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MarsSentinel
2016-Jul-25, 09:05 PM
HI all, I suppose this is obvious to the pro's, but I can't figure it out on my own.

SO, Venus is slowing down. (link to article). The story is that it is due to friction with a super dense atmosphere. Wind on mountains and such.

OK, but that makes me ask myself this: is the atmosphere not rotating with the planet?

If I stand next to a merry-go-round and reach out to grab it to slow it down (one of those playground merry-go-rounds) unless my feet are on the ground I cannot slow it down. The friction with my hand is transferred as momentum to my body which is overcome by friction against the ground by my feet/mass.

So if the atmosphere of Venus is slowing the planets rotation, what "ground" is the atmosphere's feet touching to absorb the energy required to slow the planet?

Seems to me that the more likely explanation would be magnetic friction between Venus and the sun and that the energy of slowing would be turned to heat somewhere (inside Venus?).

Also, whatever the answer is, how do you know?

Thanks

Noclevername
2016-Jul-26, 08:48 AM
Venus' atmosphere, driven by heat, circumnavigates the planet every 4 Earth days, retrograde. Because of the atmosphere's mass, this drags the rest of the planet around with it against Venus surface. Venus has no natural magnetic field and a very weak atmospheric magnetosphere.

https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation

grapes
2016-Jul-26, 09:50 AM
Did you intend to link to an article, MarsSentinel?

Venus' atmosphere, driven by heat, circumnavigates the planet every 4 Earth days, retrograde. Because of the atmosphere's mass, this drags the rest of the planet around with it against Venus surface. Venus has no natural magnetic field and a very weak atmospheric magnetosphere.

https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation
Venus rotation is also retrograde, so that wiki explains that the atmosphere is super-rotating?

Solfe
2016-Jul-26, 12:01 PM
Venus does a lot of odd things:

the retrograde rotation,
the relatively fast rotation of the atmosphere,
observation of the surface is sort of new to humans, perhaps 50-60 years (1961, I think**)
it seems to have flipped over* at some point in the past,
its more spherical than other planets because of the slow rotation,
it's year is two solar days on the surface (not that you'd want to go there),

Anyway, I think the slowing is due to a tidal locking effect with the sun. Venus had a few odd days in it's past, I can't even image how it got that retrograde spin, and those days are catching up with it.

*Not a technical term.
** (edit) I think I'm wrong, I want to say this was the late 60's or early 70's. Which leads to another oddball set of facts - Maxwell Montes, Alpha and Beta Regio aren't named for women, like every other discovered feature of Venus.

George
2016-Jul-26, 10:23 PM
Though small, it may be worth noting that Earth's air mass movements cause our days to vary slightly from day to day, but it mostly averages out, IIRC.

Grant Hatch
2016-Jul-30, 06:32 AM
Seems to me that Venus has seen relatively recent violent orbital disturbances which we see echoes of today....

dtilque
2016-Jul-30, 07:34 AM
As I understand it, the massive atmosphere of Venus is why it has a retrograde rotation. Something called thermal tide, I believe. Otherwise it'd already be tidally locked with the sun.

Noclevername
2016-Jul-30, 07:41 AM
Seems to me that Venus has seen relatively recent violent orbital disturbances which we see echoes of today....

Evidence?

cjameshuff
2016-Jul-30, 12:54 PM
As I understand it, the massive atmosphere of Venus is why it has a retrograde rotation. Something called thermal tide, I believe. Otherwise it'd already be tidally locked with the sun.

This is modeled here:
https://www.imcce.fr/fr/presentation/equipes/ASD/preprints/prep.2002/venus1.2002.pdf

Though I've never dug into it enough to work out how the torque is ultimately applied. Angular momentum has to be exchanged with something, but how? The YORP effect seems inadequate for anything larger than a small asteroid.

publiusr
2016-Jul-30, 05:04 PM
I thought I read hear about its molten interior having something to do with the spin over time
http://cosmoquest.org/forum/showthread.php?67782-Why-does-Venus-have-no-(or-virtually-no)-magnetic-field

Drat that wasn't it--but post 22 has this quote:

"Also it is thought, by some, that the slow rotation of Venus might be the result of a similar collision."