JUPITER鈥檚 central heating system may be fuelled by helium, according to
data from the Galileo probe. Planetary scientists may now have to completely
rethink their theories of the giant planet鈥檚 structure.
Researchers believe Jupiter is made almost entirely of hydrogen and helium
from the primordial gas cloud that condensed to form our Solar System more
than four billion years ago. The Sun contains about 26 per cent helium by
weight, yet when the Galileo probe dropped into Jupiter鈥檚 upper atmosphere
last December, it found only half as much.
The Galileo probe measured the refractive index of samples of Jovian
atmosphere 鈥� an extremely accurate way of determining their helium
composition. The gas contained only 13.7 per cent helium, with a margin of
error of just 0.8 per cent.
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鈥淲e are missing every second helium atom,鈥� says Ulf von Zahn of the
University of Rostock in Germany, chief scientist for the probe鈥檚 helium-
measuring experiment.
The missing helium has not just vanished. 快猫短视频s believe that it has
condensed into droplets under the intense pressures of Jupiter鈥檚 lower
atmosphere. The droplets must then have sank towards the planet鈥檚 dense heart.
This 鈥渞ainout鈥� would release large amounts of energy, says von Zahn 鈥� and it
is this heating that is upsetting planetary scientists鈥� theories.
Jupiter must have an internal heat source, because it radiates more energy
out into space than it receives from the Sun. Planetary scientists had always
assumed that this was because Jupiter had not yet cooled completely, and was
simply being warmed by this leftover heat. But now all bets are off, says
David Stevenson of the California Institute of Technology in Pasadena, who
calculates that the helium rainout could explain most of the planet鈥檚 internal
warmth. 鈥淭his low helium value requires us to adopt a different view of the
energy budget of Jupiter,鈥� he says.
This means that scientists may have to recalculate their models of
Jupiter鈥檚 structure, which could produce more surprises. One possibility, says
Stevenson, is that Jupiter may not actually have a dense core of rock and ice.
Current theories predict that Jupiter formed by the gravitational attraction
of gas towards a dense core. If the planet has no solid core, scientists will
have to think up a new explanation. 鈥淭hat would be profoundly important,鈥� says
Stevenson.
As most scientists struggle to come to grips with the new results, Daan
Frenkel of the Institute for Atomic and Molecular Physics in Amsterdam and his
colleagues in Northern Ireland and Germany are feeling vindicated. Last
September, they published a paper in Science suggesting that Jupiter鈥檚 upper
atmosphere might contain less helium than everyone thought.
The researchers calculated the velocity of sound in Jupiter鈥檚 dense
interior using the equations of quantum mechanics and molecular dynamics. But
astronomers can also deduce the velocity of sound within Jupiter by observing
the planet鈥檚 鈥渙scillations鈥� 鈥� natural vibrations similar to the ringing of a
bell. And this figure was at odds with the value calculated by Frenkel and his
colleagues.
However, Frenkel believes the disagreement will disappear once astronomers
modify the theoretical models they use to calculate the velocity of sound in
the Jovian interior to account for the fact that much of the planet鈥檚 helium
has sunk towards its core. 鈥淭he low atmospheric helium abundance is exactly
the kind of effect that would solve the discrepancy,鈥� he says.