Fatigue, that's what.
As a particularly frigid winter recedes across the north and east of the United States (we've become accustomed to milder weather in past years), the abuse suffered by asphalt roads is becoming apparent. If you've taken any form of surface transport recently in a place like New York City you'll have experienced the crunching, whumping, metal-scraping presence of potholes, slumps and fractured streets. I've already lost count of the number of differently inventive expletives I've heard cab drivers use as wheels plunge into six inch voids, where axles grind and repair bills skyrocket.
The cause of this extensive cratering is a combination of factors, including the repeated freezing and thawing of moist earth and strata beneath street asphalt, and of water that's permeated cracks or holes in the tarry layer itself. Add on the bombardment of vehicles and the whole thing starts to fracture and crumble.
Interestingly, new work on the nature of asteroid surfaces in the solar system suggests that a not dissimilar process is at play there too. Thermal fatigue may be crumbling the outer layers of kilometer-sized bodies, turning them into a loose regolith of centimeter sized nuggets and dusty fragments. In the research, reported in Nature by Delbo et al., meteoritic material here on Earth was subjected to the simulated environment of a spinning asteroid out in space, facing the glare of stellar radiation. Without an atmosphere to bathe in, and to slow down the loss or gain of thermal energy, the surface of objects in a vacuum can get alternately roasted and frozen with abrupt temperature shifts of 100-200 Kelvin. This places stress on rocky material from expansion and contraction. Microfractures can form and grow in outer layers from the constant thermal shifts, breaking the solids into smaller and smaller bits.
The relentless switch between deep winter and high summer with every day-night cycle helps answer a long standing question about what causes the loose outer layers of asteroids in the first place. Previously it had been proposed that impacts from other smaller bodies and micrometeoroids gradually pound and loft the material into pieces that fall back to cover the asteroid surfaces. But the maths doesn't seem to quite work out for that, a lot of material should get ejected to escape velocity, rather than coating these bodies. Now it seems that the cosmic equivalent of spring potholes may be responsible.
The experiments also suggest that thermal fatigue could be so efficient at breaking down small asteroids that, if their elliptical orbits carry them between the Earth and the Sun, they can be crumbled to dust and blown to interplanetary space by the solar wind in as little as only 2 million years. By cosmic standards that's not long, and would help explain the apparent scarcity of small (100 meter) carbonaceous bodies passing inward of Earth's orbit.
So the next time you hear a cabbie cursing at the crumbling, pothole-filled street, spare a thought for all those lonely asteroids, baking and freezing in the depths of interplanetary space, slowly reducing to dust.