The Clementine spacecraft took this portrait of the Moon’s near side in the mid-1990s. Bright lines emanate from Tycho crater (near bottom). Credit: Stereograph frame courtesy of R. Cowen; Inset Images: NASA

Gone. Vanished. Lost.

When it comes to the early history of the solar system, planetary scientists must contend with a case of nearly systemwide amnesia.

Although the solar system formed nearly 4.6 billion years ago, researchers have a pretty good record that goes back only 3.9 billion years. Yet those first 700 million years proved critical to all that followed. That’s when the planets coalesced and water and other compounds essential to life were delivered to the inner planets.

What’s more, according to a leading theory now being explored in detail, that early era was capped by a truly cataclysmic event. About 3.9 billion years ago, the movement of the most massive planets dramatically rearranged the outer solar system. The shifting planets freed rocky and icy bodies from the solar system’s edge, commencing a bombardment of the entire retinue of planets.

Filling in the details of this violent era in the solar system’s development has met serious obstacles. On Earth and many of the other planets, billions of years of volcanic eruptions, quakes, erosion and burials have all but erased solid evidence of the solar system’s earliest chapters. But Earth’s crater-scarred moon, quiescent and lacking an atmosphere that could destroy incoming space debris, appears to be a rare and nearby exception.

Now new observations and reanalyses of old moon data, along with progress from theorists, have renewed interest in reconstructing the events of the solar system’s preadolescence.

“If we don’t understand the [earliest] years of the solar system, then we don’t really understand how the planets formed, and where we came from,” says Bill Hartmann of the Planetary Science Institute in Tucson. “And that applies to both biology and how planetary systems look, and what fraction of the planets [beyond the solar system] might be habitable.”

Planetary scientists, says Bill Bottke of the Southwest Research Institute’s Boulder, Colo., office, had initially conjectured that the solar system grew up in a hurry, “with everything you see today already in place just a few million years after the solar system’s birth. But we’re now considering the possibility that the solar system literally rearranged itself about 3.9 billion years ago.”

At the Lunar and Planetary Institute in Houston last November, theorists and observers convened for a rare meeting in which they hashed out what each had gleaned about the solar system’s early history. “We have two communities coming at the same problem from very different perspectives,” says Don Bogard of NASA’s Johnson Space Center in Houston.

At the meeting, the observers presented new analyses of data first gathered nearly 40 years ago when the Apollo spacecraft landed on the moon and brought back moon rocks — one of the best preserved records of the solar system’s tumultuous first 700 million years. Theorists presented their latest version of a theory that could account for several unexplained features of the solar system, including the violent era between 4 billion and 3.9 billion years ago known as the late heavy bombardment, when the planets were pelted with debris. And researchers reported evidence that if life had existed on Earth before that, the cataclysm might not have wiped out all organisms but could have spared primitive forms that thrived in hot, water-rich environments.

Bombardment of data

Many of the new studies focus on events that gave the final touches to the architecture of the solar system — events that took place just a few hundred million years after the planets had coalesced from the primordial disk of gas, dust and ice believed to have swaddled the infant sun. The planet-forming process probably took only a few tens of millions of years, and by 50 million to 60 million years after the birth of the solar system, the orbs had pretty much grown to their present size.

Hartmann says it’s unclear whether the solar system suffered a high rate of bombardment by space debris continuously during its first 700 million years or whether activity suddenly spiked at the 700-million–year mark. But mounting evidence points to an abrupt lunar smack down. Several huge impact basins on the moon, dating to about 3.9 billion years ago, bear witness to this bombardment.

“When the Apollo astronauts brought back samples, it was quickly realized that all the circular features up there were in fact impact craters,” says David Kring of the Lunar and Planetary Institute. Moreover, the ages of a large variety of moon rocks date from 4 billion to 3.9 billion years ago. In addition, the rocks suggest the moon’s crust underwent intense heating in the same time period.

The combined evidence prompted researchers to coin the phrase “lunar cataclysm” for the pummeling the moon apparently received.

Initially, some researchers worried that because all the Apollo craft, as well as the Soviet Luna missions, had landed in the same general area on the moon, the lunar samples collected could reveal what had happened only over a small region, about 4.5 percent of the lunar surface.

But that earlier criticism, Kring says, was superseded by a mother lode of lunar meteorites found in Antarctica in the 1990s. Those chunks of rock, presumably from all parts of the moon, showed the same time signature as the Apollo rocks, Barbara Cohen, now at NASA’s Marshall Space Flight Center in Huntsville, Ala., and her colleagues found.

Click here for larger image. Credit: Illustration: Mark A. Gerlick /; Graph: D. Kring; Simulations: Adapted from Gomes et al./Nature 2005

Birth of a cataclysm
A simulation of the outer solar system’s early evolution known as the Nice model tracks the planets’ orbits from soon after the birth of the solar system through the period of late heavy bombardment and beyond. At the simulation’s start, a massive reservoir of icy debris, a forerunner of today’s Kuiper Belt, lies beyond the outer planets.

4.5 billion years ago
Early on, the four outermost planets follow circular orbits, packed closely together within a large disk of icy debris (shown in gray), leftovers from planet formation.

3.9 billion years ago
As the planets spread out, the orbits of Jupiter and Saturn fall into lockstep. Ultimately, the changed orbits of these two giants greatly alter the orbits of Neptune and Uranus.

3.9 billion – 3.8 billion years ago
Uranus and Neptune ram into the icy reservoir. The debris zooms into the inner solar system, where it catastrophically collides with the moon, Earth and the other planets (artist’s depiction at far left).

3.65 billion years ago
The outer solar system settles into its current configuration. The impacts left behind on the moon are now providing clues to the history of the solar system.

Theory enters the act

Early this decade, theorists Harold Levison of the Southwest Research Institute’s Boulder office and his colleagues, including Alessandro Morbidelli of the Côte d’Azur Observatory in Nice, France, were puzzling over a different problem. They were trying to understand the formation and evolution of the outer solar system. The team’s computer simulations showed that two of the planets plowed into a ring of icy leftovers from the planet-forming process, scattering the debris. This disruption and its aftermath, the researchers later realized, would have had a profound effect on the entire system.
At the Lunar and Planetary Institute meeting, Levison and Bottke presented the latest version of the theory, known as the Nice model (SN: 5/28/05, p. 340) because of Morbidelli’s contributions.

In the theory, the four biggest planets — Jupiter, Saturn, Neptune and Uranus — initially had sedate, circular orbits and were packed into a region only about half the diameter of Neptune’s average modern orbit. Gravity then caused these bodies to spread out and break into a planetary version of bowling that not only violently rearranged the outer solar system but also led to an avalanche of debris pelting the inner planets and their moons.
Prompting this melee, scientists propose, was a series of gravitational interactions between the planets and the hefty disk of debris that lay just beyond. This disk, a forerunner of the Kuiper Belt, contained as much mass as 35 Earths.

For a while, not much happened. Jupiter moved slowly inward while the three other planets moved slightly outward. Then, at about 500 million to 600 million years after the birth of the solar system, Jupiter and Saturn hit a gravitational sweet spot, with Jupiter going around the sun twice for every orbit of Saturn.

In this configuration, known as an orbital resonance, the mutual gravitational influence of the two giants strengthened, elongating their orbits over time. The changed paths of Jupiter and Saturn eventually jumbled the orderly, circular orbits of the two lighter-weight, outermost giants, Uranus and Neptune. And that’s when all hell broke loose, Levison says.

Within a few million years, Uranus and Neptune were kicked so far out that they plowed into the surrounding disk of icy debris. Like bowling balls scattering pins, the two planets scattered the debris all over the solar system.

Some of the debris became trapped by Jupiter’s gravity and could account for the planet’s retinue of Trojan asteroids, a group of objects that lead and trail the planet today, and have not been explained by any other theory, says Levison.

Some of the scattered material traveled farther, penetrating deep into the inner solar system, the simulation suggests. It was this debris that pummeled Earth’s moon during a geologically brief window of time that lasted only 100 million to 200 million years.
Indeed, this onslaught may well have generated the cataclysmic late heavy bombardment, in which Earth’s moon and the inner planets were blasted with debris, Levison says. The cataclysm generated by the Nice model “is consistent with the magnitude and duration of the late heavy bombardment inferred from lunar craters,” Bottke and his colleagues noted in an abstract from the meeting.

In this way, a fracas originally limited to the outermost regions ended up affecting the entire solar system.

Planetary scientists have recently gathered evidence that the asteroid belt, located between the orbits of Mars and Jupiter, also took a direct hit during the late heavy bombardment. An analysis of meteorites believed to be fragments of Vesta, the second largest asteroid, reveal that they, too, suffered an intense bombardment about 3.9 billion years ago, Bogard notes. In addition, the famous Mars meteorite ALH84001, which dates from about 4.5 billion years ago and was once believed to contain fossils of nanobacteria, also shows signs that it suffered a major impact 3.9 billion years ago.

All about zircons

But even on Earth, not all of this early history was erased, new research shows. The first era on our planet is called the Hadean period as in Hades, or hell. However, studies now show that this might not have been such the hellish period — impossibly hot and dry — that many researchers had imagined, says Stephen Mojzsis, a geochemist at the University of Colorado at Boulder.

The clues come from ancient zircons — durable and chemically inert minerals that are remnants of Earth’s first rocks.

Discovered about 25 years ago in the Jack Hills region of western Australia, the zircons are no bigger than the size of President Lincoln’s eyeball engraved on a penny. They are up to 4.38 billion years old, predating by several hundred million years the era of the late heavy bombardment (SN: 1/3/09, p. 10).

“These zircons are our only [terrestrial] record keeper … because Earth is constantly trying to recycle itself,” says Mojzsis. Withstanding weathering, erosion, burial, subduction and remelting, these hardy minerals are filling in a missing era of Earth’s early history when the crust, atmosphere and oceans were established, along with, perhaps, the very first biological systems.

The chemical information encoded in the zircons, says Mojzsis, suggests that Earth not only had crust during the Hadean, which some researchers had doubted, but that the crust was derived in part from granite. The formation of granite, he says, requires liquid water.

“The picture that’s now emerging about the early Earth, based on the analysis of about 100,000 grains from the Jack Hills area, is a watery world with more similarities than differences with today,” Mojzsis says. That means, he adds, that even in that early era, Earth may have possessed properties conducive to life.

Recent studies have revealed that the zircons have tiny zones, or mantles, about 2 to 4 micrometers across, which suggest the minerals were shock heated 3.96 billion years ago. That shock event may be evidence of the late heavy bombardment as it played out on Earth, Mojzsis says.

At the Houston meeting, he reported that such heating can best be explained by a massive impact event that cauterized the outermost parts of the zircons over a matter of days. This kind of heat loss, he says, has only been seen in lunar rocks that have undergone shock heating.

Mojzsis and his colleagues are planning to reanalyze ancient zircons found in moon rocks, using tools that can resolve features 100 times finer than previously possible, to look for evidence that these lunar zircons were also shock heated.

All this feeds back into the Nice model. “The model is extremely powerful … but it doesn’t tell us when the [late heavy bombardment] happened,” says Mojzsis. “That’s where people like me come along, to say, ‘Here is the time at which we see events consistent with the first wave of impactors.’”

The zircon studies on Earth provide “an emphatic no” to the question of whether the late heavy bombardment destroyed all life, he says. In recent simulations of Earth during Hadean times, Mojzsis and Oleg Abramov, also at the University of Colorado at Boulder, “have bombarded the crust with basically everything we could throw at it within reason, based on the Nice model and the lunar record,” Mojzsis says. “We cannot sterilize the Earth, even at 10 times the accepted bombardment rate” associated with the late heavy bombardment, he says.

The late heavy bombardment didn’t destroy organisms wholesale, “but it may have pruned the tree of life,” he says, selecting for organisms that could survive high-temperature, hydrothermal environments. Indeed, many believe the oldest forms of microbial life “are things that live in hot spring environments, so-called hydrothermophiles,” he says.

To learn more about how the era of late heavy bombardment put the finishing touches on the assembly of the solar system, spacecraft will have to return to the moon, says Levison. A manned mission isn’t necessary, he says, but a robotic craft that can bring back rocks from regions of the moon not yet directly sampled will be essential to better probe the solar system’s early history. Under President George W. Bush, NASA was directed to head back to the moon with both robotic and manned missions, but the fate of that program is now uncertain.

“I think the next few years is going to be a lot like the years preceding the Apollo program, when money was spent to educate people about what was going to happen in July 1969 but also to prepare the laboratories and invest in new techniques,” Mojzsis says. “We have a huge collection of [lunar] Antarctic meteorites at NASA, and I plan to investigate those until we get samples back from the south pole of the moon.”

Posted by: Soderman/NLSI Staff

Ron Cowen, February 14th, 2009; Vol.175 #4 (p. 26)

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