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A STICKY coating on the dust from which Earth and its neighbours formed could
explain a long-standing mystery—why there’s so little rubble left in the
asteroid belt between Mars and Jupiter.

The planets and asteroids of the Solar System started to build when dust
grains—typically silicates with a coating of carbon compounds, encased in
ice—stuck together in a swirling disc round the young Sun. Experiments
have already simulated this process by colliding particles made of silicates and
ice.

But Akira Kouchi of Hokkaido University in Japan suspects that this may be
only part of the story. He reckons that the ice on the dust grains would
probably have partially evaporated as the young Solar System warmed up, exposing
the carbon coat and making the particles stickier.

To investigate, Kouchi and his team dropped a 1-centimetre copper sphere onto
a copper surface coated with the kinds of carbon-based molecules typically found
in space, then measured the force needed to pull the sphere away. They repeated
this for different impact speeds and temperatures from 200 to 300 kelvin.

The organic compounds turned out to be surprisingly sticky—and were
stickiest at about 250 kelvin, roughly the temperature of the young asteroid
belt. This suggests that 1-millimetre interstellar grains would have stuck
together even if they collided at speeds up to 5 metres per second, they say in
a paper to appear in Astrophysical Journal Letters. Particles of a
similar size made only of silicate and ice have never been seen to stick, even
when moving slowly. So Kouchi concludes that dust grains in the asteroid belt
could have globbed into mini planets earlier in the Solar System’s history than
anyone thought, while gas and dust were still plentiful in the region. Slowed by
this material, the new-born rocks would have been tugged towards the Sun by its
gravity. The exodus could explain why there is much less material in the
asteroid belt than theories predict.