Space dust gives asteroids shape

Asteroid surfaces are heavy with bombardment and are being shaped by impacts at even a microscopic level, researchers have found. Credit: JAXA

CAMBRIDGE: Tiny particles of space dust bombard asteroids and shape their surface at a microscopic level, scientists have found.

A study published today in Proceedings of the National Academy of Science analysed material from the surface of a near-Earth asteroid. The features discovered provide unprecedented detail about the history of primitive solar bodies.

“Our findings show the landscape itself of the very surface of an asteroid,” said Eizo Nakamura, who worked on the study with colleagues at Okayama University in Misasa, Japan and the Japan Aerospace Exploration Agency (JAXA). “An asteroidal surface is not a quiet place, but is heavy with bombardment.”

Exploring the surface of an asteroid

Asteroids are considered to be intermediate products of processes that formed large solar bodies. Studying them therefore tells us about the formation of our Solar System and the processes that have shaped the planets and moons.

Previous information gathered about asteroids has come from remote observations measuring properties such as the reflection of light from the surface. Such techniques have provided only limited insight into their chemical compositions and surface properties.

However, in 2010 the Hayabusa mission – conducted by JAXA – collected a sample of asteroid Itokawa, the first ever asteroid material to be brought to Earth. The new study used this sample to reveal asteroidal surface features never seen before.

Space weathering

Nakamura and his team examined five dust grains from Itokawa’s surface, each measuring just 50 micrometres across. As well as analysing the grains’ chemical compositions, the team used electron microscopy to look at surface features. They found evidence of hundreds of collisions with tiny specks of space dust. “Discoveries included adhered objects and nano-sized craters,” said Nakamura. “The existence of the craters is evidence of 10 nanometre-scale projectiles.”

Characteristics of the features indicate that nanoparticles were travelling at hypervelocities – tens of kilometres per second – resulting in high-energy impacts sufficient to change the chemical properties of the asteroidal surface and produce enough heat for localised melting. Impacts would also have scattered particles, adding to interplanetary dust.

The work identifies the exact processes underlying ‘space weathering’, a term that had previously been used to explain differences in general properties of asteroid surfaces. Knowing exactly how this space-weathering occurs helps in understanding how asteroids and other solar bodies form and develop.

Complex asteroidal process

John Bridges from Britain’s University of Leicester welcomed the study, commenting, “[The authors] have gathered some fascinating new data, showing the complexity of asteroidal processes. The Itokawa craters show the importance of impacts in shaping asteroids.”

Bridges also sees similarities between the new findings and those of the NASA Stardust mission, on which he also works. “The hypervelocity impact features identified in the samples [from Itokawa] are reminiscent of the hypervelocity craters analysed on the surfaces that the Stardust mission returned from Comet Wild2.”

Nakamura added that asteroids are the “most primitive solar bodies”, and can tell us about the formation of the Solar System, its history and the processes that have shaped it, but more work needs to be done. “Age determination using isotopes is the most important step to anchor the processes,” he said. “To do so, a reasonable amount of sample is required. To collect rocks from an appropriate asteroid in an appropriate amount is the next step. Our analysis is the initial analysis only, using five tiny particles. There remain thousands of particles preserved [by] JAXA, so further analyses following our methodology will enable better understanding of space environments on small asteroids.”