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ScienceWise - Autumn 2010

Cosmic Time Machine

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Dr Marc Norman and Simeon Hui with the SHRIMP mass spectrometer at the Research School of Earth Sciences
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Comets are rare and spectacular visitors to our part of the solar system today, but once they rained down on the forming planets creating numerous massive impacts
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One interpretation of lunar soil analysis suggests that four billion years ago the inner solar system was a violent place, gravitational perturbations of the orbits of the gas giants initiated a comet shower which in turn deflected many asteroids into the paths of the Earth and Moon. Erosion has removed most of the craters from the Earth but the Moon’s ancient landscape still bears the scars.

How Scientists are Able to Look Back on the Early Solar System

If we were able to travel back in time about four and a half billion years, we’d be able to see the early planets of the solar system beginning to form around an infant sun. This was a time of massive and enormously violent impacts, as forming planets swept debris and each other out of their orbital paths. It was around this time that scientists believe that a body about the size of Mars smashed into the Earth which such force that enough matter was ejected to create the Moon.

Over time things settles down. Planets gradually found their present-day orbits and the frequency of major collisions declined. Then around four billion years ago, just about the time life was beginning on Earth, something odd happened. The orbits of Jupiter and Saturn shifted slightly which in turn perturbed Uranus and Neptune.  The gravitational effects of this reshuffle extended out into the Oort Cloud – a halo of comets in the outer solar system.

This gravitational reshuffle was enough to induce a shower of comets to enter the inner solar system, impacting asteroids and catapulting them like billiard balls across the orbits of the inner planets. The result was a second period of massive bombardment which undoubtedly had a profound effect on life on Earth.

But perhaps equally as interesting as these events are in themselves, is the way humans living four billion years later have been able to figure out just what did happen in the distant past. Dr Marc Norman and his PhD student Simeon Hui are two scientists doing just that. They’re working with tiny quantities of lunar soil gathered by the Apollo astronauts and using the most modern of analytical techniques to unravel the forensics of those distant times.

“Often people think of asteroid impacts like bullets hitting solid matter, after which the bullet comes to rest mostly intact inside the material,” Dr Norman says, “But the velocities of asteroid impacts are so great that the energy released is enormous. This often causes the original impactor to simply vaporise so the physics of these impacts is a lot more like that of underground nuclear explosions than conventional ballistics.”

These massively energetic impacts gave the Moon the rugged cratered appearance it has today. Of course the Earth was also battered in the same way but unlike the Moon, craters on Earth have been all but eradicated by millions of years of plate tectonics, erosion by atmosphere and oceans and even human activity.

But how exactly can scientists extract such information about the history of the solar system from lunar soil?
“We know from looking at igneous lunar rock – solidified lava flows from deep in the mantle - that elements like gold and platinum are almost non- existent in normal lunar rock” Dr Norman says “but we also know from meteorites that fall on Earth that these elements are actually quite common in asteroids. By using a technique called mass spectrometry we’re able to separate out lunar samples that were formed by impacts from non-impact material by measuring the gold and platinum content.”

Of course the really interesting question to a solar system historian is not just where the impacts occurred but when? To work that out, scientists take advantage of radioactive decay. Over very long periods an isotope of potassium (40K) decays into argon (40Ar). When an asteroid or comet impacts the Moon the enormous energy that’s liberated tends to turn rock into a fine spray of molten glass. This cools into the form of tiny glass spherules a fraction of a millimetre across. These spherules contain small traces of potassium 40 but essentially no argon because the melting releases it. As the spherules age, the radioactive decay of potassium 40 begins to introduce argon again.

“In effect, the impact resets the potassium-argon clock” Dr Norman explains. “From the moment of impact there is essentially zero argon in the glass. However as time passes the potassium slowly decays creating new argon. If we measure the ratio of potassium to argon in a given sample, we can get a pretty reliable estimate of the age of the impact feature the samples come from.”

Although the samples were collected almost 40 years ago, this is the first time it’s been possible to perform advanced chemical analysis and isotope dating on the exact same spherule. “This is really important to our data as it means we can determine the likely region where each of our glass spherules were formed and the time at which they were formed. That’s a really big advantage in a place like the Moon where there have been so many impacts that a lot of the material is heavily intermixed. With this data, it is possible to narrow down unique impacts and build up a history of lunar bombardment,” Simeon says.

Quite apart from filling in details about the early solar system, their work is also shedding light on the nature of life on Earth. “We now know that comets contain vast quantities of water. One of the really interesting questions that raises is, was the water in the Earth’s oceans brought here by comets impacting the surface or did it condense around the early Earth from the protoplanetary disc? If the Earth was indeed hit by a Mars-sized body about four and a half billion years ago, much of any water it had may have been lost in that process.  If that was the case, life on Earth as we know it may be due to comets!” Comets may also explain the recent discovery of water ice at the lunar poles.

Dr Norman and Simeon’s work is contributing to a large body of scientific knowledge that enables us to build up a history of the frequency of impacts in our part of the solar system. One of the interesting and perhaps alarming things to arise from these studies is that the number of lunar impacts has risen slightly in the recent past raising the question of wether another comet shower may be coming soon? But it’s not time to sell up and move to another planet. “Soon” on a cosmic scale is unlikely to be soon on a human scale!

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