Nisbet EG. (2012) Processes on the young Earth and the habitats of early life. Annual Review of Earth & Planetary Sciences 40, 521-549. Abstract: http://adsabs.harvard.edu/abs/2012AREPS..40..521A

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Bottke WF, Vokrouhlicky D, Minton D, Nesvorny D, Morbidelli A, Brasser R, Simonson B. (2012a) The Great Archean Bombardment. Abstract 4036, Early Solar System Impact Bombardment II.

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