Puzzling Science: Answering Questions About Comets And Asteroids
July 26, 2012

New Research Helps To Solve Mysteries Of Hot And Cold Comets And Asteroids

Lee Rannals for redOrbit.com - Your Universe Online

Astronomers have started to answer questions about two unsolved puzzles about the building blocks of our Solar System.

Two questions that have puzzled scientists about comets and asteroids are: How did icy comets obtain particles that formed at high temperatures? And, how did these refractory particles acquire rims with different compositions?

Researchers from Carnegie Institution for Science have modeled the trajectories of these types of particles in the unstable disk of gas and dust that formed the Solar System.

The team found that these refractory particles may have first been processed in the hot inner disk, and then travelled out to the outer regions of the Solar System to end up in icy comets.

Moving from one hotter part of a region to a cooler area could help explain the different compositions of their rims, according to the researchers.

During the earlier years of the Sun, scientists believed that our local star experienced a series of outbursts that were caused by the rapid infall of disk gas onto the Sun. They believe these outbursts were caused by disk instability.

The Carnegie researchers modeled the trajectories of several hundred centimeter-sized melilite mineral particles during a phase of disk instability.

The disk model had a marginally gravitationally unstable disk, with a mass of about 5 percent of today's Sun and temperature ranging from -350 degrees Fahrenheit to 2,240 degrees Fahrenheit. The team's calculations allowed the calcium-aluminum-rich inclusions (CAIs) to orbit the disk while being subjected to gas drag and gravity of both the disk and the Sun. These refractory particles are often found in well-preserved meteorites, as well as the comet Wild 2.

The particles started out orbiting in unison, but after about 20 years their trajectories started to diverge. Most struck the inner boundary of the disk, while others went to the outer boundary, where they could be swept up by a growing comet. About 10 percent migrated back and forth in the disk before hitting one or the other boundary, according to the research.

The team then modeled the evaporation and condensation processes that the particles would experience during their migrations, finding that these particles most likely acquired outer rims with varied isotopic compositions that characterize CAIs.

“CAIs are thought to have formed at the very beginning of the Solar System. Our results show that they must have experienced remarkably complex histories as they were transported chaotically all over the disk,” cosmochemist Conel Alexander wrote in the study published in Earth and Planetary Science Letters.

These migrations may help explain the different oxygen isotopes that were found in particles from meteorites. These types of oxygen atoms have a different number of neutrons that point to different processing conditions for the particle rims.

Previous work done by Carnegie´s theoretical astrophysicist Alan Boss, who was also a part of the current research, showed that an abundance of oxygen isotopes could vary in an unstable disk by the range that have been found in meteorites.

Taking Boss' discovery into account, the latest models show that several puzzles may have been solved. Having an unstable disk can explain both large-scale outward transport or refractory particles, as well as the peculiar rim compositions acquired during their journeys.

“It´s nice to solve two problems at once,” Boss said. “But there are still many more puzzles about meteorites for us to work on.”