Date: May 10, 2010
Title: The Late Heavy Bombardment — Was It Real?
Podcaster: Emily Lakdawalla
Organization: The Planetary Society Blog: http://www.planetary.org/blog/
Description: According to some researchers, the inner solar system was pummeled by a sudden rain of solar system debris only 700 million years after it formed, with cataclysmic results to the earliest crusts of the Moon and Earth (and any life that may have formed here by then). Is the LHB real? What’s the evidence for it, and can we explain why it might have happened?
Bio: Emily Lakdawalla is a planetary geologist and writer who works for the world’s largest space interest group, The Planetary Society, as its blogger, web writer, and contributor to the weekly Planetary Radio podcast. She is also a contributing editor for Sky & Telescope magazine. She lives in Los Angeles with a 3-year-old who can list all the planets for you, a new baby who has yet to learn their names, and a husband who likes to pretend he doesn’t know anything about space.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Brett Duane–just another back yard astronomer who’s learned a lot from these podcasts. Thanks to Pamela and the Slackers!”
I’m Emily Lakdawalla, from The Planetary Society, and today I’d like you to consider the turbulent teen years of our nearest neighbor, the Moon. Look up at the Moon with your unaided eyes and you’ll see a brilliant globe, a perfect circle in the sky whether it’s fully lit or, as it is on May tenth, twenty-ten, a skinny crescent. Throughout most of human history the Moon was thought of as a partner to the Sun, dimmer but still a symbol of heavenly perfection. So Galileo received quite a shock four hundred years ago when he turned his newly built telescope to the Moon and discovered that its surface was pockmarked with circular craters, building steep mountains that cast long shadows across the ground. Actually, Galileo wasn’t the first to observe the lunar mountains — that was an Englishman, Thomas Harriot — but Galileo was better at marketing, and first to publish his iconic drawings.
Looking through a telescope quickly convinced people that the Moon’s surface was uneven. But what made all those circular scars? Today, it seems self-evident that the scars are impact craters, made by the violent collision of asteroids and comets with the Moon. But just fifty years ago, the established view was that all those circular craters were the scars of ancient volcanoes.
Things changed around 1960. That’s when a visionary geologist named Eugene Shoemaker published a doctoral thesis on Meteor Crater, in Arizona. He had been fascinated with Meteor Crater for ten years and believed that it, as well as the Moon’s craters, had an impact origin. In the 1950s, he’d worked on the Plowshare program, an American project intended to explore engineering uses of nuclear explosions. The Plowshare program performed controlled explosions of nuclear bombs near Lawrence Livermore National Laboratories. When Shoemaker examined the rocks near the explosion sites, he discovered new minerals and rock textures that had been created by the high shock pressures and low temperatures of the nuclear blasts. Shoemaker found some of these minerals in Meteor crater as well. These minerals are never found in even the most violent of volcanoes, where temperatures are high but pressures are relatively low. The minerals and textures Shoemaker discovered allowed geologists to tell volcanic craters from impact craters, and eventually led to the understanding that the Moon’s craters mostly formed from impacts, though a few definitely are volcanic.
During the 1960s, astrogeologists started mapping the Moon and the terrestrial planets, Mercury and Mars. At the same time, Earth geologists fanned out across the globe, identifying more and more impact scars. Astrogeologists realized that impact cratering is a geologic process that operates on all the terrestrial planets. Geologists also realized that impact craters provided a way to figure out how old the surfaces of these worlds were. The longer a surface has been exposed to space, the more craters it has accumulated. Therefore, the densely cratered highlands of the Moon are older than the more sparsely cratered dark regions of the lunar lowlands. But how old are they?
To answer that question without samples of lunar rock, geologists looked at Earth. Earth and the Moon occupy the same position in the solar system, so they must have seen the same flux of asteroids. But Earth bears many fewer craters than the Moon. That’s because Earth’s active geology wipes its surface clean of the evidence of impact craters. The erosive action of flowing water and glaciers destroys the crater rims and fills in their floors, while volcanism may cover up craters with lava. And plate tectonics eventually destroys all of the ocean floor by sinking it into Earth’s interior. But there are places on Earth, the stable interiors of the continental crust on tectonic plates, that do record many ancient impacts, if you know how to look for them. In the mid-sixties, geologist Bill Hartmann compared the density of the impact craters on the Canadian shield to the density of craters on the Moon. We know how old the Canadian rocks are from isotopic age dating, so Hartmann was able to estimate that the lunar lowlands, otherwise known as the maria, were quite old, about three and a half billion years old.
If the maria are three and a half billion years old, then the highlands, which are very densely cratered, must be older than that. Hartmann mapped the highlands and found that they contained thirty-two times as many craters as the maria. The highlands accumulated most of those craters in the time before the maria even formed. Hartmann realized that, early in the Moon’s history, it was getting pummeled with asteroids at a rate hundreds of times higher than in the present day. This demanded an explanation.
Both Shoemaker and Hartmann were involved in training the men who would become the Apollo astronauts in the fundamentals of astrogeology. Astrogeologists expected that lunar rock samples, particularly those taken from the lunar highlands, would record a dramatic and dynamic early history of the Moon, with samples ranging in age from the beginning of the solar system around four and a half billion years ago to the three and a half billion year old age of the maria.
As is so often true in science, they didn’t get what they expected. The Apollo astronauts applied what they learned from the geologists and brought back a diverse array of rock and soil samples, about four hundred kilograms or eight hundred pounds in all. When the Earthbound geologists performed the isotopic analysis of these rocks to figure out how old they were, they were shocked to find that nearly all of them were just about exactly four billion years old. Geologist Don Wilhelms used the age dates of the scattered samples to calibrate the time scales derived from geologic mapping of the Moon. He figured out that nearly every one of the giant basins on the near side of the Moon, including Nectaris, Serenitatis, Crisium, and Imbrium, had all formed within a time span of only seventy million years. That may seem like a long time, but it is less than 2 percent of the age of the solar system. Not only was this basin-forming period brief, it was late; it happened six hundred million years after the solar system formed. This violent bashing of the Moon by a pulse of huge impactors wasn’t part of the Moon’s birth; it happened when the Moon was a teenager. And if it happened to the Moon, it had to happen to Earth as well, and very likely also happened to the other terrestrial planets.
This violent period in the Moon’s history is now called the late heavy bombardment. Do a literature search on “late heavy bombardment,” or its initials, L-H-B, and you’ll find that lots of people take the late heavy bombardment for granted as a proven event in the history of the solar system. Actually, though, the hypothesis of the Late Heavy Bombardment is controversial. Historically, astrogeologists have had three main objections to the idea. One is the question of how you could form a nice peaceful solar system and then somehow, long after the solar system formed, you could suddenly send a huge pulse of asteroids or comets to bombard the planets. Where were all of those impactors hanging out for the first six hundred million years, and what could possibly have caused them to suddenly change their orbits to Earth-crossing ones?
This objection has recently been answered by something called the Nice model. A model, in physics terms, is a set of mathematical rules that can be used to predict the behavior of a physical system. The Nice model was developed by Hal Levison, Alessandro Morbidelli, and several other astronomers, and was originally written down to attempt to explain some oddities about how Uranus and Neptune formed. Levison and his coworkers figured out that Uranus and Neptune likely formed much closer to the Sun, and as they interacted gravitationally with Jupiter and Saturn, they were kicked outward into a disk of icy planetesimals. This outward migration sent millions of comets toward the inner solar system, bombarding the planets and also disrupting the asteroid belt, which bombarded the planets some more. The Nice model can’t predict when this happened, but it can be tuned with reasonable choices of starting conditions to force the event to happen at the right time, six hundred million years ago. So there is now an explanation for how it would have been possible for impactors to wait around for six hundred million years before causing the bombardment.
A second objection to the Late Heavy Bombardment idea is the possibility that there were biases lurking in the Apollo data. All of the Apollo landings were clustered within a fairly small area of the lunar equatorial nearside. The youngest of the big basins is the Imbrium basin. It sprayed ejecta all over the Moon. It’s possible that, no matter where the astronauts picked them up, most of the Apollo samples represent Imbrium ejecta, and the coincidence of their ages reflects the age of the Imbrium impact. Corroborating this, Lunar Prospector maps of the Moon showed that there is an unusual elemental signature to rocks excavated by the Imbrium impact, and most of the Apollo rock samples do bear signs of that unusual elemental signature. Barbara Cohen and David Kring have tried to resolve this question by age dating lunar meteorites, which should represent a more random sample of the lunar surface than what Apollo brought back. These studies also failed to turn up any rocks older than four billion years old, but the lunar meteorites don’t show the same tight age clustering that the Apollo samples do.
A final objection is that while there may have been late heavy bombardment, this bombardment may have been no heavier than the bombardment that was happening for the previous six hundred million years. But that earlier history is invisible to us, because the youngest craters obliterated the record of what happened before.
That’s why the hunt is on for old lunar rocks. Astrogeologists would love to locate any lunar rock that is more than four billion years old. One place to look for such a rock would be the oldest basin on the Moon, a place called the South Pole Aitken Basin, on the far side of the Moon. The South Pole Aitken Basin is also the largest impact crater on the Moon, and must have dug deeply into the lunar interior. A future mission to go grab a sample from what should be the oldest rocks from the deepest interior of the Moon is a top priority, according to the scientific panel that advises NASA on what it should do next. Until that mission is flown, and probably after, scientists will continue to argue about whether or not the late heavy bombardment ever happened.
Whenever there’s a new development, I’ll write about it at planetary dot org slash blog. This has been Emily Lakdawalla of The Planetary Society. Thank you for listening!
End of podcast:
365 Days of Astronomy
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