Yousuf Khan ) writes:
Mike Jr wrote:
"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."
http://www.nature.com/ngeo/journal/v...s/ngeo746.html

--Mike Jr.

Adding sci.astro to the list.

So why exactly would Ganymede be bombarded heavily but not Callisto?

Because it is deeper in Jupiter's gravitational well, and therefore (a)
the density of attracted impacters was greater and (b) they struck with
greater speed.

--John Park (with thanks to _Quirks & Quarks_ on CBC radio yesterday)