Comets “Melted” Jupiter’s Biggest Moon

If Earth’s moon is made of green cheese, Jupiter‘s biggest moon is made of refrozen ice cream.


False-color view of Ganymede — mmmmm, planetary Drumstick!

—Image courtesy NASA/JPL/DLR

According to a new study in Nature Geoscience, the Jovian moon Ganymede used to be similar in structure to its neighbor Callisto.

But then, about 3.8 billion years ago, both moons began getting pummeled by comets sent in their direction by Jupiter’s gravity.

Jupiter today is thought to act like a sort of gravitational shield for the inner planets, flinging space rocks that fly too close out of their normal orbits.

It’s likely the gas giant did something similar during the ancient cometary stampede, but there were so many freakin’ comets that scads of the were sent screaming toward the nearest Jovian moons.

When a comet hit Ganymede or Callisto, a little bit of either moon’s rock-ice crust would melt, and rock would sink toward the bottom of the meltwater pool.


Incoming! Fragments from comet 73P/Schwassman-Wachmann 3

—Image courtesy NASA/JPL-Caltech

Since Ganymede is the closer to Jupiter of the pair, it got hit by twice as many comets, and those incoming bodies were hurting toward the moon much faster than those slamming into Callisto, the new study says.

The onslaught locked Ganymede into a period of near-constant melting, according to study co-author Amy Barr, of the Southwest Research Institute.


False-color view of Callisto

—Image courtesy NASA/JPL/DLR

“Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed,” Barr told

“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.”

Callisto, meanwhile, bore less of the brunt and was therefore mush less melty.

The study helps explain why Ganymede and Callisto—like Earth and Venus—started out so similar but grew up so different.

Ganymede shows evidence of tectonic action like Earth’s, and it seems to have a clear separation between lighter material at the surface and heavier stuff in the core.

Callisto, by contrast, shows no signs of tectonics and is much more of a jumble on the inside.


The theoretical insides of Ganymede (left) and Callisto

—Image courtesy NASA/JPL

Examples of different worlds with similar origins excite planetary scientists for the same reasons that geneticists love to study twins: the “nature vs. nurture” debate.

How much of what makes a world the way it is was influenced by its early development? And what can that tell us about how young worlds in general react to various, uh, impacts?

On a grander scale, the findings add to proof for the so-called Late Heavy Bombardment, a period of a few hundred million years or so when swarms of large impactors are thought to have gone flying through the solar system.

The Late Heavy Bombardment has been linked to heavy cratering on the moon, as well as to theories that asteroids shut down Mars’s magnetic field and icy comets delivered Earth’s oceans.

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