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 fftion 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 >TIMhondrules 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.