Be­cause it is squeezed and stretched by grav­ity, the body in the So­lar Sys­tem that JHQHUDWHV KHDW PRVW HI FLHQWO\ LV not the Sun but Jupiter’s moon, Io

Sky at Night Magazine - - MYSTERIOUS GRAVITY -

On 8 March 1979, NASA’s Voy­ager 1 space probe, hav­ing streaked through the Jupiter sys­tem faster than a speed­ing bul­let, turned its cam­era back the way it had come. Nav­i­ga­tion en­gi­neer Linda Mora­bito be­came the first per­son to see, sil­hou­et­ted against the black of space, the su­per-vol­ca­noes (strictly speak­ing, gey­sers) of the moon Io. The most ge­o­log­i­cally ac­tive body in the So­lar Sys­tem is heated by nearby moons Europa and Ganymede, and Jupiter. The moons tug its or­bit into an elon­gated el­lipse. As it moves close to and far away from Jupiter, the grav­i­ta­tional stretch­ing ef­fect of the gi­ant planet changes. Io is heated for ex­actly the rea­son a squash ball gets hot when squeezed re­peat­edly in your hand.

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