# Gravity on Io **URL:** \ **Category:** Factual Questions\ **Created:** [August 9, 2019, 7:54pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425 "2019-08-09T19:54:18Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [August 10, 2019, 4:36pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/21 "2019-08-10T16:36:26Z")
> [@Chronos](#): > > I think that it’d be the other way around: The equipotential surfaces are slightly further apart on the near side, so the field should be slightly weaker, so you should weigh less on the near side. > > But, again, it’d be a tiny effect. I think we are in agreement, assuming that near side refers to the side facing Jupiter. By tiny, I’m not even sure what order of effect it would be, as it would be coming from the air mass that is raised by Io causing tides on Jupiter. But then, not so tiny, in that it is that imbalance that keeps Io from reaching a point of equilibrium and causing volcanism. Even though it would be an inverse-n[sup]th[/sup] power effect, that effect is what provides the energy to stretch Io back and forth and keep it hot enough for all the volcanoes. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [August 10, 2019, 4:42pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/22 "2019-08-10T16:42:59Z")
I think if it was just Io and Jupiter the vulcanism would be considerably less. Io’s orbit would rapidly circularize and the tidal effects on Io would become constant. In the current situation, Io’s orbit is eccentric (due to the effects of the other big moons), leading to varying tidal effects - and the variable squeezing is what drives the volcanoes (I think) ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [August 10, 2019, 4:50pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/23 "2019-08-10T16:50:35Z")
> [@Andy\_L](#): > > I think if it was just Io and Jupiter the vulcanism would be considerably less. Io’s orbit would rapidly circularize and the tidal effects on Io would become constant. In the current situation, Io’s orbit is eccentric (due to the effects of the other big moons), leading to varying tidal effects - and the variable squeezing is what drives the volcanoes (I think) Yeah, the whole thing’s a big engine widening the orbit of all the moons, and poor Io is the transmission. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 10, 2019, 5:39pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/24 "2019-08-10T17:39:32Z")
Even if both Jupiter and Io were both perfectly rigid, you’d still see some slight difference in weight on the two sides of Io, due to those high-order effects in the shapes of the equipotential. Though, yes, we are in agreement on the sign of the effect, and I’m not sure how I misread that. ---
**Author:** ![Hypno-Toad](https://avatars.discourse-cdn.com/v4/letter/h/b2d939/32.png) [@Hypno-Toad](https://boards.straightdope.com/u/Hypno-Toad)\ **Post date:** [August 16, 2019, 2:36pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/25 "2019-08-16T14:36:50Z")
So there’s an effect, but too small to really make a difference to an observer? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [August 16, 2019, 2:58pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/26 "2019-08-16T14:58:02Z")
As shown in that fine documentary, **Io** (1981, with Sean Conery and Peter Boyle), on Io blood drips up. And in the main settled part, they have earth-normal gravity. Scientists have struggled to explain this, but filmmakers just _know_. I tried to find clips of this, but a quick search doesn’t turn up anything. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [August 16, 2019, 3:00pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/27 "2019-08-16T15:00:20Z")
> [@Hypno-Toad](#): > > So there’s an effect, but too small to really make a difference to an observer? Over a long time, an observer would see Jupiter slowing down and the orbits widening, but yeah, to an observer with human like perceptions and human like time scales, there is no difference. ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [August 16, 2019, 3:08pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/28 "2019-08-16T15:08:57Z")
> [@CalMeacham](#): > > As shown in that fine documentary, **Io** (1981, with Sean Conery and Peter Boyle), on Io blood drips up. And in the main settled part, they have earth-normal gravity. Scientists have struggled to explain this, but filmmakers just _know_. > > I tried to find clips of this, but a quick search doesn’t turn up anything. “Outland”, I think. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [August 16, 2019, 3:59pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/29 "2019-08-16T15:59:28Z")
> [@Folly](#): > > “Outland”, I think. Sorry – meant to type “Outland” but it came out “Io” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 16, 2019, 4:18pm UTC](https://boards.straightdope.com/t/gravity-on-io/838425/30 "2019-08-16T16:18:20Z")
Obviously, gravity was pulling your fingers towards the wrong keys. [Previous page](https://boards.straightdope.com/t/gravity-on-io/838425.md?page=1)