American Scientist

By gazing into slices of primitive meteorites, planetary scientists have reconstructed the tumultuous early days of the Solar System, when huge shock waves boiled and then froze minerals in the swirling cloud of dust around the newborn Sun.

These molten mineral aggregates, called chondrules, accreted with other materials to form carbonaceous chondrites—a type of nonmetallic meteorite—during the first two to three million years of the Solar System’s formation. Such meteorites still fall to Earth, providing direct evidence of conditions from 4.6 billion years ago. Isotope geochemist Nicole Xike Nie of the Massachusetts Institute of Technology studies the chemical composition of the chondrules to identify minerals that were present in the nascent Solar System and to hypothesize about the environment in which they formed.

“In the early Solar System, there were a lot of chondrules floating around,” Nie said. “But we don’t know how they were formed. That question has been there for maybe more than 50 years.”

In a search for clues, Nie and her team slice sections of meteorite thin enough for light to pass through and capture images of the embedded chondrules using a petrographic microscope, a specialized kind used to study minerals. To the naked eye the meteorites look gray, but under the microscope with polarized light the chondrules shine like colorful stained glass scattered throughout the rock. Each mineral reflects and refracts light differently; for example, olivine creates shards of green.

“You can think about it like a chocolate chip cookie,” Nie said. “You have the chondrules there that are like the chocolate chips, and the cookie dough would be the matrix that glues these chondrules together.” The rocky matrix gives the meteorite its gray appearance and encloses the microscopic rainbow of chondrules.

Nie and her team recently made an intriguing discovery. They found that the isotopic composition of the chondrules indicates that objects were rapidly heated and cooled. For the chondrules to form, the temperature of the Solar System must have spiked to about 1,500 degrees Celsius, which melted the space dust and caused the more volatile elements to evaporate. Nie’s team determined that the chondrules then cooled at a rate of about 500 degrees Celsius per hour—too fast for some of the elements to condense back into the chondrule.

These findings support the theory that chondrules were created by large shock waves. Planetesimals (small planets in the early Solar System) about the size of Mars’s moons could have produced the shock waves as they moved through the gas and dust field of the solar nebula. The fluctuations in heating and cooling required to create the observed chemical compositions of the chondrules rule out competing theories, such as lightning discharges crackling through the nebula.

Understanding how chondrules were created provides a window into the formation and composition of the Solar System. “Carbonaceous chondrites are one of the most important building blocks of our planets. They were the building materials of the larger planetary bodies,” Nie said. “By studying them, we can probably get a lot of information on the conditions of the early Solar System and the physical and chemical processes that happened.”