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Cosmic Time Machine

Moon craters

Craters reveal the Moon's turbulent history.

By Marc Norman and Tim Wetherell

Precise dating of impacts on the Moon might contribute to a better understanding of life on Earth.

The planets of our solar system formed about 4.5 billion years ago from a collapsing disk of dust and gas rotating around the axis of an infant star. Within this cosmic colosseum, violent events of enormous magnitude played out. The early Sun flared hot magnetic jets deep into outer space; supersonic shock waves swept across the disk; and portions of the disk were heated to temperatures in excess of 1500°C.

As the particle density inside the disk increased, clumps of debris grew rapidly into kilometre-sized boulders, or planetesimals. Like speeding cars in a roundabout, these planetary embryos began to collide, collecting more mass and clearing their orbits of smaller bodies.

Within a few million years, protoplanets the size of Mars and the Moon had formed, and the consequences of collisions among these bodies had increased in scale proportionately. Rather than simply breaking apart, as happens when asteroids collide, close encounters between protoplanets released enough energy to cause wholesale melting, creating oceans of magma that took millions of years to cool. In one of these late-stage collisions, a body about the size of Mars is thought to have smashed into the proto-Earth, melting the Earth and ejecting the material that formed the Moon.

Once this early stage of planet formation was complete, relatively stable orbits were established and the inner planets became more or less benign places to live. To a surprising degree, however, our research is showing how the geological evolution of the terrestrial planets continued to be shaped by celestial events long after their early crusts had solidified.

But perhaps equally interesting is the way humans living four billion years later are able to figure out what did happen in the distant past. Working with tiny quantities of lunar soil gathered by the Apollo astronauts and using the most modern of analytical techniques, the goal of our research is to unravel the forensics of those distant times. The key to being able to do this is isotope chemistry coupled with the physics of large impacts.

When we think about impacts we tend to think of things like cars colliding or boulders falling from cliffs. In this kind of scenario there’s a lot of kinetic energy released, the most obvious effect of which is crumpling and shattering of the bodies involved.

However, the speeds involved in these common terrestrial impacts are very, very small compared with something like an asteroid impact. Kinetic energy increases with the square of velocity, so the energy released when an asteroid or comet hits a planet at tens of thousands of kilometres per hour is massive.

The asteroid almost instantly decelerates on impact with the planet’s crust. It becomes vapourised by the energy released, causing a colossal increase in temperature and volume. The nearest thing to this phenomenon on Earth is an underground nuclear explosion.

You can see clear evidence of this impact explosion mechanism by looking at the Moon through a small telescope. The many thousands of asteroids and comets that have hit the Moon over the past 4 billion years came in at every possible angle. If you imagine throwing stones into mud, the ones that hit at a grazing angle leave long elliptical indentations; it’s actually quite difficult to create a circular carter. However, when you look at the Moon, all the craters and basins are essentially perfect circles because the initial impact scar is wiped out a few milliseconds later by the explosive release of kinetic energy.

Apart from creating craters that we can see today, these impact explosions also turned the lunar rock into a fine spray of molten glass, which cooled to form tiny glass spherules a fraction of a millimetre across. These glass beads, which come in all colours and shapes ranging from near-perfect spheres to dumbbells, were recovered in lunar soil collected by the Apollo and Soviet Luna missions.

By measuring both the chemical composition and age of formation of individual spherules, it’s possible to associate them with specific impact events on the Moon. That’s exactly what our PhD student, Simeon Hui, is doing at the Australian National University’s Research School of Earth Sciences.

Igneous lunar rocks that solidified from volcanic magmas produced deep within the mantle contain almost no gold or platinum. Asteroids, on the other hand, have a high abundance of these elements. Consequently when scientists find platinum in a lunar soil spherule they can be pretty sure that it was created by an asteroid impact.

However, piecing together the history of the solar system also requires information about when these impacts occurred . To discover this we look at the record of radioactive decay of another element, potassium, which is also carried within these tiny glass spherules.

Over very long periods of time, an isotope of potassium (40K) found in most lunar rocks decays into argon (40Ar) and becomes trapped within the rock. During an impact the rock is superheated and liquefied, allowing all the argon to escape. Once it solidifies into a glass spherule you have a concentration of potassium isotopes but essentially no argon remaining.

Over the course of millions of years the radioactive decay generates more argon, which again starts to accumulate within the glass spherule. So, by measuring the ratio of potassium-40 to argon-40 within the spherule we can determine how much time has passed since the spherule was created.

The highly sensitive mass spectrometers at ANU and Curtin University in Western Australia have enabled us to undertake analyses that until recently had been impossible even though it is almost 40 years since the lunar material was originally collected by the Apollo missions. For the first time we can measure the detailed chemical composition and potassium–argon impact age of a single glass spherule.

This yields information about both the location and time of a given impact, with certainty that both measurements relate to the exact same impact event. That’s a big advantage in a place like the Moon, where there have been so many impacts that most of the material at the surface is heavily intermixed.

However, the picture of lunar impacts that has been built up from this and other work is far from simple. Our work has shown that the ages of lunar rocks created during the largest impact events are clustered within a narrow range between 3.75 and 3.95 billion years ago, suggesting that intense bombardment of the inner planets had persisted for hundreds of millions of years after the initial formation of the solar system.

The explanation for this Late Heavy Bombardment remains controversial. In one scenario, the impact flux died down soon after the planets formed, then increased dramatically again in a short-lived spike about four billion years ago.

Alternatively, the impact flux may have declined more steadily, with relatively minor fluctuations, since the formation of the Moon’s crust. In this latter scenario, the apparent clustering of impact dates may simply be caused by the destruction or burial of older deposits by ejecta from the younger basins, or by a sampling bias due to the small collection areas of the Apollo and Luna missions.

Scientists are very interested to get to the bottom of this puzzle since the two different scenarios have radically different implications for solar system dynamics, the geological evolution of the Earth, and possibly the origin of life.

Computer modelling of the dynamics of the solar system has linked a late spike in the bombardment rate to shifts in the orbits of Jupiter and Saturn, which in turn perturbed Uranus and Neptune. The gravitational effects of this reshuffle would have extended out into the Oort Cloud, inducing a shower of comets into the solar system, impacting asteroids and catapulting them like billiard balls across the orbits of the inner planets.

This happened around four billion years ago, just about the time that life on Earth was getting started, and may have influenced the course of biological evolution, either by an “extinction filter” that favoured specific habitats populated by organisms that thrive in extreme environments, or by creating a unique set of conditions that kickstarted biological evolution.

An intense, short-lived burst of large impacts may also have modified the physical, chemical and thermal structure of the Earth’s crust sufficiently to alter the tectonic evolution of the planet. It may be no coincidence that the Earth’s oldest preserved continental crust is about the same age as the proposed Late Heavy Bombardment.

Alternatively, a longer-lived decline in the bombardment rate may have helped regulate environmental conditions on the early Earth by moderating early icehouse conditions. This may have created more widespread conditions favourable for life, and allowed a longer interval for life to get organised.

Although things seem to have settled down considerably over the past four billion years, the most recent results from our studies of lunar spherules suggest an interesting twist. Hui has discovered what seems to be a sharp increase in the number of impact events that occurred during the past 500 million years or so. The onset of this increase corresponds to meteorite evidence for a series of large collisions among minor planets in the asteroid belt that sent a shower of meteorites toward the Earth and Moon, and may have increased the rate of impact events across the inner solar system.

It seems that we are living in the most intense period of meteorite strikes since the Late Heavy Bombardment. Although the current phase is gentle compared with the assault 3.9 billion years ago, it still could have had profound and possibly catastrophic consequences for biological evolution. For example, the impact of a 10 km asteroid in southern Mexico about 65 million years ago apparently wiped out the dinosaurs and allowed the emergence of mammals as a dominant species.

The prospect that dramatic changes in the course of biological evolution were caused by seemingly random cosmic events, such as large asteroid impacts, is an intriguing and profoundly troubling idea. The largest mass extinction event known to science occurred 250 million years ago at the boundary between the Permian and Triassic periods, when more than 90% of existing species died out. Although a combination of environmental and geological factors probably contributed to that event, there is emerging evidence that one or more large impact events may have added to the environmental stress.

It’s an interesting possibility that more precise dating of impacts on the Moon might contribute to a better understanding of life on Earth.

Marc Norman is a Senior Fellow with the Research School of Earth Sciences at the Australian National University, where Tim Wetherell is Science Editor.