Expert: Jayendra Upadhye - 5/11/2008

Question
Jayen,
Thank you so much for your answer explaining phase locking, etc. I'll resubmit the rating for your answer to my original question about habitable conditions on a moon. I must have screwed it up somehow.
Concerning the bulge that occurs on a satellite, does it cease to travel, then, once the satellite is in a synchronous orbit?  
Given a moon of oceans and land, would the gravitational pull of the planet have a greater influence on the tides than the moon's pull has on the planet's tides? Would that gravitational pull have an impact on the incidence and intensity of quakes or volcanoes?
And to check my understanding: the planet would always be in the sky of this moon and its day would be one revolution about the planet, but depending on the speed of the orbit and the distance from the planet, the length of that day could vary greatly. Is that about right? (I'm trying to get a detailed picture of the possibilities so I can develop the setting for a story I'm working on.)
I appreciate your input.

Thanks,
Mary  

Answer
Hi Mary,
Somehow you appear to have read my answers without even registering them as "read" on this site!
That is something very very strange indeed.
I was wondering how your question went to the pool even though i had answered it in the first place! (i saw it there after i had answered).

Funny things going on on this site i must say.

Coming to your question...
1 - Yes, once the satellite is "phase locked", the bulge gets fixed with respect to the satellite's surface, and gets localised permanently midway to the face pointing towards the main planet. (along the line joining both the bodies).
That also stops the dissipative forces due to the travelling bulge, and any volcanisms associated (prime example is Io the inner satellite of Jupiter) with such heating.

2 - Tides are "mutual" always. Meaning both bodies would face same tidal intensity. (But the main planet being more massive, the braking effect is lesser as its angular momentum is greater). Also the bulge related deformation is less in the main planet and more in the smaller planet/satellite. (Again Io is an example where its sulphur volcanoes are driven by tidal heating by jupiter).

3 - Something interesting happens in phase locked moons.
As one changed longitude, then depending on ones longitude, the "altitude" of the main planet "above the horizon" varies with longitude alone. meaning for some guys, the planet would "always" be permanently overhead, for some, always on eastern horizon, for some always on western horizon and so on. Some would never see it.

4 - Yes! depending on orbital distance, the length of day would vary greately.

5 - The "mean sea level" would vary from point to point as a function of longitude, and would be highest in the bulge region & lowest in the inter-bulge region.

6 - In a phase locked moon, the main planet ceases to cause any "damage" associated with a travelling bulge, so no moon-quakes would occur. But since the moon still has a differential surface velocity with respect to the surface of the main planet, it would trigger quakes on the main planet! (On the earth it is a known fact that the moon acts as a trigger for quakes, and many quakes occur early morning or around moon-rise/set times. (associated with travelling bulge due to sun + moon combo).

7 - Do not under-estimate the effects on cyclonic weather systems on the moon! As the sun and the main planet would both "have their say" even on air currents!

Please do rate the answer Mary!
regards
Jayen