Researchers examining meteor showers have discovered that not all comets break apart in the same manner as they get closer to the Sun. A recent study indicates that the differences in behavior stem from the conditions present in the protoplanetary disk from which comets originated 4.5 billion years ago.
An international team of 45 researchers investigating meteor showers has found that comets exhibit varying behaviors when they approach the Sun. In a paper released in the journal Icarus this week, the researchers attribute these differences to the environment in the protoplanetary disk where comets were formed 4.5 billion years ago.
“The meteoroids we observe as meteors in the night sky are similar in size to small pebbles,” stated lead author Peter Jenniskens, a meteor astronomer with the SETI Institute and NASA Ames. “In fact, they are the same dimensions as the pebbles that eventually coalesced into comets during our solar system’s formation.”
As our solar system developed, tiny particles within the disk surrounding the young Sun gradually grew until they reached pebble size.
“Once these pebbles became sufficiently large, they could no longer drift with the gas and were destroyed by collisions before they could increase in size,” explained Paul Estrada, a planetary scientist at NASA Ames and co-author of the study. “Comets and primitive asteroids formed when groups of these pebbles collapsed into larger bodies, sometimes reaching kilometer sizes.”
Fast forward 4.5 billion years: today, when comets get close to the Sun, they break down into smaller fragments called meteoroids. These meteoroids orbit alongside the comet for a period and can later lead to meteor showers when they enter Earth’s atmosphere.
“We hypothesized that comets break apart into fragments that are similar in size to the pebbles they are composed of,” Jenniskens noted. “If this is the case, the size distribution and physical and chemical characteristics of young meteoroid streams can still provide insights into the conditions in the protoplanetary disk during their formation.”
Jenniskens and his team of both professional and amateur astronomers utilize specialized low-light video cameras in networks worldwide to monitor meteors through a NASA-supported project called “CAMS” – or Cameras for Allsky Meteor Surveillance (http://cams.seti.org).
“These cameras track the paths of meteoroids, their altitude upon ignition, and their deceleration in Earth’s atmosphere,” Jenniskens said. “Certain specialized cameras also analyzed the composition of some meteoroids.”
The research team examined 47 recent meteor showers. Most of these showers stem from two categories of comets: Jupiter-family comets originating from the Scattered Disk of the Kuiper Belt, situated beyond Neptune, and long-period comets from the Oort Cloud encompassing our solar system. Notably, long-period comets travel along much broader orbits than their Jupiter-family counterparts and are less strongly influenced by the Sun’s gravitational pull.
“We discovered that long-period (Oort Cloud) comets tend to break apart into sizes that suggest they formed under milder accretion conditions,” explained Jenniskens. “Their meteoroids have a low density and exhibit a fairly consistent 4% presence of a specific type of solid meteoroid that underwent heating in the past, which only illuminates at deeper levels in Earth’s atmosphere and generally contains low sodium content.”
Conversely, Jupiter-family comets usually disintegrate into smaller, denser meteoroids that, on average, contain a higher 8% of solid materials and display greater variety in their composition.
“We concluded that Jupiter-family comets consist of pebbles that had reached a stage where their fragmentation became significant during their size evolution,” Estrada remarked. “The increased mixture of previously heated materials is expected to be found closer to the Sun.”
Primitive asteroids, which formed even nearer to the Sun but still beyond Jupiter’s orbit, produce meteor showers with even smaller particles, illustrating that their pebble-like building blocks underwent more severe fragmentation.
“While there are exceptions within both groups, the overall implication is that most long-period comets were created under gentler conditions for particle growth, possibly near the 30 AU boundary of the Trans Neptunian Disk,” Estrada mentioned. “In contrast, most Jupiter-family comets developed closer to the Sun, allowing pebbles to reach or exceed the fragmentation threshold, and primitive asteroids originated in the region where the giant planets took shape.”
How did this occur? During the formation of the giant planets, Neptune migrated outward, scattering both comets and asteroids from the remaining protoplanetary disk. This outward movement likely gave rise to both the Scattered Disk of the Kuiper Belt and the Oort Cloud. Although one might expect both long-period and Jupiter-family comets to share similar characteristics, the research team found otherwise.
“It’s possible that stars and molecular clouds in the Sun’s origin region disturbed the broad orbits of Oort Cloud comets early on, and the long-period comets we observe today were only scattered into such orbits at a time when the Sun had moved away from that area,” Jenniskens explained. “On the other hand, Jupiter-family comets have always maintained shorter orbits and interact with all objects scattered by Neptune during its outward journey.”
