Science Spin July 2007

Meteorites: Clues to Solar System's origins

By Marie-Catherine Mousseau

At 10:10pm on the 28th November 1999, people in County Carlow saw a remarkable fireball, reported "as bright as the full moon", which lit up the sky and sent out booming explosions. Shortly afterwards, a grand-mother who wished to remain anonymous found a golf-ball sized rock on a local road, "dark black and wonderful!" She had found what would be known as the Leighlinbridge meteorite.

Space rocks, or meteorites, do fall from the sky, even in Ireland, and the Celtic inhabitants of Gaul may not have been that crazy when claiming their biggest fear was the sky falling upon their head. Falling meteorites do cause damage to property, livestock, and even people. Not to mention the very big ones; those that strike the ground with cosmic velocity, vaporised by the impact and leaving behind huge craters, like the one in Arizona, 47,000 years old and nearly a mile across, or the Sudbury crater in Ontario, Canada, which reaches 200 to 250 km in diameter.

The force of such collisions has the potential to cause widespread destruction. Most people have heard about the Chicxulub crater in the Yucatan Peninsula (Mexico), 175 km across, caused by a meteorite that might have been responsible for the extinction of the dinosaurs - and in fact most life then on Earth.

But meteorites are not all that bad. Only few of them are large enough to create impact craters.

According to astronomers' calculation, such major collisions are quite rare -- one every 300,000 years on average. Instead, most meteorites typically arrive at the surface at their terminal velocity (free-fall) and, at most, create a small pit. And these bodies coming from outer space can be of inestimable value in learning about our Universe.

Indeed, what is so fascinating about meteorites is that they give us direct clues about what the cosmos is made of. Think about it; what palpable material other than meteorites do we have to get to know our surrounding Universe? A few samples from the Moon, or comet dust; but so far, we have never been able to bring back any sample even from Mars, the closest planet to Earth, (and according to NASA experts we're unlikely to do so till 2011 at least). So most of our direct evidence - as far as alien chemistry is concerned - remains with meteorites.

Origins

Fortunately we are not short of them. Although more likely to be spotted in populated areas, such as Europe, Japan, or northern India, meteorites fall virtually everywhere. The vast majority come from asteroids (small celestial bodies that revolve around the Sun). Indeed, there are lots of bodies out there, especially in the so-called asteroid belt (between the orbits of Mars and Jupiter), fragments of which might get ejected due to collisions and cross the Earth's path. A few Antarctic and hot dry desert meteorites also come from our closest neighbours the Moon and Mars. They might have been ejected by the explosive impact of some asteroid.

To be called meteorites, however, these fragments need to survive an impact with the Earth's surface. This is not as straightforward as it may seem. If they are too small (less than a few grams), or are going too fast, they do not get through the Earth's atmosphere because they are burned up as a result of friction with the air. They become incandescent and appear as a streak of light - those are the shooting stars which sometimes fill our sky at night.

Overall, an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the Earth's surface each year. And of those, only about six are recovered and made known to scientists. Of course this means very few have been found in Ireland. According to Dr Mike Simms of the Ulster Museum, there may be 10 meteorites a year of walnut size or larger that strike the Irish surface; but most of them are lost.

"Most meteorites contain at least some iron metal and so rust quickly in humid climates such as Ireland", he said. "Since most geologists would be hard pushed to actually recognise them, they are unlikely to be found before they rust away completely." Therefore, only the meteorites which are actually seen to fall -- like the fireball seen in Carlow mentioned at the beginning -- can be quickly retrieved. The 1999 Leighlinbridge meteorite was in fact the last meteorite ever discovered in Ireland; the one prior to Leighlinbridge was discovered 30 years previously.

Chondrules

So what do meteorites tell us about our Universe? Obviously, they can tell us something about its composition. The vast majority of meteorites (90 per cent) are made of stone. Most of these stony meteorites are called chondrites, because they contain small, round particles called chondrules, composed mostly of silicate minerals that appear to have been melted. There is also a small proportion of meteorites composed chiefly or partly of iron; but chondrites are by far the majority, accounting for about 84 per cent of all existing meteorites (see box below for meteorite classification).

Meteorite composition tells us about their parent bodies -- that is, for most of them, asteroids. And believe it or not, it is particularly interesting to know about asteroids, because asteroids are like ancient witnesses. Chondritic asteroids in particular, which contain the small spheres called chondrules, are some of the oldest and most primitive materials in the solar system. We think they are fragments of small planets created as a result of collisions when the solar system was forming -- chondrites are actually considered to be "the building blocks of the planets." Elucidating how chondrites are formed, and more specifically how chondrules inside them are created, would thus give us fascinating clues about our history i.e. the origin of our Solar System and the creation of planets.

Brian McBreen, Associate Professor at UCD School of Physics, comments about the scientific importance of these melted pieces of rocks: "the best discovery about meteorites are undoubtedly chondrules," he said. "You would never have guessed that the material has been melted in the manner that it has. It's only once we looked inside the meteorites and found these melted droplets of material, the chondrules, that we got a clue as to what occurred." Because, as he pointed out, for rock to melt and chondrules to form you need a temperature of 1600ºC. So the question is: what caused such heat?
Professor McBreen did not actually set out to determine the origin of chondrules. However, as often happens in science, his research led him there while he was studying some a priori unrelated phenomenon - gamma ray bursts.

Gamma rays and planet formation

Working in the area of gamma ray astronomy, Prof McBreen and his team made the association with meteorites and came up with an original theory, which they published in the Astronomy and Astrophysics journal. Their theory put lightning strikes, caused by gamma ray bursts, on the central stage.

"Explosions that occur at the endpoint of stellar evolution [the death of a star] generate huge fluxes of gamma rays," he says. "If you're anywhere near them - within 300 light years - they are sufficiently intense to melt grains of sand or various oxides. When these melted grains cool down, they will form lovely spherical shapes, the shape of the chondrules."

So, according to Prof McBreen, huge lightning strikes caused by a dying star could account for the formation of chondrites and consequently rocky planets, some 4.6 billion years ago. To test the theory, one of his PhD students, Paul Duggan, did an experiment using a 'wonderful facility' as Prof McBreen describes it, the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Paul irradiated some dust particles (a precursor material made of various oxides) with a gamma ray beam to simulate a gamma bursts. And the results were just like the theory had predicted. The gamma ray beam physically melted the particles, giving rise, on cooling, to the remarkable chondrules.

This was all very good, but for this process to take place in reality the theory needs an intense source of gamma ray burst near a place where planets were formed (a protoplanetary system). "This was a phenomenon that could only occur in a nearby protoplanetary system that got blasted by gamma rays," Prof McBreen says. Well, don't get the wrong idea, when astronomers say nearby, it has not exactly the same meaning as for the rest of us. "By nearby we mean from distances from here to 10 times beyond the nearest stars," he indicates. "As far as we know, there are not enough gamma ray bursts to account for all planetary systems" he adds.

In fact, Prof McBreen's theory may not account for the formation of our Solar System, but he thinks it can account for a minority of other systems. "Astronomers are now aware of that theory. Sooner or later the chondrule signature will be found near gamma ray bursts in other protoplanetary systems," he concludes.

As for our planetary system, there is still no consensus as to what heated it. While many still believe that lightning could account for chondrule formation, not everybody agrees. As pointed out by Brian, there are a number of other theories - for instance phenomena such as shockwaves or X-ray flares from the sun could have been involved in the creation of planets "at least in our Solar System, and presumably in other similar systems."

But there is one particular theory that seems to fit very nicely with our observations...
The key role of radioactivity

A major proponent of this theory is another Irish scientist, one of the most prominent meteorite experts in Ireland - Dr Ian Sanders, geologist and lecturer at TCD. Dr Sanders has been working on meteorites for 14 years, and his research is precisely about the first few million years of the Solar System. He has his own idea about what happened. "Personally, I do not believe that flash heating of dust clumps (to make chondrules) was an important process, and may not have happened at all."

His theory is actually an old theory that he has resurrected. "The theory goes back to Harold Urey (Nobel Laureat), but was first clearly expressed by a guy called Herb Zook in 1980," Ian Sanders comments. "It claims that planetesimals [very small planets] became substantially molten, then splashed on mutual collision, releasing copious volumes of molten droplets that cooled to become chondrules."

Dr Sanders' theory is also the one favoured by Dr Mike Simms. Dr Simms enthusiastically explained the whole process in simple terms. Very early in the history of the Solar System, gravity was pulling the dust into bigger lumps; once it got into lumps bigger than about 10 km across (small planets), heat built up and melted them. "So the first million and a half year of the Solar System, orbiting the sun you had only small planets mostly melted with a thin solid crust", he says.

He continued: "There would have been so many of these molten planetesimals that they would have been crashing into each other all the time." And every time they crashed they would generate showers of molten rock droplets. These droplets would have frozen pretty quickly in space and then gravity would have pulled them together again. That's the best explanation for the chondrules, Dr Simms concluded.

But why would planets spontaneously heat up and melt in the first place? The reason is given by an interesting clue: the magnesium isotope, 26Mg. Meteorites have been found to have more 26Mg than you would expect. And 26Mg is what the radioactive isotope aluminium 26Al decays to, so this leads to a single cause: radioactivity.

"Aluminium 26 is radioactive with a half life of about three quarters of a million years," Dr Simms explained. "During the first million year and a half of the Solar System, there was a lot of 26Al in the dust. The heat generated by radioactive decay would melt the small planets once they had reached a big enough size. What we know of the dates of the chondrules fits very nicely with what you would expect from the half life of aluminium decay."

Bovedy meteorite

Interestingly, an Irish meteorite served as an important clue for Dr Sanders' revival of the theory. Not the Leighlinbridge meteorite (the one recovered in 1999 by the anonymous grandmother); this one turned out to be quite plain. No, he used the previous one, which fell on the 25th April 1969. It is in fact called Bovedy/Sprucefield because the larger stone fell at Bovedy while a smaller fragment crashed through the roof of a RUC store at Sprucefield, 60km to the SSE. "Bovedy/Sprucefield fall is easily the most scientifically important Irish fall," Mike Simms comments. "All the other Irish meteorites are pretty dull, equilibrated ordinary chondrites". "Bovedy/Sprucefield is highly unequilibrated."

Dr Simms explained what he means by equilibrated. Two million years after the beginning of the Solar System, the chondrules are already formed and a lot of the radioactivity has gone, but there is still faint radioactivity. While not enough to remelt the chondrules, there is however enough to heat them and cause transfer of some of the elements. "And if you heat up a chondrite to a few hundred degrees, after a few million years what happens is that the chemistry of all the chondrules end up being pretty similar" Dr Simms said. "This is what is called equilibrated."

Bovedy is different, in so far as it was never really cooked; so each chondrule has its particular chemistry. Mike explains why: all chondrite meteorites are parts of a bigger planet, or some planetisimal that was smashed to pieces by collision. Most fragments come from the middle and would have been heated up. But there are a few bits at the edge or at the surface which would not have been heated much. Bovedy probably comes from the edge.

As a result, "you can see the chondrules very clearly in Bovedy, much more clearly than what you can see in other meteorites," Mike says. "It's very important because they are pretty rare and tell us much about how chondrules and chondrites form."

Dr Sanders explains how looking at Bovedy's texture under the microscope helped elaborate his thesis. "Two aspects of Bovedy initiated the line of reasoning from which the theory emerged," he says. "First, back in 1992 it was one of only four chondritic meteorites that had been shown to have bits of planetary rock alongside the chondrules." Dr Sanders said this clearly shows that planetary bodies had first evolved and become fragmented before the chondrules formed. "Secondly, the chondrules in Bovedy are not spherical, but have shapes that indent one another, as though the chondrules were still hot and plastic at the time of their accretion. This observation implies local production in batches - a feature consistent with planetesimal splashing."

Dr Simms accepts Ian Sanders' theory. It is "by far the most plausible and parsimonious theory for chondrule formation. "The various other theories, such as lightning strikes or the X-wind, to name but two, fall down on many other aspects," he adds. "Ian's theory can account for many of the observations of what we see in meteorites."

And beyond

So because meteorites are very old, and because they all come from within our Solar System, they tell us a lot about what happened during its formation. But they might even bring us beyond that.

"Ultimately, the matter the solar system is made of must have come from something earlier, something around since the beginning of the Universe," Dr Simms noted. He pointed out that a lot of meteorites contain ancient tiny crystals of diamond, silicon carbide, etc older than the Sun. "These presolar grains have actually survived when most of the other compounds may have been homogenised," he explained. They are microscopic traces, but with many interesting sources. Ian Sanders comments about their origin: "presolar grains within unheated chondrites are fascinating because they contain the stellar sources of the atoms from which we are made." Experts actually believe these ancient grains come from various supernovae - not less than 40 different ones, according to Dr Simms. But, "the origin and consequences of short-live radioactive isotopes is an unsolved puzzle," Ian Sanders said.

Actually, a supernova might also be the source of all the radioactive aluminium thought to be involved in planet formation. "There must have been a supernova not that far away which blasted all this 26Al into the Solar System and set the whole thing going" said Dr Simms. "Without that things would have been very different."

So whether it be through gamma ray bursts or radioactive aluminium, the whole process seems to come down to a universal life cycle- a dying star transferring its modelling imprint to a new born star system.

More about Meteorites

Definition: a piece of rock or metal that has fallen to the Earth's surface from outer space.
Only a small fraction of the initial meteorite mass remains to reach the ground.
Meteorites are named after the geographical localities in which they were found.
About six new fallen meteorites are recovered each year around the world.
One every 30 years on average are recovered in Ireland.

Origin

Most meteorites are believed to be fragments of asteroids, but quite a few lunar and martian meteorites have been found in the Antarctic or in dry deserts of North Africa and the Middle East.

Composition and weight

Traditionally, meteorites are divided into three main types.

(1) 90 per cent are made of stone. Including 84 per cent of chondrite and 8 per cent that are achondrites, i.e. stones that have no chondrules and resemble the rock called basalt (could come from the crust of Mars or the Moon, or the large asteroid Vesta)
(2) Seven per cent have an exceptionally high iron content (with a small percentage of nickel)
(3) One per cent are stony-iron meteorites (made of both iron and stone in varying proportions)

Stone meteorites have masses between about 100 grams and 1,500 kg and iron meteorites between a few grams and 20,000 kg.
Those much larger than the largest recovered meteorites produce craters in the Earth's surface and are blown apart on impact.

Parent bodies

Planetesimal: minute planets created when the solar system was forming
Silicate crust with an iron core is what we think a number of asteroids are made of (differentiated asteroids).
Achondrites might come from the crust of asteroids while iron meteorites come from their core (and stony-iron from the frontier core/crust).
Chondrites come from undifferentiated asteroids.
Asteroid belt: Doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter.
Asteroids are some of the oldest and most primitive materials in the solar system.

Questions and answers

Do meteorites mostly fall during meteorite showers? What do these showers consist of?

There is no link between meteor showers and meteorite falls. The former are mostly sand-sized bits from comets; the showers occur when the Earth passes through the trail of debris left behind by the passage of a comet, and they burn up long before reaching the ground. Meteorites are fragments of asteroids and fall randomly.

What are the methods geologists have at their disposal to tell where a meteorite comes from?

In a few cases if enough people have observed the fall and noted its direction, or the fireball is captured on film, we can work out its trajectory. The few for which this has been done show they come from the asteroid belt. As for specific asteroids, we look at the chemistry of the meteorite and compare with reflectance spectra for asteroids.

(Answers provided by Dr Mike Simms)

For even more information on meteorites click here.