The differing appearance of Jupiter's two largest moons, Callisto and Ganymede, may be due to the number and speed of comets hitting them early in their evolution, say US researchers.
Ganymede and Callisto are similar in size and are made of a similar mixture of ice and rock, but data from the Galileo and Voyager spacecraft show that they look different at the surface and on the inside.
Explaining why these two moons are different has eluded scientists for more than 30 years ago.
Using computer modelling Dr Amy Barr and Dr Robin Canup of the Southwest Research Institute believe the two moons diverged in their evolutionary paths about 3.8 billion years ago, during the Late Heavy Bombardment, a phase in the solar system's history dominated by large impact events.
The research appears today in the journal Nature Geoscience
"Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed," says Barr. "All of Ganymede's rock sank to its centre the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream."
"Callisto received fewer impacts at lower velocities and avoided complete melting."
In the model, Jupiter's strong gravity focuses cometary impactors onto Ganymede and Callisto. Each impact onto Ganymede or Callisto's mixed ice and rock surface creates a pool of liquid water, allowing rock in the melt pool to sink to the moon's centre.
Ganymede is closer to Jupiter and therefore is hit by twice as many icy impactors as Callisto, and the impactors hitting Ganymede have a higher average velocity.
Modelling by Barr and Canup shows that core formation begun during the late heavy bombardment becomes energetically self-sustaining in Ganymede, but not Callisto
The study sheds new light on the 'Ganymede-Callisto dichotomy,' a classical problem in comparative planetology, a field of study that seeks to explain why solar system objects with similar characteristics have radically different appearances.
"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," says Barr.
"Our study shows that Ganymede and Callisto record the fingerprints of the early evolution of the solar system, which is very exciting and not at all expected."