Even the mighty can lose heart. New calculations
suggest that Jupiter's rocky core is dissolving like an antacid tablet
plopped in water.
The work could help explain why its core appears smaller and its atmosphere richer in heavy elements than predicted.
Giant planets like Jupiter and Saturn
are thought to have begun their lives as solid bodies of rock and ice.
When they grew to about 10 times the mass of Earth, their gravity pulled
in gas from their birth nebula, giving them thick atmospheres made mainly of hydrogen.
Curiously, some studies have suggested
that Jupiter's core may weigh less than 10 Earths, while the core of
its smaller sibling Saturn packs a bigger punch at 15 to 30 Earths. Last
year, researchers led by Shu Lin Li of Peking University in China
offered a grisly explanation – a rocky planet bigger than Earth slammed into Jupiter long ago, vaporising most of the giant planet's core.
That scenario could also explain another mystery – why
Jupiter's atmosphere contains a higher fraction of heavy elements than
the sun, whose composition is thought to mirror that of the nebula that
gave birth to the solar system's planets.
Now Hugh Wilson and Burkhard Militzer
of the University of California, Berkeley, suggest a competing – though
no less macabre – explanation: Jupiter's core has gradually been
dissolving since its formation 4.5 billion years ago.
Quantum calculations
Other researchers had propoposed that
the intense pressures and temperatures at Jupiter's heart might cause
its core to dissolve into the surrounding atmosphere, which is at such
high pressure that it behaves somewhat like a liquid.
"We sat down to figure out, does this actually happen?" says Wilson.
The researchers used the equations of
quantum mechanics to see how the mineral magnesium oxide – thought to be
a constituent of Jupiter's core – behaves at Jupiter-like pressures of
about 40 million Earth atmospheres and temperatures of 20,000 °C. Such
conditions cannot be recreated in Earth labs – some experiments can approximate the pressures but overshoot the temperatures by factors of a hundred or so.
Mixed up
They found that magnesium oxide does
indeed dissolve into its fluid surroundings in those conditions. "You
can think of it as if you have some salt in the bottom of a glass. Pour
warm water on the salt and it will start to dissolve in the glass, with
salty water in the bottom and less salty water at the top," says Wilson.
He suspects that the dissolved rock
might get mixed into the rest of the atmosphere over time. "It could at
least partially explain both the enrichment of heavy elements in the
outer atmosphere and also the fact that its core may be smaller than
[formation] models would suggest," Wilson says.
The calculation also suggests why
Saturn – which is about one-third the mass of Jupiter – seems to have a
heftier core. Conditions inside the ringed planet are simply not as
extreme as they are inside Jupiter, so if Saturn's core is dissolving at
all, "it will be a lot slower", says Wilson.
Heartless planets
The process probably happens much more rapidly in planets more massive than Jupiter, the team thinks. Dave Stevenson
of the California Institute of Technology in Pasadena agrees. "Core
erosion is probably more effective as the mass goes up," he says.
"I suspect that for very large
'super-Jupiters', you would have no core at all," says Wilson. If so,
this should boost the concentration of heavy elements in their
atmospheres, which future telescopes might be able to detect, he says.
Does the fact that Jupiter's heart is
dissolving sadden Wilson? Quite the opposite, he says. "It's kind of a
sign that Jupiter is still forming – it hasn't yet settled down into a
steady state."
