"The meteoroids we see as meteors in the night sky are the size of small pebbles," stated lead author and SETI Institute and NASA Ames meteor astronomer Peter Jenniskens. "They are, in fact, the same size as the pebbles that collapsed into comets during the formation of our solar system."

As the solar system was forming, minute particles within the disk around the nascent Sun gradually merged into larger bodies, eventually reaching the size of small pebbles.

"Once pebbles grow large enough to no longer travel along with the gas, they are destroyed by mutual collisions before they can grow much bigger," explained NASA Ames planetary scientist and co-author Paul Estrada. "Comets and primitive asteroids instead were formed when clouds of these pebbles locally collapsed into kilometer-sized and larger bodies."

Today, 4.5 billion years later, comets approaching the Sun disintegrate into smaller fragments called meteoroids. These meteoroids can temporarily share an orbit with the parent comet before eventually colliding with Earth's atmosphere, resulting in meteor showers.

"We hypothesized that comets crumble into the sizes of the pebbles they are made of," Jenniskens noted. "In that case, the size distribution and the physical and chemical properties of young meteoroid streams still contain information about the conditions in the protoplanetary disk during this collapse."

Jenniskens and his team, composed of both professional and amateur astronomers, monitor meteors using specialized low-light video cameras in a NASA-sponsored initiative called "CAMS" - Cameras for Allsky Meteor Surveillance (http://cams.seti.org).

"These cameras measure the meteoroids' paths, how high they are when they first light up, and how they slow down in Earth's atmosphere," Jenniskens added. "Specialized cameras measured the composition of some of these meteoroids."

The researchers analyzed 47 young meteor showers, most of which originate from two distinct types of comets: Jupiter-family comets from the Scattered Disk of the Kuiper Belt beyond Neptune and long-period comets from the Oort Cloud surrounding our solar system. Unlike the more tightly bound Jupiter-family comets, long-period comets follow much wider orbits, held loosely by the Sun's gravity.

"We found that long-period (Oort Cloud) comets often crumble into sizes indicative of gentle accretion conditions," Jenniskens remarked. "Their meteoroids have a low density. The meteoroid streams contain a fairly constant 4% of a type of solid meteoroids that were heated in the past and now only brighten deeper in Earth's atmosphere and typically are poor in the element sodium."

Conversely, Jupiter-family comets tend to break apart into smaller, denser meteoroids, which contain a higher 8% of solid material on average and display more variety in their composition.

"We concluded that these Jupiter-family comets are composed of pebbles that had reached the point where fragmentation became important in their size evolution," said Estrada. "The higher admixture of materials that were heated in the past are expected closer to the Sun."

Primitive asteroids, which formed even closer to the Sun but still beyond Jupiter's orbit, generate meteor showers with even tinier particles, suggesting that their pebble-sized building blocks underwent even more intense fragmentation.

"While there are exceptions in both groups, the implication is that most long-period comets formed under more gentle particle growth conditions, possibly near the 30 AU edge of the Trans Neptunian Disk," Estrada commented. "Most Jupiter family comets formed closer to the Sun where pebbles reached or passed the fragmentation barrier, while primitive asteroids formed in the region where the cores of the giant planets formed."

As Neptune migrated outward during the growth of the giant planets, it scattered comets and asteroids from the remaining protoplanetary disk, contributing to the formation of the Kuiper Belt's Scattered Disk and the Oort Cloud. Although this outward movement would suggest similar properties in long-period and Jupiter-family comets, the study's findings challenge this assumption.

"It is possible that stars and molecular clouds in the birth region of the Sun perturbed the wide orbits of Oort Cloud comets early on, and the long-period comets we see today were scattered into such orbits only at a time when the Sun had moved out of this region," Jenniskens suggested. "In contrast, Jupiter-family comets have always been on shorter orbits and sample all objects scattered by Neptune on its way out."

Research Report:Properties of outer solar system pebbles during planetesimal formation from meteor observations

Related Links
SETI Cameras for Allsky Meteor Surveillance
Asteroid and Comet Mission News, Science and Technology