George W. Wetherill

Formation of the Asteroids

On the same day that I sat down to write this brief essay, I learned by e-mail of the death of the Russian astronomer Victor Safronov, who during the 1960s created the modern epoch in the study of the physical processes by which planets are formed. I wouldn't be doing this work if it had not been for his inspiration. One of his minor contributions was to introduce the biological term embryo to describe the bodies that were to grow into planets. This tradition can be extended to describe the development of this science itself. It has passed through the stages of infancy and childhood, when these processes seemed relatively easy to understand. On the way to maturity, we are now experiencing a confusing and difficult period of "adolescence," in which nothing seems to work as well as it did a decade earlier. Ironically, this is a natural consequence of the great advances that have been made in understanding the dynamic processes involved, the ability to test complex models with readily affordable desktop computers, the new theoretical and observational discoveries in the area of star formation, and the first actual observation of the planetary systems of other stars, a field in which our new Staff Member, Paul Butler, is a pioneer. Many enigmas have been revealed by these more advanced discoveries.

A major difficulty is that in our solar system all of these events took place 4.5 billion years ago. All of the larger planets have experienced major changes since their formation, and even for the Earth, the only planet that has been studied in much detail, the essential record of its earliest history has been very badly obscured by later events. Thus the planets provide little "ground truth" for testing theories.

Nevertheless, well-preserved samples of rocks that were formed within the first few million years of the formation of our Sun are readily available and are being studied by use of precise technology in laboratories throughout the world, including the isotope cosmochemists in this department. These samples are the meteorites. A small number of these are known to be rocks blasted from the Moon and Mars by impacts of asteroids and comets. The great majority are fragments of asteroids themselves. Asteroids are small planets, ranging up to about 1,000 km in diameter. Most of their orbits lie between those of Mars and Jupiter. Because of their small size, they have been spared the geological processes that have obscured the early history of the larger planets. As a result, they still have a "memory" of the circumstances under which they and the other planets were formed.

It is not possible to simply "invert" the meteoritic data and infer events in the early solar system. Rather, it is necessary to "forward model" quantitative alternative plausible scenarios of asteroid formation, and compare the consequences of these models with the record preserved in the meteorites.

Rather little attention has been given to this opportunity. One such study is an extension of earlier calculations I published in 1992. These have now been repeated by the use of greatly improved computational facilities, as well as software developed by a former DTM postdoctoral associate, John Chambers, who is now on the staff of the Armagh Observatory in Northern Ireland. This work has been published during the past year. Chambers and I have written a paper concerning the processes in the asteroid region during the early solar system. It examines the consequences of the "standard model" of planet formation and shows that not only the inner planets, but unobserved planets in the asteroid region as well, would grow to about the size of Mars in less than a few million years. In this standard model, Jupiter and Saturn become gas giant planets somewhat later. We then find, thank goodness, that gravitational interactions between these "asteroid planets" and Jupiter and Saturn quickly remove these presently nonexistent large bodies in the asteroid region, leaving only residual small bodies that represent the observed asteroids, the sources of the meteorites.

At the same time, present DTM postdoctoral associate Stephen Kortenkamp and I are developing an alternative model to permit comparison with these consequences of the standard model. This model makes use of the computations made by Alan Boss that imply that the giant planets discovered by Paul Butler and others could have formed very rapidly, during the later stages of the formation of their central star. Boss has suggested that Jupiter and Saturn may have formed by this same mechanism (see above), thereby avoiding some serious difficulties with the standard model. We are also examining whether this alternative model also permits the formation of small asteroids and the inner terrestrial planets. At present our answers to these questions are "maybe" and "probably." We are trying to provide better answers and to infer the consequences of such alternative models, with regard to the unique information preserved in the meteorites, as well as their asteroidal parents that are at present targets for observation by spacecraft.