Thursday, August 28, 2014

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


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

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:
Bottke WF, Vokrouhlicky D, Minton D, Nesvorny D, Morbidelli A, Brasser R, Simonson B. (2011) The Great Archean Bombardment, or the Late Late Heavy Bombardment. Abstract 2591, 42nd Lunar and Planetary Science Conference.
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.
Bottke WF, Vokrouhlicky D, Minton D, Nesvorny D, Morbidelli A, Brasser R, Simonson B, Levison HF. (2012b) An Archaean heavy bombardment from a destabilized extension of the asteroid belt. Nature 485, 78-81.
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:
Hand E. (2008) The hole at the bottom of the Moon. Nature 453, 1160-1163.
Levison HF, Morbidelli A, Tsiganis K, Nesvorny D, Gomes R. (2011) Late orbital instabilities in the outer planets induced by interaction with a self-gravitating planetesimal disk. Astronomical Journal 142, 152.
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.
Morbidelli A, Marchi S, Bottke WF. The sawtooth-like timeline of the first billion year of lunar bombardment. (2012a) Abstract 4014, Early Solar System Impact Bombardment II.
Morbidelli A, Marchi S, Bottke WF, Kring DA. (2012b) A sawtooth-like timeline for the first billion years of lunar bombardment. Earth and Planetary Science Letters 355, 144-151. Abstract:
Ryder G. (1990). Accretion and Bombardment in the Early Earth-Moon System: The Lunar Record. Abstracts for the International Workshop on Meteorite Impact on the Early Earth, LPI Contribution 746. Lunar and Planetary Institute.
Ryder G. (2002). Mass flux in the ancient Earth-Moon system and benign implications for the origin of life on Earth. J Geophys Res Planets 107, E45022.