FRED AN­SWERS YOUR QUES­TIONS

Australian Geographic - - Geobuzz -

Q: In Di­a­monds are a world’s best friend (AG 145), you say plan­ets were formed by elec­tro­static at­trac­tion of dust and sub­se­quent col­li­sions of pro­to­plan­ets. If so, how are the lay­ers and molten core of Earth ex­plained? Ar­nav Mathur, Concord, NSW

A: I should have clar­i­fied: elec­tro­static at­trac­tion just starts off the process; the main agent of planet for­ma­tion is grav­ity. As the mass of a pro­to­planet in­creases, so does its tem­per­a­ture, due to the en­ergy of the col­lid­ing ob­jects. As it be­comes molten, heav­ier ma­te­ri­als, such as iron, sink to the cen­tre due to their own grav­ity. On Earth, resid­ual heat (as­sisted by nu­clear fis­sion) keeps the outer core molten, while the in­ner core is solid due to the enor­mous pres­sure it is un­der.

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