Callisto, Ganymede & Comets

Jupiter's second largest moon, Callisto

Four hundred years ago to the month, Galileo trained his telescope on the planet Jupiter and discovered the four largest moons of the solar system’s largest planet.

The very great differences between the two largest of those moons have puzzled scientists for decades. Ganymede, the largest moon in the solar system, and Callisto, are nearly equivalent in size, but only Ganymede has a magnetic field, a highly differentiated composition, and evidence of past tectonics. Now, a pair of scientists from the Southwest Research Institute have proposed in the journal Nature Geoscience that the contrast can be explained by the differential effects of cometary bombardment. (Here is the abstract of their article.)

Here’s how Scientific American explains their thesis in the article Early Cometary Bombardment May Explain the Divergent Path of Jupiter’s Biggest Moons:

“Each time a comet strikes an icy satellite, Barr explains, a portion of the moon’s surface melts from the heat of the impact; the heavier metallic and rocky constituents mixed in sink to the bottom of the melt pool. With enough impacts providing sufficient melting, the sinking rocks’ gravitational potential energy is released as heat, producing more melting, and the separation of rock and ice becomes self-sustaining, a process known as ‘runaway differentiation.’

“During the solar system’s period of intense impacts about 3.8 billion years ago known as the late heavy bombardment, tremendous amounts of cometary material would have been flying around Jupiter and the outer gas-giant planets. Barr and Canup estimate that Ganymede’s proximity to Jupiter, the latter of which acts as something of a gravitational sink, led to Ganymede’s experiencing double the impacts of Callisto, and at higher velocities, to boot. ‘Ganymede gets 3.5 times as much energy in the late heavy bombardment as Callisto,’ Barr says. That energy differential, Barr and Canup realized, could account for Ganymede’s much more complete state of differentiation—the so-called Ganymede–Callisto dichotomy.”

Visit the article in Scientific American, or see the original (subscription required) in Nature Geoscience for greater detail.

Jupiter's largest moon, Ganymede

Published in: on January 27, 2010 at 4:46 pm  Leave a Comment  

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