According to a team of scientists co-led by Prof Maria Zuber of Massachusetts Institute of Technology and Prof Jay Melosh of Purdue University, meteorites are byproducts of planet formation rather than planetary building blocks.
Chondrules in meteorites are the remnants of early protoplanetary collisions, scientists say. Image credit: NASA / JPL-Caltech.
Meteorites have long been regarded as relics of the early Solar System. These objects are studded with chondrules – millimeter-scale, previously molten, glassy spherules.
Planetary scientists have thought that chondrules represent early kernels of terrestrial planets: as the Solar System started to coalesce, these molten droplets collided with bits of gas and dust to form larger planetary precursors.
However, Prof Zuber, Prof Melosh and their colleagues have now found that chondrules may have played less of a fundamental role.
Based on computer simulations, the scientists conclude that chondrules were not building blocks, but rather byproducts of a violent and messy planetary process.
“Understanding the origin of chondrules is like looking through the keyhole of a door; while we can’t see all that is happening behind the door, it gives us a clear view of one part of the room and a glimpse into the very beginnings of our Solar System,” said Prof Melosh, who is a co-author of a paper published in the journal Nature.
“We’ve found that an impact model fits extremely well with what we know about this unique material and the early Solar System, and this suggests that, contrary to the current opinion among experts, meteorites are not leftover planet-building material and clumps of chondrules are not prerequisite to a planet.”
Prof Melosh added: “chondrules are identical in size, shape and texture to spherules on Earth and spherules found in the lunar soil. The only difference among chondrules, impact spherules and lunar soil particles is in their chemical composition, which fits because they are made of different starting materials from impacts on different bodies.”
“Impact spherules are small droplets of solidified molten rock found embedded in rocks on Earth. It is widely accepted that impacts created the spherules, which formed from droplets of molten rock in the plume of debris ejected when large asteroids crashed into the Earth. The droplets condensed and solidified to form the spherules, which then fell back to the surface creating a distinct layer on the Earth.”
The method of chondrule creation proposed by the scientists is slightly different and focuses on a small portion of debris ejected at the earliest moments of impact created by a process called jetting.
Jetting occurs at the beginning of impact as the surfaces of the two objects meet. The rock caught in the pinch between the two colliding objects is compressed to high pressure and intensely heated, which is responsible for the initial bright flash seen in laboratory impacts. The heat created by jetting is enough to melt rock and create droplets in the ejected debris that could become chondrules.
“Impact origin theories proposed in the past had been dismissed because they could not explain the melted material found in chondrules,” Prof Melosh said.
In the early Solar System, collision speeds were much lower than they are now. The planetary embryos were no larger than the Earth’s moon and their collisions were relatively gentle, occurring at a speed of a few kilometers per second. For the most part, impacts at this speed would blast rock into broken fragments, but not melt it.
“Jetting allows a low-velocity impact to melt a small quantity of the target rock. The melted material, but not the broken rock, is then ejected at high speed, such that the molten droplets can escape their parent bodies and depart into space, to later loosely bunch together. Millions of years of additional impacts and other compression mechanisms then created the asteroids and meteorites we know today,” Prof Melosh explained.
“The debris ejected at high speed escapes the gravitational pull of the planetary embryo, while the majority of the debris plume falls back to the surface. The dust and molten droplets quickly slow to relatively low velocities due to the nebular gas in the early Solar System. The gas provides a soft catch for the chondrules that allows them to accumulate into smaller bodies that eventually become asteroids.”
“Chondrules have long been a puzzling feature of meteorites and, if they weren’t observable in meteorites, scientists would likely never have predicted their existence,” Prof Melosh said.
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Brandon C. Johnson et al. 2015. Impact jetting as the origin of chondrules. Nature 517, 339–341; doi: 10.1038/nature14105
