It's stressful being a small asteroid. They fly unguided around the solar system, get bullied by gravitational fields and can smack into a planet at any time. Now these space rocks can add fatigue to their list of stresses, thermal fatigue that is.
In a study published online today (April 2) in the journal Nature, an international team of astronomers have presented their findings that the majority of fine debris that collects on the surface of small asteroids isn't formed through asteroid collisions, but is mainly formed through a kind of space erosion.
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Regolith is found on planetary surfaces such as the moon or even Mars where the atmosphere is thin or even non-existent. Regolith is the fine, grainy material that is formed through the pulverization of rock over aeons of meteorite impacts. Earth doesn't have regolith as frequent meteorite impacts are mitigated by our thick atmosphere and weathering processes break down any broken rock into other mineral deposits and cycled through biological and chemical processes to form soil.
Asteroids are known to possess dusty regolith on their surfaces and scientists have always assumed that the material - composed of grains of rock under a centimeter in size - is formed through asteroid and micrometeorite impact debris settling onto the asteroids.
But there's a problem with this model.
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Over asteroid evolution timescales, there is too much regolith on small asteroids' surfaces to have been deposited there solely through impacts. The impact energies of these collisions, the asteroid spin rate and the very low gravity asteroids possess means the debris should be flung away into interplanetary space.
So how did all that regolith get there?
While carrying out tests on meteorites found on Earth, the researchers modeled the space environment to see how these samples deteriorated.
"We took meteorites as the best analog of asteroid surface materials that we have on the Earth," said Marco Delbo of the Observatoire de la Côte d'Azur, France. "We then submitted these meteorites to temperature cycles similar to those that rocks experience on the surfaces of near-Earth asteroids and we found that microcracks grow inside these meteorites quickly enough to entirely break them on timescales much shorter than the typical lifetime of asteroids."
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This process is known thermal fatigue and is caused by the rapid day-night cycle of a small spinning asteroid. The rapid heating and cooling creates thermal expansion and contraction in the asteroid material, initiating cracking and eventual fragmentation.
Using the meteorite data, the researchers were able to extrapolate over long timescales and found that the fragmentation of asteroid material through thermal fatigue was more rapid than fragmentation through micrometeorite impacts over the life of a given asteroid.
Perhaps unsurprisingly, thermal fatigue is more significant for asteroids that orbit close to the sun than those that orbit at greater distances, but "even asteroids significantly farther from the sun showed thermal fatigue fragmentation to be a more relevant process for rock breakup than micrometeoroid impacts," added collaborating scientist Simone Marchi, of the Southwest Research Institute.
Through a combination of thermal fatigue fragmentation and solar wind pressure, the researchers also found that small asteroids with compact orbits around the sun (coming as close as 28 million miles from the sun, well inside the orbit of the planet Mercury) will be completely eroded away within 2 million years.