杏吧原创

IN ITS youth the Sun may have swallowed vast numbers of asteroids and comets,
leaving its surface enriched with heavy elements, according to a British
astronomer. Mark Bailey of the Armagh Observatory in Northern Ireland says that
his finding could radically alter astronomers鈥� ideas about the Sun鈥檚 life
history. It could also explain why tiny particles called neutrinos, created by
nuclear reactions in the Sun鈥檚 core are detected at Earth at a lower rate than
expected.

Bailey, who studies near-Earth asteroids and comets, notes that far more
comets and asteroids are nudged into Earth-crossing orbits than anyone thought
until a few years ago. 鈥淭he natural endpoint of bodies moving inwards through
the Solar System is to crash into the Sun,鈥� says Bailey. 鈥淚t is now recognised
that the Sun is being bombarded by about ten times as much material as anyone
suspected five years ago.鈥�

Today, the amount of material falling into the Sun is very small. But Bailey
says that when the Solar System was young there was far more debris around, left
over from the planet-building process. These fragments would be rich in heavy
elements like carbon and oxygen, and the amount gathered up by the Sun could
have been 10 to 100 times the mass of the Earth. 鈥淭his is still small compared
with the Sun鈥檚 mass,鈥� says Bailey. 鈥淗owever, it could have significantly
enriched the Sun鈥檚 outer convection layer with heavy elements.鈥�

If the newborn Sun鈥檚 surface was enriched with heavy elements, our ideas
about solar evolution will have to change. Astronomers who model the evolution
of the Sun assume that the surface has remained unchanged since its birth, and
that the surface composition we observe now matches the original composition of
its interior. 鈥淚nstead, there may be fewer heavy elements in the interior than
anyone expected,鈥� says Bailey.

Heavy elements play a crucial role in 鈥渂ottling up鈥� solar radiation. If there
are smaller quantities of these elements in the Sun鈥檚 interior, heat will escape
more easily. This in turn would mean that the core needs to generate less energy
to create the observed solar brightness. Less energy also means nuclear
reactions that are not as active. 鈥淪ince neutrinos are associated with these
reactions, it may explain the shortage of solar neutrinos,鈥� says Bailey.

The possibility that the solar neutrino problem could be resolved if the
Sun鈥檚 core had fewer heavy elements than expected was suggested two years ago by
Eugene Levy and his colleagues at the University of Arizona (Astrophysical
Journal, vol 431, p 881). However, they were unable to imagine a plausible
way in which the Sun鈥檚 outer layers could become richer in heavy elements than
the core.

Levy now agrees that if 100 Earth masses of asteroid-like material did fall
into the Sun, as Bailey suggests, this would solve the neutrino problem.
鈥淗owever, I would be surprised if as much as 100 Earth masses of planetesimals
entered the solar convection zone shortly after formation,鈥� says Levy. Bailey
hopes to pin down the mass estimate more precisely with further studies of the
behaviour of asteroids and sun-grazing comets.

According to Douglas Gough of Cambridge University, the way the Sun vibrates
rules out the possibility that the average atomic mass in the core is currently
less than at the surface. However, Bailey says this does not necessarily
contradict his claim, since the conversion of hydrogen to helium over the Sun鈥檚
lifetime has boosted the core鈥檚 average atomic mass. This effect could ensure
that today it is above that of the surface, despite its shortage of heavy
elements.