Smacks in the Face Explain Unique Looks of Two Moons
Each of Jupiter's more than 60 moons has its own unique
character, but scientists have often wondered at the striking differences
between the surfaces and interiors of two of the gas giant's largest moons,
Ganymede and Callisto.
A new study, detailed in the Jan. 24 issue of the journal
Nature Geoscience, might have found an explanation
for the disparate features of these Galilean moons: Ganymede
was pummeled by more and faster comets impacts than its sister moon billions of
years ago.
While Ganymede and Callisto are
similar in size and both made up of a mixture of ice and rock, data from both
the Galileo and Voyager
missions show that they sport different looks, both on the inside and outside.
But just why the two moons looked so different is a problem
that planetary scientists have been grappling with for 30 years. The solution
to the problem could shed light on how our solar system, and planets in
general, evolved.
"Similar to Earth and Venus, Ganymede and Callisto are twins, and understanding how they were born
the same and grew up to be so different is of tremendous interest to planetary
scientists," said Amy Barr of the Southwest Research Institute Planetary
Science Directorate.
Ganymede has a surface that shows evidence of resurfacing by
tectonic processes ? the same forces that continually reshape the surface of
the Earth. The moon also has a large rock/metal core, showing that its
constituent materials separated out over time, with the heavier stuff settling
to the interior of the planet (just as the iron present in Earth settled to the
core, while the lighter rocky materials floated to the surface).
The surface of Callisto, on the
other hand, shows no signs of resurfacing, and the separation of rock and ice
within it seems to be incomplete.
Barr and her colleague Robin Canup
created a model that looked at the possible role of comet impacts in the
evolution of these two moons. The model simulated the impacts and rocky core
formation and found that Ganymede and Callisto's
evolutionary paths diverged around 3.8 billion years ago, during a period in
the solar system's life called the Late
Heavy Bombardment. (The pockmarked surface of Earth's moon shows that this
period was dominated by large impacts).
In the model, Jupiter's strong gravity focuses
comets that swing into the neighborhood into the paths of Ganymede and Callisto.
When a comet impacted either moon, the mixed ice and rock
that made up the surface would have created a pool of liquid water, allowing
rock in the melt pool to sink to the moon's center.
Because Ganymede is closer to Jupiter, it was hit by twice
the number of impactors as Callisto
was. The proximity to Jupiter also meant that the comets colliding with
Ganymede were going faster than those that hit Callisto.
The model shows that if the impacts to Ganymede released
enough energy, the process of rock sinking and core formation could have become
self-sustaining.
"Impacts during this period
melted Ganymede so thoroughly and deeply that the heat could not be quickly
removed. All of
Ganymede's rock sank to its center the same way that all the chocolate chips
sink to the bottom of a melted carton of ice cream," Barr said. "Callisto received fewer impacts at lower velocities and
avoided complete melting."
These model findings help link the evolution of Jupiter's moons to the overall evolution of the solar system and the history of bombardment of Earth's own moon.
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