Is there a physics major on here willing to help me with a few things?

[InvaderSquoosh]

Paul, I regret to say that I'm unemployed and bouncing between careers at the moment (programming now). I have only a B.S. in Astro--I was too scared of the tight situation for postdocs to try grad school.

 

[Alzrius]

Mars is a 10th of the size of Earth

According to what I read here ( http://ic.arc.nasa.gov/ic/projects/bayes-group/Atlas/size/Mars/Earth.html ) Mars is a little bigger than half the diamater of the Earth (and thus about twice as big as the Moon), not a mere tenth the Earth's size (which would make it, size-wise, about the size of a large asteroid). Even Pluto, the planet that is so small that many astronomers don't consider it a real planet at all, is just under one-fifth the Earth's size. See the fourth paragraph here: http://www.wired.com/news/technology/0,1282,41328,00.html

 

[tleilaxu]

Ok, I haven't the time to double check my math, but here it is:

If Mars were to orbit Earth and appear the size of our Moon (1/2 degree), Mars would be around 1.5 million kilometers from Earth compared to 380,000 kilometers for the Moon. Because Mars is pretty massive with respect to Earth, the center of mass of the system is somewhat beyond the earth's surface. I didn't have time to do the math for that part, but it was estimated from the center of mass of the Earth-Moon system being about 1600 kilometers below the surface of the earth. Ignoring the center of mass, the period of Mars' orbit around Earth is 1.8 years. Because this is longer than the Earth's year, Mars would appear to circle the sky in the opposite direction of the Moon (E to W instead of W to E).

The implications of this setup on tides is simple: Tides would occur with the same daily regularity due to the sun, but the high and low tides would occur nearly yearly instead of monthly. So the port at Procampur would experience a high tide once every two years and those conditions would persist for almost half an year!

A moon like phobos would probably not be in a stable orbit at only 10,000 km out. It would need to be at a lagrange point in the system, but then that would be boring as it would appear to be locked in step with Mars.

If you want Mars to orbit the Earth monthly, then there will be incredible consequences. Mars would be the distance of the Moon, roughly speaking, and around twice the angular size. The impact on tides would be devastating. Having Mars at 1.5 million km out is already stretching it. I haven't thought out the gravitational conseuqences of that setup in full, especially the part of having the center of mass of the system beyond the Earth's surface.

Excellent!

So the phases of the moon, New Moon, gibbous, waxing, waning etc... They are dependent on how often the moon orbits the earth? I thought they were dependent on something else? So it would take .9 years to go from new moon to full moon?

 

[InvaderSquoosh]

So the phases of the moon, New Moon, gibbous, waxing, waning etc... They are dependent on how often the moon orbits the earth?

Yep. Hmm.. let's fudge things and make it an even two years for orbit. That'll make this discussion easy.

Don't forget that the lunar eclipse phenomenon is dependant on a tilted orbit. If Moon's orbit were perfectly on the plane of the ecliptic (plane of Earth's orbit), we would never have a complete full moon, just lunar eclipses. A full moon requires the Earth's shadow to miss the Moon, which is typical of our system. So to keep things interesting, tilt the big moon's orbit. Then a lunar eclipse will become a very rare event. However, because this Mars as Moon is farther out, the Earth's shadow won't cover it entirely. What you get is a solar eclipse happening on the Mars moon! If you have seen pictures of what a total eclipse looks like from space, you'll get an idea of how the Earth's shadow would look on the Mars moon.

Another strange thing about that setup is that full moon and solar eclipse both occur at the same time of the year. You can have a lunar new year celebration or set of events that have a dual nature. Maybe something becomes good aligned on the lunar new year, then evil the next year. A lunar eclipse would signify something special.

Edit: I fixed the blue/new/lunar moon confusion.

 

[tleilaxu]

so, would such a system be possible or would they fling out of each others' orbit like dragongirl suggested?

 

[tleilaxu]

also... from what you posted it seems like you said new moons are dependent on eclipses. i thought they were just new because the sun was shining on the non-visible side?

also, how big would the earth's shadow be on mars during eclipses?

also, if the phases of the moon are dependent on how often it rotates the earth, then wouldn't pseudo-phobos go through its fazes more quickly than pseudo-mars? i thought that because the light on the moons came from the sun they would always be in the same phase, even if they rotated at different speeds.

also, how often would phobos go around the earth and how long would it take to cross the sky?

if you get sick of these questions just tell me so...

 

[InvaderSquoosh]

Well, that's a hard question. The first thing to consider is how such a system could be formed in the first place. It seems highly unlikely to occur in capture or collision scenarios. Capture makes sense only if one body is far more massive than another. Two masses within an order of magnitude might fling each other out of the solar system. Collision is too ridiculous: something that can split the Earth like that would probably pulverize it. Perhaps you can just resort to divine intervention and leave it at that.

The second consideration is the effect of other massive bodies in the solar system, particularily a Jupiter-sized object. This can easily be resolved by placing the Jupiter object far enough away that we don't have to worry about its gravitational influence.

 

[tleilaxu]

what if the mars object is a moon of the jupiter object, is hit my a massive collision which knocks it towards an orbit near the earth shaped object. over a billion years they fall in step with each other and start rotating each other...?

possible?

either that or space aliens moved it there

 

[Numion]

so, would such a system be possible or would they fling out of each others' orbit like dragongirl suggested?

Both earth and the "mars moon" orbit around their common center of mass, with their angular velocity enough to counter their gravitational pull. Think it like this: sun and earth have even more mass than the earth and mars combo, and neither is flung out of orbit.

It's all in the distance, mass and velocity.

 

[tleilaxu]

If there are any geologists or areologists on the boards, I am also interested in how a feature like the Tharsis Plateau (the huge uplift on mars that includes Olympus Mons, Pavonis Mons and Ascraeus Mons) is formed and what it would be like if it existed on a planet with a hydro and biosphere...