Meteorites tell of shocking experience in planetary formation
Posted: March 26, 2002

The search for Earths around other stars is one of the most pressing questions in astrophysics today. To home in on what conditions are necessary for Earth-like bodies to form, however, scientists must first solve the mystery of how our own Earth arose. The formation of the dominant constituent of meteorites -- tiny, millimeter-sized spheres of melted silicate rock called chondrules may hold the clue to this puzzle. A new model published in this month's journal, Meteoritics and Planetary Science, by Dr. Steven J. Desch of the Carnegie Institution of Washington's Department of Terrestrial Magnetism and a member of NASA's Astrobiology Institute, and Dr. Harold C. Connolly, Jr., of CUNY-Kingsborough College in Brooklyn, NY, represents a huge step in understanding chondrule formation and thus what went on in our early solar system. And it answers a series of problems that have plagued theoreticians for years. The model determines how chondrules melted as they passed through shock waves in the solar nebula gas. As chondrules melted, they changed from fluffy dust to round, compact spheres, altering their aerodynamic properties, and enabling the growth of larger bodies. Because shocks would melt chondrules early in the solar nebula's evolution, the results are consistent with the common idea that chondrule formation was a prerequisite to the formation of planets in general.

"This model may be the key that unlocks the secrets of the meteorites," says Desch. "It is the first model detailed enough to be tested against the meteoritic data, and so far it has passed every test. At the same time, it is providing a physical context for all that meteoritic data, and is giving us fresh insight about chondrule formation."

Researchers have long thought that the interstellar dust coagulated to form the planets, but they have not understood what the physical conditions were that led to centimeter-sized particles sticking together in the first place. Without understanding the origin of chondrules, the data-rich meteoritic record could not be used to assess the probability of Earth forming, which is essential information in the search for other life-bearing planets.

"Astrobiology is about the progression from planetary 'building blocks' through the formation of planets, their habitability, and the origin and evolution of life," adds Dr. Rosalind Grymes, Associate Director of the NASA Astrobiology Institute, a research consortium that provided funding for the study. "This work is at the early end of that progression, and is fundamental to understanding life on Earth, and life beyond Earth."

Meteoriticists have determined a wide body of rules that models of chondrule melting must obey. For instance, scientists know that chondrules reached peak temperatures of 1800 to 2100 K for several minutes; that they almost melted completely; and that they cooled through crystallization temperatures of 1400 to 1800 K at rates slower than 100 K/hr, which kept them hot for hours. To prevent the loss of iron from the silicate melt, pressures had to be high -- greater than 0.001 atm -- which is orders of magnitude higher than the expected pressures in the nebula. A few percent of the chondrules stuck together while still hot and plastic. These "compound chondrules" tend to be more completely melted and to have cooled faster than the average chondrule. Satisfying all of these conditions simultaneously has been a challenge to theorists. In a 1996 review article by Alan Boss of the Carnegie Institution of Washington, nine possible mechanisms were reviewed, including lightning, shock waves, and asteroid impacts. More recently, the "X-wind" model has been introduced by Dr. Frank Shu of UC Berkeley, in which chondrules are melted near the protoSun. Even melting by a nearby gamma-ray burst has been considered. None of these ideas, however, has been developed to the point to calculate cooling rates precisely enough to match what is known about meteorites.

The model proposed by Desch and Connolly is the most detailed physical model yet of chondrule melting by any mechanism. It exactly correlates the cooling rates of chondrules -- a key meteoritic constraint -- with physical conditions in the solar nebula. The model includes several previously ignored effects, such as dissociation of the hydrogen gas by the shock wave, the presence of dust, and especially a precise treatment of the transfer of radiation through the gas, dust, and chondrules. According to the model, chondrules experience their peak heating immediately after passing through the shock front. Even though the gas is slowed almost instantaneously, the chondrules continue to move at supersonic speeds for minutes until friction slows them down. During this stage, chondrules emit intense infrared radiation. This radiation is absorbed by chondrules that haven't reached the shock front yet, and by chondrules that have already passed through it. This transfer of radiation is important to be calculated accurately, since the gas and chondrules cool only as fast as they can escape the intense infrared radiation coming from the shock front. With this effect included, typical cooling rates are 50 K/hr, which is exactly in line with what is known about the average chondrule. Moreover, Desch and Connolly predict a correlation with the density of chondrules: regions with more chondrules than average will produce chondrules that are more completely melted and cooled faster. This is because in dense regions radiation from the shock front cannot propagate as far before being absorbed and chondrules can escape the radiation from the shock front more rapidly. Compound chondrules are overwhelmingly produced in regions with high chondrule densities, so the extra heating and faster cooling of compound chondrules is easily explained by this shock model. Since the time a chondrule spends in a semi-melted, plastic state is also calculated by the model, even the frequency of compound chondrules can be determined -- it is on the order of a percent, satisfying another key constraint. Finally to satisfy another condition, shocks compress the gas to pressures orders of magnitude higher than the ambient pressure.

The source of the shock waves is not specified by Desch and Connolly, but they do identify gravitational instabilities as a likely candidate, assuming the solar nebula protoplanetary disk was massive enough. And there are sound theoretical reasons for believing it was. More importantly, observations of other protoplanetary disks in which planets are forming today indicate that sufficiently massive disks may be common. If shock waves triggered by gravitational instabilities are taking place in other protoplanetary disks, then the odds of chondrules melting and planets forming, including Earths around other stars are greatly increased.