# Europa and the Unified Theory **URL:** \ **Category:** Factual Questions\ **Created:** [July 11, 2001, 5:20pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110 "2001-07-11T17:20:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [July 11, 2001, 5:20pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/1 "2001-07-11T17:20:27Z")
I know this question is about as obscure as they come, but seeing as my friends are a bunch of morons (“no, Matt, Europa is not a new micro-brew”), I thought I would cast a line into the teeming sea of millions and see if I can’t turn up a physicist. I was sitting in traffic a while back and a question occured to me. I’ve always heard that Europa is only covered by ice and that there is liquid water beneath. I know all about pressure being largely the same as heat and that the moon has volcanic activity, but the prevailing theory seems to be that this water is supposedly heated by tidal forces stemming from Jupiter’s gravity. Ostensibly, the gravity and the moon’s orbit “kneads” Europa and the friction is what causes the heat. But I remember the law of conservation of energy states that this heat (i.e. energy) has to come from somewhere. Europa’s orbit is not decaying (as far as I know) and gravity is not a form of energy, per se. So where does this energy come from? Am I coming up against the fact that we don’t as of yet have a unified theory of the univerese? (as in " we know the energy DOES come from Jupiter, we just don’t know how.") ---
**Author:** ![Opengrave](https://avatars.discourse-cdn.com/v4/letter/o/919ad9/32.png) [@Opengrave](https://boards.straightdope.com/u/Opengrave)\ **Post date:** [July 11, 2001, 5:37pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/2 "2001-07-11T17:37:46Z")
I used to have a ton of links on this but they all expired. IIRC gravity causes movement of Europa’s crust and movement causes friction. See [http://galileo.jpl.nasa.gov/moons/europa.html](http://galileo.jpl.nasa.gov/moons/europa.html) Maybe The Bad Astronomer will poke his head in and provide more info. Welcome to the boards. Judging by your screen name you will fit in well here. ---
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [July 11, 2001, 5:41pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/3 "2001-07-11T17:41:11Z")
Could it be taking it from Jupiter’s rotation? i.e. slowing Jupiter down? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 11, 2001, 5:58pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/4 "2001-07-11T17:58:40Z")
I think gravity is a form of energy. It takes energy to accelerate an object. As an object (say a meteor) approaches a large body (say the earth) it accelerates. IIRC in _A Brief History of Time_ Stephen Hawking mentions that the earth is losing energy in the form of gravitational waves much like a cork loses energy bobbing up and down in water. Eventually the earth will spiral into the sun. However, if memory serves, he said the energy the earth loses in this fashion would be about enough to power an electric toaster. As a result we won’t be hitting the sun anytime soon (indeed…not till long, long, long after our sun has burned out anyway). Since Europa circles Jupiter I imagine that they give and take energy from each other. Sometime Europa is pulling Jupiter forward a bit and other times it’s dragging it back (and everything in between). I assume that this is close to a perfect balance so even if there is a discrepency in favor of one or the other it is minicule compared to their overall energy states. In the meantime all that pushing and pulling heats the surface via friction as others have said. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [July 11, 2001, 6:07pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/5 "2001-07-11T18:07:56Z")
The energy is mainly from the rotation of Europia with some comming from a non-circular orbit and still more from the orbit not being in the plane of Jupiter. Basically any change in the vector force of gravity causes mass of Europa to change orientation (and move slightly towards it.) The energy converted to heat will cause Europia to always have the same ‘face’ pointed towards Jupiter if it doesn’t already (like the moon) also remember that there will be an equal effect on Jupiter but very small. or such is my wag ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [July 11, 2001, 6:24pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/6 "2001-07-11T18:24:04Z")
People, listen to the question again: Friction causes the heat, but what causes the friction? Gravitational forces cause the friction (supposedly) but garvity is only POTENTIAL energy (Stephen Hawkings waves aside). So where is the kinetic energy input to this system? BlinkingDuck might be right about the Jupiter losing rotational motion. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [July 11, 2001, 6:38pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/7 "2001-07-11T18:38:01Z")
Point of clarification: Friction causes the heat, but what causes the friction? Gravitational forces cause the friction (supposedly) but gravity is only POTENTIAL energy (Stephen Hawkings’ waves aside). So where is the kinetic energy input to this system? BlinkingDuck might be right about Jupiter losing rotational motion. K2Dave also might be right, but that requires Europa’s orbit to be decaying. I didn’t think that was the case but I might be wrong about that. My personal guess has now become that the rotations of both objects are slowing down. This was something I hadn’t thought about when thinking of the original question. ---
**Author:** ![Adithya](https://avatars.discourse-cdn.com/v4/letter/a/e99b99/32.png) [@Adithya](https://boards.straightdope.com/u/Adithya)\ **Post date:** [July 11, 2001, 7:18pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/8 "2001-07-11T19:18:01Z")
Gravity is a force,not energy.So there has to be something else that is also responsible,unless Jupiter is cooling down,which violates thermodynamics. ---
**Author:** ![Adithya](https://avatars.discourse-cdn.com/v4/letter/a/e99b99/32.png) [@Adithya](https://boards.straightdope.com/u/Adithya)\ **Post date:** [July 11, 2001, 7:25pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/9 "2001-07-11T19:25:28Z")
Friction is also a force.So the real question is much simpler,why does friction cause heat?That’s all.Gravitational force is ‘converted’ to frictional force(it actually induces it)and friction causes heat. So why does friction cause heat?I’m at a loss for that but I’m sure there’s a simple answer because that’s what happens on Earth as well and everywhere. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [July 11, 2001, 7:30pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/10 "2001-07-11T19:30:44Z")
Here is a good starting point on tides. Note it is discussing Earth and the Moon, but the principles are applicable. [http://www.badastronomy.com/bad/misc/tides.html](http://www.badastronomy.com/bad/misc/tides.html) Essentially, tidal forces are due to uneven stretching of the object because of the distances involved. When we first do physics in beginning classes, we simplify everything to point masses. However, the real life situation doesn’t work quite so cleanly. Since the pull of gravity varies inversely with the square of the distance, the farther away an object is the less force is pulling on it. With large bodies like planets and moons, the force on one side of the moon is less than the force on the other side. This approximates as a squishing force on the body - it is getting pulled outward on the sides and flattened on the top and bottom. Now the dynamics - the bodies are moving around each other. In this case, Europa is moving around Jupiter. It is getting pulled on as it passes the other large moons (Ganymede, Io). They add differential stresses. That is where the energy is coming from. You asked about gravity not being energy. Think for a minute about what an orbit is. An orbit is a constantly changing path. What makes the moon circle instead of traveling in a straight line? Gravity. Ergo, that is energy. Yes, it is a force, but energy is merely a force acting through a distance. Look at the units. Force = Newtons = kg_m/s^2 or kg_m/s[sup]2[/sup] Energy = Joules = N_m = kg_m^2/s^2 or kg\*m[sup]2[/sup]/s[sup]2[/sup] The energy from tides is the changes in the pulls, either due to motions of Europa (spinning, elliptical orbit), or by other bodies moving closer and farther away. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [July 11, 2001, 7:34pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/11 "2001-07-11T19:34:59Z")
Adithya, force is defined as mass times acceleration. Acceleration requires energy. Therefore force requires energy. Also, gravity isn’t a “force”, per se, it’s more of a property of matter. That’s what is screwing up the unified theory. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [July 11, 2001, 7:43pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/12 "2001-07-11T19:43:14Z")
Europa’s orbit is not a perfect circle. At the point of closest approach to Jupiter (perijove) the tides distort Europa’s shape to the maximum extent. As it moves away, toward the farthest point (apojove) it relaxes a bit–the crust changes shape to be a bit more like a sphere. Then it moves in again (squash), then it moves out again (relax), and now we’re cha-chaing. The friction occurs in Europa’s crust as it changes shape. (Take a paperclip, straighten it out, and bend and unbend it–you’ll feel the bend warming up.) The energy comes out of Europa’s orbit, tending to circularlize the orbit. (I’m trying to think of an accurate, easy-to-grasp physical analogy and failing.) However, gravitational interactions with the other large moons of Jupiter prevent Europa’s orbit from losing all its eccentricity. Europa’s already tidally locked to Jupiter, so there’s no more energy to taken from Europa’s rotation. Europa does raise small tides on Jupiter, decreasing Jupiter’s rotational speed (albiet slowly). This is a separate effect. Io’s experiencing exactly the same tidal heating, so if you’re having trouble finding information about Europa, you can look under Io instead. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [July 11, 2001, 7:51pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/13 "2001-07-11T19:51:15Z")
Oooo! Oooo! Thought of an analogy! Europa starts out at perijove, lower in the gravitational potential, and moves up to apojove, higher in the gravitational potential, trading off kinetic energy for potential energy. In that way, it’s a bit like a bouncing rubber ball! As the ball gets deformed, some of its kinetic energy is transformed to heat energy through friction, so that it doesn’t bounce as high the next time. Thus Europa’s “high point,” apojove, is less high each orbit, leading to a more circular orbit for, in a circular orbit, perjove is at the same distance as apojove. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [July 11, 2001, 8:11pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/14 "2001-07-11T20:11:53Z")
Thanks, Podkayne, I think you nailed it. Thank you everyone -Beeblebrox “In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.” ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [July 11, 2001, 8:47pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/15 "2001-07-11T20:47:31Z")
> [@](#): > > \*Originally posted by \*\*\*Irishman \*\* > However, the real life situation doesn’t work quite so cleanly. Since the pull of gravity varies inversely with the square of the distance, the farther away an object is the less force is pulling on it. With large bodies like planets and moons, the force on one side of the moon is less than the force on the other side. This approximates as a squishing force on the body - it is getting pulled outward on the sides and flattened on the top and bottom. I think you have the distortion backwards. The pull of gravity is strongest at the bottom and centrifugal force is strongest at the top. Therefore, the sides are squished in and the top and bottom stretched out like a football ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [July 11, 2001, 11:13pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/16 "2001-07-11T23:13:03Z")
> [@](#): > > \*Originally posted by Irishman \* > \*\*Here is a good starting point on tides. Note it is discussing Earth and the Moon, but the principles are applicable. > [http://www.badastronomy.com/bad/misc/tides.html](http://www.badastronomy.com/bad/misc/tides.html) > > Essentially, tidal forces are due to uneven stretching of the object because of the distances involved. When we first do physics in beginning classes, we simplify everything to point masses. \*\* Did you folks read the link that **Irishman** posted? From what I understand, you would get tidal friction even in a circular orbit. As far a conservation of energy is concerned, the system is converting kinetic and gravitational potential energy to heat. BTW, the gravitational force is not energy. Some of you are mixing this up with gravitational potential energy. > [@](#): > > \*Originally posted by DrMatrix \* > \*\*The pull of gravity is strongest at the bottom and centrifugal force is strongest at the top. Therefore, the sides are squished in and the top and bottom stretched out like a football \*\* This has come up in the past. Centrifugal force does not exist. The **centripetal** force keeping Europa in its orbit **is** gravity, and it does indeed fall off with the square of the distance. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [July 11, 2001, 11:20pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/17 "2001-07-11T23:20:28Z")
> [@](#): > > \*Originally posted by robby \* > **Centrifugal force does not exist.** Well…I’ll throw in a caveat. Centrifugal force does not exist in a Newtonian (inertial) frame of reference, which is the frame of reference most people are thinking of when they devise explanations for physical phenomena. That’s why it’s often called a “fictitious” force. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [July 12, 2001, 5:21pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/18 "2001-07-12T17:21:35Z")
> [@](#): > > \*Originally posted by \*\*\*robby \*\* > From what I understand, you would get tidal friction even in a circular orbit. You would get a constant tidal force in a circular orbit. The body would adjust its shape to the force and there would be no friction. In a non-circular orbit, the tidal force is maximum at the closest approach and decreases with distance. It is the variation that causes friction > [@](#): > > This has come up in the past. Centrifugal force does not exist. The **centripetal** force keeping Europa in its orbit **is** gravity, and it does indeed fall off with the square of the distance. Sigh. When I posted, I just _knew_ somebody was going to say that. Centripetal force is the force keeping Europa in orbit. It is gravity. Centrifugal force is the force that keeps it from falling. It is inertia. I’ve never understood why it is called non-existent or “fictitious” when it _is_ felt by the body. Why would it not exist for a rotating body in an inertial frame? ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [July 12, 2001, 5:43pm UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/19 "2001-07-12T17:43:26Z")
You _can_ have tidal friction in a circular orbit as long as the body is not tidally locked. Now, if all the little children in the world will clap their hands, the centrifugal force won’t die! (Centrifugal forces can be confusing sometimes, but in this case I think that thinking of tides in the co-rotating frame, including the centrifugal force, is helpful.) ---
**Author:** ![JS\_Princeton](https://avatars.discourse-cdn.com/v4/letter/j/57b2e6/32.png) [@JS\_Princeton](https://boards.straightdope.com/u/JS_Princeton)\ **Post date:** [July 13, 2001, 3:23am UTC](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110/20 "2001-07-13T03:23:37Z")
Here’s the deal. You have a body with non-trivial size in a gravitational field. That means there is a gradient in the field. The gradient is what we define as force. What the President of the Galaxy is asking really is what is the couteracting force that allows for conservation of energy. (Newton’s Third). By the way, this has nothing to do with field dynamics. Let’s not jump to GUT conclusions when one isn’t necessary. This is what we have… The tidal force is there regardless of orbit. Why? Well, detailed mechanics of the thing is difficult to transcribe in a thread=reply so I refuse to do it. Let’s just say what happens is that the tidal force is a force that acts superficially, that is we’ll ignore the deeper shells of our Europa for now. The gravity that is felt by a particular point on this surface which deals with the gravity of it’s parent body + the gravity of the body it’s orbitting is the tidal force in actuality. Surprising, but there are aspects of the gravity of the parent body that come into play with tidal forces. Well, in response to a force, something must change (acceleration) to get to a more stable potential. The acceleartion begins, pulling tidally (both toward and away the body, strangely enough) until the impulse is satiated. But then it is converted into potential energy as it is pulled away from the parent body. This can’t be good, so then we do the switcheroo again and back down the tides go. Only, we gotta look at the whole picture… not only are the tides moving, but Europa is too! What? You say. Yep, I say, slightly, the center of mass of Europa is pulled toward the tidal budge (in effect toward Jupiter). Yeppers, tidal forces cause rotational momentum spin-downs (or more specifically here, rotate-downs) of things when the potential energy is converted into a nonconservative thing macroscopically (of course if we could see every molectule in the entire system, ang. momentum would be finely conserved, but don’t expect me to do that, just pretend it’s non-conservative for now and leave it at that). Since Europa is now lower in the potential field of Jupiter and still experiencing tidal forces (albeit different ones… we can get into the nitty-gritty math, but that’s really not neccessary, I don’t think. Get a classical mechanics book and find the problem for yourself and then look at the slight changes that occur if the thing experiencing tidal strain is nudged slightly toward the center of mass of the system… as truly it is, but only slightly.) Point of this is Jupiter’s mass is SOO big that the energy from a tiny change in position down the potential is a tremendous gain. Enough to keep some heat down there. Yeppers. That’s it. [Next page](https://boards.straightdope.com/t/europa-and-the-unified-theory/72110.md?page=2)