New research has shed light on the collisional history of one of the most famous asteroids.
Itokawa’s curious, varied terrain and lack of impact craters indicate it is a rubble pile asteroid. Image credit: Japan Aerospace Exploration Agency.
Asteroid 25143 Itokawa is a near-Earth, rubble-pile asteroid, measuring 535 m x 294 m x 209 m.
It was discovered in 1998 by astronomers with the LINEAR project and, seven years later, became the first asteroid from which samples were captured and brought to Earth for analysis.
Japan Aerospace Exploration Agency’s Hayabusa spacecraft touched down twice on Itokawa and collected a small amount of dust. It delivered about 1,500 dust particles to Earth in 2010.
Now, Curtin University planetary researcher Fred Jourdan and colleagues have studied two small particles from Itokawa.
The scientists used the Argon-Argon dating technique to investigate when impact crater events happened on Itokawa.
“Itokawa was no ordinary asteroid, with fly-by pictures taken by Hayabusa prior to sampling in 2005 showing it had a peanut-like shape and resembled a rubble pile of boulders and dust more than solid rock,” Dr. Jourdan said.
“Prior to being a rubble pile, Itokawa was part of a much larger asteroid that was destroyed by a collision with another asteroid. Our job was to try to find out when that collision happened.”
“Using our noble gas mass spectrometer, we were able to measure tiny amounts of gas and analyze these fragments from Itokawa.”
“The impact-shocked particle indicated a small-scale collision that occurred 2.1 billion years ago, whereas the other non-shocked particle preserves a very old age, similar to the formation age of the Solar System itself.”
According to these results and a series of models, Dr. Jourdan and co-authors concluded that asteroids do not always break up due to a single cataclysmic impact.
Instead, they can internally fragment due to the medium-sized collisions that constantly batter large asteroids until they shatter from impact.
“The final impact could be seen as ‘the straw that broke the camel’s back’,” Dr. Jourdan said.
“Our results tell us that Itokawa was already broken and re-assembled as a rubble pile about 2.1 billion years ago, showing that ‘rubble pile’ asteroids can survive a much longer time in this state than researchers previously thought.”
“This is due to their cushion-like nature and the abundance of dust in between the boulders.”
The findings are published in the journal Geology.
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F. Jourdan et al. 2017. Collisional history of asteroid Itokawa. Geology 45 (9): 819-822; doi: 10.1130/G39138.1
