Location Made Twin Moons Different

A new explanation behind why Jupiter's twin moons ended up so different reveals it's all about location, location, location.


- Comet showers likely account for surprising differences between the moons, Ganymede and Callisto.

- Jupiter's gravity dealt Ganymede a heavier blow when it came incoming comets.

- The explanation offers a new way of thinking about planetary evolution.

Scientists have long wondered why Jupiter's once-twin moons, Ganymede and Callisto, came to be so different.

Ganymede, the larger of the two, has a rocky core and scars from geologic trauma spread across its face. Callisto's center is a mix of ice and rock. Its ancient surface is heavily pocked with impact craters.

What happened?

Comets, says Southwest Research Institute's Amy Barr, lead author of a paper published in this week's Nature Geoscience that shows how differences in the frequency and strength of comet impacts explains the moons' distinctive histories.

The finding on the twin moons also bolsters a new way of explaining how planetary bodies (including our own) evolved, by examining possible external factors, rather than focusing solely on intrinsic characteristics.

In other words, if there were a planetary nature vs. nurture debate, score one for nurture.

Ganymede, located about 500,000 miles closer to giant Jupiter than Callisto, bore the brunt of its parent's heavy hands. Jupiter's extreme gravity (a 150-pound person would weigh 355 pounds on Jupiter) tugged more comets toward Ganymede and caused them to crash at higher speeds than it did for Callisto.

Ganymede got so hot that its ices melted, leaving rocks to pool in the moon's core like chocolate chips at the bottom of a container of melted ice cream.

Barr and colleagues built a computer model that takes into account the differences in energy from comet impacts and came up with an explanation that neatly matches the present-day properties of Ganymede and Callisto.

Previous theories, such as tidal heating, held true only for specific periods of time or temperature conditions on Ganymede.

"The advantage of our explanation is that we don't require any fine-tuning of the model to get Ganymede to go one way and Callisto to go another," Barr told Discovery News. "What is creating the difference between the satellites is the difference between gravitational focusing at Ganymede and at Callisto.

The research adds another toehold into the idea that external factors, like comet impacts, can play significant roles in planetary evolution.

"This is really looking at external factors as the driving force for different evolution," Barr said.

The perspective has implications beyond understanding how Jupiter and the icy moons of the other outer planets evolved.

Earth and Venus, for example, were once much more alike than they are today. Venus, the second planet from the sun, grew too hot for liquid water, leading to a carbon-laden atmosphere that retains heat. Earth, 26 million miles farther from the sun, stayed cooler.

The planets also weathered the period of heavy bombardments differently, with Earth getting a moon out of the process, while Venus had its spin flipped around.

"A lot of the research on adjacent bodies are very much related to chance," said planetary scientist Mark Bullock, also of the Southwest Research Institute. "We can't qualify these yet and probably can't until we study many, many planetary systems."

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