Peter Goldstein wrote:

>This one has been the subject of some speculation.  The four-note phrase
>of the Jupiter is something like a motto for Mozart: it appears in a number
>of his works, going all the way back to the Symphony No. 1, and is treated
>contrapuntally several times.  It's been suggested that he first saw
>Haydn's 13th Symphony as a boy while in Paris.

Neal Zaslaw suggests that this four-note motto is based on or an actual
transcription of the beginning of a Gregorian chant, but I've never seen
it indicated exactly which chant is being referred to.

Mark K. Ehlert