Using Isotopes to Probe the Earliest History of the Solar Nebula

The age of the solar system is set at 4.567 billion years, and the new work traces some of the history of these small grains during about 300,000 years, before the formation of comets, asteroids, or planets. All the solid objects in the solar system formed within a circumstellar protoplanetary disk called the solar nebula. Among the oldest materials are tiny calcium- and aluminum-rich grains or inclusions (called CAIs) found in some meteorites. The earliest history of these grains can be probed by studying their magnesium isotopes, since part of this magnesium was produced by the rapid radioactive decay of aluminum-26, an isotope with a half-life of less than a million years. Somewhat ironically, we have better time-resolution on these earliest events than for most of the later history of the planetary system.

A recent paper by Ed Young and other members of the UCLA NAI team, done in collaboration with colleagues in England, examines this early history of CAIs. The absolute ages of these oldest grains, determined by the decay of uranium into lead, is 4567 million years (4.567 billion years), which is usually defined to be the age of the solar system. Part of the new high-precision analysis of meteorites reported in this paper leads to a re-evaluation upward of the amount of aluminum-26 present when the solar system formed. Knowing this initial quantity of radioactive aluminum allows scientists to make better use of the aluminum-decay "clock" to understand events at the time dust was condensing in the early solar nebula.

This new work looks at the history of the CAIs as revealed by the balance between aluminum and magnesium, which can be altered by heating. They conclude that over about 300,000 years temperatures were high enough for transfers to take place between the minerals melilite and anorthite. However, these temperatures cannot have been elevated for that entire time. Rather, the grains must have experienced many episodes of heating and cooling.

Two mechanisms are suggested for this episodic heating. One possibility is that the orbits of these millimeter-sized grains within the solar nebula brought them very close to proto-sun perhaps as many as 15 times, where temperatures up to 1600 K persisting for 10-20 years would have caused partial melting of the grains. To achieve such high temperatures, the grains would have needed to come much closer to the young Sun than the present orbit of Mercury. Alternatively, part of the grain heating could have come about from shock waves generated by turbulence in the protoplanetary disk.

Studies like this, based on analysis of meteorites, can tell us something about the conditions in the solar nebula before the planets, or even the comets and asteroids, were formed. These data can also be compared with remote sensing of circumstellar disks seen today, where other planetary systems are being born in our Galaxy.

The paper by Young et al., titled ?Supra-Canonical Al-26/Al-27 and the Residence Time of CAIs in the Solar Protoplanetary Disk, was published on-line in Science Express for March 3, 2005.

Source: NASA Astrobiology Institute

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