From NASA Ames: “NASA Researchers Find Evidence of Planet-Building Clumps”

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Aug. 21, 2018
Darryl Waller
NASA Ames Research Center, Silicon Valley

Noah Michelsohn
NASA Johnson Space Center, Houston

False-color image of Allendale meteorite showing the apparent golf ball size clumps. Credits: NASA/J. Simon and J. Cuzzi

NASA scientists have found the first evidence supporting a theory that golf ball-size clumps of space dust formed the building blocks of our terrestrial planets.

A new paper from planetary scientists at the Astromaterials Research and Exploration Science Division (ARES) at NASA’s Johnson Space Center in Houston, Texas, and NASA’s Ames Research Center in Silicon Valley, California, provides evidence for an astrophysical theory called “pebble accretion” where golf ball-sized clumps of space dust came together to form tiny planets, called planetesimals, during the early stages of planetary formation.

“This is very exciting because our research provides the first direct evidence supporting this theory,” said Justin Simon, a planetary researcher in ARES. “There have been a lot of theories about planetesimal formation, but many have been stymied by a factor called the ‘bouncing barrier.’”

“The bouncing barrier principle stipulates that planets cannot form directly through the accumulation of small dust particles colliding in space because the impact would knock off previously attached aggregates, stalling growth. Astrophysicists had hypothesized that once the clumps grew to the size of a golf ball, any small particle colliding with the clump would knock other material off. Yet, if the colliding objects were not the size of a particle, but much larger – for example, clumps of dust the size of a golf ball – that they could exhibit enough gravity to hold themselves together in clusters to form larger bodies.”

Mosaic photograph of the ancient Northwest Africa 5717 ordinary chondrite with clusters of particles. Credits: NASA/J. Simon and J. Cuzzi

The research provides evidence of a common, possibly universal, dust sticking process from studying two ancient meteorites – Allende and Northwest Africa 5717 – that formed in the pre-planetary period of the Solar System and have remained largely unaltered since that time. Scientists know through dating methods that these meteorites are older than Earth, Moon, and Mars, which means they have remained unaltered since the birth of the Solar System. The meteorites studied for this research are so old that they are often used to date the Solar System itself.

The meteorites were analyzed using electron microscope images and high-resolution photomicrographs that showed particles within the meteorite slices appeared to concentrate together in three to four-centimeter clumps. The existence of the clumps demonstrates that the meteorites themselves were produced by the clustering of golf ball-sized objects, providing strong evidence that the process was possible for other bodies as well.

The research, titled “Particle size distributions in chondritic meteorites: Evidence for pre-planetesimal histories,” was published in the journal Earth and Planetary Science Letters in July. The publication culminated six years of research that was led by planetary scientists Simon at Johnson and Jeffrey Cuzzi at Ames.

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Ames Research Center, one of 10 NASA field Centers, is located in the heart of California’s Silicon Valley. For over 60 years, Ames has led NASA in conducting world-class research and development. With 2500 employees and an annual budget of $900 million, Ames provides NASA with advancements in:
Entry systems: Safely delivering spacecraft to Earth & other celestial bodies
Supercomputing: Enabling NASA’s advanced modeling and simulation
NextGen air transportation: Transforming the way we fly
Airborne science: Examining our own world & beyond from the sky
Low-cost missions: Enabling high value science to low Earth orbit & the moon
Biology & astrobiology: Understanding life on Earth — and in space
Exoplanets: Finding worlds beyond our own
Autonomy & robotics: Complementing humans in space
Lunar science: Rediscovering our moon
Human factors: Advancing human-technology interaction for NASA missions
Wind tunnels: Testing on the ground before you take to the sky

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