Science Spin March 2008

Meteorite evidence challenges view of birth of Solar System

By Ian Sanders

For as long as we have existed, the Solar System has been flinging evidence about our origins at us. Only now, however, are we beginning to realise just how much information about our ancient past is preserved in meteorites.

These are those chunks of rock that fly into us from the Asteroid Belt, located between the orbits of Mars and Jupiter. The meteorites become extremely hot when plunging through our atmosphere, but because the period of passage is so brief, the inside remains relatively cool, so the core minerals arrive unchanged, neatly packaged in an outer skin of glass.

The chemistry and physical appearance of these minerals can tell us a great deal about how these rocks originated, and amazingly, they have been found to incorporate material that is much older than our Earth.

Solar System

One of Ireland's leading experts in this field is geologist, Dr Ian Sanders, TCD, and his interest in the early Solar System was originally sparked when the late Robert Hutchinson, one time Curator of Meteorites at London's Natural History Museum, published evidence that threatened to upset the prevailing view on how and when planets were formed.

As Ian commented, the scientific community reacted to the news with horror. The details of how the Solar System began remain controversial, yet, as Ian explained to a recent meeting of Astronomy Ireland, there is now compelling evidence to suggest that "the books have got it wrong."

Meteorites, said Ian, can bring us back more than 4,500 million years, to a time, in fact when the Solar System was taking shape. Of particular interest are those meteorites known as chondrites. In the July 2007 Issue of Science Spin, in a feature entitled, 'Meteorites: Clues to Solar System's Origins', Marie-Catherine Mousseau explained that there are many different types of meteorite, but most of those that reach us are classed as chondrites.

Geologically, said Ian, these are cosmic sandstones, in which most of the grains, known as chondrules, are frozen droplets of once molten rock."A most remarkable feature of chondrites," said Ian, "is their chemistry." In almost all respects this chemistry is identical to that of the existing Solar System, and this chondrites are widely regarded as representing the original 'primitive' materials.

Challenge

However, Ian challenges this view on the antiquity of chondrites and the chondrules within them. On some essential points there is no dispute, such as the evidence of early heating. When thin slices of a chondrite are examined under a microscope, it can be seen that the condrules have fuzzy edges. Geologists, explained Ian, are familiar with that feature, which shows that the grains were heated to a temperature of about 900ºC to 1,000ºC after they had already come together.

No one has any problems accepting this as evidence of an extremely hot environment, but this is where the accepted explanations began to get into trouble. To understand why, it is necessary to look at the chondrite slices in more detail.

Alongside the chondrules are rare fragments, called calcium-aluminium rich inclusions, or simply CAIs for short. Dating methods, based on the delay of uranium show that CAIs were formed 4,567 million years ago, and are the oldest objects ever dated from the Solar System. That's not all. CAIs show peculiar ratios in their magnesium isotopes. Magnesium occurs in three isotopic forms, 24, 25, and 26.

Throughout the Solar System the proportions are always identical, except in CAIs. In CAIs there is more 26Mg than normal. This is interesting since 26Mg is produced from the decay of radioactive aluminium, 26Al, so the elevated levels indicate that when the CAIs were formed the Solar environment was highly radioactive.

As 26Al decays heat is generated, and it was this factor that made scientists such as Robert Hutchinson and Ian Sanders realise that the prevailing views on how and when the planets formed must be flawed.

Sun

All scientists, said Ian, agree that cosmic dust formed a great disc around the newly formed Sun. The conventional view holds that this dust first began 'somehow' to stick together, forming fluffy clumps. These then became rapidly heated by solar flares or shocks, causing them to form chondrules, which in turn became the building blocks for the first generation of planets.

Hutchinson disagreed with this view, arguing instead that the first generation of planetary bodies must have existed before chondrules were made. "Hutchinson's key evidence," said Ian, "was rare pieces of basalt rock, sitting side by side with chondrules in several different chondrite meteorites, one of which fell in Ireland in 1969." Basalt can only be made on an existing planet. "A planet must have existed, and been broken up to make basalt fragments, before the chondrules came together," said Ian.

This evidence, he added, is quite convincing, and he points out that an early heat-driven phase of planetary evolution makes more sense of what is seen. We know from the 26Mg evidence that the level of radioactive 26Al was high. "There was enough 26Al around to completely melt any small planetary bodies," said Ian, who added that 6 to 7 kilojules of radioactive energy would have been locked into each gram of original dust, and this could have produced about the same energy as the cat food that keeps our pets running around. Whether from cat food or 26Al, this release of energy would have resulted in the melt down of any object over 20 km in radius.

Under these conditions, any planets larger than this, forming from coalescing dust, would very quickly have melted. Basalt would have come to the surface, and molten iron would have flowed into the centres. These molten bodies would then have collided and splashed into each other, resulting in huge cascades of molten drops, which would have cooled to form chondrules.

Those early chondrules, said Ian, would still have been radioactive, so the planets into which they were added would, again, have melted, and that process would have continued as long as the level of 26Al fuel remained high. The half-life of 26Al is a relatively short three quarters of a million years, so after about 2 million years most of the fuel would have been spent, and chondrule formation by splashing could cease.

Overturned

"This two-million-year time interval is totally consistent with recently measured ages of chondrules," said Ian. Measurements, based on decay of uranium, show that chondrules are close to two million years younger than the time when CAIs were formed. Residual radioactive heating, while unable to cause melting, would, however have caused a high temperature of about 900°C to 1,000°C, producing the distinctive fuzziness in chondrule edges. "Thus, it seems highly likely that chondrules came from the first planets, and not, as the books say, that the first planets were made from chondrules." The conclusions in the books should, thus, be overturned.

In most places gravity tidied up a lot of the debris from all these collisions, and small planets coalesced to make the larger ones we have now. However, in the Asteroid Belt, it seems that the pull of Jupiter interfered with the process, so millions of rocky fragments preserving frozen droplets from the molten interiors of earlier planets, remain in orbit.